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?

Paul

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 surface

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

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.

Home

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

EP – when truly on the plane the rear of the boat will lift and a hole will be formed behind the transom and so the water jets will be firing above the water level without the nose having to point down which it won't anyway. If your boat is at full throttle it needs a bit more omph!!

Jeff – sorry to go off at a bit of a tangent but that's what usually happens on here and hopefully all relevant. Hopefully you are clear now. Keep posting about your build.

Chris

Edited By Chris Fellows on 29/07/2020 15:15:40

There is some confusion creeping in here. Jets are nearly always underwater when the boat is on its static waterline but when a boat moves its waterline changes. On a planing boat the water break away from the transom so although the jets are still below the static waterline they are not now working underwater. That the difference and Chris was on the right track

In principle a jet is a precision pump whose function is to squirt water out the back. Some of the water going through the pump is due to suction especially at low speeds but the ram effect contribution increases with speed. Jets do not need to push against anything to make them work. They work on a reaction principle and in that way they are different from propellers, ducted propellers and thrusters. The impellers also operate at very high speeds as usually there is direct drive with no reduction gearboxes. The outlet nozzles are nearly always convergent as this accelerates the flow. This is important.

One of Newton's famous laws is that if you double the speed applied to any given mass the force is quadrupled. Velocity if the jet is therefore paramount to maximising the applied force (reaction). While jets do work under water, they coming into their own venting into atmosphere as this provides the least possible hindrance to the velocity.

Think of a fire hose. A plain hose will pump out lots of water but it won't travel very far. Add a normal tapered nozzle and you now have a very powerful jet that may require four firemen to control. The reaction has increased enormously. If you now stick the nozzle in water. The force will dramatically reduce. A simple experiment that you can do with a garden hose and a bucket.

True water jets are not used on displacement vessels for all the above reasons,. Other methods are more efficient. Even the lifeboat which is likely to be semi displacement will probably have the water breaking away from the transom at cruising speed but there may be other compelling reason such as being able to operate in shallow water. Most lifeboats are not jet driven.

This was discussed in Bob Abell's Brutus thread and Paul T did not agree with me. I did not want to hijack that thread and this one presented a better opportunity for a fuller explanation.

Tim R

an intresting jet boat to look at is the marine alutech jet boat with a patria mortar fitted, these were designed for the finnish navy to patrol and land troops on the hundreds of islands they have to patrol, or just google m16 watercat they did use rroyce drives but i think they they now use scania,s they also use some very interesting weaponary on them. I have just finnished my AMV18 marine alutech watercat ihave used twin kerber jet drives, will post some photos when i get a chance.

Neil. One has to assume the RNLI has a clever design for these units? Sometimes you cant tell exactly how these things work by looking at them statically.

As an agreement to jets working above water, Trevor`s little fireman SAM boat has its very own bow thruster in the form of Sam`s squirty hose which is well clear of the deck let alone the pond, and this turns the boat amazingly well.

Ashley

Neil

They are under the water when stationary and at low and medium speeds but at full chat when max. thrust and efficiency is required they are pretty much out of the water.

Chris

……and that just the bath test!,

Ashley