Peters On (Fast) Powerboats The author’s eponymous Florida-based yacht design firm has developed some of the most successful high-performance production and custom powerboats in the world, across a broad range of categories: flats skiffs, sportfishermen, motoryachts, and offshore raceboats. Here, in Part 1, he presents his office’s consistent approach to the design process. by Michael Peters In the 18 years that Professional BoatBuilder has been producing, or Artwork courtesy of MPYD co-producing, the IBEX trade show, we have rarely repeated a seminar topic, even though the seminar program has offered upward of five dozen sessions per show. One notable exception was Michael Peters’s IBEX ’02 presentation titled “High-Speed Boat Design.” Audience response was so enthusiastic, we asked Peters to present it again, which he finally agreed to do…at IBEX ’08. But he refused to simply reprise his ’02 talk, because technology had evolved, and so had Peters. Having designed several of the fastest powerboats on the planet, he decided to back off from that scary end of the spectrum. What follows is Peters’s IBEX presentation in Miami Beach in 2008, adapted for print publication—Ed. hat exactly is high speed ? The reason I’m showing you this W In 1973, I spent the day wet- comparison is because I think our sanding the bottom of a sloop, get- idea of high speed is ill-defined. Take, ting the boat ready for a race the for example, a 34' [10.4m] Contender next day with a builder I was then outboard-powered center-console that working with. I came home and told does 50 knots. We’d probably all agree my dad what I’d been doing, and he 50 knots is a fast boat; note how the just laughed at me. “That’s absolutely Contender compares to the Reynolds ridiculous—wet-sanding the bottom Number of an F-16 fighter jet. Now, of a boat to make it go faster. You take our firm’s fastest design—a know, Michael, in aeronautics we Tencara raceboat fitted with turbine don’t even start worrying about rivets engines—and it has essentially the until about 250 knots.” same Reynolds Number as the U.S. My dad, being a mechanical engi- government’s SR-71 high-altitude, neer and having worked in the high-speed [2,000+ knots], long-range wind tunnel for NACA—the National reconnaissance plane. Advisory Committee for Aeronautics— So my question is: What is a high- during World War II, calculated the speed boat? Where do you think Reynolds Number for the sailboat to the line is when people start talking see if it made any sense to wet-sand about high speed ? Fifty miles per hour it [Figure 1]. The first thing he found [80.5 kmh]? Well, at my office—which was that the Reynolds Number of a I know is different from the way sailboat at racing speed of 8 knots some other design offices work—we compared favorably with a P-51 compare speed to the length of the Mustang single-seat fighter, one of boat. We take the waterline length, the war’s fastest prop-driven planes. as you would for a conventional sail- He also discovered that the surface boat, and view the waterline in terms smoothness of the bottom made a dif- of percentages. With that, we can ference almost as soon as the sailboat judge all boats, from large to small, in started moving. some sort of nondimensional way. 38 Professional BoatBuilder Slide 2 Figure 1. Reynolds Number 3 ρ x V x L Re = µ This equation non-dimensionalizes vessel speed and length by accounting for fluid properties: density (ρ) and dynamic viscosity (µ) Slide 2 Slide 6 Reynolds Number Figure 2. Speed/Length Ratio ρ x V x L V (knots) Re= S/L = µ L (ft) 4 High speed: S/L > 5.0 Non-dimensionalizes vessel speed and length by accounting for fluid properties: density (ρ) and dynamic viscosity (µ) If you accept Renato “Sonny” Levi’s definition of high speed, it would be a speed/length ratio of 5 and greater [2]. For the purposes of today’s talk, I’m using 5—though six years ago, at that IBEX, I said 4. Since then, I Two highly successful, and influential, large yachts—not designed by Peters—that ran across someone else’s definition meet the marine industry’s generally accepted definition of fast: the Mulder-designed of high speed, which loosely agreed 120'/36.6m Moonraker (top), and the Blount-designed 220'/67m Destriero. with mine, and I decided to change to Moonraker’s top speed is 67 