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water jet boat

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jeffrey mundell 128/07/2020 16:04:37
21 forum posts

i am building a guided missle boat and i am going to use a water jet propulation simular to the Grapner ones.

what i want to know is ,the outlet pipe should this be under the water,or half way or what is recommended

regards Jeff

Paul T28/07/2020 16:36:39
7185 forum posts
1214 photos
2 articles

Hello Jeff

We will need a little more information such as:

Are you building from plans?

What is the type / name of the full size ship?

Have you tried researching water jets on the internet?


Chris Fellows28/07/2020 17:08:41
796 forum posts
518 photos

Hi Jeff

From the little I know of jet drives the height of the outlet is governed by the position of the inlet which is set into the bottom of the hull which results in the outlet being close to the bottom of the boat. So the position of the outlet is determined by the manufacturer of the drive.

The outlet is usually a quite large diameter and so at low speed the outlet will be partly or completely immersed in water but up to speed/on the plane the outlet will be completely above the water.


Edited By Chris Fellows on 28/07/2020 17:28:39

Empire Parkstone28/07/2020 19:02:57
241 forum posts
8 photos

on My Pibber at low speed the jets are completely below surface and as speed in creases and planes they are deeper. This creates a furrow astern the jets are firing down at whatever the hull angle the the Plume you see is coming from below surfacecapture.jpg

ashley needham28/07/2020 22:27:20
6833 forum posts
200 photos

I think Chris has it. If you buy jet units, you do not have a say in where the outlets are if you have the inlets on the bottom skin.


Empire Parkstone28/07/2020 22:52:18
241 forum posts
8 photos

How can the outlets be OUT of the water Ashley think again

Chris Fellows28/07/2020 23:13:51
796 forum posts
518 photos

You're not thinking that the water jet has to be below the water level for it to drive the boat forward are you, as in it pushing against the water!

Your boat doesn't look to be fully on the plane. If it was it would be riding more level and the water jets would be partly or fully out of the water. Plenty of videos on YouTube showing this.

neil howard-pritchard29/07/2020 00:30:38
1573 forum posts
947 photos

hi, the nozzle for the outlet of the waterjet propulsion is almost always under the water level for maximum efficiency, with the inlet to the jet tunnel on the bilges of the boat.

below is my Shannon lifeboat which used twin waterjet drives.

don't touch graupner jet drives, as their reversing buckets are very haphazard and not very efficient. go to a company called ocean works......they are as good as the ones on my Shannon, and half the price.



Malcolm Frary29/07/2020 09:26:19
918 forum posts

A read of the wiki about jet boats indicates that, however counter to intuition, having the outlet above water works better

One of the most significant breakthroughs, in the development of the waterjet, was to change the design so it expelled the jetstream above the water line, contrary to many people's intuition. Hamilton discovered early on that this greatly improved performance, compared to expelling below the waterline, while also providing a "clean" hull bottom (i.e. nothing protruding below the hull line) to allow the boat to skim through very shallow water. It makes no difference to the amount of thrust generated whether the outlet is above or below the waterline, but having it above the waterline reduces hull resistance and draught.

Empire Parkstone29/07/2020 09:37:01
241 forum posts
8 photos

With a draft of 40 mm centre 30 mm outer she is nearly flat bottomed the jets are 20 mm D going more level well she can This a small boat 21'' and will bounce easily but for the jets to fire above water she'd have to be nosediving.

Anyway the OP asked if the jets had to be below or partially above the waterline I say below the internal plumbing layout also determines it as the props need to be submerged

Very nice Neil I was tempted to get some buckets but so far touch wood as she will turn on the spot

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