Prop Choice for Liveaboard

[Bajansailor]

Agreed it does look rather good - no sharks have been nibbling on it...... /forums/images/graemlins/grin.gif
Re going down an inch in both diamter and pitch, were you not getting full design revs on the engine before when under way? If you were, you do not want to go down in revs. But if you were not, eg 100 or 200 revs short, then your prop is slightly too big. But that can be a good thing in a way, as then it makes sure you dont over rev the engine.
As mentioned above, wallop it soundly with rubber mallet on the tips next time you haul out, and listen.... and look to see what happens.... if it doesnt disintegrate, and rings sweetly, then odds are it should have a fair bit more life in it, IMHO.
You could probably also try it underwater, while snorkelling..... sound travels well underwater...

 

[Tranona]

sorry Jonny, Your original post said brittle at the edges, so I assumed bits had come off. Anyway, you have fixed the anodes so hopefully no more electrolysis. Back to the original suggestion - tap it and if it rings it is OK (that is what the surveyor will have done plus looking for pink bits, not obvious on your pic). The Aquapoise is a good shape and if you are able to pull full revs then the size is right. Only go down (in pitch, not diameter) if the engine assuming it is in good nick, loads up and puts out black smoke.

If you want to check for size then there are websites with charts etc, although probably the best is Dave Gerr an American Naval architect www.gerrmarine.com

If you do buy a new prop either plain or fancy, the supplier will size it for you based on data you give him.

 

[roly_voya]

Dia (and therfore blade area) governs the amout of power the prop will transfer to the sea and needs to match the engine hp. Pitch is the distance forward the prop theorectically moves whith each rev. You need to match boat speed to shaft speed and pitch so that the engine reaches full revs at about 1.3 time square rout of water line length (theoretical hull speed) Best way to test it is to take the boat out on flat water and check you get full revs at full speed then do the same with a stiff head wind and some chop, if you still get hull speed all is fine and you have a good power reserve.

As a very rough gide
Engine wont rev and black smoke - Pitch to high
Engine revs easily but boat is easily stopped by wind/waves without loosing revs - Dia to small
Engine tends to over-rev but boat wont reach hull speed - pitch to low
Engine is fine on calm flat water but overloads as soon as conditions deteriorate - Dia to large

If you want to check size there is a good prop calculator here http://www.thepropellershop.co.uk/index.html but remember this is a guide 10% either way may work fine or even better as it depends on the exact water flow pattern around your hull

Generally if the prop is badly dezinctified it will show a pinky discolouration particularly on the edges and will eisily dent. If you tap it with a steel hammer it will have a dull tone where a good one will have a bell like ring. From looking at the pic your prop dosnt look about to fall apart so why not give it a seasons use and then look again. Its unlikely to break but may errode on the edges and become unbalence so keep an ear out for increasing vibrations. It could be that the survayor was just covering his back in case by reporting minor/possible damage then if you have a problem 'he told you so'

 

[charles_reed]

Reduced drag is only a part of the advantage of a self-pitching prop, and in my experience (admittedly only 16 years) a diminuitive part.

The greatest benefit I obtain is the ability to motor-sail at little above tickover revs and get 5-6 knots out of 6-7 knots true, and use less than 1 litre an hour.

The prop I have is an early Brunton Autoprop. Had its first new bearings 2 years ago and didn't need changing when I re-engined the boat. Previously I had a folding and then a 2-blade fixed props - performance with the Autoprop was markedly better than the folder (especially in reverse) and (apart from the reverse-lag) comaparable to the fixed prop in ideal (for the fixed prop) conditions. I can't comment on the other props (except to say that the Maxprop has marginally less resistance than the Autoprop, but inferior drive in adverse conditions), but in this life you usually get what you pay for.

 

[cladwalk]

having fitted a folding Gori prop and been in the Med for 2 years.....I would stongly advise sticking with 3 blade fixed.....what has not been mentioned is the fact that the blades sometimes get fixed in mid fold with barnacles etc.....requiring numerous dives to scrape clean

 

[FAITIRA]

Definitly, for me too, KIS.

 

[Chris_Robb]

Alternator on Shaft

Has anyone used the freewheeling prop to generate power?

I sailed accross the Pond in 76, on a Sailar 40, with an alternator attached to the shaft. It generated all the power we needed. We did'nt run a fridge and no autopilot, as it was bust.

 

[FAITIRA]

Re: Alternator on Shaft

Friend had one on a Westerly Conway, it was very effective, I saw it in action on a trip to Gib with him, he ran a fridge and autopilot, not certain if it kept up mind you, but I think he reckoned 3amps at 5knts???
Down side was it was bl##dy noisy, and it was a tidy enough installation.