Calculating thrust

[drmiller100]

I have spent many hours playing with your spreadsheet OLAV. In my opinion, the spreadsheet closely follows observable behaviors. I can't say the exact numbers are right, but it sure predicts well.

as an example, set your speed to about 80 feet per second, 350 horsepower, and start playing with nozzle size. If you make your nozzle size too large, you end up with negative thrust in a big hurry, which follows the real world.

Further, if leave nozzle at .25 feet (realistic high speed nozzle), change speed down to 20 mph or so, and note the thrust. Then change your nozzle size to 1 foot, and note the HUGE change.

Different nozzle sizes make a HUGE difference in pump characteristics.

I am currently reading frantically about propellers. I think there might be some efficiencies to be found in them.

[Doc Nozzle]

I've decided to take a crack at thrust formula and how/why a jetboat or PWC moves through the water. You can view or download my tech handbook on jetpump thrust at: www.mdotfx.com

I welcome any and all thoughts, questions, challenges, et cetera. I don't profess to know anything - I just profess to want to try!

Cheers!

[drmiller100]

I can't get it to download!!!

[Doc Nozzle]

Sorry that the site is so basic! I think you just have to right click on the funny little thing with the red arrow above it next to the file name: Jetpump Technical Handbook Vol1 vA.

When you right click, you can select "Open Link in New Tab" (or window). That should get it displayed for you and then you can print or save.

Hope that helps!

[Guest625101138]

Originally Posted by Doc Nozzle

I've decided to take a crack at thrust formula and how/why a jetboat or PWC moves through the water. You can view or download my tech handbook on jetpump thrust at: www.mdotfx.com

I welcome any and all thoughts, questions, challenges, et cetera. I don't profess to know anything - I just profess to want to try!

Cheers!

There is a simple formula back at post 6 that will get you a much more accurate answer than the 6 tonnes of thrust you arrive at for the jetboat.

I will work with metric using the formula because it does not have the confusion of units that has created your error. (I will let you work it out)

The 2360gpm through the 3.125" (0.079m) nozzle produces a flow of 30m/s. This is all you need to know to get the thrust. From the previous post #6:
thrust = rho x pi x D x D x .25 x Vjb x (Vjb-Vbw)

We are considering static thrust so Vbw is zero.

thrust = 1000*pi*0.079^2*.25*30^2 = 4411N (990lbf)

I think you will find this much more realistic than 13,293lbs. That sort of thrust would push the transom through the bow. With a boat weight of say 1000lb you would pull 13g - not plausible.

Rick W