Personal Jet Boat Water Craft Thrust Question

[ruko]

Assume a jet boat or personal water craft (JetSki, etc) has a movable propulsion nozzle. This nozzle can be moved vertically so that the water jet driving the boat forward can be either above the surface or below the surface of the water.

Further clarification. You are speeding along on your Person Water Craft (PWC) at 40 mph. Let's say the propulsion jet is above the water surface at this time, that is the propulsion water jet is exiting the nozzle above the water surface. Your PWC has this above mentioned movable nozzle. 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 for convenience water drag on the craft stays the same.

 

[BvU]

Just assume for convenience water drag on the craft stays the same

Funny assumption. What base? Do you think the PWC would pretend everything stays the same ?

 

[anorlunda]

The angle of the jet flow is very important to performance. You have centers of gravity, of buoyancy, of thrust, and of drag to deal with.

The angle is related to whether the jet flow is above or below the water line, but it is not identical to the angle.

 

[jrmichler]

Looks like you answered yourself back in 2009, Posts #3 to #5: https://www.physicsforums.com/threads/personal-water-craft-jetski-etc.308883/

 

[ruko]

Funny assumption. What base? Do you think the PWC would pretend everything stays the same ?

I'm not asking the PWC to pretend, just you to pretend because there are those who will erroneously bring up drag changes and angles, etc. Really doesn't have anything to do with my question: Does the thrust increase or decrease?

 

[BvU]

erroneously

Sic !

 

[berkeman]

I'm not asking the PWC to pretend, just you to pretend because there are those who will erroneously bring up drag changes and angles, etc. Really doesn't have anything to do with my question: Does the thrust increase or decrease?

What do you think and why?

When you hold a garden hose in the air, and compare the force that takes to what happens as you approach a wall with it, what happens to the force of your hand and why?

 

[jrmichler]

This thread brings back memories. My boat project from high school days, about 1969:

The idea was to build a boat that I could sit on similar to a snowmobile. I designed and built the hull shown. Then bought an old 22 hp outboard motor, removed the powerhead and converted it to run with the crankshaft horizontal. And finally got in over my head when I tried to build a jet pump, which was a significant factor in my decision to go to engineering school.

The project never got finished, but it was a superb learning experience.

 

[berkeman]

My boat project from high school days

Wow. I think I built a California Mission out of popsicle sticks or Ivory soap bars in high school.

(I feel so small now...)[/size]