Roostertails Unlimited: [1973]
Chapter 4 - The Principle

Introduction
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
Glossary
Bibliography
Appendix A Unlimited Class Speed Records
Appendix B National High Point Champions
Appendix C Major Races

What, you might ask, does a 3-point suspension mean? It's self explanatory really ...when the hydro is running only three points touch the water - the very rear of each sponson, which resemble pontoons, and half of the propeller. The basic premise is a combination of the airfoil and the hovercraft.

As the hydro picks up speed the shape of the hull creates lift, much like an airplane wing. The principles of aerodynamics are complicated, but simply, because the air flow is slower going beneath the hull than over the top a certain lift is accomplished. The boat rises and sits atop the water, or is "on the step", and a cushion of air gets trapped beneath to keep it there. A downward air pressure on the deck keeps the hull from continuing its climb and remain on the surface. The downward pressure and the upward lift compensate for each other in a properly balanced boat.

Behind each sponson is a plate that helps in the containment of the air between and behind the sponsons. If this space holds the air too well and overcomes the downward pressure, the boat will tend to fly, or "kite". If the "air trap" does not hold enough air the boat will ride so that she is not flying and thus an excess amount of water must be pushed aside which in turn causes drag and a reduction in speed.

Some variations of this basic idea have been used from time to time. Many of the modern hydros sport what is known as a "fork lift" bow or "pickle-fork", which means the sponsons extrude beyond the deck. Because the leading edge of the airfoil does not come to a point, as in the conventional bow, but to a blunt end, as in an airplane wing, the boat will get greater lift and will ride higher, thus a smaller air trap is needed. It also reduces the weight in the bow, shortens the boat's length and makes the boat less susceptible to disintegration from nosing down.

As the hydro goes into the turn the air beneath the boat escapes as it slows down and she begins to ride with greater resistance from the water. With the fork-lift bow the boat will not "spill" as much air thus it retains more of the 3-point suspension and travels faster through the turn. It also will accelerate faster coming off the "exit pin" for it does not have to pick up as much lift and can get-up-and-go in less time. The drawback? What the fork-lift gains in the turns it loses in the straightaway for it rides much too light for the higher speeds. In 1972 Bob Gilliam introduced his Valu Mart which is designed as a compromise between the standard bow and the pickle-fork.

Since the success of Slo-Mo nearly all hydroplanes use her hull design but there have been attempts to the contrary. In 1957 a Seattleite, Armand Swenson, designed a "one-point" hydro which he called Miss U. It had a horizontal stabilizer, much like an airplane wing, and had pulling, contrarotating propellers instead of the pushing variety as in most other boats. It sank during its launching. As late as 1967 a boat named Gale’'s Roostertail was built in a "four-point" or "step" configuration, much like Miss Pepsi. It failed to qualify and also never raced.

In addition to the sponsons and the air trap other smaller details add to the better performance of the boat. A look at the transom, the boat's stern, will show a trapezoidal shape, the top, the bottom and the "non-trip" which angles toward the bottom. Though it looks unassuming the non-trip, which is incorporated into the hull, prevents the boat from destroying itself. As the boat goes through the turn it slides sideways to a certain extent, if the side of the boat were square it would catch and flip the craft. Decide for yourself which barge would go through the water easier, the one with a vertical bow or a slanted bow ...it follows the same principle.

There are two basic transom designs: the low profile and the standard transom. The later type was used in most early hydros because it was thought to be more stable. But recent wind-tunnel data indicates that with the deck area at the stern almost equal to that in the bow the nosing down tendency is reduced. Thus the low-profile transom was introduced. It is much wider than the standard and also gives the stern better lift with the additional area, and is usually no more than a foot in height.

There are also two kinds of sponson: the wet and the dry. A wet sponson is one which water may enter through a large hole in the back of the member, whereas the dry sponson has this hole covered. The dry has lately been considered the more efficient and is usually designed into the modern models.

A few boats have what is called a "non-trip chine"; Ron Jones, for one, always includes one in his designs. On the inboard sponson (the left one) the inside edge is beveled at the bottom. This lets the boat slide through the turns better and reduces the pitching effect. It was popular to have "Hallet drop sponsons" in the early 60s, this consisted simply of the top of the sponsons stepped below the level of the deck ...some designers felt this would give a smoother ride. Many boats also include a "spoiler" between the sponsons and below the bow or on the side of each sponson. This member distributes the air evenly beneath it and also provides a smoother ride, some push the bow out of the water in case of a nose down while others prevent the boat from bouncing from side-to-side. There are as many variations in the design of spoilers as there are purposes.

In 1956 Ted Jones designed a boat named Thriftway Too which deviated from the standard hydro in a big way, the driver sat in front of the engine. The idea was first tried in George Sarant's Etta in 1952, a catamaran with a split stern, but mechanical problems halted the operation before it really started. The principle is known as a "cab-over" which actually is superior in many ways. For the driver it reduces engine noise, heat and fatigue, he has better visibility, a smoother ride and greater general safety.

Because the engine sits behind the driver the center of gravity for the hull is moved back accordingly. This shift provides higher speeds through the turns and quicker acceleration coming out of them. As with the fork-lift, however, the chute speed is hampered so it therefore provides a balance between the two basic configurations. Many drivers dislike the cab-over because they cannot relate the handling of the boat to the short distance they must look over. In a standard design the driver can see the reactions of his hull better as his sight covers a greater length of the boat.

