Prop Efficiency

[Halvern]

I have a 1979 11A, and recently when I had her on the hard (actually as we lifted her) observed there is 2 bladed prop. For the size of boat and wet surface I was expecting a 3 blade. How would I determine what type of a prop to install. Seems there is a calculation that I could use that would help.
Or, just leave it alone because the drag is less.

 

[sail sfbay]

Theoretically, a propeller with fewer blades is more efficient. Most props have three blades to provide balance, improve efficiency, minimize vibration from the water, move faster, improve manueverability under power, and backing control. More on propeller selection: 6 Tips to Choose the Correct Boat Propeller (triumphboats.com) and Choosing the Proper Sailboat Prop | Cruising World. However, if you are happy with your current 2 blade fixed propeller performnace then keep it and avoid the expense.

 

[JamesG161]

How would I determine what type of a prop to install.

There is a stamp on the hub of the propeller, that when decoded, tells you all you need know.

From the Crusing World linked by @sail sfbay

“What matters is to get the right combination of blade area and pitch for the engine and gear box,” he says. “On flat water, a two-blade propeller can be just as efficient or even slightly better than a three-blade."

Jim...

 

[Hollyberry]

2 blade fixed prop much less drag than a 3 blade... if the boat performs satisfactorily under power and sail without significant vibration you might want to leave it be....

 

[jssailem]

2 blade fixed prop much less drag than a 3 blade..

While this is a possibility it is not alway factual. Much depends on the prop design and the orientation of the prop when not spinning.

My own experience with two 3 bladed props. Two totally different performance profiles.

Both about the same overall diameter but the one on the right provides a knot and a half better through the water thrust and nearly a knot better sailing performance.

If these were 2 bladed props the orientation of the big eared prop would be critical to the sailing performance. You would need a way of orienting the blade to hide in the wash of the keel not stick out in the horizontal position. That would be a real drag....

 

[Hollyberry]

While this is a possibility it is not alway factual. ....

Of course, I should have qualified.. assuming similar blade shape and area. We have had, in the past, a 3 blade Campbell Sailor which caused less drag than the same diameter michigan wheel, of course. Performance was not too bad but reverse was not significantly better than a good folding prop.
Having also had a 3 blade MAX prop, and now a Kiwi, IMO feathering is the way to go. (prop change was a boat change - wouldn't have gone from MAX to kiwi otherwise!!)

 

[kappykaplan]

feathering is the way to go

Love my MaxProp!

 

[Hollyberry]

Love my MaxProp!

What's not to love??