ruko71
Jet boat Misunderstanding
The sentence, " It makes no difference to the amount of thrust generated whether the outlet is above or below the waterline, but being above the waterline reduces hull resistance and draught design", is wrong in part. With the same horsepower applied to the water pump, the thrust developed will be considerably more with the propulsion water exiting the nozzle above the water surface compared to below it. The same principle applies to jet boats as it applies to rockets. NASA says their rockets develop more thrust in the vacuum of space than in the earth's atmosphere. Why you ask? Because air slows the propulsion gasses exiting the rocket nozzle. Air must be pushed out of the way. Thrust is determined only by volume and velocity of the thrust medium and anything that slows this thrust medium reduces thrust.
In the case of the jet boat, if the thrust nozzle is located under the surface of the water it is very inefficient because it has to move water out of the way as opposed to air. Rockets or jets do not rely on "pushing" against anything external for thrust development. It is action and reaction.
The sentence, " It makes no difference to the amount of thrust generated whether the outlet is above or below the waterline, but being above the waterline reduces hull resistance and draught design", is wrong in part. With the same horsepower applied to the water pump, the thrust developed will be considerably more with the propulsion water exiting the nozzle above the water surface compared to below it. The same principle applies to jet boats as it applies to rockets. NASA says their rockets develop more thrust in the vacuum of space than in the earth's atmosphere. Why you ask? Because air slows the propulsion gasses exiting the rocket nozzle. Air must be pushed out of the way. Thrust is determined only by volume and velocity of the thrust medium and anything that slows this thrust medium reduces thrust.
In the case of the jet boat, if the thrust nozzle is located under the surface of the water it is very inefficient because it has to move water out of the way as opposed to air. Rockets or jets do not rely on "pushing" against anything external for thrust development. It is action and reaction.