The placement of the engine in the hull is very important, one inch forward or behind a certain location and the boat can prove inoperative. When a new boat is brought into the world the power plant will always sit well forward in the engine compartment. As the boat is tested the engine is continually moved back until it reaches that point where it rides perfectly. If a mistake is made and the engine ends up too far to the stern the entire prop-shaft must be replaced because one can only take away from a shaft, not add on. The propeller shaft is made of 1" diameter"K" Monel and the shaft bearings are lubricated with water pumped under pressure from the engine.

Before each race much testing is done to find the best prop for the water ...it can make all the difference in the world. To a driver the prop, or "wheel", is the most important single piece of equipment; they are protected and babied. When he travels, some drivers are known to keep their wheel with them at all times, take it on board the plane and not let anyone tamper with it.

To ride properly the boat must be balanced correctly. The weight distribution can be altered by a couple of methods: the most drastic effect comes when the engine is moved within the engine compartment, but changing the length of the prop shaft or changing the angle of the propeller blade would also balance the boat correctly. Testing before the season is usually for the purpose of finding the correct combination of engine placement and prop blade that will give the best ride.

The props are made of forged steel and cost upwards of a thousand dollars apiece; they are usually built in Italy and must be perfectly balanced. When the propeller spins at over 12,000 r.p.m., roughly 200 times a second, the slightest nick can throw it far enough off balance to twist the shaft like a pretzel - some boats have sunk when their prop was tossed through the bottom - and yet these propellers are over 25 times stronger than those on a pleasure craft. Depending on the condition of the water, be it rough, choppy or sticky, different blades must be used. (The term sticky is used when the water is smooth which does not let enough air beneath the boat and saps valuable power from the engine and speed from the hull.) The basic prop is about 13" in diameter and has a pitch, the twist of the prop blade, anywhere from 19" to 23".

It is the propeller that creates the symbol of the hydroplane ... a roostertail. As I mentioned before, the propeller is only halfway in the water when it is up to speed; this concept is known as "prop-riding". Because of this particular situation the blades of the prop, or "flukes", are repeatedly going in and out of the water. Each time the blade comes out it kicks spray high into the air and by the time it falls and hits the water again the boat has traveled a good many yards beyond that point. The combination of the spray rising and falling causes the formation of an arch of water known as a "roostertail".

The roostertail is not only beautiful, it can be a fearsome weapon in the hands of an expert driver fighting for an advantage over another driver and is a very good indicator of the boat's performance. The longer the 'tail', the faster the boat's going. A smooth looking 'tail' denotes a smooth ride and conversely a jagged one tells that it is pretty rough and the boat is bouncing. A thin, wispy 'tail' says the hydro is riding too light because not enough prop is in the water, a heavy short roostertail means it is riding too heavy and that the shaft is probably dragging.

The principle of prop-riding also causes a problem known as "propeller torque". Because the prop rotates in a clockwise direction, every time the blade strikes the water it pushes the transom to the right which causes a constant left turn. To counteract this effect the rudder is placed on the left side of the transom. The rudder is also made of forged steel and is coated with Cadmium in many instances ...they also cost more than a thousand dollars. In competition the hydro will only turn towards the left, which is another reason for the rudder's placement. An evasive move to the right is possible but cumbersome and unresponsive in handling, as one must fight both the prop torque and the left sided rudder to do so.

When the boat goes through a turn it slides sideways a little, this cannot be stopped. Near the rear of the inboard sponson is what looks like a butcher knife blade hanging down about 6", it is called the "skid fin". It prevents the boat from sliding sideways too much; instead, it shifts the centrifugal force toward the front and in turn gives better control and keeps the boat on the apex of the turn. The placement of the skid fin is very important to the hydro's performance as well as is the size of the fin. Lately some camps have been using a very deep skid fin, 13" to 15", which is located on the outside of the sponson; this idea has been quite successful and is sure to be used more in the future.

On the stern is a "tail fin". Not only does this provide more surface on which to paint advertising to catch the eye of the spectator, it also serves as a stationary air rudder. Its purpose is the same as the tail on an airplaneto let it proceed straight ahead and prevent the stern from bouncing back and forth. Some boats have included an adjusted surface, known as a "trim tab", on the tail fin to help counteract the propeller torque and keep it on a straight course - Slo-Mo IV and Thriftway used one for example Whether or not it actually works is subject to debate.

As in most aspects of Unlimited design there have been variations in tail fin types too. The third Miss U.S. sported a dovetail and in 1968 the Schoeniths introduced their Smirnoff. Wind tunnel tests show that by taking a pair of tails upward, as in a Bonanza airplane, the boat's horizontal and vertical stability are improved. This theory was used on the Smirnoff but has not been used since.

In 1973 a boat was being built which uses many different ideas. Besides having twin turbine engines the boat (U-95) has triple tail fins with an air rudder and a horizontal stabilizer across the top of them that uses a trim control. The new Pay 'N Pak that was built the same year also uses a racecar-type stabilizer over the twin tails. This wing brings the stern up out of the water in the turn which increases the cornering speed.

The engine noise in the cockpit of the turbine craft is quiet enough to permit radio communication between the shore and driver during a race and the crew also use audio-visual equipment, with data linkage, that gives continual read-outs and keeps a record of the boat's performance for use by the crew.

(Reprinted from Roostertails Unlimited by Andy Muntz, 1973)


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Leslie Field, 2000