Explaining 1/8th Scale Hydroplanes

One eighth scale Unlimited hydroplanes are scaled down versions of any full size Unlimited hydroplane that has run from the 1940s to the present. In UNW the boats are registered to a particular owner and no one else may race that boat in UNW competition. The owner of a registration pays an initial, developmental and yearly registration fee to keep his boat eligible for competition. Most registration fees are paid at the UNW winter meeting in January. All boats that move from the developmental to the competition stage are inspected. The UNW inspector pays particular attention to the dimensions, paint and graphics as to their scale authenticity. The boat will also be inspected to make sure that the equipment installations are adequate for the rigors of racing.

The models are constructed on a 1 and a half inch to the foot formula. The boats can be 42" to 57" long depending on the length of the full size boat it is being modeled after. UNW allows a 5% plus or minus deviation in length and a 10% plus or minus deviation in width, afterplane length, pickle fork depth and depth. There are plans available for almost any boat one would want to build. Most boat builders find that a boat of minimum length and depth coupled with the maximum allowable width make the best performing models. Our boats carry a much higher power to weight ratio than the full size boats.

The boats are usually constructed of wood, fiberglass or a combination of both. The cowlings, wings and uprights are nearly always made from closed cell foam and fiberglass. Almost all of the boats are either hand constructed from wood or hand laid up with fiberglass. Some structural elements are often made of Kevlar and fiber or aluminum honeycomb. While Dumas makes some Unlimited hydroplane kits, the sport has evolved to a point where these hulls are for the most part no longer competitive because of their narrow width and balance. Most boats that are currently being built weigh in the neighborhood of 11 to 14 pounds. When the sport first started the average boat weight was twenty pounds or more. Naturally the current crop of boats far out perform their older cousins in terms of top speed, acceleration and cornering ability. Almost any of the current front line equipment will have a top speed of 60 M.P.H. or more depending on how they are propped.

The boats are controlled via radio control. The radio control (or r/c) system is composed of a hand held transmitter (there are two varieties, stick or wheel) and an on board system which includes the receiver, two to three servo motors (rudder, throttle, fuel mixture control), an on/off switch harness, battery and antenna. The on board system is contained in a waterproof box with mechanical seals for the pushrods or cables that control the various functions. All of the radios have replaceable frequency crystals (up to 20 different frequencies) in the transmitter and receiver. Boats on the same frequency never run together at the same time. The club has a frequency tree, which has pins that represent the various frequencies. A transmitter is turned on only if the person has the pin for that frequency clipped to their transmitter antenna. In the event that there is a conflict the driver with less accumulated points must change their frequency and then inform the contest director as to the frequency they are now using.

Engines have a maximum displacement of .67 cubic inches or 11 c.c.s. The engines are two cycle, water-cooled engines especially made for model boat racing. The fuel is a mixture synthetic or natural castor oils, methanol and 40%-65% nitromethane. The engines can run such high ratios of nitromethane because they are water-cooled. Put this same fuel in r/c airplane air cooled engine, and you would melt it down in a couple of minutes, (it would be one fast airplane for a little while!). The water is provided from a tube that is suspended down in the water from the boats transom and is forced through the engine and overboard, or by a wedge rudder that is drilled with a inlet water hole for the same ram effect. What many people find most surprising is the power these little motors put out - 6.5 plus horsepower at 25,000 to 28,000 r.p.m. or 400 + revolutions per second. All the boats use a tuned pipe that is concealed inside of the boat to help achieve these phenomenal r.p.m. ranges. One of the primary tuning methods is to vary the tuned pipe length by sliding the pipe in and out of the exhaust header and coupler. The engine flywheel has a groove cut in it to accept a quater inch wide starting belt. An electric starter that has a similarly grooved pulley is used to start the boat on a bunk (a wooden cradle the boat sits on to start and transport it). All bunks must be enclosed in the back in case a propeller should come loose. The boat is then picked out of the bunk and launched by an assistant after checking with the driver that the rudder is functioning.

These boats pack a big wallop in terms of speed, sound, spectacle and fun. The competition is steeper than the full size boats when one takes into consideration of how many boats (25 is not uncommon) arrive to compete at every event.