Personal Jet Boat Water Craft Thrust Question

Join the discussion
Registration is free. Ask a follow-up in this thread, or start your own.
Sign up Log in
8 replies · 2K views
ruko
Messages
57
Reaction score
1
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.
 

Discussion

Physics news on Phys.org
ruko said:
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 ?
 
BvU said:
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?
 
ruko said:
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?
 
This thread brings back memories. My boat project from high school days, about 1969:
Boat.jpg


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.
 
Last edited:
  • Like
Likes   Reactions: BvU, anorlunda and berkeman
jrmichler said:
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]

:smile: