|MadSci Network: Engineering|
Hi John, Good question... And I have two parts to the answer that are inter- related. The first answer is that boats do in fact have transmissions, however it's not quite like what we have in cars... you'll notice that boats have a neutral, forward, and reverse. This is in fact a simple transmission. As for why there aren't more gears than just one, is a little more complicated. Unlike a car's tires, which are fairly efficient at any speed, a boat's (or even a fixed bladed airplane) propeller (whether it's exposed or internally housed as an impeller on a jet boat)(and engine) is really designed and set to be truly efficient at one speed. This is called the pitch of a propeller. If it spins too fast, it "slips" (and "cavitates") in its rotation through the water, if it spins too slowly, you save gas, but take forever to get there and thus waste gas in the time you take to get there. Usually, the engines are set for their peak performance at the "ideal" speed or RPMs, and thus they really don't need more gears like a car. A car on the other hand, has a lot of friction to overcome and the tires aren't supposed to slip at all. Thus, you want to start with a lot of mechanical advantage (in first gear) and work your way up to lots of speed (in say 5th or sth gear). So basically a boat engine can do what a car engine can't, go from a stand-still to maximum efficient RPM's and let the boat catch up as the propeller first slips and then bites into the water and accelerates to speed... this would be like throwing your car into it's top gear and dropping the clutch at a stop light. Instant stall. As for higher speed, if you look in a motorboating magazine, you'll see dozens of ads for "high performance" propellers... These are simply propellers that take a bigger bite out of the water with each rotation and thus give you higher speed, at the cost of fuel economy. Hope I answered your questions well enough for you, if you have more, write back to us and we'll get back to you as quick as we can with the answers. Cheers, John Metcalfe
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