Jet drive water level

[Trevor HollowayParticipant @trevorholloway99134]

Does anyone have an idea of the optimum water level for a boat with jet drive ie should the outlet nozzle be out of the water, fully submerged or half way ?

I am sgoing to make a boat of my own design and want to get the waterline somewhere about right at the sketching stage.

Thanks

[Trevor HollowayParticipant @trevorholloway99134]

[Paul TParticipant @pault84577]

Trevor

It might be best to design the boat around the drive.

Paul

[Trevor HollowayParticipant @trevorholloway99134]

I have the drive, battery, controls etc so can workout the overall weight and therefore displacement.

I want to have an idea of how much of the hull I need underwater to get the optimum drive performance.

[Paul TParticipant @pault84577]

Hello Trevor

As you correctly surmised different drive units require differing levels of water to operate in, the drive manufacturer would be able to supply the correct information.

However if you cant get hold of the correct specifications you could post a picture of the drive on here and we might be able to identify the unit and then answer your question.

Paul

[Trevor HollowayParticipant @trevorholloway99134]

Its the NDQ-6024 >> **LINK**

[ARTHUR COLLETTParticipant @arthurcollett48040]

Hi.

Depends on the speed required. The intake must be situated on the area always in contact with the water(planing Area). The outlet of the jet for starting would be submerged. As the boat reach optimal speed and thus be planing the jet drive outlet will be nearly out of the water. The length of the jet between the outlet and inlet usually determines the distance underneath the hull and thus the planing area.

Regards

Arthur

[ARTHUR COLLETTParticipant @arthurcollett48040]

Hi

Looks like a Graupner drive. Will be submerged on take off. Had a look at my Graupner jet drive boat.

Remeber to incorporate the weight of the batteries when determining the water line.

Regards

Arthur

[Trevor HollowayParticipant @trevorholloway99134]

Yes, similar but lots cheaper (£27 the pair with motors and delivery from Hong Kong).

Did a similar excercise when determining the size of the James Bond Lotus Esprit to get the right stance in the water – works a treat.

[Paul TParticipant @pault84577]

From looking at the unit on the Internet it would appear that the exhaust is fully out of the water.

[TelstarParticipant @telstar]

Hi With water jet units the impeller needs to be below the water level when stationary . Otherwise the impeller spins in air and gives no thrust to start, a bit like priming a water pump.

Cheers Tom

[Paul TParticipant @pault84577]

I did see that in some cases this particular unit had primers fitted which does raise a question over starting problems.

Paul

[ashley needhamParticipant @ashleyneedham69188]

Trevor, I dont think youi will have any trouble with this to worry about it, as long as the jet impeller is below surface level all should be ok.

If you are making your "normal" balsa hull(s) arrangement, then a plan might be to make the hulls first, seal them with sanding sealer and then carry out a float test ballasted with your running ger and see where the level is. Obv you dont want to have to ballast up to acheive the required depth, so slimming the hull(s) could be used instead to get a higher waterline..not so much of a problem at an early stage of construction.

i was wondering if a self bailing system could be used, a compartment open to the water (at the rear) that empties under the affect of acceleration and boat attitude..? has weight at the start, where you need to dig the drive in, and lightens progressivly as you get going.. is this an idea or just rubbish??

Ashley

[Trevor HollowayParticipant @trevorholloway99134]

"Normal" hull construction ie quick, easy and cheap but 6mm balsa bulkheads and I have some lite ply lurking in the cupboard under the stairs that will be used for skinning.

Waterline will be acheived by working out the volume of the hull required then drawing up scale(ish) sketches until I like the layout.

No such thing as a "rubbish" idea, however having a self ballasting compartment is an option worthy of consideration – works well on my tug barge.

[ARTHUR COLLETTParticipant @arthurcollett48040]

Hi

I had two frives from China that broke due to small debris and stones sucked into the drive. Very brittle. Could not get replacemet impellers. It is the same length and dia impeller as the Graupner drive but much thinner shaft. Make sure of replacement parts as it is difficuld to remove the drive once fitted in the boat.

Might be useless info. just advise

Regards

Arthur

[ashley needhamParticipant @ashleyneedham69188]

Trevor. As above, a ploy might be to fix the drives in using silicone sealant, rather than epoxy/whatever, should be easy enough, the bottom of your hulls will be flat and there should be an overall flat mounting surface on the drives.

This will fix the drives in place very securely but will not be too difficult to cut out with a scalpel or whatever, if the worse happens. A slatted grilll in the water inlet would be good, but a bit diff to arrange in such a small size. Mesh no good as will obstruct water inlet too much at speed.

Ashley

[Trevor HollowayParticipant @trevorholloway99134]

Above comments noted, the drive units will be screwed and removable / replaceable and a complete new unit less the motor is under £13 its not too bad should anything happen.

The "protection from ingress of debris is very crude so may well replace wit brass bars (as per the Graupner unit).

As Ashley would attest my models are built for fun not scale !

[ashley needhamParticipant @ashleyneedham69188]

Trevor, so right.

Fun, the only way to build `em.

Ashley

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