prop size

[jerryvan]

I repowered my 18' Harbor Pilot with a 21hp Westerbeke diesel.  It had a 60hp Gray Marine gas.  I never changed the prop.  At 2500 rpm's I'm getting 7 mph.  At 3000 rpm's I'm still getting 7 mph. 

That original prop has the following markings ... Equipoise, t-36, 9-53,606 RH, and R33.  Can anyone tell me what those markings mean and what their prop size is if they have a diesel? 

Jerry

[Viking North]

I realize it's been some time since this posting and surprised there hasn't been an earlier reply. Often gas and diesel powerplants require opposite turning props Left Hand verses Right Hand rotation. Possibly the origional gas engine ran a left hand prop and you are now unknowingly running the diesel engine in reverse to go forward, compensating for not having installed a Right Hand turning prop.The other factor at play is Prop Pitch. From the description it sounds like a cavation problem resulting from have too little pitch on the origional gas prop to match the prop required for diesel engine. Generally there should be sufficient pitch on your diesel engine prop to stall engine RPM at about 3600. My interest in this topic is, I am re building and converting a badly damaged keel to hull join,older Herreshoff 18 cat rig into a power boat installing a 3GM Yanmar diesel. If there is someone out there that has a diesel powered version of the 18 (Pilot,Launch,Bay Fisherman,) would you be so kind to pass on your prop diameter and pitch as stamped on the props hub. I want to compare with my calculations before ordering,  With Thanks --VN   

[Viking North]

Did a little searching on the postings in the different categories and have found the answer to my prop question contained in the marine survey on Casabel,  13in.dia prop 10deg. pitch driven by a 3gm Yanmar. I calculated 14 to 15in. dia prop 8deg. pitch which should be a ok. I like the idea of a larger dia. prop allowing more blade area out into undisturbed water. And for all you purists out there, yes I am converting an 18ft. cat rigged version into a custom power boat.  However being an experienced boat builder I promise to be kind   

[Adam]

I got a chuckle out of that. There have been several folks that have converted one of the sailing versions of that hull into a powerboat - no harm no fowl!