Selecting a propeller - your help would be appeciated

Sailing on a shoe string? When I was a boy you were bloody lucky to have shoe strings.... My Dad used to lace his boots with left over bacon rind... we lived in the box the shoes came in until I was 12. Ar... them were the days.... /forums/images/graemlins/grin.gif
 
I should, perhaps also said that our types of boats are not good at motoring into wind and seas. They are fat, flat bottomed and light so do not carry their way well. As Kenjohnson says, perhaps better to back off a bit and be more comfortable rather than fight the laws of physics.

A 3 blader will use your available power better and probably give the same SOG at lower revs in heavier conditions. It will almost certainly be better in reverse than a 2 blade.

Most people looking at folder/featherers already have 3 blade props and want to retain motoring performance while reducing drag. The Autoprop as Boatmike says does different things and makes sense if you do a lot of motorsailing - but at a cost!
 
Re. changing the prop whilst the boat is in the water: even with a fixed two bladed/three bladed prop. there is a nut to unscrew that probably has some sort of locking feature/split pin to undo first. Then you will need a puller to draw the prop off the taper. Take care not to lose the push-fit key that is fitted in a slot in the shaft and located in another slot in the bore of the prop. The new prop needs to have identical tapered bore and keyway to the old prop. The new prop needs to be lined up with the key and pushed onto the tapered shaft, nut replaced and tightened and locking feature secured.
To deal with all of this under water without dropping anything or getting it wrong is a tall order in my opinion.
I would definitely go for a 3 bladed folding/feathering/autoprop rather than trying to swap the props in the water when changing between racing and cruising trim.
 
Thanks for the answers. A few more points:
I have no idea what the pitch and the diameter of the prop is. It's the one installed by Beneteau factory when new, if this answers the question.
When going against headwinds RPM don't fall, just the SOG while still going around 2500-2600 rpm (of course at higher rpm SOG will increase). So yes I am reluctant to exceed the 3/4 like "boatmike" mentioned.
Do I get black smoke at full rpm? Honestly, I don't know. At least up to 3000 rpm I have never seen any smoke.
I am not an expert, but comparing to similar boats with 2 bladed fixed propeller I think performance is more or less the same.
 
We had a similar dilemma 2 years ago - Wauquiez 35 (about 7 tonnes displ, 2 blade folding prop, VP 2003 28hp, 6,000 hours). It would make 5 kts in calm water but this dropped to 2 kts in a Force 5 headwind (Med sailing) so one day we had to abort a 10 mile voyage, then the wind changed direction and headed us again so we were 10 hours at sea. (Why not sail you might ask - SHMBO not keen on thrashing to windward).
Having spent 800 Euros on a top end engine overhaul I was dismayed when advised to spend the same again on bottom end. Fortunately I had just received a small bequest so blew it on a new engine _ VP MD40 (40 hp) and a Volvo 3 blade folding prop. Result: bliss! For the first time we can now reverse reliably. She does 6 kts @ 2,000 revs in calm to moderate seas. given that I had the cash I've never regretted the investment.
 
Yes that's what I thought. In that case I would not bother spending any money at all. There is now't broke so no need to mend it. A three blader will only be very marginally better and cause much more drag when sailing. If you want to go faster open the throttle! You will certainly use more fuel but it will probably do your engine more good than harm. Diesels thrive on hard work and most die of a combination of neglect of basic maintenance and running at low throttle for short periods than overwork.... A good full throttle blast is good for it now and again!
 
Go ahead and fit a 3 blade fixed prop if you are not concerned to loose half a knot under sail. I just fitted 3 blade fixed along with new 21hp engine, and against a large sea can still make 7kts under motor. Well worth having if cruising. I may splash out however when i go racing next year for a 3 blade feathering. good luck with your 300mile passage.
 
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