Duo-props v Folding

[boatmike]

I have a "wing engine" in my Prout Snowgoose Catamaran in addition to the main central engine which has a lifting leg. It's a tiddly little Volvo 2010 (2 cylinder 10 HP) with a saildrive. I run it mainly as a generator to charge batteries and give me manouverability when docking. Both of which it does superbly. The "duck bill" 2 bladed prop fitted as standard is inefficient though and I want to get a bit more thrust from it so that it will be useful to run it when motor sailing etc as an alternative to the main engine. At the LBS I looked at Bruntons duoprop which appears to adjust pitch to suit conditions and should therefore give good account of itself when motor sailing. Volvo themselves have a 3 bladed folder which they claim to be more efficient too and there is a 3 bladed self feathering prop from Darglow to consider too.
They are all expensive of course. So the question is will I get more useful thrust out of 10HP on a boat of this size with any of these and is the amount I would get worth spending £600-£900?
Anyone with experience of any of these? I hate spending money!

 

[Noggin80]

I think you mean Autoprop from Bruntons, duoprop is the volvo z drive thingy. Your saildrive is probably a 2001 which is a 10hp engine. It is difficult to quantify efficiency with propellers as your engine is controlled by a governor which adjusts the fuel flow to match the load demanded by the propeller. If you swap your 2 blade for a 3 blade you may well get 50% more thrust but you will also use 50% more fuel.

If you want to use it for motorsailing and hate spending money then consider the alternative cost. If you do fit a 10hp prop to match your engine, say an Autoprop, it will cost you about £1500 quid inc tax. That woud buy you 1000 gallons of red diesel at £1.50 per gallon. My Nauticat 42 uses just under half a gallon per hour at 4 knots at 10 hp (I have measured it), your cat should do even better. That works out at 4000 nautical miles, enough to get you across the Atlantic and past the Galapagos with out bothering with all the sails. Or if you go the med where you won`t need the sails you could motor for years.

As for the props I use a Maxprop (brilliant) and from where I sit I hear the Autoprop is exellent in fact my old friend Mr John Coxon who invented the Autoprop was here just a few days ago.

 

[dulcibella]

I was lucky enough to buy a second-hand boat with an autoprop already fitted, but this might be too radical a solution. However you get one, they are excellent.

 

[boatmike]

I did mean Autoprop from Bruntons. Correct. However the engine is as I stated a 2010. There is no such thing as a 2001.
The point is to get a more efficient propellor than the 2 bladed which is very inefficient. Your point regarding the set up of the engines fuel system is not valid. The fuel system is set up to deliver a set output in terms of KW at a certain RPM. The engine does not know what prop is on the shaft. Obviously fitting a prop that is over diameter or over pitch would result in the RPM not being reached. You can't get a quart out of a pint pot. However with a more efficient prop I should get higher speeds or the same speed at lower revs. My question was, does anyone have first hand experience of the Autoprop and be able to tell me how much improvement I might expect?