knots; Destriero’s, 60. For a detailed account of the the higher number. latter’s design development, see Professional BoatBuilder No. 109, page 100. Okay: Take the waterline length, take the square root of it, and you can calculate the speed/length ratio. bottom the way Moonraker is up on For a 100' [30.5m] waterline, the hers. This observed phenomenon is square root is 10; if you’re doing 30 purely indicative of relative size. knots, the S/L ratio is 3. In theory, speed varies as the cube Let’s look at a boat most people of horsepower divided by weight. If would consider fast: the Mulder- all factors are equal, eight times the designed 120' [36.6m] Moonraker [3], power is needed to double the speed. capable of 67 knots. Her S/L ratio In reality, things don’t stay the same; is almost 7. Or consider the Blount- we can do much better. As we’ll see designed 220' [67m] transatlantic in Part 2, a modern racing catama- record-holder Destriero [4]; her S/L ran can achieve a speed equal to the ratio is 4. Destriero does 60 knots; square of horsepower divided by I think we’d all agree that’s pretty weight. fast. But if you put Destriero on a There are several ways to calculate graph and actually chart out what S/L the speed of a planing boat. Crouch’s ratio you’re sitting in, you find that formula is commonly used by many Destriero does not fit our definition of in the industry as a shorthand method high speed [8]. for predicting speed [5]. Sonny Levi’s Also, notice the way Destriero is formula is what I started with, in the running compared to Moonraker. 1970s [6]. But the formula that we Moonraker has much more boat employ in our office today is one out of the water than Destriero has. taught to me by Eduardo Reyes, a Remember, there are different speed Cuban who escaped the island when zones: an S/L ratio of 5 is something Fidel Castro took over [7]. of an arbitrary line, and if you lower Actually, what we find in our practice the ratio to 4, Destriero is clearly is that any one of these formulas will inside it. My point is, because of her work; it’s a matter of becoming fully size, because of her length, relative to familiar with the one you’re using. that length, Destriero is planing, even They’re all fairly simple. But in all though she’s not really up on her three formulas, understanding what august/sePtemBer 2010 39 Slide 2 Slide 10 Reynolds Number Figure 5. Performance Prediction Figure 8. ρ x V x L Crouch’s Formula: Re= 75 µ C 70 mph = Moonraker 65 S/L = 5.0 Non-dimensionalizes vessel ∆ HP 60 speed and length by accounting High Speed Destriero 55 for fluid properties: density (ρ) Where: and dynamic viscosity (µ) ∆ = Displacement of vessel (lbs) 50 Full Planing C = Constant based on 45 S/L = 3.0 40 Slide 2 boat type (180–200) Slide 11 35 30 Reynolds Number Figure 6. Performance Prediction Transition Region 25 S/L = 1.4 Sonny Levi’s Formula: 20 ρ x V x L Speed (knots) Vessel Re= µ 15 mph = K x SHP 10 Displacement Non-dimensionalizes vessel ∆ 5 speed and length by accounting Where: 0 for fluid properties: density (ρ) ∆ = Displacement of vessel (LT) 25 50 75 100 125 150 175 200 225 250 and dynamic viscosity (µ) K = 3.9–4 for single shaft 3.5–3.6 for twin shaft Waterline Length (feet) 3.2–3.3 for three shafts Slide 2 3.0–3.1 for four shafts A graphic representation of performance designations based on vessel speed-to- Slide 12 length ratio. Note the relative positions of Moonraker and Destriero in this context. Reynolds Number Figure 7. Performance Prediction where you want and you’ve got your around, Where do you think this boat ρ x V x L engines where you want and all the ought to balance? One guy will say, I Re= Eduardo Reyes’s Formula: µ rest. You end up with a longitudi- think it ought to balance around 6.2. HP x 46.4 nal center of gravity, and you say, And another one says, I think our V (knots) = K Okay, now I’m gonna design my boat drawing puts it at 6.5. Non-dimensionalizes vessel ∆ around this. Well, what if that LCG It doesn’t matter what length boat speed and length by accounting Where: is just plain wrong to begin with? we’re talking about. What matters is C for fluid properties: density (ρ) Because the center of gravity varies the speed-to-length ratio. The physics and dynamic viscosity (µ) ∆ = Displacement of vessel (LT) ∆ K = Constant based on with the speed.