Chapter 2 Single-Strut Hydrofoil

CHAPTER 2 SINGLE-STRUT HYDROFOIL

Pursuing maneuverability – a bike on water

MY DREAM OF DESIGNING HYDROFOILS I did not feel like using the same system. Even the function for deciding the foil borne altitude, which There was a time in my youth when I dreamt of affects the performance of the boat, is a load spending a lifetime designing and building imposed on the boat. It leads to degradation in its hydrofoils. That was the time when Supramar of performance, and I could not accept this. I was also Switzerland started commercial service of hydrofoil apprehensive that the original performance of the craft in the Leman Lake in Switzerland. hydrofoil would not be obtained because of Incidentally, I was collecting historical data on ventilation of the surface-piercing part. Moreover, hydrofoils at that time, and I was certain that the the contradictory requirements of large angle of hydrofoil was the boat of the future. Since attack and large chord of the foil near the waterline childhood, I had dreamt of becoming an prevented me from proceeding in the same aeronautical engineer, and I loved rowing and direction. sailing. After four to five years of work experience, I was confident of designing and building a good Practically all successful hydrofoil boats until now hydrofoil boat by myself. are of the surface piercing type. It was extremely important to perform the overall design using the As I looked for data on hydrofoil boats, I technology available at that time, to sidestep the discovered that this boat had a very long history. contradiction, and push the design to the extreme Various theoretical studies had been carried out on within the bounds. I was still young and had too hydrofoils since the past. I also found that many many dreams, I guess. I could think of nothing kinds and shapes of hydrofoil boats dreamed of by except fully submerged foils. engineers had appeared and disappeared along with them; however, they had left behind a large number HOW DOES A HYDROFOIL WORK? of photographs of these boats. Let me describe a hydrofoil here (see Photo 2-1). A During the Second World War, a few large hydrofoil achieves high speeds by significantly hydrofoil boats carried tanks and heavy equipment reducing frictional resistance and wave making across the Mediterranean Sea for Field Marshal resistance of the hull by supporting the boat’s Rommel, but subsequently the difficulty in building weight on foils. Since water is 800 times denser such boats for general use hindered their than air, if a foil (wing) with an area 1/800 that of development. While I was working for Yokohama the area of an airplane wing is used underwater, the Yacht Works, I heard that Shin Meiwa Industries same lift can be obtained at the same speed as that had started work on plans to build hydrofoils. Its of an airplane. On the other hand, a foil that team included Kenji Sugawara, a model airplane receives that much of a lift must have adequate colleague; so I could easily exchange data, see the strength to withstand the lift, and it is more difficult prototypes being built there and sometimes ride on to make the adjustment required in the flying height them with Kenji. The hydrofoil boats built by Shin when the hull separates from the water but the foils Meiwa Industries adopted the same system as remain under the water. It is also very difficult to Supramar for surface- piercing foils (presently, the maintain the optimum flying height especially in system being used in hydrofoil boats all over Japan waves. except in the Sado area) I personally felt, however, that this system was somewhat incomplete and did not feel like adopting it.

PHOTO 2-1 Author testing a submerged foil system

A hydrofoil boat with surface piercing foils is THRILL OF FLYING OVER THE WATER designed such that the foil is inclined at an angle of about 30°. The angle of attack and the chord width A magazine published in the USA in 1952 carried both increase going up the foil, so that when the foil an article on a boat named Sea Hazard, the is fully immersed in water and the boat picks up prototype of a submerged type hydrofoil boat. Since speed, the lift force increases continuously and it was a really fascinating boat, I convinced steeply. This enables the boat to run in a stable President Shiro Chiba to permit me to build a manner above water at a high position at high prototype. speeds at a low position at low speeds. The main I had adequate data from this magazine, so I was issue is that the upper parts of the foil come in able to complete the drawings in my own free time. contact with water in waves so that lift increases Luckily we had an old operating Johnson outboard temporarily, which may lead to an uncomfortable motor in the company, so we used this motor with ride. propeller that came with it. The transom height to On the other hand, since the foils are immersed in suit this motor was about 26 inches, and the motor water in a fully submerged type foil system, the was painted in khaki color. The engine was a problem mentioned above is reduced; however, the horizontally opposed twin-cylinder engine of 25 HP. “flying” height needs to be controlled from A starter rope was wound around the flywheel, and somewhere other than the foil, and the difficulty in this rope was pulled fully to start the engine each doing so hinders the realization of this kind of boat. time.

