Prop sizing

[GBurton]

Does anybody know the formula for prop sizing?

Thanks for any help

[capngregg]

If you're into calculus it can be a great exercise to determine the proper characteristics for a propeller for any given boat. There isn't any simple formula to produce that information.

There are lots of software programs out there to solve the problem www.hydrocompinc.com is one site or you can call the manufacturer to get the original installation for your vessel. If you're re-engineing, typically the engine/transmission guys have software they will run your boat information through to come up with a "recommended" prop for your installation.

If you're just experiencing prop performance problems in general there are all kinds of propeller repair sites that offer basic formulas to vary engine RPM based on prop pitch, cup, rake, diameter and so on. Usually your local prop guys can recommend changes based on your information.

Good Luck,

Gregg

[Tartan34C]

It’s a book not a formula. One good one is written by Stanton and published by Cornell Maritime Press and the other standard work is written by Dave Gerr. Stanton is specific to sailboat propellers and Gerr is a general propeller book. Most texts on design also have a chapter on propeller selection with Skene’s Elements of Yacht Design being the easiest one to understand. What do you want to know? By the way most propeller shops and all propeller manufacturers will make a recommendation if you give them a call.
All the best,
Robert Gainer

[JohnRPollard]

In addition to the good suggestions above, I would also poll other Westsail 32 owners to see what they are using as an engine/prop combination (assuming this question is for your new boat).

[GBurton]

Thanks for all the help! I guess I was oversimplifying it a bit.
I was under the impression that if you had LWL, weight, engine HP and torque it could be easily done.

The engine that I plan to use (Bukh) is not common over here in the States so I need to plan from scratch.

[Tartan34C]

It’s somewhat of an art and not straightforward at all. The shape of the area around the propeller and in fact the shape of the hull will affect the slip of the prop as just one factor. That factor alone makes a great difference and if you have a Westsail 32 the slip is considerably greater then on a Person Flyer for instance. The idea of checking with the Westsail forums is a good one and you might want to start there. You can get enough information to calculate the slip from other boats and use that to enter the formulas to calculate for your own engine. It doesn’t matter who made the engine the only thing that matters is the HP, RPM’s and propeller pitch and diameter. And the slip is the same for a given hull and all engines/ propeller combinations and only varies by the shape of the hull.

The diameter is clearly related to HP and RPM’s so that’s easy to figure out. The pitch is controlled by slip and the shape and number of blades and the RPM of the shaft.
Good luck and all the best,
Robert Gainer