Making an Fast Body by Minimizing the Volume of Hydrodynamic and Aerodynamic Elements

Tom Augenstein, Mechanical Engineering Class of 2017 Sara Gregg, Mechanical Engineering Class of 2016 Jacob Langenbacher, Mechanical Engineering Class 2016

Biorobotics and Locomotion Laboratory Cornell University United States May 14, 2016 Abstract

Our research is to design and manufacture a small, cheap, and mass-producible autonomous robotic sailboat that can intelligently navigate the ocean and collect data. The project is meant to decrease the cost of environmental research by designing a research vessel that can cheaply collect data such as salinity, turbidity, fluorescence, or acoustic patterns. Our boat has a symmetrical airfoil sail, a thin, bulbed , and a new and unique control surface: a controlled tail fixed to the trailing edge of a controlled sail. Our simulations suggest that this set-up has directional stability. Directional stability is a boat’s ability to maintain any stable heading without electrical or mechanical actuation. Directional stability will help our smaller sailboat intelligently navigate high water current environments such as the ocean. This is important because smaller like ours are slow and more susceptible to motion dominated by these currents. This report documents the design and fabrication processes of the ”body” of sailboat, addresses the discovered shortcomings of our designs, and discusses our plans to improve. The body of the boat is everything that keeps the boat afloat and upright, like the hull, , and keel. The body also includes elements inside the body such as the controlling system. The primary objective of our designs was to reduce drag on each body element while preserving each components role. For the hull and deck, this means eliminating as much volume above and below the as possible to reduce aerodynamic and hydrodynamic drag, respectively. We accomplished this by scaling an existing racing yacht hull shape to a one-meter . With the new, small shape, we eliminated the majority of the hull’s volume above the waterline, leaving us with a very thin and narrow hull with very little exposed volume above or below the waterline. The keel is very thin and deep to reduce drag with a heavy ballast at its deepest point to maximize its righting moment. The keel tapers as it moves towards the ballast, with a longest chord near the hull, to keep the side force close to the hull and reduce unwanted heeling moments. Preliminary testing indicates that our designs should be improved to increase the pitching and heeling stability of our sailboat in high winds. During testing, we found that higher winds sometimes result in unwanted behavior such as oscillatory motion and the boat finding a stable equilibrium position with the tail upwind to the sail. We plan to improve our sailboat’s high wind behavior by implementing one or more of the following elements: Heavier ballast, outer rigging, active keel to increase righting moments, and an active mast to keep the sail’s driving force above the keel’s center of pressure during heeling. In addition, in high wind situations, the sailboat will nosedive while sailing downwind. This will likely be remedied by elongating and sharpening the prow (the above-water part of the ), similar to a viking , to more effectively cut through the water and stay afloat. Contents

1 Introduction 2

2 Selecting the Shape of Our Boat Components 2 2.1 Selecting our Hull Shape ...... 2 2.2 Designing with our Hull Shape ...... 4 2.3 Mast Control Design ...... 7 2.4 The Deck ...... 10 2.5 Mass Projection ...... 11 2.6 Designing the Keel Shape ...... 11 2.6.1 Theory ...... 11 2.6.2 Design Specifics ...... 13

3 Fabrication of Our Boat Components 17 3.1 Fabrication of Our Sailboat Hull/Deck ...... 17 3.2 Fabrication of Our Keel ...... 20 3.3 Fabrication of Our Mast Control ...... 21

4 Analysis 22

5 Future Work 24

6 Conclusions 25

7 Appendices 26 7.1 Appendix I: Detailed Hull Fabrication Process ...... 26 7.2 Appendix II: Detailed Keel Fabrication Process ...... 30 7.3 Appendix III: Epoxying Procedure ...... 31 7.4 Appendix IV: Vacuum Bagging ...... 31 7.5 Appendix V: Wind Vane ...... 32

1 1 Introduction

It is logical to think of a sailboat as a sideways airplane moving in two different fluids. Both vehicles are governed by the same by physical laws, harness similar lift forces, and fight similar drag forces. This thought experiment allows us to imagine a “physically-optimized” sailboat, or a sailboat with only lift forces and no drag forces. A truly optimized sailboat has a thin, near-infinitely deep keel with small point mass at its end. The sail is the same, very thin and reaching very high. The last source of drag on our perfect sailboat to eliminate is that on the hull moving through the water. Because a hull does not provide a lift force, our perfect boat’s hull is nonexistent (assuming that the sail and keel are massless). A non-existent hull is not feasible, but a long, narrow, and thin hull is feasible and a reasonable substitute because it minimizes the drag forces from both the air and water. Our hull is one meter long, narrow, and very flat, with only three-quarters of an inch vertically above the waterline. The shape is meant to be fast and minimize both air and water drag while maintaining rolling and pitching stability, supporting our mass budget, and providing space for an effective mast motor control system. The keel is very thin, starting with a long chord near the hull and tapering to a very small chord at its end. There is a torpedo shaped ballast fixed to the bottom. Both the hull and the keel were manufactured using a smoothed foam surface shaped into our desired shape. For the hull, the smooth surface was used to take both negative and positive molds with fiberglass.

2 Selecting the Shape of Our Boat Components

2.1 Selecting our Hull Shape

Before choosing the curvature of our sailboat, we first had to define what we considered to be a “good” shape. We define a good hull as fast, stable, and supportive of our mass budget. We chose an existing hull shape and scaled the shape down to one meter long instead of designing our own due to time constraints. A fast boat hull is typically long and very narrow. Also, fast hulls typically have very little volume below the waterline to further decrease drag. A stable hull has a low to ratio i.e. small depth compared to width.

Seeking a narrow and shallow hull barely filtered our pool of potential hulls because there are an uncountable number of existing monohulls, , and that satisfy our criteria. Our mass budget drastically reduced the number of useful hulls. We expected that our sailboat would weigh approximately five kilograms, so we chose a hull with a long, narrow shape that, when scaled to our sizes, could support our mass budget. It is important to note that “supporting” a mass budget is different than floating with a given mass. Professionally made sailboats have a “” characteristic, measured in units of mass. This displacement value is the mass of the water that the hull displaces when it is sailing under normal circumstances (expected number of passengers, constant velocity, no tipping, etc.). In other words, it is the mass that the boat was designed to sail with. However, when scaling a hull’s length by some factor, according to Archimedes’ Principle, the displacement scales by the cube of the same factor. This relationship is shown mathematically below. In the following equations, we assume a boat hull is a perfectly submerged rectangle, with its surface flush with the waterline. In addition, L is the length of the rectangle, W is width, H is height,

2 and C is an arbitrary scale factor. ρwater is the density of water, g is the gravitational acceleration, and V is the hull’s volume.

