Stepped Hull and Hydrojet

Hello

I have got a question about the possibility to associate stepped hull with hydrojet propulsion on a fast motoryacht.

I am planning to build a 1m long model boat looking like a Sunseeker Predator, but I would like to combine both stepped hull and hydrojet.

I have drawn my hull on MaxSurf and then on Rhino3D, with sprayrails obviously, and one step located a bit aft amidship. As you know, step creates an air cushion below the hull, and this is precisely my problem: is not that going to make my hydrojet cavitate ? If the inlets suck more air than water, it may be a problem...

As I said, my model is 1m long, the inlets are located 22cm from transom, and the step is located 40cm from transom. Do you think the 18cm clearance between them are sufficient for the air cushion ?

Thank you for your advices

Rémy

[mmd]

You have selected a very complicated design project. The model is a good testing tool - arrange to be able to see the hull bottom as you run your experiments. I'd suggest scouring the 'net for technical papers on the subject, and possibly contact the tech department of the candidate jetdrive manufacturer for their recommendations.

[nevd]

I agree with mmd on the complexity of this project. At some speed and weight combination (which you shoud be able to calculate), the jet will suck air. Have you considered having the jet suction at the rear section of the step or alternatively having the suction as close to transom as it will fit?

Where did you get your design data on spray rails?

Regards,

nevd

Thank you for your answers

The jet suction is located as close as possible to the transom. The air cushion should therefore be located between the jet inlet and the aft of the step.

This is all prediction, there nothing sure about it.

To design the spray rails, I used the informations provided during my course at the Southampton Institute, and the datas provided in the book Principles of Yaht Design, 2nd Edition, by Lars Larsson and Rolf Eliasson.

Rémy

[Morgig]

First I assume when you talk about hydojet you mean waterjets?

I also assume that as you are talking about using steps you are talking about a high-speed craft.

The problem with running waterjets on high-speed craft is that as speed increases the more important the intake angle of the Waterjet is. Such that at high speed the intake will need to be shallower that at a lower speed. The effect of this is to push the intake fwd, closer to your step.

It’s also worth considering what happens at low speed. It’s fair to say that at low speed the cause a fair amount of turbulence, which in turn will cause problems with cavitation.

Another consideration is proximity of rails; these also can have an adverse effect on waterjet performance.

[nevd]

Originally Posted by Rémy

Thank you for your answers

The jet suction is located as close as possible to the transom. The air cushion should therefore be located between the jet inlet and the aft of the step.

This is all prediction, there nothing sure about it.

To design the spray rails, I used the informations provided during my course at the Southampton Institute, and the datas provided in the book Principles of Yaht Design, 2nd Edition, by Lars Larsson and Rolf Eliasson.

Rémy

Thanks for your spray rail suggestions Remy. I have looked again at the book reference you provided but I need more info than this provides. Is the Southhampton Institute course notes available anywhere on the net or is there any other way I could get access to the information you referred to covering spray rails?

Regards,

nevd

No Nevd, I'm afraid there is no way for you to have access to the Institute course's notes, as they are not on line on Internet. This is all paper notes given to us by the lecturers, and oral explanations...

Sorry about it...

But maybe I could answer your questions about spray rails, what do you need to know ?

Kind regards

Rémy

[nevd]

Hello Remy,

Thanks for your offer to help. Most of this is probably best covered between the 2 of us, so if you sign in as a member I can send you private messages. Alternatively, can you provide an email address?

On your water jet application, have you considered placing the inlet below the planing surface like that of the jet outboards?

Regards,

nevd

I am also interested in learning how to design spray rails.

[miles p m]

Remy the correct term you are looking for is ventilate cavitation occurs when an extremely high vacuum is formed under water , like around a nick on a prop which will actually cause a burn mark on the prop. yes the jet could ventilate but an intake qahead of the step would solve this. you may get a loss of efficiency from surface friction on the long jet intake. My 50' double stepped boat weighs fifteen tons has twin man diesels and does vfery well at 60 plus knots. the secret is tri-max surface drives mile p m