Prop size and pitch-CD30MKII

[Richard Feffer]

In the interest of improved performance, I would like to remove my two-bladed prop and replace it with a three-bladed one. Does anyone have any recommendations as to diameter and pitch? Engine is three cylinder Westerbeke 21A.

Thanks,

Richard Feffer
CD30MKII
Adamarie



RichFef@Prodigy.net

[Tom]

In the interest of improved performance, I would like to remove my two-bladed prop and replace it with a three-bladed one. Does anyone have any recommendations as to diameter and pitch? Engine is three cylinder Westerbeke 21A.

Thanks,

Richard Feffer
CD30MKII
Adamarie

I'm not familiar with your engine, Richard, but no one has jumped in here with any help for you so let me give you some general information. If you've got the correct two blade prop on there you can probably go to a three blade with the same diameter and pitch. This is stamped on the prop near where the shaft goes in. The first number is the diameter and the second is the pitch. If it's not stamped there you can measure the diameter and the prop shops have a machine that they run it through which will give you the pitch. A 3 blade makes a noticeable difference in push maybe 10 or 20 percent, but it isn't going to make a 4 knot boat into a 6 knot boat or anything dramatic like that.

If you want more push you can go to a larger prop, but you don't want to put any more strain on your clutch plates or engine and presumably you've got the largest 2 blade that your engine can handle. Nobody ever puts a prop on that's too small because they want to go slower. So you can go to a prop that is one inch larger in diameter, but you then have to remove one unit of pitch to keep from overloading the engine. So in a way it's one step forward and one step back. However, a larger prop will give you more push even with less pitch. The catch is that it also gives you more drag when you're not motoring, because you're putting a bigger and bigger sea anchor so to speak in your slip stream.

Thus to some extent it depends upon what kind of sailing you do. People who sail a lot and don't like to motor like the 2 blade because it hides in the shadow of the keel when not running and gives minimal drag. People who sail in an area where they have to motor against the current a lot often give up the sailing performance to get a bit more push when motoring and they accept the drag when sailing from at least one blade always being exposed.

I've had 6 different props on my CD 31 in the 12 years I've owned her. She came with a two blade 11 by 11. After much experimentation I've settled on a three blade 12 by 10. I do a fair amount of motoring in my area and have a Universal 3 cylinder 21 horsepower diesel. I hope this will give you something to go on. Of course, if someone has already over propped you in the past you don't have any zero line and have to start from scratch. Any prop shop can give you specs for the correct prop for your engine, or it could be that your manual has the info for you. Getting the right prop for you and your kind of sailing is a journey not a destination. As always, Good Luck!

Tom Coons
CD 31 "Heiress"



TomCambria@mindspring.com