Lnew = C ∗ L (1)

Wnew = C ∗ W (2)

Hnew = C ∗ H (3)

Displacement = ρwater ∗ g ∗ V = ρwater ∗ g ∗ (L ∗ W ∗ H) (4)

Displacementnew = ρwater ∗ g ∗ (Lnew ∗ Wnew ∗ Hnew) = ρwater ∗ g ∗ (C ∗ L ∗ C ∗ W ∗ C ∗ H) (5)

3 Displacementnew = Displacement ∗ C (6)

Many modern racing sailboats, when scaled down, have mass budgets too small to fit our needs. For example, The Comanche, one of the fastest racing yachts in the world, can reach up to 40 knots (46 mph). The Comanche has an overall length of 100 ft and displacement of 35 metric tons [1]. When scaled down to a meter length, the Comanche has a resulting displacement of only 1.23 kilograms. This drastic drop in mass budget compared to size removed all racing trimarans and catamarans, or at least the ones we could find. It also eliminated most modern racing yacht hulls, typically possessing a scaled displacement between one and two kilograms. The small displacement of these hulls arises from their narrow, flat shape, which displaces very little water compared to passenger yachts of comparable sizes.

We decided that the Luja, a modern racing yacht by Sparkman & Stephens, is a suitable fit. An image of Luja’s hull line plans are shown below.

Figure 1: Line Plans of the Luja

3 The hull is long, narrow, and has flatter curvature below the waterline. This hull design is notably older than the other hulls we initially looked at (The Luja debuted in 1929 while the the Comanche was debuted in 2015). Older racing yacht hulls are better for our needs because they are typically much wider than more modern models. Although this wider shape makes them slower, it increases their length to displacement ratio, giving us a higher displacement when scaled down. With a one meter waterline length, the Luja has a displacement of six kilograms.

2.2 Designing with our Hull Shape

We scaled the Luja by its waterline length instead of its overall length to maximize our mass budget. We then removed most of the hull’s volume above the waterline. We did this because we are only concerned with the hull’s curvature below the waterline. Volume above the waterline is a source of air drag and has no influence on the hull’s hydrodynamic performance. In addition, because our sailboat is autonomous, is unnecessary because we do not need large clearance to protect passengers. We only use the portion of the hull between its lowest point and, after scaling, three-quarters of an inch above the waterline. The three-quarters of an inch above the waterline provides an ample safety factor for our hull’s performance during heeling.

This hull shape is the design foundation of our boat, from which all other aspects of the boat are designed around. The keel slides into a slot in the bottom surface of the hull, shown below.

Figure 2: Keel Block with End Plates to Support Keel, Cross Sectional View Shown in Lower Bubble

The keel support is located at the center of mass of the hull so the boat floats correctly. The slotted piece of aluminum is called the “keel block”. The segment of the keel which enters the hull fixes into the keel block. The keel is fixed by two bolts threaded vertically into the top of the keel. The access

4 points for the bolts are shown in Figure 2 by the two small holes in the top of the block. The keel is constrained with two end plates adhered at either end of the keel block.

The keel block is positioned in the hull by both the small slot in the hull, through which the keel enters, and the lower deck. The lower deck is shown below.

Figure 3: The lower deck and how it supports the keel block

The lower deck is the floor of the hull with a slot cut into it to secure the keel block. The lower deck’s main purpose is to provide support to the keel block and distribute the stress from the large torque of the keel throughout the hull. The lower deck is flush with the top of the keel block to maximize this support. The lower deck is held along the inner surface of the hull with fiberglass and an epoxy- colloidal silica mixture. Colloidal silica is fiberglass ground into a fine powder and is typically used to either provide additional structural strength to an epoxy or polyester adhesive or a waterproof filler in marine environments.

5 An aluminum ring is bolted to the top of the keel block, shown below.

Figure 4: Mast Supporting Ring Attached to the Keel

The center of the aluminum ring and the center of lift of the keel are vertically aligned. The aluminum ring serves two purposes: Locate and support the mast. The mast of our boat is vertically aligned with the sail’s center of pressure. By using the keel block’s ring to locate the mast, we align the center of pressure of the sail with the center of pressure of the keel. This alignment eliminates any unwanted yaw moments that our electrical systems would be constantly fighting to stay on path. In addition, the ring provides a reaction torque braced by the lower deck. The ring’s support provides relief to the motor control system, which vertically aligns and supports the mast.

6 2.3 Mast Control Design

The mast control system is shown in Figure 5, below:

Figure 5: The mast control servo is the black block on the left of the image (A), and the mast (B) is the long vertical piece on the right side of the image, linked to the servo by a chain and sprocket system (C).

The mast control servo (a HiTec HS-785HB Servo) rotates a sprocket that drives a chain. The chain connects the servo sprocket to a second sprocket on the mast. To contain the mast control system, we designed a housing made of multiple pieces. Two hatches fit into the top and bottom of the mast control housing. Four holes at the corners of the hatches align with the four bolts at the corners of the sail control housing, shown in Figure 6, below:

Figure 6: Mast control housing

7 We insert the hatches of the mast control assembly into the boat as shown in Figure 7, below.

Figure 7: Inserting hatches into the mast control housing (A is the upper hatch, B is the lower hatch)

We use the bolts shown in Figure 5, to align the hatches. We place an O-ring under the lip of the lower hatch (B in Figure 7) for waterproofing. The upper hatch (A in Figure 7) is a clear 0.25” acrylic sheet. We can visually inspect the mast control system through the acrylic upper hatch without disassembling the mast control housing.

The mast extends through both hatches, as shown in Figure 8,below.

Figure 8: The mast extend through the upper and lower hatches. A flanged ball bearing (A) and a nylon bushing (B) the mast in place in the mast control assembly

A flanged ball bearing mounted (A in Figure 8) in the upper hatch of the mast control housing supports the mast. A nylon bushing (B in Figure 8) mounted in the lower hatch keeps the mast aligned perpendicular to the hatches.

8 We cut a hole in the lower hatch to fit the mast control servo. The servo sits on a thin aluminum plate, the design of which is shown in Figure 9, below:

Figure 9: Servo plate part drawing, thickness of plate is 0.065 inches

9 2.4 The Deck

The final version of our constructed deck is shown below.

Figure 10: The Deck. Labelled (1) is the GPS, IMU, and battery housing, (2) is the motor control housing, and (3) is the central electrical system housing

The shape of the deck is convex, or bowed outward, to provide extra space within the boat for elec- tronics and the motor control system. The deck has three compartments. (1) houses the inertial measurement unit, GPS sensor, and battery, (2) is the motor control system, and (3) the central electronics . The windvane, equipped with a rotary encoder to read wind direction, sits at the bow. The sensors and electronics compartments are waterproof boxes made of polycarbonate plastic and adhered to the slots in the deck by colloidal silica mixed with vinyl ester. Vinyl ester is an adhesive used in composite work, similar to epoxy, to make composite fibers rigid. It is weaker than epoxy, but was used to adhere the waterproof boxes because the deck is also made from vinyl ester. The motor control compartment is an assembly “ cut” plywood pieces and then adhered together using epoxy. Laser cutting is a form of rapid prototyping in which two-dimensional shapes are cut from wood, plastic, or metal (depending on the type of cutter). The assembled wooden compartment was then adhered to the deck with colloidal silica and vinyl ester.