The appeal of the hydrofoil has been acknowledged You can see the general view of the boat in Photo all over the world since the past; its history is older 2-2. The control arrangement consisted of a control than that of the airplane. The history of prototypes stick exactly similar to that in an airplane. By of these boats being built everywhere suggests that moving the stick forward or aft, the angle of attack there is everlasting interest in this boat everywhere. of the submerged foils at the bottom of the two There are many submerged type hydrofoil boats struts changes simultaneously, and the foils fitted with automatic height adjustment devices. function like elevators. If the control stick is moved Fortunately, I have two or three good “hydrofoil- to port or starboard, the port and starboard foils loving friends” in overseas countries who feed me move in opposite directions. For instance, if the information. stick is moved to port, the angle of attack of the port foil reduces, while the angle of attack of the during a turn, the bottom hull on the inward banked starboard fin increases by the same degree. If the side would touch the water or the foil on the starboard side of the pedal is depressed, the outward side would come out of the water. outboard motor turns to port so that the boat makes a turn toward the starboard side. In this fully If the accuracy in height was to be about 10 cm, the submerged hydrofoil boat, the flying height is banking had to be reduced slightly. On the other controlled by a fully manual control system. hand, the boat had to bank to a certain extent, otherwise the strut would break laterally due to the The development of the prototype progressed side force. smoothly. Mr. Koji Chiba, President Chiba’s second son and I, started tests on the prototype. When President Chiba came to see the hydrofoil While working for Okamura Corporation on the boat along with Mr. Seiichi Niwa (the author of designs of N-52 (light plane manufactured for the High Speed Boat Technology (in Japanese), first time after the Second World War, basic design torpedo boat designer, and presently the Chairman by Mr. Kimura) and the LBS-1 (the two-seater of the All-Japan Boat Association), we were on our Soarer for Tokyo University, basic design by Mr. return run with the hull in contact with the water Yamana), I was too heavy to participate in the test after the foil broke during a steep turn. Even in flights. However, since I wished to deepen my Japan, during the War, the hydrofoil boat had been understanding on control systems on airplanes, I studied for military use, but witnesses had not taken was allowed to try out the control stick operations photographs of hydrofoil boats in the foil borne of the US liaison aircraft Ryan Navion (which was state in those days; we can only take their word for being imported and assembled at Okamura it, and it is unfortunate that no records remain. The Corporation) and the TT-10 of Toyo Aircraft at that foil borne runs of this hydrofoil boat and Shin time. Although I was not a very good pilot, the test Meiwa’s boat were witnessed by many probably for flights were a dream come true for me. the first time in Japan.

I remember that the hydrofoil boat started to “fly” This boat was operated in waves as high as 1 m. It over a distance of 40 to 50 m in two to three days. was extremely interesting to reduce the speed, and My happiness knew no bounds at that time. The drag the tail bottom on the surface. The impact of length of the forward strut was 70 cm. The length the waves was received near the transom, but the of the aft strut was such that even if the propeller driver was unaffected being seated near the forward was raised and exposed, the transom would be no strut. If the boat was driven low when at the crest of more than 35 cm approximately from the water the wave, and driven such that the forward strut was surface. Accordingly, for the boat to remain foil almost fully exposed so as to stand on tiptoe when borne, height control within 30 cm was necessary at a wave trough, the vertical amplitude at the so that the driver could view the water surface as seated position became very small, and riding far as 10 m from the seated position. While riding comfort was excellent. After tests in rough weather, the boat, the height could be controlled to an the tail bottom finally broke, and we returned home accuracy of about 10 cm, thus endorsing the as water filled up the hull, but I felt satisfied that I appropriateness of this system. had fully utilized the benefits of a fully submerged type hydrofoil. The turning ability however was a bit problematic. There were struts on both sides of the hull of For building such a small craft, my interest always breadth 2 m, with foils below these struts. If the turned to maneuverability. It was also natural to boat was banked (inward) by over 15 degrees consider the same from the

