Propeller Sizing for Yachts

Nobody can tell you what the right size propellers are for your boat because there are too many variables involved. There are hundreds of thousands of boats of all sizes and shapes, as well as power plants. You may be thinking, "Oh, but my boat is just an ordinary 32' so and so, much like all the others out there."

Okay, to you it is, but not to anyone trying to figure out which propeller will offer the best performance. Propellering, you see, is more of an art than science. All boats are not alike, not even two of the very same model, at least not in terms of the possible variables that affect performance. Without writing a book on the subject, take it on faith that every boat presents an incomprehensible set of variables which are more or less unpredictable. You and your neighbor may both have a Trojan F32, but it is entirely possible that there are factors that make those two boats sufficiently different that each will need a different propeller. Our problem is that we do not know what those factors are, and we probably can't find out.

To obtain the technical solution to your propellering problem, we need a formula for each and every boat. A formula that is not available.

Engineers have been attempting to develop sure-fire propeller size selection tables right from the very first propeller. And right up to this day of computerization, no set of charts or tables has ever proved completely effective.

To obtain the technical solution to your propellering problem, we need a formula for each and every boat. A formula that is not available.

Yes, we would all like easy answers, but there aren't any, and least of all from someone who knows nothing about your boat. Builders and designers first choose props from calculations and charts. Rarely do they hit it right the first time. Back in the heyday of Bertram Yachts, there wasn't a day go by that I wouldn't see a Bertram out zooming around on Biscayne Bay at full speed. What they were doing was constantly testing propellers and speed. Bertram, you see, used to certify their boat speeds upon delivery. I don't know of any other builder who does or did that. So when you think about all those boats that are built in plants that are nowhere near the water . . . . well . . . it's not hard to see why there are so many performance problems.

The only way to efficiently resolve the problem is to hire an expert. But there are not many experts around, and it may take him a long time to find the problem, especially if it is a combination of factors, which it usually is. One ends up going through a process of elimination, which is very time consuming and costly. The sad fact is that people with major performance problems usually end up stuck with them, even after spending a lot of money to find out why.

Source:

David Pascoe,