Personal Jet Boat Water Craft Thrust Question

In summary, a movable propulsion nozzle on a jet boat or personal water craft can be moved vertically so that the water jet driving the boat forward can be either above or below the surface of the water. 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?
  • #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.
 
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  • #2
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 ?
 
  • #3
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
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?
 
  • #6
ruko said:
erroneously
Sic !
 
  • #7
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?
 
  • #8
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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Likes BvU, anorlunda and berkeman
  • #9
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...)

:smile:
 

1. What is a personal jet boat water craft?

A personal jet boat water craft is a small, high-speed watercraft that is powered by a jet propulsion system. It is designed for recreational use and can typically accommodate 1-3 passengers.

2. How does a jet boat work?

A jet boat works by drawing water through an intake, compressing it with a high-pressure pump, and then forcing it out through a nozzle at the back of the boat. This creates a powerful thrust that propels the boat forward.

3. What is the advantage of using a jet boat over a traditional propeller boat?

One major advantage of using a jet boat is its ability to operate in shallow water. Since there is no exposed propeller, there is less risk of damage to the boat or injury to passengers. Jet boats also have quicker acceleration and better maneuverability compared to propeller boats.

4. How fast can a personal jet boat water craft go?

The speed of a jet boat can vary depending on the model and engine size, but most personal jet boats can reach speeds of 40-60 miles per hour. Some high-performance jet boats can even reach speeds of over 100 miles per hour.

5. What safety precautions should be taken when operating a jet boat?

As with any watercraft, it is important to wear a life jacket and follow all safety guidelines and regulations. Jet boats should also be operated with caution in shallow or congested areas, and passengers should be aware of the location of the jet nozzle to avoid potential injury. Regular maintenance and proper training on how to operate a jet boat are also crucial for safety.

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