Propeller 2 blade v 3

[Trident]

I have a 50 foot catamaran with Yanmar 3YM engines and saildrives

I put 2 blade folding props on a couple of years ago and found a good 0.5 or more increase in sailing speed and of course no noisy sail drives under sail

Previously I had 3 blade fixed props. I appear to have lost about 1 knot under engine at any given RPM - does that sound about right?

Would I expect to gain some of that back with super fancy 3 blade folding props when under engine?

To avoid unhelpful comments both the old and new props were matched to the engine and legs so are not the wrong ones etc

Does anyone know about this arcane magic and have experience that would help decide whether to change again?

 

[Tranona]

No - in terms of straight boat speed you should see virtually the same because your 2 blade is likely to be at least 1" larger in diameter than the 3 blade to achieve a somewhat similar blade area. Where you might see some difference is in reverse and in adverse conditions. Your loss of 1 knot suggests that the 2 blade is not the correct size (despite what you say!). Just as an example I have had 2 saildrives on 30hp and the 3 blade was a 15*12 replaced with a 2 blade 16*11 with no measurable difference in performance. Your Yanmar sizes may be different but the principle is the same.

A loss of 1 knot at a given rpm suggests the 2 blade is too "small", either diameter or pitch, or both. Fairly easy to check with a monohull as you can easily calculate the hull speed and (assuming you have enough power available) the prop size to achieve hull speed at near maximum rpm. Can you achieve maximum rpm easily and if so how does the speed compare with what you got with the 3 blade. Would the 3 blade have gone faster?

In terms of changing to 3 blade then suggest you look at feathering rather than folding propellers such as the Featherstream, partly because of the better overall performance under motor and partly because they can be fine tuned to suit your boat including having different pitch in reverse. Drag reduction is essentially the same as a folder.

 

[johnalison]

Although a very different boat, the received wisdom for my class is that 2 & 3 will give the same speed but the 3-bladed will do so at about 400 rpm less.

 

[geem]

I have a 50 foot catamaran with Yanmar 3YM engines and saildrives

I put 2 blade folding props on a couple of years ago and found a good 0.5 or more increase in sailing speed and of course no noisy sail drives under sail

Previously I had 3 blade fixed props. I appear to have lost about 1 knot under engine at any given RPM - does that sound about right?

Would I expect to gain some of that back with super fancy 3 blade folding props when under engine?

To avoid unhelpful comments both the old and new props were matched to the engine and legs so are not the wrong ones etc

Does anyone know about this arcane magic and have experience that would help decide whether to change again?

I know you are planing to do some long distance sailing. Definitely worth considering folding props as opposed to feathering props. The amount of discarded fishing gear that it out there needs to be considered. Folders don't catch stuff so easily as feathering props.
Very happy with our Brunton 3 blade folder.

 

[Tranona]

Although a very different boat, the received wisdom for my class is that 2 & 3 will give the same speed but the 3-bladed will do so at about 400 rpm less.

That does not make sense. There should be little difference in revs for any given speed. As I explained above the load should be similar because the "size" should be the same. Crudely diameter multiplied by pitch 15*12 is 180, 16*11 is 176. A change of 1" in pitch or diameter on these engines (Yanmar or Volvo 30hp) changes revs for a given speed by 250-300

 

[Tranona]

Agree, although fitting a rope cutter deals with the potential fouling issue. The advantage of feathering props is as I described, but not necessarily valuable for long distance sailing where my choice with those engines and drives would be a properly sized FlexOfold 2 blade.

 

[geem]

Agree, although fitting a rope cutter deals with the potential fouling issue. The advantage of feathering props is as I described, but not necessarily valuable for long distance sailing where my choice with those engines and drives would be a properly sized FlexOfold 2 blade.

Most of the time long distance sailing is not motoring. Feathering props will catch nets, line and any debris when sailing. They are a little more vulnerable. Rope cutters won't always deal with a net. The engine will often stall due to the volume of net in the cutter.

 

[Trident]

No - in terms of straight boat speed you should see virtually the same because your 2 blade is likely to be at least 1" larger in diameter than the 3 blade to achieve a somewhat similar blade area. Where you might see some difference is in reverse and in adverse conditions. Your loss of 1 knot suggests that the 2 blade is not the correct size (despite what you say!). Just as an example I have had 2 saildrives on 30hp and the 3 blade was a 15*12 replaced with a 2 blade 16*11 with no measurable difference in performance. Your Yanmar sizes may be different but the principle is the same.

A loss of 1 knot at a given rpm suggests the 2 blade is too "small", either diameter or pitch, or both. Fairly easy to check with a monohull as you can easily calculate the hull speed and (assuming you have enough power available) the prop size to achieve hull speed at near maximum rpm. Can you achieve maximum rpm easily and if so how does the speed compare with what you got with the 3 blade. Would the 3 blade have gone faster?

In terms of changing to 3 blade then suggest you look at feathering rather than folding propellers such as the Featherstream, partly because of the better overall performance under motor and partly because they can be fine tuned to suit your boat including having different pitch in reverse. Drag reduction is essentially the same as a folder.

I can't do a like for like comparison as I never went near full speed under engine with my last props - max revs is 3600 and I'm not sure we've ever been above 3000 . I'll start by contacting a different supplier of the same prop and see if they work out the size as the same - however the 3 blade fixed was a 16 inch and so is the two blade so by the sound of it they may have worked things out incorrectly - though its also possible there would not be clearance for a 17 from the hull - I'll check that when I dry out to do anodes. Thanks for the help

 

[Tranona]

Most of the time long distance sailing is not motoring. Feathering props will catch nets, line and any debris when sailing. They are a little more vulnerable. Rope cutters won't always deal with a net. The engine will often stall due to the volume of net in the cutter.

