Personal Jet Boat Water Craft Thrust Question

  • Thread starter ruko
  • Start date
  • #1
ruko
57
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.
 

Answers and Replies

  • #2
BvU
Science Advisor
Homework Helper
15,273
4,252
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 ?
 
  • #3
anorlunda
Staff Emeritus
Insights Author
11,209
8,607
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.
 
  • #5
ruko
57
1
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?
 
  • #6
BvU
Science Advisor
Homework Helper
15,273
4,252
  • #7
berkeman
Mentor
64,179
15,398
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?
 
  • #8
jrmichler
Mentor
1,978
2,524
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 BvU, anorlunda and berkeman
  • #9
berkeman
Mentor
64,179
15,398
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...)

:smile:
 

Suggested for: Personal Jet Boat Water Craft Thrust Question

Replies
2
Views
392
  • Last Post
Replies
16
Views
504
  • Last Post
Replies
28
Views
477
Replies
6
Views
401
Replies
12
Views
308
  • Last Post
2
Replies
37
Views
1K
  • Last Post
Replies
2
Views
876
Replies
24
Views
689
Replies
16
Views
502
Replies
8
Views
737
Top