2 Blade vs 3 blade prop for 30' yacht

[ruvane]

Hi,

I have a 30' Moody with a Volvo MD11C and need a new prop. Assuming the new prop (fixed blade) is correctly sized what are the advantages/disadvantages of 2 or 3 blade props?

I currently have a 3 blade and the prop-walk is vicious!

Any additional info would be appreciated.

Thanks,
Ruvane.

 

[lw395]

Get a folding prop, it's supposed to be a sailing boat!

 

[NormanS]

The only experience that I have with a two bladed prop was with a Folkboat, many years ago. If trying to butt into any weather, it was hopeless. I changed it to a three blade, and reckoned that I was then able to make use of the available engine power (Volvo MD1). There was probably more drag when sailing, so as has already been said, to get the best of all worlds, get a folding or feathering or Autoprop.

 

[johnalison]

It might depend on the type of cruising you do, and the distances you motor. I have a 2-bladed folder on my 34 footer and generally manage well, but the same boat with 3 blades will do the same speed with about 400 rpm less. It would be a mistake to go for another fixed prop these days since the advantage of having a folder/feathering prop is so marked. Anyway, you should be looking at prop walk as an advantage to be exploited, not as a problem.

 

[Tranona]

In general terms a 2 blade prop is less efficient in producing thrust than a 3 blade but creates more drag when sailing. Expect poorer motoring performance, particularly in adverse conditions. A change may not make any difference to prop walk.

 

[ruvane]

Thanks for the input.

I don't want to go for a folding prop for various reasons. Looks like a 3 blade is on the agenda.

 

[Pagetslady]

How do you put your boat alongside or turn around without propwalk?

 

[lw395]

How do you put your boat alongside or turn around without propwalk?

Rudder.
Springs.
Lots of boats don't have much propwalk.

 

[Tranona]

How do you put your boat alongside or turn around without propwalk?

Just as you make allowance for prop walk you make allowance for no prop walk. Many modern boats, particularly saildrives have very little prop walk which means they generally go where you point them. You can of course make it all so much easier by fitting a bow thruster.

 

[RichardS]

In general terms a 2 blade prop is less efficient in producing thrust than a 3 blade but creates more drag when sailing. Expect poorer motoring performance, particularly in adverse conditions. A change may not make any difference to prop walk.

That seems counter-intuitive. If the 2-blader moves less water under engine then surely it would present less resistance under sail?

Imagine a two blader with blades 1 cm wide. Hopeless drive but minimal resistance.

Richard

 

[Tranona]

That seems counter-intuitive. If the 2-blader moves less water under engine then surely it would present less resistance under sail?

Imagine a two blader with blades 1 cm wide. Hopeless drive but minimal resistance.

Richard

Not what I meant to say! The "but" should be "which" - although could have written the whole sentence better.

 

[seumask]

Not sure why a folder is out. 2 years ago we replaced an average 2 blade fixed prop on our similar sized Fulmar with a Flexofold 2 blade folding Cruising prop. Discounting the the obvious improvement in sailing, under engine we had a great improvement with the new prop having much more thrust due to its larger blade area and better pitch. So I can recommend you talk to Darglow Engineering.