More efficient prop for Penn Yan "Pocket drive"

[RGJ]

Looking for recommendations and thoughts on a more efficient propeller for my brother-in-law's 1990 23ft Penn Yan Avenger (it has a pocket drive). The numbers stamped on the current prop are 13 * 12 1 1/4. We are assuming it is a 1 1/4 diameter shaft but don't know how to read the first two numbers. (13 degree/12 inch or 13 inch/12 degree??) We did a WOT test on calm water a couple of years ago and achieved 26.7 mph at 4400 rpm with the 305 Chevy engine. Normal cruise is ~ 17mph at 3300 RPM and ~ 1 MPG. It seems to be just barely "on plane" at that speed. I've seen some stuff about "cupped" props but don't understand the language or technology. I think he would be willing to trade starting torque for speed and efficiency (aka, better gas mileage). Any thoughts and/or recommendations you might have on an optimum prop for this boat would be appreciated.

 

[porscheguy]

The pocket drive is some kind of inboard which I think they call a tunnel drive. On most props the first number is the diameter of the prop. I know very little about these boats other than their use in the Vietnam conflict.

im not sure who, if anyone makes inboard type props. I was always under the impression that for inboard props, you generally had them custom made at the prop shop. I’m not sure of your location, but I know in Most areas that have any kind of port or commercial boating industry, they’ll have prop shops who can take care of your needs. Penn yan went the way of the dodo 20+ years ago, so whichever place you chose, hopefully they’ll have a guy whos old enough to remember them.

 

[RGJ]

Thanks for the input. I hadn't thought about the potential of the commercial boating industry.

 

[garbageguy]

..Normal cruise is ~ 17mph at 3300 RPM and ~ 1 MPG. It seems to be just barely "on plane" at that speed...

if at that RPM the boat is barely on plane, why is that "normal cruise"? (that IS about the RPM I think of as normal cruise for my boat). Maybe that's a reason tunnel drive didn't really catch on