Wheels on Water
Total success in any motorized competition, whether automobile, motorcycle, snowmobile, or personal watercraft, is governed by a set of very important rules. First, there is the desire to win. Second, the knowledge of how to win. Third, having plenty of usable power and proper traction.
Traction gives your vehicle the ability to translate the power generated by the engine into forward motion. In other words, the better your traction, the faster you'll go. For cars and motorcycles, improving traction means keeping better contact between the tires and the track surface. For personal watercraft, traction is largely a matter of keeping your jet pump in the water, and making sure the jet pump is processing water properly.
All of the power developed by an engine is useless if it is wasted on an improperly maintained or badly constructed jet pump system. Whether you are a sport rider or pro racer, the objective is the same - achieve 100 percent traction to build forward momentum.
The construction of any modified craft should always include the jet pump, and this can be a frustrating and expensive project. The large variety of parts such as intake grates, ride plates, impellers, and nozzles produced by so many manufacturers can cause a newcomer to seek out psychiatric help. By the time you figure out what works or doesn't work for you, a second mortgage on your home may be necessary to pay the bills ... just kidding.
Because of the cost factor, it is important to have periodic testing and evaluation of these parts by independent watercraft builders. When purchasing any item, make sure the seller will stand by his product and its performance, preferably with some type of guarantee.
There are a large number of totally useless parts on the market. Be wary of smooth sales tactics, because most salesman do not have mechanical training or knowledge.
To build a reliable, cost effective, practical drive unit, we first have to recognize that there are a number of manufacturers who produce truly outstanding products. These people have dedicated their life and resources to produce excellent parts, and I have seen this in companies like Skat Trak, Ocean Pro, Westcoast, Coffmans, and Jet Dynamics.
When modifying a jet pump, I don't take any shortcuts. If you do a professional job the first time, you won't have to update yourself later. Don't be lured into installing less expensive parts - remember, you get what you pay for. It's better to pay a little bit extra now than to pay a lot later.
To give you an overview of how a jet performs its designed function, there is a lot you need to know. The whole idea of the jet pump is to flow a desired amount of water through it at any given speed or pressure, and this is done by impellers, which come with pitches between 13 and 20+ degrees. For most applications, impeller selection is made according to available horsepower, rider weight, and top speed desired.
In general, lower pitched props permit higher engine rpm. Consequently more torque, used for acceleration, is developed, which is better for closed course or slalom racing.
For drag racing, high top speed is desired, so a higher pitched prop is used. The jet pump nozzle is selected according to the desired pressure. A longer nozzle will produce more pressure, resulting in more speed. A shorter nozzle will develop less pressure, which is good for lower speeds.
Variations in nozzle diameter also control water pressure. Larger diameter nozzles develop less pressure, but more flow.
Intake grates also have a great effect on jet pump efficiency, because if insufficient water is delivered to the prop, cavitation will occur, causing a loss of power.
Part of the process is handled by ride plates that determine the horizontal position of a watercraft's bottom. Lowering the plate in the rear will cause the nose to dive deeper. The ride plate has a significant effect on watercraft speed and stability.
Begin your jet pump work with impeller inspection and modification, followed by the blueprinting of the duct section, which involves matching related parts to the same diameter to ensure minimum flow turbulence. Then, bore the nozzle to produce the desired pressure.
Upon installation in the boat, the intake grate is also matched to ensure maximum flow. Before installation, you may take this opportunity to install an extra cooling line, if needed. The extra line can be installed on the opposite side of the pump, 180 degrees from the factory installed unit.
Finally, the pump must be sealed with silicone to prevent cavitation and air leaks. Do this by spreading silicone around the edge where the pump meets the inside of the hull and also on the bottom next to the ride plate. Once the jet pump and ride plate are securely fastened, wipe off the excess silicone.
And I can't overemphasize the importance of using high quality impellers. Aluminum props can cause serious damage to the jet pump. When a watercraft is in stock form, an aluminum prop does an adequate job. However, if the engine is modified, jet pump velocity increases significantly. The increased water pressure can eat into an aluminum prop near its center hub, weakening the blades. If not replaced frequently, a blade can break off, causing complete destruction of the jet pump unit - a costly occurrence. A wise move would be to install a stainless steel impeller right away.
The second investment should be the purchase of an intake grate and ride plate. There are a number of good products which perform satisfactorily, but the one that I'm using comes from Jet Dynamics. There are two parts which must be set up together, the top loader intake grate and a specially made ride plate. The ride plate and intake grate angles effect handling and speed. To increase speed you can take off 1/32 of an inch at one time from the small rests in the bolt area, test and re test until satisfied. When properly set up,you get superior handling
The final stage in jet pump modification consists of selecting proper nozzle diameter. Increased performance demands precision jet stream control. There are many excellent products available. Remember, quality parts cost more but if you project that over a long period of time, they are the best investments you can make in your watercraft. Take your time and shop around when looking for performance. Ask a lot of questions. Compare products to ones you may already be familiar with. There is no substitute for on the job training.
George Grabowski HPT Sport USA