To waterproof the connection of the hull and the deck, we adhered a shelf along the inside of the hull along the top edge. We then adhered an identical one along the bottom edge on the inner surface of

10 the deck. These two shelves are joined by silicon caulk to waterproof the connection. The silicon caulk does not provide structure to the connection because it is very flexible, even when fully cured. The structure of the connection is provided by the mast-keel block connection and the waterproof boxes, which are fixed to the deck and connect snuggly within the hull.

2.5 Mass Projection

We project that the hull and deck will weigh 3 kg total after fabrication. This mass is the sum of three layers of 7.5 oz fiberglass at a 50-50 ratio with vinyl ester (1.02 g/mL) [3] and the surface areas of the hull and deck found with the CAD representation. Including the sail and the keel, we expect the total mass of the assembled boat to be 5.1 kg, giving us approximately a 1 kg buffer zone between our assembly mass and our hull’s designed displacement rating.

2.6 Designing the Keel Shape

2.6.1 Theory

The purpose of a keel is twofold: to provide stability and to prevent sideways motion. The keel stabilizes the boat, preventing rolling by adding a large mass to the system far below the waterline. This large weight keeps the boat’s overall center of mass low. Boat stability is governed by the position of the center of mass, center of buoyancy, and metacenter. The effect of boat heeling, or rotating about its longitudinal axis, is shown in Figure 11. As the boat heels, the center of buoyancy and center of gravity no longer align, producing a moment about the metacenter based on the the lever arm created between them. This is called the righting moment, and returns the boat back to its stable equilibrium, or the upright position with a heel angle of zero. The metacenter is located along the boat’s vertical centerline, at a position exactly defined by the heeling angle. The heeling angle is the angle between vertical and the boat’s vertical centerline. A center of mass as low as possible is important, to increase the lever arm about the metacenter to create the righting moment. Therefore, optimum stability is achieved when the weight of the keel is designed as far from the hull as constraints allow.

11 Figure 11: Force diagram of contributing factors to the righting moment. The hull buoyant force originates from the center of buoyancy, which isn’t directly labeled. The metacenter is located along the vertical center-line of the boat defined exactly by the heeling angle. The segment d represents the righting lever arm. The righting moment is created by the action of the weight and hull buoyancy on this lever arm to right the boat. When the boat is perfectly upright, the forces are in line, so there is no moment produced and the boat is in stable equilibrium. Since the keel and sail side force contribute to the moment perturbing the boat in the first place, designing the keel side force to act as close to the hull as possible will shorten the lever arm, lowering the destabilizing moment. [Michalak, Jim. ”Jim Michalak’s Boat Designs” http://www.jimsboats.com/15nov12.htm]

Additionally, the keel counteracts the sideways forces generated by the sail. The keel is shaped as an airfoil and acts similarly to the sail, only it uses the water’s relative velocity and direction to produce lift and drag. Since water currents in lakes are negligible compared to wind speeds, the speed of the water is based on the forces on the sail due to wind. As a boat initially at rest begins to move due to the forces on the sail, the keel comes into play as the it develops a velocity with respect to the water. Figure 12 shows the forces of the sail and keel, and the resulting velocity of the boat. The sideways wind force creates an angle of attack between the water and keel, leading to a pressure differential between the sides of the keel. This creates lift and drag forces, which act opposite to the sideways wind forces, pushing the boat back towards the wind.

12 Figure 12: The force generated by the keel to balance the sail creates a force for- ward with a few degrees of leeway. [Gladstone, Bill. ”Introduction to Trim”. http : //waypointamsterdam.nl/performance racing trim 3.html]

Overall, the boat moves forward with only a few degrees leeway, which is necessary to provide some angle of attack for the keel to keep the boat on course. It is better for the area of the keel that will produce the lift and drag forces to be placed close to the hull to decrease the moment that perturbs the boat from its stable position, described in Figure 11.

2.6.2 Design Specifics

The keel was designed as a deep fin with a lead-filled bulb ballast attached to the end. Using theory discussed in Section 2.6.1, the overall design objectives were to have most of the surface area of the keel high up, near the hull, with the ballast attached very low by the thinnest possible connector. A rectangular aluminum core was used for structural purposes to run throughout the entire keel, attaching to the ballast and sliding into the hull to secure the keel in the boat. Airfoil shaped foam surrounded the aluminum core, creating a low drag surface with a chord length a quarter of the boat length near the hull that quickly tapered to the minimum chord length necessary to completely encase the aluminum core within an airfoil. The foam and aluminum were covered in fiberglass, primer, and finally paint. A side view of the finished keel is visible in Figure 13. The ballast was created using a revolved airfoil shape, made of 3D-printed plastic filled with epoxy and lead. The whole assembly attaches to the hull such that the quarter chord of the entire keel airfoil is aligned with the quarter chord point of the sail, so no tipping moment is created.

13 Figure 13: Finished keel. The long chord length near the hull leads to the total lift force of the keel acting near the hull, lowering the heeling moment.

Design of Airfoil-Shaped Foam:

The keel’s airfoil-shaped foam was designed in Solidworks and is based on three lofted airfoils. The total length of the keel is 0.68m, which Jesse Miller’s simulation found to be the optimum for our boat [2]. The thinnest section of the keel, which is most of the keel’s length, is a NACA 0021 airfoil with a chord length of 0.05m. See Figure 13 for a side view of the keel model. This airfoil was chosen because it entirely encloses 0.25in x 1.25in thick aluminum and has a short chord length. See Figure 14 for view of how the foam model was shaped to fit closely around the aluminum core.

Figure 14: View near bottom of airfoil-shaped foam model. The foam barely covers the rectangular aluminum core, which corresponds to the rectangular cut visible at the bottom of the model.

14 The optimum chord length from Miller’s simulation is 0.04m using a NACA 0015, but the size of the aluminum gave the keel structural strength, and adequate strength was placed ahead of low drag in terms of importance.

Figure 15: Side view of upper section of finished keel. Note the aluminum tab visible in Figure 13 at the top of the foam core has been slid into the corresponding slot in the bottom of the hull, so is now out of sight.