Why are you telling me this when I have spent 40 years or more involved with drives, propellers and rope cutters?

I know long distance sailing is not about motoring hence my suggestion for a simple low cost 2 blade folder

 

[geem]

Why are you telling me this when I have spent 40 years or more involved with drives, propellers and rope cutters?

I know long distance sailing is not about motoring hence my suggestion for a simple low cost 2 blade folder

Take a chill pill Tranoma. You will get your blood pressure up

 

[Tranona]

I can't do a like for like comparison as I never went near full speed under engine with my last props - max revs is 3600 and I'm not sure we've ever been above 3000 . I'll start by contacting a different supplier of the same prop and see if they work out the size as the same - however the 3 blade fixed was a 16 inch and so is the two blade so by the sound of it they may have worked things out incorrectly - though its also possible there would not be clearance for a 17 from the hull - I'll check that when I dry out to do anodes. Thanks for the help

Think you are OK to go up to 17" Assume you have SD20 drive drives which have a 2.64 reduction so a 16" 3 blade and 17" 2 blade would make sense (Volvos for the 3600rpm engines are 2.47 so generally run smaller diameter props) However I note that they have a standard or long reach leg size so best to check.

Worth, if you can doing a series of runs from, say, 2400 upwards at 200rpm intervals up to the maximum you can go logging speed against RPM as then you have some hard data to work with. Combined should give you a maximum of somewhere around 9-10 knots

 

[Tranona]

Take a chill pill Tranoma. You will get your blood pressure up

Really relaxed - just get get bored with you stating the obvious that adds absolutely nothing when I have just explained something. Please improve your typing ability and learn to spell my name correctly.

 

[geem]

Really relaxed - just get get bored with you stating the obvious that adds absolutely nothing when I have just explained something. Please improve your typing ability and learn to spell my name correctly.

Yes, dyslexia does have its drawbacks, but I try hard. Not always successfully.
Other people than you read these posts. Filling in the gaps for others to read, not necessarily you.

 

[johnalison]

That does not make sense. There should be little difference in revs for any given speed. As I explained above the load should be similar because the "size" should be the same. Crudely diameter multiplied by pitch 15*12 is 180, 16*11 is 176. A change of 1" in pitch or diameter on these engines (Yanmar or Volvo 30hp) changes revs for a given speed by 250-300

It may not make sense, but is what I gather is known from practical experience in the same 34’ boats with Volvo 2 vs 3-bladed props. I can only suppose that the propellor parameters are arranged to give the best all-round performance with each type. I myself have a 2 but friends with a 3 and almost the same boat seem to have an easier time keeping the same speed.

 

[Tranona]

Yes, dyslexia does have its drawbacks, but I try hard. Not always successfully.
Other people than you read these posts. Filling in the gaps for others to read, not necessarily you.

What gaps were there?. The question was about speed with different props not about fouling and anyway were already dealt with in post#6 along with the reason for folding rather than feathering as motoring was less important in this style of sailing.

 

[Tranona]

It may not make sense, but is what I gather is known from practical experience in the same 34’ boats with Volvo 2 vs 3-bladed props. I can only suppose that the propellor parameters are arranged to give the best all-round performance with each type. I myself have a 2 but friends with a 3 and almost the same boat seem to have an easier time keeping the same speed.

Still puzzling. The only reason with the same power/boat for revs to be lower is because the prop is "bigger" - that is the 3 bladers is overpropping. If props are matched correctly the number of blades makes little difference. Maybe the 3 blade owners think they are using less fuel when they are not because the engine is producing the same power, just at lower rpm. The only difference a 3 blade makes is in adverse conditions where the greater disc area might provide more thrust or for stopping power in reverse. Generally boats like yours (and my Bavaria 33 which had similar basic dimensions and engine) are well powered with 30hp and don't need 3 blade props.

 

[noelex]

The main advantage of 3, 4 and 5 blade props is that they reduce vibration compared to 2 blade alternatives. They can also produce better performance when the blade tip to hull clearance is inadequate, especially in rough water or when stern propulsion is used.

As your main issue is a lack of speed, and tip clearance is probably OK, it is likely that the two blade propeller is undersized or does not have enough pitch. The other possibility is that your previous three blade prop was over-pitched. See if you can achieve the maximum engine RPMs before the throttle is advanced all the way.

The propeller manufacturers can sometimes (for a fee) increase the pitch on an existing folding propellor.

This is a reasonable article on how various designs of propellers perform.

Folding and feathering propeller test

 

[johnalison]

Generally boats like yours (and my Bavaria 33 which had similar basic dimensions and engine) are well powered with 30hp and don't need 3 blade props.

I have not actually used a boat such as my triple friend, so I don't have first-hand experience. I'm fairly sure that my boat is correctly propped, but there are some situations such as motoring into a head-sea and wind when I feel that the extra bite would be of benefit.

 

[Supertramp]

Am I alone in being a little scared of folding or feathering props? I have this vision of it refusing to play just when I need it. Irrational I know.

 

[geem]

Am I alone in being a little scared of folding or feathering props? I have this vision of it refusing to play just when I need it. Irrational I know.

I can't speak for all folders as this is the first one we have had, but the Brunton 3 blade mechanism is very robust. It's super simple to strip down. The only draw back of it is we seem to have a lot of prop walk. It's a 22" prop. Throwing it into reverse wll send us sideways. Getting going in reverse is best with bursts of power then steer where you want to go in between. We can't steer where we want to go with the prop turning in reverse as it just goes to port.
The prop has huge forward thrust as the blades look like mickey mouse's ears. Lots of surface area. It will push us in to big seas and wind without issue. I would find it hard to go to a fixed prop and lose the sailing performance.