Inboard Issues

[mbd]

My engine specs:
Yanmar 2GM 13 HP @ 3400 RPM, 15 HP @ 3600 RPM
Trans: 2.62:1

Michigan Wheel: http://miwheel.com/MIWheel/PropAnaly...is/InboOne.asp

Results: "Mike, according to my calculations, your current 13 x 11 three blade prop should allow the engine to reach its maximum rated RPM (it is nearly the same as the theoretical size, based on an engine rated at 15 HP @ 3600 using a 2.62 to 1 reduction gear, and a top speed of 5.5 knots)... I am reluctant to suggest a 13 x 6 or 13 x 7, which it appears is what you need based on the reported current performance and propeller size..."


Gori Propeller: http://www.gori-propeller.dk/

Results:
Calculated Propeller
Two Blade : 15" x 12" RH
Three Blade: 15" x 11" x 3 RH
Warning: There is an unusual big difference between the estimated hullspeed of your boat, your previous topspeed under power and the expected speed. The propeller sizing you get here might need modification, please contact us before ordering.


Carl's Sailing Calculator: http://www.image-ination.com/sailcalc.html

Results: This was an easy one, no emails, no browser forms, just punch in the numbers. Bottom of the page, pick your boat, enter HP and prop type. I entered 15 HP and a 2-bladed prop and got 21.4" on this one!!

Something is not adding up. I must be doing something wrong here...

[mbd]

The latest: I spoke with an engineer at a local prop shop who is the supplier for the new Sabers. Very informative. I'm sure I won't do it justice, and have probably already forgotten more n this topic than I'll ever learn about it.

Ideally, you start with an engine with the HP appropriate for the boat. My 15HP @ 3600 RPM is too big, but that won't be changing anytime soon (hopefully).

Most likely, the tachometer is incorrect, and/or my observations are incomplete. I never ran it at full throttle long enough to see if it would get to 3600 RPM, and I'm also not sure if I reach RPM in idle.

But, bottom line, it is better to "under work" the engine than "over work" it, and based on the size and prop, that's what it looks like I'm doing. He said it could be pitched more to make better use of the RPMs, but it's probably not necessary given how I use to boat - i.e. simply getting in and out of the marina.

I'll see how it does next season and watch it closer now that I'm more aware of the issue.

BTW, he didn't recommend bottom painting props because of the additional metals you're introducing to the area. The PO painted mine, and it'll be cleaned up before the season starts. My zinc was obliterated after just a few months in the water...

As always, thanks everyone for your input!