Physics of Personal Watercraft

Physics of Personal Watercraft

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Personal Watercrafts or "jet skis" are basically Personal Watercraft (PWC) are basically small inboard boats able to travel at high speeds due to large amounts of power and very light weight. Alomst all PWC's are under 600 lbs and most of todays PWC's have at least 90 hp.Not only are PWC's some of the fastest water vehicles they are also some of the most maneuverable water vehicles. This is because PWC's propultion is based on a jet that also is it's turning mechanism. When the driver turns the handlebars the jet (via cables) turns in the direction of the handlebars so the stern is pushed in the opposite direction. This allows the driver to turn at a much tighter angle than traditional boats with keels and rudders.The main drawback to this maneuverability is the fact that if there is no thrust coming from the engine the ability to turn is effictively zero meaning that anytime the driver presses the kill switch (a large red button) they lose all ability to steer. This is extremely dangerous whenever an inexperienced person may drive the PWC back to dock or into shore. PWC's have no brakes and have no ablilty to stop other than turning around. They have an extremely efficient ability to hydroplane (when most of the PWC is above water) and it takes most PWC's a few hundred feet to come to a stop after being at full throttle. This is because 600 lbs + a rider is traveling at a very high speed with only minimal friction to slow them down (since PWC's are made to travel with very little friction).

PWC's also have a problem with turning. When a PWC is at full speed there is a great amount of force produced by the jet and the vehicle is therefor very difficult to turn. The main way to turn sharply during high speeds is to let of the throttle temporarily. This slows the amount of water flowing through the jet so the jet can be turned. Once the jet is turned the driver can depress the throttle again. Although being very effective this tecneque can also be quite dangerous. When the PWC turns it resists it's change in motion, however the driver does not recieve the same change in motion. The driver's body still wants to continue in the previous path of the PWC. Unless the Driver holds on very hard the driver is likely to fall of the PWC at a very high speed.

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When the PWC starts it's motion it is just floating on the water. After a short amount of accelleration the PWC will become "on step" or begin hydroplaning, which means the majority of the vehicle is traveling above the water. The effect of hydroplaning is that the effective friction from the water is greatly decreased(there is less surface area contact between the two bodies). Since more of the PWC is above the water the drag is increased. But, since air is less resistant to motion than water, the overall frictional force is decreased.

The "flying boat" or hydroplanes uses aerodynamic lift to hold the majority of the boat above the water so only the propeller is underwater. The most common type of flying boat are the ones with tunnel hulls seen here.

The basic idea of the tunnel hull is to have the center piece of the boat act as an airfiol to lift the boat up and reduce drag on the outside hulls. When designing a tunnel hull with an airfoil three things must be taken into consideration because the boat needs to be in equlibrium. First Drag needs to equal or less than the thrust, if the drag is greater than the thrust then the boat won't go anywhere. Second Lift needs to equal weight, if this does not occur then the boat will either flip over backwards or the boat will stay on the water.

Thirdly and also very important is the fact that the tendency to pitch about the center of gravity needs to be nonexistant. If there is a high tendency to pitch the likley tip over on one of it's axis'.

If all these things are true for a set velocity then the boat will sucecssfully begin to fly.

The amount of thrust(water coming out of the back of the) is based on three things; them being the propeller pitch, the RPM, and the combination of gear ratios and horsepower since they directly effect each other. The RPM is the rate at which the engine is cycling. The gear reduction ratio is based on the amount of horsepower the engine has. The propeller pitch is the distance between the inside and outside of the propeller blades.

The RPM divided by the gear ratio gives the value for the turning rate of the driveshaft of the impeller. To calculate the theoretical speed created by the jet take the Driveshaft RPM and multiply it by the propeller pitch this gives a value for how far the propeller will push the PWC based on the RPM. So it gives a value of distance per time or simply speed.

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