The upper quarter of the keel consists of two other profiles. This section is visible in Figure 15. A short NACA 0021 shape widens into a NACA 0012 with a chord length of 0.15m at a vertical distance pf 0.06m from the hull. This transitions to a NACA 0012 shape with a chord length of 0.25m at the very top of the keel, where it meets the hull. The keel maintains a thin, hydrodynamic shape throughout its length, even as the chord length increases. This low drag profile is visible from the front in Figure 16.

Figure 16: Front view of finished keel.

15 Design of Ballast:

The ballast was designed to be a 1.1kg heavy, hydrodynamic shape. A plastic shell in the shape of a revolved NACA 0024 airfoil was designed to hold the lead shot and attach securely to the keel. The very front of the ballast was placed flush with the keel airfoil in order for seaweed to slide off easily. The plastic shell has an airfoil-shaped hole in the top so that the keel can slide halfway into the ballast. A 1/4in diameter aluminum rod was slid through the holes in the ballast and keel, reinforcing the connection between the two. The holes in the ballast shell for the rod and the keel are shown in Figure 17a and 17b.

(a) Side view (b) Top view

Figure 17: 3D model of ballast shell.

In the future, the ballast should be made into the shape of a torpedo, which is much longer and thinner than the current ballast in Figure 18. This would have lower drag, though could cause structural problems as the center of mass of the ballast would be further back from the supporting connection to the keel.

Figure 18: Isometric view of finished ballast.

16 3 Fabrication of Our Boat Components

3.1 Fabrication of Our Sailboat Hull/Deck

Note: The process described below is discussed in much higher detail in Appendix I, the section below addresses the main points and reasoning behind each decision. In addition, the process described below to manufacture the hull was also employed to make the deck.

To make our hull we started by cutting the outer surface of our hull into high-density, polyurethane foam. A computer numerical controlled (CNC) mill cut the shape using a CAD rendering of our hull to create a positive mold of our hull, also known as a “foam plug”. The foam cut shape is shown below.

Figure 19: Foam plug of Luja

We layed up fiberglass over the foam plug to harden the surface and protect the foam while making the surface finish. We then spray-painted Duratec Grey Surfacing Primer over the foam plug. Duratec Grey Surfacing Primer is a surface finish controller. Once it is sprayed on it can be sanded to the surface finish the maker desires. The primer is sprayed using a high-velocity, low pressure (HVLP) paint gun with 40 psi of pressure. This process often takes several applications of primer because it is common to sand through the Duratec to the underlying fiberglass. After each coat of primer, we sanded from 80 to 220 grit sandpaper. On the last coat, indicated by a smooth grey surface after 220

17 grit sanding with no exposed fiberglass, we sanded up 2000 grit all over the surface in approximately 100 grit increments. After the primed surface reached 2000 grit, we then sealed the surface using Frekote FMS-100. “Sealing” a surface is a preparation step for using the shape to make a mold. Even after 2000 grit sanding, there are still microscopic voids and dips in the surface that can reduce the quality of your mold’s surface finish, create voids and dips in your mold that will carry through the final shape, and damage the foam plug during mold release. The sealer melts these voids and dips away, both sealing the mold and removing residual dust left over from sanding. The sealer was rubbed on using a rag wetted with the FMS-100 and then buffed using a dry rag. The primed, sanded, and sealed plug is shown below.

This final plug is what we used to make the negative mold of our hull. To prepare the plug for taking a mold after it has been sealed, we applied three coats of release wax. After, we rub polyvinyl alcohol (PVA) over the surface. Both the release wax and the PVA are mold release agents which create a thin, impermeable film over the foam plug so that the mold releases nicely. PVA is by far the more robust of these two products. PVA can be used on its own without the release wax, but the release wax must be used with PVA or else there is a high risk of damaging both the foam plug and the mold during release. An example of a plug damaged by mold-making is shown below. We only used release wax on the plug without PVA.

Figure 20: Damaged Foam Plug from not using PVA

After the plug was coated with the release wax and PVA, we then coated the surface with gel coat. Gel coat is a vinyl ester surface compound with a variety of applications such as tooling, mold-making, repair, or surface finishing, depending on the type of gel coat. For our needs i.e. spraying gelcoat and then laying-up fiberglass over the gel coat layer, surface gel coat without wax worked best. Waxed gel coat is used in repair because gel coat is vinyl ester and therefore air-inhibited, so will remain tacky for a very long time. The wax in waxed gel coat rises to the surface to seal the gel coat and allow it to fully cure. For mold making, however, the gel coat is sealed in between the mold and the fiberglass, and therefore does not need the wax. After the gel coat was sprayed, we waited for the gel

18 coat to become tacky. After the gelcoat became tacky, we layed-up 4 layers of 7.5 oz standard weave fiberglass with vinyl ester and vacuum-bagged the shape. After the last coat of vinyl ester was applied, we vacuum bagged the shape to come close to the ideal resin-to-fiber ratio (1:1), eliminate air-bubbles in the mold, and match tough contours like the nose of the hull. We allowed the shape to sit in the vacuum for twelve hours.

After the shape has finished in the vacuum, we popped the new mold from the plug. The mold released with relative ease because of the PVA and release wax. This released shape was the negative mold of our hull. The negative mold is shown below during and after release.

Figure 21: Popping Negative Mold from Plug

Figure 22: Negative Gel Coat Mold of our Hull

We then used the negative mold to make our hull. The process to make the hull from the negative

19 mold is very similar to using the foam plug to make our negative, with only a few differences. For the final shape, we used three layers 7.5 oz standard weave fiberglass in 0-90 degree orientation with the longitudinal (length of the boat) axis, then 45-45 degrees, and then 0-90 again. 7.5 oz fiberglass is standard for boat manufacturing. The 90-45-90 orientation is to establish multi-directional strength and increase the stiffness of our boat. The release shape is our final boat hull. Shown below our complete hull and deck with the sail, tail, and keel attached.

Figure 23: Finalized Hull and Deck

3.2 Fabrication of Our Keel

A detailed step-by-step list of our manufacturing process is found in Appendix II (section 7.2). The keel primarily consists of a machined aluminum core, CNC cut airfoil-shaped foam, and a 3D printed, lead-filled ballast. The aluminum core was machined to the specified dimensions in Figure 24.

Figure 24: Top and side view of aluminum core, with dimensions. The 26.77in long section of the aluminum core is encased by the airfoil-shaped foam, and the slightly wider 1.2in long section at the top fits into a slot in the bottom of the hull. The two 0.12in diameter holes in the top view are threaded to fit screws that secure the keel to the keel block within the hull.