Photo 2-2: The author (front) and Mr. Chiba “flying” on three struts marketability aspect. From this aspect, a turn with a drawings for a new hydrofoil boat while working in 15-degree bank results in too large a turning radius the tank testing laboratory (Fig. 2-1). This boat hull and lacks the “sporty” feel. The maximum G felt by would not come in contact with the water surface the body at a 15-degree bank is about 1.035G, and and the tip of the foil would not emerge out of the the increase is only 3.5%. This is nothing compared water surface even at a 45-degree bank. to the 2G experienced when an airplane banks by 50 degrees during a tight turn, unless the boat banks Just like a slalom water ski with one ski, a single by 30 to 45 degrees, the thrill is absent. If a skilled forward foil has to be located on the center line of water skier slaloms on water, the banking would the boat to bank the boat to a large angle. The probably be a maximum of 70 degrees (3.0G). If a operating principle of a bicycle or motorcycle may normal person slaloms on a single ski, the be used to attain lateral stability, instead of the acceleration would be 1.3G to 1.4G, and will this operating principle of the aileron. I thought it would not give the person a real thrill? be ideal to combine this concept with the height control mechanism obtained in the prototype HUNDREDS OF SIMULATIONS mentioned above. The chine breadth was made narrow in line with the objectives. One strut was At that time, I was given the responsibility of the provided forward and aft respectively, the aft foil tank testing laboratory. I used to perform tank tests was fixed to the outboard motor, and the steering based on both in-house and requests from outside mechanism of the outboard motor was fixed to the the company, and prepare test reports. I made hull. The assembly from the handle to the foil was improvements on the test equipment too. After a constructed such that it could be turned like the while, I had nothing much to do; so I prepared front wheel of a motorcycle using a handle bar.

FIG. 2-1: Submerged hydrofoil system designed by the author

When this handle bar was pushed in the forward collect funds for building the prototype. However, direction, the effect was to bring the boat down. President Chiba found that the All-Japan Motor The starboard-side grip was made similar to the Boat Association had the means to put up the throttle grip of a motorcycle, and had the same money for research on this prototype, and went function of the engine throttle. The seat used was a ahead with the discussions to procure funds on his double seat, similar to that used in a motorcycle. It own. The result was that we received 100,000 yen enabled the rider to move freely to the forward and for developing the boat; we could go ahead with to the aft, and also served to seat two persons if building the prototype. necessary. (See Fig. 2-2, 2-3) The prototype work progressed smoothly, but I Nobody believed at first that this boat would run worked uneasily, without much confidence. If this successfully. Even President Chiba was not project failed, I would no longer be able to work on convinced at first. For this reason, it was difficult to other interesting projects again. At this stage, I remember performing many simulations before new long shaft were fitted to obtain a long-shaft actually building and testing the prototype. outboard motor. To avoid strength issues related to the bracket and above the spacer, wires were fitted I had run this hydrofoil boat hundreds of times in from the spacer toward the fore and aft direction for my mind while lying in bed. Simulations cannot be support. The front part of the wire taking up the performed with eyes closed; I did lots of load was a rope, which would break if a strong calculations and verified them practically. These impact was received and the strut would fold included a whole range of related topics such as toward the aft. Submerged foils were held by the launching the boat and boarding it, releasing the skeg under the lower unit. suspension rope, shifting the position of the boat using the paddle, getting an assistant to hold and The throttle had to be opened to a certain extent maintain the boat in position, considering the before starting the engine. There was no clutch. I position of the knee when starting the engine to the was worried that the boat would spurt suddenly quick movement of hand from rope trip to the before the throttle was gripped, and if the elevator throttle grip just after starting the engine, and so on. was not in the neutral position, the boat would The night before testing the prototype, all doubts behave uncontrollably and turn over. except two were cleared up.

