Can someone teach me about Engine Speed and Propellors ...

[Trident]

Os suggest somewhere I can learn.

I understand the basics and have read previous threads on fuel efficiency versus revs and prop specs but specifically I can't work out what is best for my boat ..

It's a catamaran so none of the usual prop calculations work well - though choosing to use one hull dimensions with half the displacement and the over all beam (not 2X hull beam) with full displacement give almost identical results - obviously with one engine for one hull and two for the overall .
I have two 3YM20 Yanmars with nominally 20bhp each and a 2.64:1 SD20 sail drive each side with currently , 15 inch two blade Eliche Radice folding props.

The props were recommended by the supplier based on all my specs. Before this he recommended three blade non folding of basically the same size , though the prop calc sites all suggest that a 2 blade should be an inch bigger than a 3 and there does seem to be room to be bigger without hitting the hull.

I am on average a knot slower under engine with the folding 2 blade props than I was with the fixed 3 bade. (Faster under sail of course)

So, my first question, given that I wont go back from folding props now, is are 3 blade props inherently better than 2 ? Are fixed props inherently better at propulsion than folding? Is the loss of speed I have seen just a by product of gaining the sailing speed or can the right folder match the fixed blade. More bluntly did the expert give me the wrong size folder?

Secondly, if the props are ideal for the motors - and diesels appear to provide power more efficiently at closer to WOT, would I get better fuel efficiency running one engine at around 3300 rpm ,where it is supposedly making 16 bhp for 4L/hr , or running two engines at 2500 rpm where for for 4l / hour combined I get two lots of 7 bhp - so 14 in total . Will one prop with 16 hp behind it move me over more distance in an hour than two with 14 behind them? I get these figures are all theoretical from Yanmar in ideal bench conditions but I notice many cat owners tend to run on one engine at a time . I find the noise of anything much over 2200rpm to be intrusive but if I find myself with limited fuel in an emergency etc would I get further if I use one engine at higher revs?

 

[Tranona]

The simple answer as to why your boat is slower with the 2 blade folder is because for propulsion purposes it is not a good propeller - poor flat blade profile compared with say a FlexOfold. A 3 blade folder of the same diameter will obviously be better, and probably so would a 3 blade 14" or a 2 blade 15" fixed. I had 2 blade Flexofolds on both my Bavarias (30hp Volvo saildrives) and no loss of performance compared with same size fixed. You are right the conventional prop calculators are not good with all non fixed being based on a "standard" blade type and blade area ratio. You don't say whether the revs were the same when there was a 1 knot difference. I suspect the folder is too flat a pitch and a pair of 12" blades might help by bringing the revs down by about 300rpm for any given speed.

As to your second question fuel consumption is almost a direct function of power used so it does not matter whether it comes from one engine or two. I say almost because if you look at the power curves you will see the specific consumption (litres/hp) rises once you get over 2500 or so. Not really enough to make a big difference with the small amount these engines use. 2500rpm is the optimum continuous running speed, right on top of the torque curve and much better on the ears. So for me it is run both engines.