Look at these figures. Is my propeller efficient?

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cmedsailor

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I often wonder if my 2 blade fixed propeller is adequate for the boat (and if it is the correct size) and what more a 3 blade propeller would offer besides more drag if it is a fixed one or more costs if it is a folding. The boat is a Beneteau Oceanis 361, engine is Volvo Penta MD2030D 29hp, gear ratio is 2.35:1, the 2 blade propeller size is 16X11 and I can get the following figures when motoring AGAINST 13-14 knots of true wind (17-19 apparent depending on speed) and a little bit of chop:
At 2000rpm speed of 4,3 knots.
At 2500rpm speed of 5,2-5,4 knots
At 2600rpm speed of 5,9-6,1 knots
At 2800rpm speed of 6,5-6,6 knots
At 3000rpm speed of 6,8-7 knots
Maximum rpm is 3400 and speed 7,6-7,8.
“Cruising rpm” I like to use is around 2600. If I have to motor against heavy seas I would not like to exceed 2800rmp. Above this there is a lot of noise and vibration plus increase of consumption, not the best if I have to make a long trip since my fuel tank capacity is only 75 litres. Of course speed drops a lot in heavy seas but I haven’t made a similar test to check it.
What do you think of these figures?
 
I have a Moody 31 with a VP 28hp and a standard 2 blader. I don't do those higher revs but at the bottom end those figures look OK to me. Around here there is not much motoring without tide so it is hard to be definitive without a specific test.
 
Please note that my test mentions speed (over the water), not SOG (that's a completely different story). Also to mention again that it was the speed against the wind (0 degrees). At any other angle, speed is of course higher.
Thanks
 
These speeds are virtually identical to my Bavaria 37 with the same engine and same size propeller - albeit on a saildrive.

Assuming your waterline length is the same as mine, that is just under 9 metres, the theoretical maximum hull speed is approx 7.4 knots and you are just exceeding this at max revs. So I guess it is propped just right. Incidentally, my engine is governed to 3600 and I have taken it there once and got 7.8 knots. I normally cruise at 2400 at 5.4 knots.

I know your interest is in potentially improving speed going into a head sea, and a 3 blade will improve that sort of performance, but at a (probably) minor cost in sailing performance.

I am going through exactly the same thought process with my boat, not for performance reasons necessarily, but because you can't put the saildrive in gear when sailing, so there is noise once you get over about 5.5 knots. However, I have come to the conclusion that a feathering 3 blade prop is the one that will achieve better motoring performance and same of lower drag, but I have better things to do with the money!

As I think I said in response to one of your earlier posts, a new fixed 3 blade is not expensive (compared with a folder or featherer) and maybe worth experimenting with.
 
Tranona thanks.
Yes indeed I was wondering whether to change or not to a 3-blade however the question this time was more to find out if I get the best out of the current combination (propeller size - gear ratio - horsepower). So it seems to be OK, and as you mentioned "I have better things to do with the money" so I may just leave everything as they are for the moment.
 
Why can't you put the saildrive into reverse when sailing? We do this to stop it from turning and encourage the blades to fold.
 
Because that is what it says in the Volvo manual if you have a fixed prop as I do. If you have a folder, you can put it into gear. As I understand it the loads placed on the transmission by a stationary fixed prop lead to premature wear and I am not sure I want to risk that.

Having said that the boat has done 7 years on charter, so I have no idea what others have been doing - guess many have put it in gear as the noise can be very irritating, but the transmission is still the original.

Would welcome others' experiences on this.
 
Your figures are very thorough. Are they with a clean bottom?
What's the cruising consumption?
I think I'd be happy with the numbers.
Personally I've only owned boats with 2 blade folders, and noisy engines that you want to switch off ASAP, so I wouldn't be motoring far in 13kts true, I'd rather be sailing! I'm now getting interested in a boat better set up for motoring, although I will always go for a folding/feathering prop.
The other issue with selecting propeller pitch is how much thrust can you get at zero hull speed i.e. when you're aground! If the engine won't rev to peak torque aground, the thrust available may be reduced more than you realise! This can be a trap in pitching for best cruising economy.
Larssen and Elliasson (sp?)'s book 'Principles of yacht design' has some good info on propellers.
 
lw395, yes they are with a clean bottom. Regarding consumption unfortunately I still haven't managed to work it out. It's difficult because every time I get the chance to use the sail (sailing or motor sailing) I decrease rpm or shut the engine. Not only this, I often try to persuade myself go out, spend a couple of hours motoring to test consumption but I never do it. It's too boring. Probably some one with same boat that calculated consumption could help.
Definately I would prefer to sail in 13 knots true but only if it was giving me at least 20 degrees deviation from my density so that total distance covered was not becoming too much especially in a long trip. However, life is not always so easy!!
 
We have been in touch before re. 361. From your figures it seems pretty standard speed expectations. We get similar with a folding prop. I do think Beneteau have done their homework with prop size and pitch.

We repitched a previous yacht because we felt underproped, it gave us better top speed but with cavitation, singing prop and a bit of overload at higher revs.

I think we overdid it a bit.

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Hey Ozzy. It's been a long time. In fact exactly a year ago when I was trying to find information about the boat I was about to buy!!!
Out of curiosity what folding propeller do you have?
Thanks
 
Hi Philippos. when we bought our 361 it already had a Gori propeller fitted.

The advantage is a small increase in speed under sail. However the big disadvantage is that there is a very strong kick to Starboard when manoeuvring in reverse. The prop acts like a paddle wheel climbing to starboard, it is virtually impossible to reverse straight out of a berth or reversing in tight areas. The kick can be used to advantage when berthing or departing from alongside you have just got to work out how to use it.

Not having tried a 361 in reverse with a normal prop it hard to compare, I probably wouldn't recommend this type, although it is I think one of the cheaper folding props.

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