help with prop selection

[galynd]

I recently helped my brother put a rebuilt engine into his 1978 model Catalina 30. We removed the old Atomic 4 and replaced it with a Yanmar 2gmf, knowing it might be slightly under powered. The engine runs well but it only pushes the catalina about 4 knots maximum speed. We're hoping to gain some speed by optimizing the prop. I don't know what prop is on it now as it's in the water. However, the transmission is a KM2A, with a gear ratio of 2.62. What would be the best prop selection for the boat?

thanks.

 

[Ross]

I should think that the Catalina and my Islander 30 are not too different in terms of hp needs. I push my Islander with 10 hp a 3:1 transmission and a 3 blade 13x13 prop and can get 5+ knots in flat water. And I have enough power to get nearly that going to the weather an a 2 foot chop.

 

[Dave Groshong]

Perfect Pitch offers great props for that boat:

http://mysite.verizon.net/resqp86c/performancepropellers/

 

[Hermit Scott]

I recently helped my brother put a rebuilt engine into his 1978 model Catalina 30. We removed the old Atomic 4 and replaced it with a Yanmar 2gmf, knowing it might be slightly under powered. The engine runs well but it only pushes the catalina about 4 knots maximum speed. We're hoping to gain some speed by optimizing the prop. I don't know what prop is on it now as it's in the water. However, the transmission is a KM2A, with a gear ratio of 2.62. What would be the best prop selection for the boat?

thanks.

I have a YSM 12 single cylinder in my C30. I had one prop that would get over 6 knots on flat water, but 2 knots in chop. I brought the pitch down from there and now I get 5 to 5.5 even if the chop is bad in Corpus Christi bay. The prop I am using now is 12LH10. How many HP is that 2GM? Do you have a 2 blade or 3?
With any engine bigger than mine I would expect the C30 to get 6 knots on a flat bay.
Edit: I checked it out, the 2GMF is rated at 13hp continuous. Mine is rated at 10 hp continuous. You should be able to achive 6 knots to 7 knots.

 

[Hermit Scott]

Off the top of my head my guesstimation I think it will be a 12-11 or 12-12. I have both of those props however my engine turns counterclockwise when going forward when viewed from astern. So the props are 12LH11 and 12LH12.
My boat is a 1979 C30. Assuming his prop/hull design is the same as mine, I don't think anything larger than a 12" prop will fit on it. The clearance above the tip of the prop to the hull should be 15%. You want the largest prop possible for efficiency.