We just got our boat, a 1985 C-36 with a stock m25 21hp engine with 900 hrs.
The previous owner replaced the stock 2 blade with a 15 X 9 RH 3 blade.
While the boat motors real well, 6 kts at 2000rpm, 2200 rpm at 6.5 kts and 7.x (I have hit 7.3 or so) at WOT, but it only goes to about 2500, maybe 2600 rpm.
Using the CPS as a speed indicator, My SOG would verify between about 5.5 and 7.5 at 2000 rpm and 6 to 8 at WOT depending on if I was running into or down wind.
At WOW in would only run about 2500 RPM upwind and a little more, maybe 2600+ down wind (wind in the 15kt range)
Am I over propped or with a 3 blade will it only run about 2500rpm.
Or am I set up good to cruise at 2200 rpm at 6.5 kts as that is where everything seems to run real smooth.
Thanks
Bob
Bob, LaRainne and McKenzie Robeson
1985 Std Rig C-36, Hull #374
San Pedro, Cal
Sailing the So Cal Islands and coastal ports from San Pedro south to San Diego.
Bob,
Either the 15x10 two blade or 15x9 three blade are what came from the factory for your MkI. You are not over-propped, but your tach is off. Both of those props will perform well, but it has often been told to me that all props are compromises. In order to do any real comparisons, you will have to calibrate your tach. Tachometers in all boats are notorious for being off. You should be able to reach about 3000 RPM in neutral, and just shy of that in gear with those props. If you are a member of C36IA, Tom's Tool Box has an optical tach which you can borrow for only the cost of postage. There is at least one article on this website which explains how to do the calibration. It is VERY easy to do with the optical tach.
Tom Sokoloski
C36/375IA Past Commodore
Noank, CT