Hi, I’ve just finished your latest Storm Tactics, it’s grate.I’ve also read and loved your others. Can you tell me how to tell if my prop is sized properly.I own and am planing on cruising my Rob Perry designed Islander 32. It has a Westerbeak 25 hrs . My engine manual doesn’t say anything about max engine RPM. My current prop is a 12/14 two blade. At full throttel the RPM is 3400. I’m not sure what my hull speed should be,the waterline is 25′, and I can’t look it up in my Cruisers Encyclopedia because I lent it out and havent gotten it back.I can get .4 gal per hour and about 5kts at 1800 RPM. Is this too low of a power setting for long periods. Do deisel do better at higher settings. Is it hard on them to run at higher settings, say 3000 RPMs for extended periods.It seems that power settings above 2600 dont produce higher boat speeds.Any thaught on all the above would be a great help. Thanks
Hi Allan: You’ve posed a bunch of questions, and as we’re leaving in the AM for Panama I don’t have to time to give you the details–but I will try and point you in the correct direction. In the Offshore Cruising Ency. you will find the formula for both hull speed and power calcs. These are easy to run through and will give you an idea of the power required. The most critical part of this equation is the propeller efficiency which can be anywhere from 20% for a lot of two bladed sailing props to as high as 60% if you have something like our Hundested controllable pitch prop. Bob Perry can probably point you in the right direction on prop size/type, or your local prop shop will have some suggestions. As to what speed to run your engine, that is a function of how it is loaded, and what sort of design it is. Here I’d go to the local rep and talk to one of their mechanics. For example, you can run most 3600 rpm Yanmars at 3200 rpm for thousands of hours, and they are happier than if you run them slow and lightly loaded. Regards–Steve Dashew
Posted by Steve Dashew (November 30, 1999)
