Propeller guesstimates

[Sheerline]

I need to re-pitch my two-blade folder, because it's not making my new diesel work hard enough.

It's a 14"x8" I think, possibly 14"x7". I can only fit a 14" in the gap between skeg and rudder.

It worked nicely with my 6hp diesel.

But the new Nanni 2.5HE diesel is 14hp. Its max is 3,600rpm, but the mechanic who fitted the motor ran it flat out and clocked 3,900rpm with the woosy prop!

The yacht weighs about 1,600kg or so and is 7m on the waterline. Gearbox has a 2.7:1 reduction (I think).

Any suggestions on what pitch I should get put into the prop?

 

[sailorman]

Here

 

[Sheerline]

Thanks, but what's prop slip? How do I calculate that? Sorry to be obtuse.

 

[Pye_End]

That's a reasonably large reduction, so you are able to have a fairly powerful prop. At least you are able to use a 14" which helps.

Have you considered going to a 3 blade prop, or maybe a Kiwi?

You should be able to get recommended props from the supplier of the engine/gearbox. Sounds like you are not completely sure whether it is a 2.7:1 - you will need to be sure about this to run it through propcalc as it makes a big difference.

 

[dukes4monny]

I cant help you with this but a quick Google search found this page

 

[Tranona]

Propcalc gives 14 x 11 3 blade or 15 x 12 2 blade as the optimum. so 14 x 13 2 blade is the nearest.

The engine is probably too big for the boat and the 2 blade has difficulty in absorbing the power.

Suggest you take it to a prop specialist and ask their recommendation and get them to re-pitch it. Then you have some comeback if they get it wrong.

Hope this helps

 

[omega2]

www.clementsmarine.co.uk

 

[bendyone]

If it helps my Nanni has a 12x8 3 bladed prop which is a little overpitched. Will be changing to 12x7.

 

[RobinBirch]

Be careful not to go to the other extreme - at idle Hanser will do 2.5 knots. Make's marinas and close quarter stuff intereresting!

 

[lw395]

A clean hull on flat water should not always be limited by the prop pitch. The thrust required to drive into a force 6 and chop is rather larger.
How fast was the boat going at these revs?
Maybe aim for quoted cruising RPM being a sensible speed.
How fast does it go at cruising rpm at the end of the season?
Your motor is quite big for the boat, but pitching the prop for max speed could be a mistake.
The most helpful book I've read on this subject is Larsson and Elliason's 'Principles of Yacht Design' (name spelling dodgy!). I would welcome other peoples recommendation for textbooks on this. Please!

 

[Sheerline]

Thanks all for sensible advice. Yes the motor's probably a bit too big - but I'm a boy... and I'd had the previous unreliable lo-po BMW 7hp up to the back teeth. The Nanni 14 was almost the same price as the 10, but with better accessories.

A friend has the same hull as mine, with a Volvo 10 and a three blade fixed... and he slowly walks away from me when we're motoring together. I can see his prop has muuuch more pitch than mine - but he doesn't know what it measures.

Yes, I'd considered the problem of too much speed at idle. Having got another warning here, I'll pay more attention. I'm leaning towards the 14x12... either way it arms me with more info when I speak to the prop people later this month. Much appreciated.