20 The foam casing was cut using a CNC router. The foam was epoxyed to the keel, then covered with two layers of 7.5oz fiberglass and vacuum bagged to remove any air bubbles in the epoxy. A hole was drilled in the end of the keel in order to secure it to the ballast later. The ballast shell was 3D printed, then filled with lead shot and enough epoxy to fill in the gaps between the pieces of shot. Immediately after being filled with epoxy and lead, the fiberglass covered aluminum and foam was inserted into the hole in the ballast, and a 1/4in diameter aluminum rod was slid through both the aluminum and the ballast to reinforce the connection. This configuration was allowed to cure, fixing the keel to the ballast.

Next, shape defects in the keel from the fiberglass or foam cutting were filled with Bondo Body Filler. The keel was sprayed evenly with a few layers of Duratec primer and sanded until smooth. The purpose of the Duratec primer is to fill in holes in the fiberglass to make a completely smooth, aerodynamics surface. Figure 25 shows the keel after one application of Duratec and sanding.

Figure 25: Keel with grey Duratec primer sanded through, revealing the fiberglass in light yellow beneath. More primer was applied after this picture was taken.

Once the keel was completely covered in a smooth layer of Duratec primer, the entire keel was painted with waterproof black marine paint because the primer is water soluble.

3.3 Fabrication of Our Mast Control

We designed the pieces of the sail control box in SolidWorks and cut them with an Epilog laser cutter in the RPL at Cornell University. We waterproofed the pieces of the sail control box and glued them together with epoxy (for proper epoxy procedure, see Appendix III). We attached the assembled mast control box to the inside of the deck using a mixture of colloidal silica and epoxy.

21 4 Analysis

We performed preliminary testing on the sailing behavior of our boat. These tests were composed of constant trajectory paths up, down, and across the wind and certain maneuvers including tacking, jibing, and ”directional stabilizing”. Directional stabilizing means setting the sail and tail angle for the next desired heading abruptly and allowing the boat to fall into the heading. We performed these tests in both high wind (about 20 mph) and low wind (about 5 mph) environments. In the future, we hope to observe our boat in a larger spread of wind speeds and performing a wider spread of maneuvers.

Our testing revealed several notable discoveries. First, our boat can quickly and successfully tack. This contradicts predictions made by former team members Robert Baker and Andrew Wilson based on their simulations. Their simulations asserted that the boat in use at the time they wrote the simulation (Spring of 2015) was too slow to tack successfully or without major ground loss downwind [4]. This turned out not to be the case for the boat constructed this semester, and in most instances tacking turned out to be a faster and more efficient maneuver than jibing while alternating between upwind headings. Based on Baker and Wilson’s research, we had planned to only jibe in all scenarios, including alternating upwind path, but based on our findings, we are now adjust our navigation algorithm to take for efficient paths which include tacking. Tacking proved to be preferable in both the high and low wind situations.

We discovered that the boat has odd behavior in high winds. The boat sails up and downwind in an oscillatory motion. We suspect that the oscillations are due to misalignment between the lift force on the sail and the side force on the keel when the boat heels. These forces are normally vertically in line, but tipping separates their alignment, creating a moment about the yaw axis, or vertical axis, of the boat. This misalignment is shown below in Figure 26

Figure 26: Rear-view Demonstration of the moment on the sailboat generated during turning

22 With upwind headings, the misalignment results in a moment that turns the boat upwind drastically. The boat then stalls and is over-corrected by the high winds, taking a trajectory farther from the wind than its original heading. This process repeats until the boat changes heading. The same oscillations occur in downwind headings, except the winds turn the boat downwind instead of upwind. The oscillations reduces the sailboat’s efficiency along certain trajectories.

Another odd behavior the sailboat has in high wind is it sometimes finds a new stable equilibrium position where the tail is upwind of the sail. A sailboat with this the substitute stable orientation is said to have ”dual stability” [2]. Our sailboat’s dual stability is shown below in Figure 27

Figure 27: Our Sailboat’s Dual Stability, Image Courtesy of Jesse Miller

This new stable position is shown belowIn this situation, it appears as though the lift force on the tail drives the boat and the sail becomes the . When this occurs, the sailboat is initially in the correct orientation, and when tipping begins, it will come about very quickly to point the tail upwind. The tail, depending on its angle before the boat came about, will then either drive the boat forward or backward. We suspect this occurs because the tipping of the boat has become so drastic that the center of pressure of the tail is approximately above the center of pressure of the keel. When this happens, the sum of forces is very close to our design, with the sail and keel’s lift forces vertically aligned and the tail purposely offset to steer, except the sail and the tail’s roles have switched.

In addition, in high winds, the sailboat will nosedive if sailing with a downwind heading. The nose diving drastically decreases the performance of the boat, sometimes bringing it to nearly a dead stop despite 20-30 mph winds. The diving does not occur in low winds.

The oscillations, unwanted stable equilibrium, and nose diving are all caused by lack of hull stability. The oscillations and unwanted stable equilibrium come from large heeling. The nose diving comes from large pitching. Decreasing these behaviors in high wind will increase the performance of our boat, both in velocity and travel efficiency. To decrease the negative effects of pitching and heeling, we will need to increase the stability of our boat in these directions. Steps to increase stability are discussed in the next section.

23 5 Future Work

For future models of this boat, I expect that we will keep the general hull shape the same and improve or replace all other components of the hull. Several design changes will likely be implemented to eliminate the negative of effects of pitching and heeling seen during high-wind sailing. To reduce the negative effects of pitching, we will install an elongated and sharpened prow. The prow is the above water portion of the bow. This strategy is inspired by viking , which were designed with a long, knife-like prow to cut through waves even when the boat was pitching forward. The extra volume at the bow will also increase buoyancy in that area, thus reducing pitching. An example of an extended prow on a viking long is shown below in Figure 28

Figure 28: Example of an Extended and Sharpened Prow on a Viking Longship. Image Courtesy of

Civilization in Contact: http : //www.schools2.cic.ames.cam.ac.uk/vikingships.html

Design elements, either passive or active or both, will be added to increase heeling stability. Passive elements we are considering include outer rigging (small pontoons on either side of the hull, making the boat a ) and a heavier ballast. A heavier ballast is slightly more difficult to implement due to our tight mass constraints. Outer rigging is much simpler and will provide more stability than a heavier ballast due to the moments their buoyancy forces will have. Each pontoon will be slightly raised above the waterline to preserve the hull’s speed capabilities.

Active elements we are considering include an active keel and an active mast. An active keel is essentially a motorized keel that will swing out to the side of the boat to increase its righting moment during heeling. An active mast is a motorized mechanism which keeps the mast normal to the water plane, eliminating unwanted moments caused by heeling. These elements are slightly more difficult to implement, and will require careful planning.

