Newton's Law (JetSkis)

[Guest Ruko]

I have a question about jet boats or personal water craft or jetskis or what ever you like to call them. It concerns the water jet propulsion unit in the back of the boat from which water exits as a jet stream and moves the boat forward. One of Newton’s laws at work here I believe. My question is: With an equal amount of power supplied to the system, forget about drag etc, is it better to have the water jet propulsion unit above the surface of the water or below it? Another way to put this is, do you get more measurable thrust, with the same amount of power applied, with the jet above the water surface or below the water surface?

[J.C.MacSwell]

I have a question about jet boats or personal water craft or jetskis or what ever you like to call them. It concerns the water jet propulsion unit in the back of the boat from which water exits as a jet stream and moves the boat forward. One of Newton’s laws at work here I believe. My question is: With an equal amount of power supplied to the system, forget about drag etc, is it better to have the water jet propulsion unit above the surface of the water or below it? Another way to put this is, do you get more measurable thrust, with the same amount of power applied, with the jet above the water surface or below the water surface?

 

Below.

[swansont]

Below.

 

Whay do you think it matters?

[jordan]

That's a bit strange. The question asks you to ignore all drag and then asks whether water or air provides more drag. If drag was in the question, I would assume you would want as much of the jetski above the water as possible.

[J.C.MacSwell]

Whay do you think it matters?

 

Assuming all other things being more or less equal (diameter of jet etc.):

 

Below the waterline the surrounding water would provide more back pressure than the air above and therefore more thrust for the same power.

 

Because you would be effectively propelling more mass aftward (relative to the body of water or freestream) you would be imparting more momentum for the same power. (the exit velocity would actually be slower). So more of the energy would go to the craft and less to the jet.

 

Hope this makes sense.

[[Tycho?]]

Also, if it was shot out below the water's surface the water would provide thrust directly forward. If it was shooting out of the water some of the thrust would be directed downwards.

[J.C.MacSwell]

'']Also, if it was shot out below the water's surface the water would provide thrust directly forward. If it was shooting out of the water some of the thrust would be directed downwards.

 

Good point. As a minimum you would lose the "head" height of the jet trajectory (or exit) above the waterline. This would also add to the displacement (effective weight) of the craft unless the jet was redirected downward which would of course lose even more forward thrust.