FIG. 2-3 Raising/lowering the boat FIG. 2-2: Steering the boat

Although this issue could apparently be resolved by One was the stability of the boat when the engine using a stopper for the handle and by adequately was started. The engine used in the prototype was testing the engine, I regretted being remiss. As a the Kinuta outboard motor with an output of 18 HP countermeasure, we secured the grip to be lashed to at 350 cc, designed for racing boats. The lower unit the shore to the transom, and started the engine of this engine was removed; a 40-cm spacer and a while it was lashed. Another concern was the strength of the hinged part above the water for about 20 to 30 m. There were of the front foil. During a 45° bank, when one side practically no lateral stability problems. My body of the foil emerges out of the water surface (in had probably grown used to the feel of riding a Photo 2-1, the starboard foil can be observed to bicycle from childhood. After I made several have come out of the water surface), the entire lift “flying” runs of the boat over short distances, I is provided by foils on one side. Calculations became firmly convinced of the ability of boat to assuming that no lift is provided by foils on one “fly.” During the first trial attempt, the boat did side showed that the strength was totally inadequate. “fly.” In practice, such a clear-cut situation does not occur, and the severity is mitigated to some extent, but it is Although we had some engine problems thereafter, not clear to what extent. We also did not like the I became accustomed to handling the boat better as thought of making the hinge larger than was each day passed, and could also make steep turns. required. We decided to consider adopting measures only if the hinge broke.

Strength calculations were carried out similar to those for an airplane, and after many simulations, I began to grow confident of the design.

FROM PROTOTYPE TO THE REAL BOAT – THE DREAM IS ALIVE EVEN TODAY

PHOTO 2-4: Entering a turn

Photo 2-3: Single strut submerged type hydrofoil Although it took some time to bank the boat to 45 “flying” on the water degrees, I could bank it through 35 to 40 degrees easily. Even if the foils on one side came out of the The day finally arrived. We floated the hydrofoil water (Photo 2-1 clearly shows the foil end plate in boat just as predicted in the simulation, and started the shape of an arrowhead emerging out of the the engine. The engine started at an unexpectedly water), the hinge did not break. The second issue low throttle position, and it could be controlled was also resolved. On the other hand, however, we easily from the shore. This suggested that it could could not predict the forces that acted on the foils. be re-started when floating the boat also. The first issue was resolved. Stability was adequate at low Once, upon entering a steep turn, the height of the speeds, and throttle response was good. After a few hull above the water was excessive, and the strut other checks, we started to test the foil borne ability came completely out of the water. The boat rolled of the boat. After a run of about 10 m, I pulled the over 180 degrees at a high position, I was thrown handle. The boat lifted slightly. I was able to run out sideways, the boat turned over upside down and the boat in this condition with the hull slightly remained floating. Yet another time, while running in shallow water of about 20 cm, I was thrown I continued trial runs of this boat thereafter, with a forward by about 6 to 7 m, the rope that acted as a passenger also. When the boat rose excessively, air fuse broke, and there was no damage to the boat. entered the upper surface of the foil (ventilation), Mr. Ryoichi Sasagawa came to see this boat at and lift was lost abruptly. However, by throwing Yokohama Yacht Works, and subsequently placed the handle forward quickly, it was possible to blow an order for three of these boats and supplied them off the air, and we could continue with the foil to the All-Japan Motor Boat Racing Association. borne run (see Photo 2-6).

