The Multiple Point Hydro
The prescription for a winning race boat surely calls for a combination of speed, acceleration and the ability to turn. In my opinion, these qualities can best be attained in a well-designed and properly balanced multiple point suspension hull.
The real nemesis of boat speed is the resistance encountered in trying to propel a boat through, or on the surface of, the water. Since your normal water highway has a density 800 times greater than air, You can really reduce that speed-stealing resistance by making your boat ride on air.
An approach to true airborne performance is only possible in a multiple point suspension hydro with its characteristic air-trap between the sponsons. Even with a multiple pointer, a proper angle of attack of the sponsons and correct weight distribution are necessary to attain flying behavior. The faster multiple point hydros ride with their entire after plane completely out of the water. The boat is supported forward only by the very trailing edges of the sponsons. The Slo-Mo-Shun IV at top speed has approximately 8 square inches of wetted area. Compare this with the many square feet of wetted area on a conventional step hydro!
This airborne ride probably explains why, in the Gold Cup and Harmsworth Races, we used less than one-third as much gasoline as some of our competitors. Our consumption was 20 gallons in each 30-mile heat of the Gold Cup event, 32 gallons in the first 40-nautical mile Harmsworth heat and 43 gallons in the second heat when our speed averaged over 100 m.p.h. Not only is fuel saving a help to the pocketbook; the resultant weight reduction in itself enhances the boat's speed.
Many times a race can be won by a boat with fast acceleration even though her top speed is not as great as her competitor's. The Slo-Mo-Shun goes from 0 to 125 m.p.h.
conventional hydro at low speeds, which accounts for its more rapid acceleration. There are other contributing factors which, however, are too technical to explore here.
There was some question in the minds of the experts at Detroit as to the turning ability of Slo-Mo. It's my opinion that it takes less intestinal fortitude to throw such a multiple point suspension boat into a fast turn than to do it with a conventional hydro. I have made a good many turns in both types of boat in the past 23 years and those that were not completed were attempted in a conventional boat. Those impromptu baths I received led me to develop hulls with sponsons. The sponsoned boat's great width and large non-tripping areas both forward and aft tend to give you stability and speed on the turns. The driver can feel when it is about to upset and has time to avert a spill with a slight turn of the wheel.
No longer being a youngster, I'm more or less looking for comfort. My choice in designs gives me a much softer ride due to the film of air between the bottom and the water. Also the contact with the water—rough or smooth—is made by either or both sponsons which have a great amount of dihedral on their forward areas to part or knife through the rough stuff. Many of the people who have been given rides in the Slo-Mo remark that she rides easier than any runabout.
Slo-Mo-Shun IV is definitely not the result of a few lucky lines drawn in a single night. Nor was it the magic touch of a good builder that gave her the speed necessary to establish a world's straightaway record and to win races. That this touch did produce was a hull that was exceptionally light and extremely rugged.
'The prototype of her speed-producing sponsons grew from the "outriggers" I built on to my conventional hydro in 1934 in an effort to eliminate clumping on a turn. Later that season I deepened and widened these projections so they could carry the weight of the boat, thereby lifting the bull out of the water's clinging drag. The increase in speed was startling.
I had built many conventional hydroplanes in previous years, yet my interest in this single step type of hull had always been weak as the only way to gain speed was to add more horsepower. Now, with the sponsoned hull, I had the secret of greater speed without increasing the horsepower. This became interesting.
Every spare moment from then on was used in building and rebuilding racing hulls; experimenting with shaft angles, rudders, struts, steering ratios, fins and propellers, and with different lengths, widths, depths and angles of attack of sponsons. Trapping air under the boat that would support the weight of the entire boat, designing a spoiler to keep the nose down, and many more factors contributed to speed and stability. Every succeeding boat was more stable; faster on the straightaway, accelerating and around the turns.
This work commanded my whole interest—made me lie awake nights with a pad and pencil nearby to jot down notes and to sketch out a new idea so that I would have it the next day. There were no text books available to yield this information. Trial and error was the method; enter as many races as possible and see what change was needed next.
By 1940 I felt capable of drawing up a Gold Cupper. Being financially comparable to the proverbial "church mouse," I had to shelve this drawing until 1948 when I convinced Stan Sayres that I could produce a boat that would move rapidly and safely over the water.
Slo-Mo-Shun IV has but scratched the surface of the eventual speed a multiple point hydro is capable of attaining. On the other hand, it is apparent that the only solution in getting more speed from a conventional hydro is by adding horsepower as proven in recently built boats of this type.
The superiority of the multiple point hydro as a racing boat is proven by the records it has established in classes from the little 48 cubic inch jobs up to the unlimiteds. In the case of the latter, I am looking forward to a change in the rule book which will encourage development of super hulls without resort to brute horsepower. Should that come to pass, the multiple point suspension design would become even more universally appreciated than at present.
(Reprinted from Yachting, April 1951)
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© Leslie Field, 2006