Roostertails Unlimited: [1973]
Chapter 5 - The Powerplant

Ch.1 A Race
Ch.2 A Little History
Ch.3 The Evolution Revolution
Ch.4 The Principle
Ch.5 The Power Plant
Ch.6 Building a Hydroplane
Ch.7 The Crew
Ch.8 The Men With the Money
Ch.9 What's an Unlimited?
Ch.10 Preparing the Race
Ch.11 The Rulebook
Ch.12 The Spectator
Ch.13 First You Get Into the Cockpit
Appendix A Unlimited Class Speed Records
Appendix B National High Point Champions
Appendix C Major Races

The most important element in the operation of the hydro is, of course, the engine. Many models have been used but only two have had any element of success -- the Allison and the Rolls-Royce Merlin. Most arrived as surplus at the end of the Second World War and became the basis of powerplants before long.

The Allison engine was built by General Motors and had powered the Curtis P-40 "Warhawk", the Lockheed P-38 and the North American P-51 "Mustang". This engine was the first one used in Slo-Mo-Shun IV and actually was the main power of the majority of hydroplanes in the early days after the war. It is slightly larger than the Rolls Merlin in displacement and has a higher stock horsepower.

The Rolls-Royce Merlin was built by Packard and served as the powerplant for most of the British fighter planes, including the "Spitfire" and the Hawker "Hurricane". This engine now serves as the power for the vast majority of the hydroplanes of today mainly because of their superior supercharger, a unit which compresses the air-fuel mixture before it is sent to the cylinder to be ignited, thus giving the engine about 500 extra horsepower.

On the Allison the supercharger has a single-stage, one-speed "blower". The Merlin has a two-stage, two-speed supercharger besides having an inner and after cooler in its intake system, which means the supercharger is surrounded and cooled by a water jacket. Between the engine and the ‘charger’ the air-fuel mixture is compressed to 44 pounds per square inch pressure, goes through the inner cooler which stabilizes the mixture temperature and prevents the expansion of the intake gases too son. This gives it more power. The infamous "quill shaft", that has eliminated so many boats through the years, is a thin rod that connects the supercharger to the power unit.

Inside the supercharger are impeller blades that whirl around at above 35,000 r.p.m. — near supersonic speed. These razor sharp fans get white hot instantly and can "grow" enough to fuse to the side of the chamber and cause a supercharger malfunction. If the boat should leap from the water there results an instant overspeeding of the engine with an accompanying increase in supercharger speed; this is when the "growth" occurs.

To further compare the two types of engines, we find the Allison has a higher low-speed torque, in fact, at 100 m.p.h. it has more power but has a slower rate of acceleration. The Rolls Merlin, on the other hand, has increasing power with rising speed, which will not slack off. The Merlin is also 250 pounds heavier and must, therefore, be mounted several inches behind the location of an Allison.

Both engines are V-12s and have about the same outside dimensions. Each cylinder of both models has two exhaust and two intake valves and are ignited by a couple of spark plugs which are fired independently by a pair of magnetos. They have either fuel-injection or a standard two or three barrel carburetor, have seven main bearings and are liquid cooled. The water is fed to the engines through water intakes that hang from the back of each sponson. Most engines are designed for a constant water pressure of 25 pounds per square inch. At high speeds, when the pick-up pressure is greater, a spring-loaded relief valve opens to keep the pressure at a constant. The engines had cost about $45,000 apiece for their respective governments to produce. They now sell for less than $10,000.

To transform the engine from airplane use to hydroplane use it must be inserted backwards, the supercharger is turned upside-down so that the carburetor sits atop the engine instead of on the bottom, as it was in the airplane, and the propeller gears are replaced by a special gear box manufactured especially for the sport.

Other engines have been tried through the years. The Rolls-Royce Griffon, which powered Canadian aircraft and is still used in some British craft, is larger and more powerful than its sister the Rolls Merlin. It was used in the Supertest entries and was very fast, then later in Harrah's Club and Budweiser Malt Liquor. They are now mostly controlled by the Budweiser organization. Austin Snell tried to drop a Daimler-Benz Mercedes engine, from the ME-109 Messerschmitt, into his Coral Reef. More than one problem was encountered in this venture, not the least of which was the necessity to mount it upside-down.

In addition to the most popular Allison models: the G-6, the-113 and the -111, the old Scooter Too had a special model W-24 Allison which had 24 cylinders, the first $ Bill had a rare -1710 model and Wildroot Charlie ran on a -117. The Gale VII used a V-16 Packard but it never panned out.

A couple owners have actively tested turbine engines for the Unlimiteds, none successful enough to compete with the rest of the field. Westinghouse and General Electric turbines were used in these tests. Early in 1973 the plans for another turbine powered hydroplane were introduced. The craft uses twin Lycoming T-53 "Huey" helicopter engines, which put out 1700 horsepower and weigh only 465 pounds.