PHOTO 2-5 Overall view

This hydrofoil boat was a sturdy sports boat and Subsequently, I did spend many years on working had a fairly stable performance; it was my creation mainly on developing and building workboats and and was like my own child. I got energy from it, pleasure boats; however, at various periods of my and later, tried out various practical designs, but all working life, I was urged to reproduce this design. of them failed in the simulation stage, and none There were many who nicknamed me “one-strut reached the prototype stage. The scale effect hydrofoil Horiuchi.” Personally, I still have the (condition beneficial to design as the boat got desire to create more sophisticated versions of this larger) was significant, and I did not like the boat some day. I have not yet seen a boat that can thought that with the success of this boat, its beat this boat’s maneuverability on water. I think construction work would pass on to large shipyards. this boat belongs to the small-boat world. The only This was an era when such boats generally meant issues I can think of are: the boat is weak against for sports would not sell. If this boat was to be sold one-sided loads since the struts are only on the as a high speed ferry boat within Japan, about 100 centreline; maintaining the height above water is of these would be the upper limit; export too was difficult and delicate particularly at high speeds, not feasible because we lacked peripheral consequently, the speed has to be restricted. It is technologies. After considering these circumstances, these issues particularly that created everlasting I graduated from my initial dream of spending an interest in the development of this design. entire lifetime on hydrofoil boats.

DESIGN AND PERFORMANCE motor strut became 40 cm long, so that the boat would rise to a high position. However, the thrust The hull design as shown in Fig. 2-1 was a box position was relatively low, and the forward part shape. The plane shape was practically rectangular, tended to rise when the boat took off. To resist this and this shape was used because it was necessary tendency, the bottom of the hull was extended aft to to: 1) minimize the maximum width at the bottom obtain more buoyancy. We could also use this part as far as possible to enable high banking angles, effectively to fasten a thrust-bearing wire from the and 2) provide as wide a bow as possible so that aft side of the strut. when the bow dropped on the water surface when the forward foil stalled (actually when it sucked up We fitted an aluminum alloy rear foil with foil area air or ventilated), it would provide a strong and of 525 cm2, taper ratio of ½ and span of 50 cm to quick lifting force. For this reason, just 2 x 2 the lower end of the outboard motor. The bottom plywood sheets of 3 feet x 6 feet joined by scarph surface of the foil was a flat surface, and the upper joints were adequate for a simple hull structure surface was arched, quite similar to the cross covering the entire bottom shell and the side shell. section of a propeller blade. The foil thickness ratio was taken as 9%. A thrust-bearing wire was fitted forward of the housing, and at its front end, the wire was joined to a rope that acted like a fuse. If an obstruction hit the strut, this rope would break and the strut would escape damage. If this rope was removed from the cleat, and the resistance-bearing wire rope was removed and pulled, the outboard motor could be tilted.

The method of supporting the forward foil and its action can probably be deduced from the detailed sketches in Fig. 2-4 and 2-5. If the resistance- bearing rope is removed, and the handle is pushed forward and down, the front foil can be stowed in a box just like a center board trunk, and can be raised together with the outboard motor, enabling the boat to be beached when necessary.

The lowermost cross section of the strut has a chord

of 8 cm, and a wedge-shaped, sharp leading edge of Photo 2-6: Even if the foils came out of the water thickness 18 mm, within which it carries a control surface, the boat could be brought to the normal rod for varying the angle of attack of the front foil. “flying condition” by quickly pushing the control stick down Photo 2-2 shows a three-strut boat having a lens- type cross section. Just behind the strut, a high See Fig. 2-4 for details of the aft foil and its support. spray can be seen, but such a spray does not occur We used an 18-HP Kinuta outboard motor in case of a strut with wedge-shaped cross section. originally used for racing boats, and fitted an This shape reduced the resistance, and enabled the extension shaft, housing and intermediate shaft rudder to respond obediently to the movement of bearing all made in-house. In this way, the outboard the handle bar. FIG. 2-4 Engine and forward strut

