My boating friend and I were having a discussion about jet boats like Jetski or personal water craft as they are called. My friend says it is better to have the propulsion water jet outlet under the water surface as opposed to above the surface. He says it will produce more thrust under the water because it has a denser medium to push against. I say there is more thrust developed above the water with the same amount of power applied to the system. Who is right?
I looked at the jet outlet on his Yamaha while we were side by side going about 40 mph and indeed the jet is above the water at this speed
I would have to side with your friend. It probably makes more sense to push directly against water, without all that energy loss that occurrs when you push it through air.
But that's just an educated guess. Most of the large jet boats do indeed lift their nozzle out of the water when they get up on plane, but that's probably just a side effect of the boat lifting.
My guess, is that the drag induced by having the nozzel under water far outweighs it being slightly above the water at cruising speed. If you think about it though, you will need the most thrust starting anyways.
I wouldn't think it would matter. I'd have to side with Binary on this one; you're throwing out the same amount of mass at the same speed so you should be generating the same thrust. Being out of the water just reduces your drag. Now there may be some exotic cavitation effects going on while you're dumping water into water, so that may have some effect, but it's probably only a concern for really high velocity drives, or large vehicles like submarines.
Jetski Question
Posted - Jan 21 2005 : 4:47:32 PM