The final mass our hull, deck, and battery together came out to 3.15 kg, representing only a 2.64%

24 error in our predictions and keeping us well within our 1 kg buffer zone in design. The mass calculation for the hull/deck/battery was based on the surface area of the hull and deck, the density of 700 Vinyl Ester Resin cured with methyl-ethyl-ketone-peroxide (1.02 g/mL), and 7.5 oz fiberglass. This mass can be significantly reduced by selecting different materials for manufacturing. In the future, we plan to fabricate the hull and deck using two layers of pre-preg 3K, 2x2 twill carbon fiber. “Pre-preg”, short for “pre-impregnated” is a type of carbon fiber which has already been pre-infused with 35% epoxy by mass. “3K” stands for 3000 fibers per bundle. “2X2” is a weaving pattern, which can be thought of it, when moving across the sheet of carbon fiber would see two vertical strands, two horizontal, two vertical, etc. This carbon fiber weave has 24% less weight per fabric area than the fiberglass (5.7 oz/yd2 compared to 7.5 oz/yd2). If we manufacture the hull and deck with two layers of this pre-preg carbon fiber, we stand to reduce the mass of our hull and deck by up to 0.76 kilograms.

We plan to replace many structural designs in the hull and deck. The connection member between the hull and deck is particularly weak. It does not provide any structure to either the hull-deck connection or the individual hull and deck. In addition, it provides very little surface area for caulk to waterproof. We will likely replace the wood ridges with a plastic structure with support members. The plastic will likely be reinforced with carbon fiber as well. The lower deck will likely be replaced with a similar plastic-carbon fiber structure to support the keel block.

6 Conclusions

The performance of sailboat is improved by decreasing both the mass and the volume appropriately. We selected a long and narrow hull shape with a small draft. With this shape, we removed excess material above the waterline to decrease drag by having a low profile above and below the waterline. Our design left only enough volume as was required to preserve the hull’s performance and house the motor controlling systems and electronics. The resulting hull and deck are fast, smooth, and lightweight, exposing very little volume above and below the waterline. The same approaches were taken with the keel, making it thin and deep into the water to maximize its righting moment while minimizing drag. However, there are areas for improvement, discovered in fabrication, assembly, and testing. Designs will be improved to make the stronger, more durable, faster, and easier to use. In addition, certain changes will be made to improve the stability and therefore its high-wind behavior. These changes will be necessary to move our boat from short distance races to trans-atlantic journeys.

25 7 Appendices

7.1 Appendix I: Detailed Hull Fabrication Process

1. Cut Foam Plug using CNC router and .STL file of desired hull

2. Sand corners/ripples out using 60 grit paper

3. Cover plug with one layer of 7.5 oz fiberglass. Paint epoxy onto the fiberglass along the fibers. Be generous with the epoxy as well. For instructions on how to mix epoxy and handle safely, refer to Appendix III.

4. Prime Surface

(a) Pour appropriate amount of Duratec grey surface primer (by mass) into mixing cup (paper cup will do). I used 300 grams for one coat of the hull

(b) Add 10% Lacquer thinner by mass to the cup (buy separately)

(c) Add 2% Methyl-ethyl-ketone-peroxide by mass to cup

(d) Mix thoroughly for 1.5-2 minutes using a paint stirrer

(e) Pour contents into the (High-Volume, Low-Pressure) HVLP spray guy cup

(f) Connect Spray gun to air pump, turn valve below HVL gun trigger to allow airflow

(g) Pull handle to initiate spray, cover foam plug surface evenly, moving the spray back and forth across the part at a slow but constant rate. Be sure the gun is giving a fan spray pattern, which you can control by adjusting the airflow through the air outflow ports on the edges of the gun. Using the fan pattern, sweep the spray across the foam plug like a brush or a scraper. he spray of each new pass should cover 50% of the previous pass. After each layer you apply, be sure the passes you make for the next layer are perpendicular to the passes you made on the previous layer (i.e. in the case of the hull, if the first layer’s spray passes were along the length of the hull, we sprayed the next layers along the beam). Use all of the primer you mixed to preserve resources and save yourself future work.

(h) When finished, unplug the gun from the pump and clean thoroughly with either PMC or acetone. Be sure to clean both the gun cup and the air holes surrounding the primer nozzle. You will need a sharp object like a knife or a razor blade to get these small holes. There are ten of them. When you are finished fill the gun cup with a cup (units of measurement) of your cleaning fluid and spray into a trash can. Either use gravity or air pressure to blast the cleaning material through the gun, but if you use air pressure, use a fume hood and where a mask. NOTE: Both chemicals are toxic and pose long term health risks if contacted, so be sure to wear gloves and long sleeves during this process.

5. Sand Surface

(a) Clamp foam plug to working surface. Be sure that area is open and well-ventilated, such as a warehouse, loading dock, or even outside.

(b) Put on mask and wear protective eyewear.

26 (c) Fix 60 grit sandpaper to a long, flexible rod or bar. Run long rod back and forth along length of part. You will need a partner for this step. After you have thoroughly sanded in this direction, sand using the long, flexible rod in the perpendicular direction. Sand any awkward/hard-to-reach spots with either an electric sander, block sander, or sand paper on a foam block. Sand until you cannot distinguish between scratches left in the primer by the sandpaper. The purpose of the long, flexible rod is to evenly eliminate bumps or ripples along your plug without creating awkward low points from point sanding with either your hand or a block. NOTE: This is especially important if you are making a hull, which is built from a series of splines. A spline, when referred to as a geometric tool, is the lowest strain-energy state of a shape. A bendy rod assumes a spline as well, so using one to sand a hull is very important.

(d) Repeat Step 5-c. using 120 grit sandpaper

(e) Repeat Step 5-c. using 220 grit sandpaper

(f) NOTE: It is expected to sand through the primer after your first coat. You should typically apply 2-3 layers of primer to guarantee a smooth surface without exposed foam or fiberglass

6. If exposed foam or fiberglass, repeat steps 4 and 5

7. If no exposed foam or fiberglass: Finish sanding surface surface

(a) Repeat Step 5-c using 320 grit sand paper

(b) Wipe down surface with water

(c) Wet sand surface using 400 grit sandpaper

(d) Repeat Steps 7-b,c using 600 grit sandpaper

(e) Repeat Steps 7-b,c using 800 grit sandpaper

(f) Repeat Steps 7-b,c using 1200 grit sandpaper

(g) Repeat Steps 7-b,c using 1500 grit sandpaper

(h) Repeat Steps 7-b,c using 2000 grit sandpaper

8. Using Frekote FMS-100 mold sealer, seal the foam plug by rubbing the FMS-100 into the sanded surface with a clean rag

• NOTE: Sealing a Mold fills the remaining microscopic voids and crevasses remaining after sanding. Unsealed molds can lead to rougher mold surfaces and damaged plugs/molds during release.

9. Prepare Plug for Mold-Making

(a) Apply Meguiars Glaze No.8 Wax using a foam brush in small, circular strokes over the entire release surface

(b) Buff wax using a clean rag

(c) Repeat steps 10a and b three times.