Thrust bearing adjuster

Connecting rod Rod

Holder

Bow cleat and rope guide

Wire Stopper Wire

Extension housing Wire Strut

Waterline mark

Main foil Rear foil

Hinge Center of lift

The two water line marks show the change in The handle was designed such that when it was height above the water and the attitude of the boat pulled by a force of about 2 kg while the boat was from the sides. In calm water, the bottom mark foil borne, the lift force generated would balance coincided with the water surface, and the boat could the head-lowering moment of the boat (see Fig. 2- run within a range of ±2.5 cm height from this mark. 4). Symmetric cross section foil was used so as to Compared to the rise of the forward strut, the aft minimize the movement of the lift centre of the strut rose higher with the increase in speed, and the poor-in-accuracy sand-cast foil made of aluminum angle of attack of the rear foil decreased. As the alloy.The handle could be moved by about 30° speed approached 50 km per hour, sometimes the forward and aft, and by its action, the front foil propeller emerged out of water surface sucking air could be turned so as to vary the angle of attack by (ventilating), so the forward strut had to be about 35° downward and upward respectively. immersed slightly. The foil borne speed was about 20 km per hour, and The forward half-tapered foil had a span 40 cm, an the distance after which the boat separated from the area of 450 cm2, foil thickness ratio of 8% and foil water was only 10 m. This is the beauty of fully with a lens-type symmetric cross section. To submerged foils. If the handle is pulled abruptly prevent trapping debris, and to accommodate the after sudden acceleration, the hull separates all at hinge within the thicker part of the wing, the once from the surface of the water, and the foil leading edge was given a sweepback angle of 20°. borne run over the water feels good. The hinge was installed 7 mm forward of the center of lift force. We tried out jumps on the three-strut boat often. After the boat ran at a low height near the surface Fig. 2-5 Exploded sketch of front leg

and picked up full speed, if the control stick was system is changed to give larger movement to the pulled back in one quick motion, the boat jumped, handle, the maximum speed can presumably be and the front foil separated by about 30 cm from the increased. Considering the performance of the water surface. The aft foil could also be made to engine and the hull, we expected speeds in excess separate by a small distance from the water surface. of 65 km per hour, but we did not test the boat for This suggested that when the hull was touching the such a condition. It is preferable to run the boat at water surface, ventilation could be prevented by low speeds to ensure a high level of controllability pushing the stick forward adequately, and in this and to maneuver the boat to suit the observed state we tried to continue with the foil borne waves. running of the boat. However, the hull did not rise but it dropped down to the water surface. We wished to control the boat just as we wanted at a speed of approximately 30 km per hour, so we did In the case of the single-strut hydrofoil, we not try to break any speed records. A speed record expected that since the front strut was long, the boat can be challenged only after installing sophisticated would remain in the foil borne state after a high automatic control devices; we felt however, that jump, but the behavior of the boat after the jump this boat had adequate potential. was unstable, and finally the time for testing the boat was over without any decisive result. Even after 25 years, when I started drawing these explanatory sketches for this paper, I realized that The maximum speed was 50 km per hour. The lift there was not one part or item that needed to be force of the submerged foil is proportional to the corrected or revised. square of the speed, and is also proportional to the angle of attack. Therefore, for attaining steady foil I had put in a lot of time into the design, and borne operation, an angle of attack inversely moreover, I had designed the boat with all my proportional to the square of the speed is necessary. passion and enthusiasm; I had looked at it from For this reason, a very small movement of the various angles and made thorough studies. I think handle at high speeds is required, and fine control is that the frantic simulations I had carried out to not possible within the range of human interactions. finish the boat had contributed significantly to the For instance, if the handle movement for level design. On the other hand, I am also surprised at flight at a speed of 25 km per hour is taken as about myself because my fundamental design concept at 4 cm, then at a speed of 50 km per hour, the the time of graduation has remained practically adjustable range should be 1 cm. If the foil surface unchanged – this is probably because I have not is made smaller, and the leverage of the steering made sufficient progress, or maybe I have not got rid of my little idiosyncrasies.