Personal Jet Boat Water Craft Thrust Question

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Discussion Overview

The discussion centers around the effects of a movable propulsion nozzle on a personal water craft (PWC) when the water jet is positioned above or below the water surface. Participants explore the implications of this configuration on thrust while assuming constant water drag for the sake of the question.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant posits that lowering the nozzle from above to below the water surface will affect thrust, but does not specify how.
  • Another participant questions the assumption that water drag remains constant, suggesting that it may not be a valid simplification.
  • A participant emphasizes the importance of the angle of the jet flow in relation to performance, noting that it involves multiple factors such as centers of gravity, buoyancy, thrust, and drag.
  • There are references to previous discussions on similar topics, indicating ongoing interest and complexity in the subject matter.
  • One participant draws an analogy with a garden hose to illustrate their inquiry about thrust changes when the nozzle position is altered.
  • A participant shares a personal anecdote about a boat project, reflecting on the learning experience rather than directly addressing the thrust question.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions made regarding drag and thrust. There is no consensus on whether thrust increases or decreases when the nozzle is lowered, and the discussion remains unresolved.

Contextual Notes

Assumptions about constant water drag and the effects of nozzle angle are not universally accepted, and the discussion includes various perspectives on these factors.

ruko
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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.
 
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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 ?
 
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.
 
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:
erroneously
Sic !
 
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:
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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:
 

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