27 (d) Using a clean rag, coat the releasing surface with polyvinyl alcohol (PVA). The PVA will leave a greenish-blue residue on the plug.

(e) Buff PVA into mold

• NOTE: PVA is very difficult to buff, so be sure to apply PVA in thin coats

10. Begin negative mold manufacturing and prepare for vacuum-bagging

(a) Prepare for application of gel coat by measuring out necessary gel coat volume. We used approximately 500 mL for the gel coat layer. The gel coat is applied using a gel coat dump gun (a.k.a. a cup gun).

(b) Pour appropriate amount of gel coat into gel coat dump gun paper refill cup

(c) Pour 2% MEKP by mass into the cup with the gel coat to initiate curing process

(d) Mix contents thoroughly for 1.5-2 minutes

(e) Examine the lid of the cup gun. Ensure that both the nozzle and the air intake valves are clear and will allow gel coat and air to pass through, respectively. If either valve is not clear, use PMC, Acetone, and a sharp object to clear opening

(f) Mount dump gun lid onto cup. Push lid firmly onto cup to establish tight seal

(g) Insert nozzle of lid into gun, secure cup with spring-loaded latch

(h) Apply gel coat to mold. Gel coat is applied in three immediately consecutive layers in one operation. Prerelease may occur if the layers of gel coat begin curing at different times. Prerelease is a when a mold releases before full cure, resulting in a scrap part. Prerelease has many sources such as large temperature changes and vibrations, and varying time frames for gel coat cures can cause the layers to pull on each other, causing prerelease

• NOTE: When using the gel coat dump gun, hold gun and initiate the gel coat spray with nozzle pointed vertically. Otherwise gel coat will drip out of the nozzle and onto your part

(i) Allow 45 minutes for gel coat to become tacky

• NOTE: 45 minutes is the time for which gel coat becomes tacky, but you can wait up to two days before the next step if using unwaxed gel coat. Gel coat is air-inhibited, so unwaxed gel coat will take a very long time to fully cure. In theory, unwaxed gel coat exposed to air will never fully cure

(j) Drape layer of fiberglass onto tacky gel coat surface. The fiberglass will create a strong, long-lasting part. Random ply fiberglass is the best for this step to improve the mold’s durability, but standard ply will also do

(k) Prepare to apply vinyl ester resin to the fiberglass. Vinyl ester must be used for this step because gel coat is a vinyl ester compound, and will therefore bond most strongly to fiberglass made rigid with vinyl ester resin

i. Measure out appropriate volume of vinyl ester resin. For four lay-ups of fiberglass on the hull’s negative mold, we use approximately one liter of vinyl ester resin

28 ii. Add 2% by volume of MEKP to the vinyl ester resin

iii. Mix Contents thoroughly

(l) Paint mixed vinyl ester resin onto fiberglass using well-made bristle brushes. Cheap bristle brushes will leave hairs in the mold. Paint along the fibers if you are using directional-ply composites so to not disfigure the weave. You are aiming for a 50-50 ratio of fiber to resin, so be generous with the vinyl ester resin

(m) Repeat Steps 10j.-10l. for each layer of fiberglass you are applying to make your negative. We used 4 layers for our negative hull

(n) Vacuum Bag Negative mold immediately after last lay-up of fiberglass. Refer to Appendix 4.4 for instructions on how to vacuum bag composites

11. Use cutoff tool to cut away regions of fiberglass bonded to non-releasing surfaces of the foam plug.

12. Using a plastic a plastic or rubber mallet, hammer the mold thoroughly. Hitting the mold with a soft hammer releases the mold in regions not near the edges

13. Release the negative mold from the foam plug using a mold-popping tool, essentially a plastic wedge, by running the wedge along the edge of the mold and the gradually pushing the wedge between the mold and the plug. If you applied PVA and the release wax well, the mold should release with relative ease

14. Clean negative mold with water to remove PVA residue and acetone to remove wax residue, dust, and other pollutants

15. Prepare negative mold for making positive mold

• Repeat all of step 9 on the negative mold

16. Begin final mold manufacturing and prepare for vacuum-bagging

(a) Repeat steps 10a-i, replacing the foam plug with the negative mold

(b) Stuff a handful of fiberglass strands or a patch or random ply fiberglass into the nose of the hull and any other sharp contours to guarantee strength in hard-to-reach places

(c) Drape one layer of fiberglass over the tacky gel coat such that the fibers are parallel or perpendicular with the hull’s longitudinal axis

(d) Repeat steps 10k,l

(e) Drape another layer of fiberglass over the first layer and orient it such that the fresh piece’s fibers are all 45 degrees with the hull’s longitudinal axis

(f) Repeat steps 10k,l

(g) Drape one layer of fiberglass over the tacky gel coat such that the fibers are parallel or perpendicular with the hull’s longitudinal axis

(h) Repeat step 10k,l

29 (i) Vacuum bag positive mold immediately after last fiberglass lay-up, refer to Appendix 4.4 for vacuuming bagging instructions

17. Repeat steps 12-14 using the positive mold you vacuum bagged, and the resulting shape that releases is the hull of the boat

7.2 Appendix II: Detailed Keel Fabrication Process

1. Cut foam core using CNC router. A CNC routers capable of cutting large foam shapes can be found in Rand Hall

(a) Fill out online form located here: http : //aap.cornell.edu/forms/aap−resource−request− form. Contact AAP (Architecture, Art, and Planning) Shop staff to learn if the request was approved by their supervisor. Here are two members of the staff we have worked with before: Chris Oliver ([email protected]) and Daniel Solomon ([email protected]). The shop takes up the entire first floor of Rand Hall.

(b) After approval has been granted, arrange a time to cut. Bring .STL files and foam that is at least an inch too long and wide for the part. Use the large CNC foam cutting machines. Stay for the entire cutting process to watch over the machine in case of errors.

2. Machine aluminum core in Emerson Machine Shop, Rhodes 116.

3. 3D print ballast shell in Rapid Prototyping Lab, Rhodes 114. Bring in .STL files in person or send via form on website. Website: cornellrpl.wix.com/cornellrpl

4. Cut 1/4” diameter aluminum rod to fit through ballast.

5. Epoxy the two halves of the CNC cut foam core to the machined aluminum, see Appendix 7.3 for epoxying procedure.

6. Cover foam and aluminum core with two layers of 7.5 oz fiberglass. Each layer should be coated with epoxy, see Appendix 7.3. Finally, the fiberglass covered piece should be vacuum bagged, which is covered in detail in Appendix 7.4.

7. Cut off excess fiberglass around Keel outline, and use Bondo Body Filler to fix any shape defects (cures in 2 hours).

8. Drill 1/4” diameter hole in the aluminum keel core so that a rod passed through the ballast will also pass through the keel, holding the two firmly together. The keel fits into the airfoil shaped hole in the top part of the ballast, extending nearly two inches into the ballast.

