Atomic 4 prop pitch

[All U Get]

Does anyone have any experience with my question from Ask any Catalina Owner, I'm fine tuning my pitch for performance. Read the following:

"Two years ago I replaced my 2-blade prop with a 3-blade 10X7.4 prop. This is on my 1981 Catalina 30 which has the Atomic 4, 30 horsepower gas engine. I would like to change the pitch to 10X10 to get more bite. Has anyone done this with their Atomic 4? There isn't a tach on the engine for RPM's, but appears to be plenty of additional throttle that does nothing for boat speed. My cruising speed is about 5 kts and reverse is slow to little response.
"I didn't find anything in the archives." I would think someone has tried a different prop or am I the last owner of an Atomic 4? "
All U Get

 

[Ross]

My boat came with an A-30 with a 13x7 two blade sailorman prop. I can't say about a reduction gear. I sold the engine to the first offer over 250 bucks. I kept the prop but it just hangs on a nail in the cellar.

 

[Hermit Scott]

Prop Size

My C30 has a 12" prop on it. If you want better performance a larger diameter prop will reduce prop inefficiency, but it has to be the right pitch for that diameter. From what I have measured there is supposed to be 15% of the prop diameter for clearance to the hull. 12" is pushing it.
Also the prop has to be the right pitch for the torque the engine produces at the max sustained rpm. I thinik the gas engines have less torque so they have lower pitched props, but they probably run faster to make up for it. However the most efficient way is to have larger props, run slower with higher torque engines.
When you say better performance I'll assume you mean while motoring. Because a 3 blade prop is less efficient for sailing.
There is only going to be one right pitch for your boat assuming you are gong to stick with a 3 bladed prop. There are prop calculators but you have to know some numbers about your boat.
I just bought another prop for my C30. I bought it on ebay so I didn't get exactly what I want. I originally had a 12x10 and I replaced it with a 12x12. I am not sure how much my data will matter to you because I have a 12hp diesel. Really I don't have any prior performance to compare it to because the prop I had was missing part of one blade and had a bent and cracked blade. By the way the recommended prop for my boat is a 12x10, so I will let you know what upping the pitch past the recommended pitch does.

 

[higgs]

CDI makes a 2 blade. Very reasonably priced so I tried one on my irwin 32. It was a great prop that improved performance greatly. I got at least 1/2 knot forward speed. Reverse was still not as much bite as I would have liked, but it was an improvement.

Indigo makes a three blade, also reasonabley priced, but I know nothing about it.

http://www.sailcdi.com/ppmain.htm
http://www.atomic4.com/propeller.html

 

[All U Get]

Thanks guys,
The old prop was 12 inch and painted with several layers of bottom paint. It would vibrate at anything above idle so I replaced it, the shaft, cutless bearing, and reworked the strut. My new 3-blade is from Indigo but I believe the 7.4 pitch is too small. Under sail I can reach 6-7 kts so their 10-inch diameter has little effect on drag and I don't do any racing. I've been happy with the gas engine since it seems to have been well cared for by previous owners burning 1-2 gallons per hour depending on conditions. Thanks for the info.

 

[Hermit Scott]

All you get, I got my latest prop on ebay and I paid $65 + $15 shipping. If you look for long enough you will probably find exactly what you are looking for. But to have a prop repitched can start out at $75 to $100.
I was told by more than one prop guy that you can pitch up or down 2", provided it's not already been repitched. I'm not sure if you know , you probably do but the pitch of 7.4 (7.4")means that is the distance the prop would travel through the water if it had no slip at all with one revolution.
So it costs the same to get another prop on ebay as it does to try a new pitch on your old prop. MIght as well have an extra for the same price.