The automobile engine has been tried on numerous occasions. In 1956, a boat named Miss Skyway was powered by two Chevy V-8s with a V-belt drive to the prop shaft ...she never went faster than 45 m.p.h. Miss U, the "one-point" boat, was powered by a six cylinder Ranger. Chrysler Crew appeared in 1966 with twin Chrysler Hemis as its powerplant, she was more successful than most but an Allison eventually replaced them because of a strike at Chrysler Motors in 1968. The idea caught on and in 1967 The Dutchman was powered by a pair of Fords, she failed to qualify. Ron Jones built a boat in 1969 that had twin Chevrolet Mercrusers, but it never raced, then in 1970 two boats appeared that were auto powered: the Pride of Pay 'n Pak and Atlas Van Lines, U-29. Both were powered by Chrysler Hemis and both had them replaced by conventional power when the season ended.

There is presently a revolution brewing in the engine development of Unlimiteds...the turbocharger. Tested a great deal at Indianapolis this concept was first introduced in a manner especially made for Unlimiteds by Bob Fendler on his Allison powered Lincoln Thrift in 1972, the same hull mentioned above as Atlas, U-29. A different version was used on Harrah's Club in 1965 but its particular set-up never evolved and was replaced. The turbocharger takes the place of a supercharger and can be adjusted to accommodate the situation, be it qualifying or racing, and is much quieter than its supercharger cousin.

Instead of compressing the air-fuel mixture with the mechanical power of its own engine's revolutions through the quill shaft, as does the supercharger, the turbocharger compresses the mix with power supplied by the exhaust. When the engine with a supercharger is throttled back, the power continues to be produced at the same rate but with no r.p.m’s to take it up. With the turbocharger, this excess power is vented off through a special valve which can be closed when all the power is again needed. To date the turbocharger idea has worked very well and is becoming more popular.

There have been many configurations of engine placement in the hull also. In 1953, the Gale III had a single engine but two contra-rotating propellers from one gear box. Later, in 1954, a boat named Miss Wayne had a pair of Allisons placed side-by-side with contra-rotating props. There have been many famous attempts at tandem mountings, few successful. Jack Schafer insisted on this design in most of his Such Crusts and it was also used in both Gale VI’s. Both applications were always being plagued with mechanical troubles - especially in the gear box. The notable exception is Miss Pepsi which was quite competitive with its twin engines.

Let's look at the performance of a typical engine used in a hydroplane. In racing trim the Rolls-Royce Merlin displaces 1650 cubic inches, has a bore and stroke of 5.5" x 6.0" and a supercharger ratio of 5.80:1. The engine can turn at 4200 revolutions per minute, though it was designed for about half that amount in airplane use, and coupled with a gear box ratio of approximately 3:1, the shaft rotates at over 12,000 revolutions per minute. As it races the Unlimited powerplant produces about 2500 horsepower.

The engines are made for the most part of lightweight metals such as aluminum and magnesium, and with certain internal modifications the performance is much higher than for what they were designed. As a result they must be overhauled completely after operation for only an hour, sometimes less, compared to every 300-400 hours while in their original role.

To feed this monster is gasoline that fills two 40-50 gallon tanks on either side of the cockpit and engine compartment. The fuel tanks are made of welded aluminum wrapped with reinforced fiberglass so that if a crack should appear it can be repaired with a little resin and cloth between heats. As the boat goes through the turns at high speeds the fuel is thrown high against the wall of the tank. This leaves a void and fills the fuel lines with air which causes a backfire with slowdown or can even blow a part of the engine off. To help eliminate this problem, there is a partition that traps some fuel over the fuel line intake within the tank.

The fuel type is categorized according to a system which refers to its comparison with the highest known anti-knock product at the same the scale was created ... iso-octane. An octane number of 100 was considered the equivalent of pure iso-octane. Fuels have surpassed that scale and a high test aviation fuel with a performance rating of 115-145 is used as the base in most hydros. The "115" refers to the lean figure while the "145" is the performance possible with a rich mixture setting. Your car probably uses a fuel rated at nearly 100 but some cheap gas can be as low as 80.

Added to this fuel are many things, including alcohol. Some crews add a couple cc's of tetra-ethyl lead to help govern the rate of burn in the engine and boost performance. The Hawaii Kai III used a "super" mixture which blended two select and pure hydrocarbons in a 60/40 ratio, this combination gave it a performance rating of 176-196. The engine loves the fuel and consumes almost five gallons per mile would never win in a good mileage contest.

To give the boat an extra boost as it exits the turn a squirt of "laughing gas" is given the powerplant. Also known as Nitrous Oxide, this substance causes a large explosion in the engine and a quick spurt of extra power. It cannot be injected for more than five seconds at any one time, however, because it would blow the engine apart with the higher manifold pressure created. This idea was first used in Miss Exide in 1963.

An Unlimited Hydroplane engine holds 15 gallons of oil to keep it well lubricated and to start the thing is an electric starter motor somewhat like the one in your car. The difference is that it takes two 24-volt batteries to do the trick - these are hidden beneath the driver's seat.

(Reprinted from Roostertails Unlimited by Andy Muntz, 1973)

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This page was last revised Thursday, April 01, 2010 .
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Leslie Field, 2000