9. Fill 3D printed ballast shell with lead shot and epoxy. Fit end of keel into slot in ballast, slide aluminum rod through ballast and aluminum to secure, then let cure for 10 hours.

10. In the Experiential Learning Laboratory (ELL), use spray gun to coat keel and ballast in Duratec primer. See Appendix I for detailed instructions on using the spray gun in the ELL for priming. Make four passes on each face, alternating between horizontal and vertical passes. Let dry for 8 hours.

30 11. Sand primed surface. Wear a mask and protective eye-wear. Sand first using long strokes going down the entire length of the part, until scratches made in the primer by the sandpaper are indistinguishable from each other. Next, sand across the width of the part, perpendicular to the previous pass. Sand until the scratches from the previous direction are covered by the current pass’ scratches.

(a) Begin with 60 grit. Sand primer till the surface is smooth with no low spots. If fiberglass begins to show, repeat step 10 and this step until a smooth surface is achieved with no fiberglass showing through.

(b) Sand with 120 grit sandpaper

(c) Sand with 220 grit sandpaper

(d) Sand with 320 grit sandpaper

(e) Sand with 400 grit sandpaper

(f) Sand with 600 grit sandpaper

(g) Sand with 1000 grit sandpaper

(h) Sand with 1500 grit sandpaper

(i) Sand with 2000 grit sandpaper

12. In ELL, use spray gun to coat keel and ballast in marine paint. Make only two passes per side. Wait 24 Hours to dry. Repeat process at least 1-2 times.

7.3 Appendix III: Epoxying Procedure

Epoxy will come with instructions on how to mix and apply it. For further instruction on general epoxy usage, go to: http://www.westsystem.com/ss/how-to-use

Always use hand and eye protection when working with epoxy. Always wear a respirator with an organic vapor cartridges when working with epoxy. For further safety information go to: http://www.westsystem.com/ss/general-safety-guidelines/

7.4 Appendix IV: Vacuum Bagging

Refer to the following video for instruction: https : //www.youtube.com/watch?v = 5jSwxEkJgM

The main points are summarized below.

1. Use Breather/Bleeder fabric to cover the non-releasing surfaces of the existing mold i.e. the base of the foam plug or the rough exposed fiberglass of the negative mold. Leave an area of exposed breather fabric to place the base of the pump hose connector.

31 2. Cover the curing fiberglass with release fabric. Be sure to push the release fabric into an corners or sharp contours that the fabric does not naturally fall into

3. Wrap the entire system, including the breather fabric, with bagging fabric. Cut a hole in the bag to connect the hose connector to the base

4. Turn on the vacuum, listen for leaks. Let the vacuum run for 12 hours

7.5 Appendix V: Wind Vane

A wind vane is a device for determining the direction of the wind. We use a wind vane on the au- tonomous sailboat to monitor the direction of the wind relative to the direction the boat is pointing. Knowing the wind direction relative to the boat is necessary for successful navigation (see Navigation Team report for more detail on navigation).

Below (Figure 29) is a schematic illustration of the wind vane design.

Figure 29: Wind vane schematic illustration

We made the plastic tail fin out of a thin, rigid plastic. We attached the fin to member A using hot glue. Member A is a 0.375” outer diameter (OD) tube with a counterweight attached to the end opposite the plastic fin. We attached member A to member B by drilling a 0.12l” hole in the middle member A and press-fitting (i.e., using friction to keep the members together) member B into the hole. Member B is a 0.125” OD, 1” long aluminum rod. We press fit member B into a waterproof tube with bearings.

We made the waterproof tube from a thin plastic tube with 0.251” inner diameter (ID) and 0.375” OD, with 0.250” outer diameter flanged bearings press-fit into both ends. The waterproof tube is press-fit into the deck and identical aluminum plates.

32 We made two identical 1” x 1” aluminum plates (see Figure 29, on Page 32) using a mill in the Emerson Machine Shop at Cornell University. With the mill, we also drilled holes in both aluminum plates, so that the holes lined up as perfectly as possible. We made matching holes for in the section of the deck where we mounted the wind vane. We attached the aluminum plates to the deck and wind vane using plastic 4-40 bolts. We used plastic bolts because magnetic materials, like some steels, could interfere with the function of the magnetic rotary sensor, leading to faulty readings of the wind direction (See Navigation Report for more detail on the rotary sensor).

We designed the wind vane frame in SolidWorks and printed the frame on a uPrint SE 3D printer in the Rapid Prototyping Lab (RPL) at Cornell University. The frame includes a shelf to hold the rotary encoder, as well as two shelves to hold bearings for member C. The frame is shown in Figure 30, below:

Figure 30: 3D-printed wind vane frame

As seen in Figure 30, the frame is separated into two pieces that interlock at a seam. We separated the frame into two pieces for ease of assembly. After we put the other components of the wind vane in place, the two pieces of the frame were assembled.

Member C is a 1” long 5-40 threaded aluminum rod. We connected member B to member C with a

33 1” long section of 0.125” ID of surgical tubing (see Figure 29, Page 32). We attached the magnet to member C using a small 3D-printed container that held the magnet in place against a 5-40 lock nut. We threaded the lock nut onto the lower end of member C, thus fixing the magnet to the lower end of member C, above the rotary sensor.

Initially, we used two ball bearings mounted in the lower shelves of the wind vane frame to support member C. However, we found that the bearings were magnetic and therefore interfered with the rotary sensor. To fix this problem, we replaced the bearing closest to the sensor with an aluminum bushing. The aluminum bushing keeps member C properly aligned and is not magnetic, so it does not interfere with the rotary sensor.

The current location of the wind vane (at the bow of the sailboat) often exposes it to water. While the design is waterproof, the water flowing over the deck can interfere with the wind vane’s operation. If waves pass over the deck or the bow goes below the water, the wind vane will deflect due to the motion of the water over the deck. To address the issue of water affecting the wind vane, future iterations of the design should place the wind vane further above the waterline of the boat.

References

[1] Comache - Built Hodgdon Yachts. (2014). Retrieved May 8, 2016, from http://www.hodgdonyachts.com/#!comanche/cejo

[2] Miller, Jesse.”A Directionally Stable Robotic Sailboat: Concept and Simulations.” April, 2016.

[3] Ashland Inc. (2011, August 30). DERAKANE 8084 Epoxy Vinyl Ester Resin Technical Datasheet. Retrieved May 8, 2016, from http://www.ashland.com/Ashland/Static/Documents/APM/DERAKANE 8084.pdf

[4] Wilson, Andrew. ”Andrew Wilson’s M. Eng Report.” May, 2015.

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