MadSci Network: Physics

Subject: Jet boat thrust measurement. Above or below water surface

Date: Thu Aug 28 16:14:27 2003
Posted by Jason
Grade level: undergrad School: U of M
City: Stacy State/Province: MN Country: usa
Area of science: Physics
ID: 1062105267.Ph

Dear Expert

Assume a jet boat or personal water craft (JetSki, etc) has a movable 
propulsion nozzle. This nozzle can be moved up or down so that the water jet 
driving the boat forward can be either above the surface or below the surface 
of the water. Applying the same amount of power to the jet unit does the 
thrust increase or decrease when the jet nozzle is under the water surface and 

Further clarification. You are speeding along on your Person Water Craft (PWC) 
at 50 mph. Lets say the propulsion jet is above the water surface at this 
time, that is the propulsion water is exiting the nozzle above the water 
surface. Your PWC has a special nozzle that can be moved up or down. You keep 
the throttle at the same setting and you lower the nozzle so that the water 
jet is now under the water surface. What happens and why? Just assume drag 
stays the same.

Thank you.


Re: Jet boat thrust measurement. Above or below water surface

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