details required to spec new prop on IP38

Hi Guys looking for some help. I am considering fitting a folding / variable prop on my IP 38. I have been all over the net looking at various blogs, pros and cons for all makes. As I cannot get access to my boat I am looking for info that I can send to the various manufactures to get quotes. I know that the engine is a Yanmar 4JH2E with max HP50 at 3600rpm, I think the gearbox is a KBW 20 (whatever that means) but I have no idea of the shaft details. She has a 3 blade fixed at the moment. Any help would be appreciated.

Call the factory, I want to think the spec I was quoted for my 38, hull #41 was an 18" x 8 prop, which is a whole lot different than a 14" pitch. My engine is a 4JHE, which I think is 44 HP?
Mine is over propped as I can only get 2600 RPM and I plan on fixing that this winter during haul out, but as I get 6.5 kts at 2000 RPM and 7 at 2200, I may just leave it alone as I do hit hull speed (7.5 kts) at full throttle.

Not at the boat now, but if memory serves I have a KBW20 with I think a ratio of 2.6 to 1, but believe there are some 2.1 to 1 -20's as well, not sure what IP spec'd.
I called the factory a couple of months back as I'm overpropped and wanted to either find another prop or have mine re-pitched during haul out this winter, they told me the factory prop for an IP 38 was 18 x 8.
That is a big difference from a 14 pitch prop and why I said call the factory, I assume they still have the build sheets from these old boats and could tell us exactly what was put on there.
You have me worried now, if I need a 12 or 14 and put on an 8, well I've wasted time and money because that can't work.
I also assume the factory diameter and pitch should be real close to perfect, or you have to assume Island Packet doesn't know what they are doing, and they are in fact wrong
Many, many sailboats are overpropped, I assume this comes from the old type of low RPM Diesels and people just not wanting to run their new Yanmars beyond 2000 RPM or so because that's how they ran their Perkins.
You can get away with overpropping without harm as long as you don't use much throttle, say never use more than 25 HP out of a 44 HP engine and that is where the perceived better fuel consumption comes from, the fact that your only using so little HP, turning the RPM down on an engine does not reduce it's BSFC (brake specific fuel consumption)it does however greatly reduce the power output.
Turning down the RPM can have a little effect as your reducing engine friction slightly, but it's much smaller than you would think, very difficult to measure.A slower turning prop is more efficient too, but again difference is very small.
Any of you pilots? With a variable speed prop you can of course change engine cruise RPM, turning the RPM down does lower fuel consumption, because it slows the airplane down as the engine loses power, but reduce the speed to the same but leave RPM up, and fuel consumption is the same.

Thanks for all the input. I have bitten the bullet and ordered a 3 Blade Autoprop H5-470mm. I will be back in the water beginning of May and will update the forum on the props performance.

Thanks

Mark

Wow...18x8. So, I'm guessing a 17x16 would be considered over propped?!?!?

I too cannot get my engine much above 2500-2600...but thought nothing of it since I can cruise along at about 7 knots with 2200RPM rung up. But with the caveat that my stern walk is, well, um, epic - to say the least. I'm thinking maybe going back to the factory spec might help with this...

Lots of good info in this thread.