folding props

[Tevens]

is it possible to get folding/feathering props for motorboats (>25m)? i need to my main engine prop to minimise the drag it causes when the vessel is going at top speed, (using the auxillary engines) or how far in terms of drag and effectiveness can CPP's lower the drag of a prop?

[gonzo]

Folding props are not too efficient, they are designed for auxiliary power. Feathering props are better, but still have more drag because of the larger hub. I don't really understand how the propeller is dragging at top speed. It should be producing thrust.

[Tevens]

basically i have a 30m vessel, with a single main engine which be used at cruising speed. And two auxillary engines to propel the vessel at 20knots max speed. The first thing i am not sure about is whether the main prop which will be at a lower speed to the others, will it still produce thrust when using the higher speed auxillary props? If you can answer that then that would be helpful.

[gonzo]

I think that the engines that make the vessel reach top speed should be called the main engines. There are a few calculations needed for that. Can you post all the specifications?

[Tevens]

30m vessel
6.1 = BOA
124 displacement
1.8m draft
cruising speed 12 knots
max 20 knots, what other specs do you mean?
WSA 127m2

[gonzo]

HP, RPM at crusing and max speed, slip percentage of both sets of props, diameter of props, pitch of props, engine specifications, transmission ratio.
Can you explain the setup a bit more? It is not clear what the whole layout is.

[Tevens]

the vessel is only in the designing stage at the moment, so much of the data as the pitch of the props I am yet to choose. this is where the problem stems from: i need the vessel to cruise efficiently and then in rare spurts accterate to 20knots. The proposed installation is one engine of 800kW to be able to do the cruising speed and then when the vessel is required to "bust a gut" the extra engines will kick into force. And so far i don't know yet if the single engine will still be able to give thrust when the faster engines are doing their thing. Can i use the main engine power along with the extra's to propel the vessel at max speed? this is the proposal, i have looked sparingly at waterjets but i am not sure if they would help the problem.

[gonzo]

Are you sure this can be more efficient than a twin engine installation? Does the client absolutely demand three engines? Jet drives have little drag and are one solution.

[jtexas]

3 engines running together need to be evenly matched as to prop RPM x pitch.

With the aux engines pushing the boat, the unloaded main engine will be able to make more RPM than it otherwise would. If it can turn that fast without overreving, it can contribute thrust. Whether that's more fuel efficient than just accepting the drag, is the question.

General rule, smaller number of larger engines is more fuel efficient than a greater number of smaller engines.

[gonzo]

If the cruising speed is 12kt and the maximum 20kt, that would be within the usual parameters for a single main engine or twins.