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- - Propeller sizing issues (https://www.moyermarineforum.com/forums/showthread.php?t=11962)

Propeller sizing issues

I have an old 31' Columbia with an atomic4 . It has an 11" dia. 2 blade prop the pitich I haven't quite figured out (but I'm working on). The boat idles at ~800-1000 rpm. I have been told that the transmission is 1:1 so I'm guessing the prop at idle speed is turning 800-1000 times/minute. The problem with this is that at idle speed the boat travels at ~5 knots and let me tell you how much fun that is pulling into my slip!! What I do is sort of ride the clutch so to speak moving from forward to reverse. I didn't put the boat in last year and this year when I put it in, it was stuck in forward gear.....Pulling the transmission cover and spraying wd40(sidebar: I want to meet the engineer who put this engine in this boat as I had to put a towel over the engine and climb over it) Working the lever back and forth freed the gear. I don't think my strategy of forward and reverse is the best solution to happy sailing. Anyway any ideas on sizing a proper propeller so that I actually have an idle speed.

800~1000 RPM at idle seems high regardless of prop size. A very precise timing exercise followed by carburetor idle mixture and throttle stop adjustments are in order. Look for around 600 RPM.
Of course, get the reversing gear shifting corrected. It may be internal in the gear box or it may be the cable and linkage or a combination.
If the engine is in fact a direct drive (1:1), common props are in the range of 12" diameter x 6 or 7" pitch. The Indigo prop is popular with some owners @ 10 x 8½. With your [assumed] full keel I'd go for greater diameter as the aperture allows and lesser pitch. 11 x 6 should be pretty sweet.
A docking technique I use may be beneficial to you. I enter my slip dead slow but coasting out of gear. I'm usually in neutral maybe 4 or 5 boat lengths before the final turn. The only time I engage the engine is in reverse for braking after I'm at least halfway into the slip. For me this method avoids prop walk getting me all crossed up and over rotating the turn into the slip. For you, coasting out of gear should check your boat speed nicely and make things less exciting.

I too pull into my slip precisely as Neil describes.
(It'll save you "entertaining" your dock mates)

Attached is the procedure for adjusting the Reverse Gear (Tranny)
Also, HERE is a good video about it.

Your current prop must have a pitch on the order of 10-12 inches based on your reported boat speed of 5 kts at 1000 RPM. Most likely your WOT RPM is only 1200-1300 RPM. That is way too much pitch all the way around!

Neil is correct in stating that a larger diameter prop will most likely function better in a "broad deadwood" application such as yours. Having said that, we have props on probably 50 Alberg 30s and Pearson Tritons with the same deadwood condition and I have had no reports of any issues. I have always been a bit concerned that reverse performance would be compromised by the deadwood issue but experience has not shown that to be the case.

A small 3 blade prop will improve the prop walk in the astern direction due to the smaller diameter and also reduce vibration in the ahead direction as only one blade at a time will be in the disturbed water flow associated with the deadwood.

BTW, the Indigo prop is 10 X 7.4 pitch and they are in stock for immediate shipment.

Tom Stevens
Indigo