Originally Posted by Pblais
Any prop shop can do the math for a sail boat knowing the engine and transmission. This stuff is pretty much precise engineering. Screw propellers have been studied to death for 150 years. There isn't any personal judgement left.
In my experience precise is anything but with prop calculations. After having worked on many power and sailboats when I worked in boat yards, and having a friend who runs a prop shop, and having purchased many of my own props, for my own boats, I can assure you that matching the prop to the MAX rated RPM on a boat is anything but precise. Many folks now ASSUME it is precise but only because they are clueless about proper prop sizing and having a prop that allows you to spin MAX RPM on a displacement
hull. Close yes, but hitting the MAX RPM within 25-75 RPM or so is actually much more rare than you think.
A number of years ago, when I was helping a friend who is a surveyor
, after his partner moved out of state, we found that nearly four out of ten sailboats were over propped. We noted this on the surveys and even checked with a photo-tach to make sure it was not a tachometer error. Most surveyors never even check this critical measurement but the good ones do. I think one of the reasons we see sooo many boats with failing exhaust
elbows is from so many boats being over propped.
This past spring I purchased a Campbell Sailor for my current
vessel. Even after discussing the closer to actual vessel displacement
, engine specs and the current prop the prop calculator still failed Campbell and myself. I went from a 16X12 Michigan Wheel
to a 16X10 Campbell and it was still over pitched and 300 RPM off MAX rated. What I wound up with was a 16X9 with less cup and I am now within 25 RPM of MAX rated. That 275 RPM made a HUGE difference as it always does.
The Campbell is a very, very efficient prop and more than once I have heard of them coming in too aggressive.
If you need to run your boat while waiting for Norm at West by North to send you a new prop just run her up to hull speed
and stop the RPM at that point. If you are 300 RPM off MAX back your cruise
speed off by 300 as well. Less smoke and less over loading of the engine..