Correct Prop Pitch?

[hdoliner]

Hello,

Please forgive me if this has been discussed before - I searched topic and didn't find it. We have a Cape Dory 25D with the original Yanmar 1GM. The previous owner swapped out her two-blade propeller for a nice bronze three-blade. I've had two friends who are licensed captains aboard who, independently, both queried whether the prop was the correct pitch, as the boat is very slow under power and the rpms at full throttle don't exceed 2,000.
>Does anyone know the correct prop pitch for the 1GM-25D combination?
>Is there a method for measuring pitch?
>Have others had issues with switching from two to three blades?

THANKS for any guidance you can provide.
Harlan Doliner
True North CD-25D #69

[fmueller]

Disclaimer: I'm not an expert ...

"Does anyone know the correct prop pitch for the 1GM-25D combination?"

... most small marine diesels rev to about 3,000 rpm or even a little higher, 3,500 is not uncommon. Do you have a tach ? If yes, put engine in neutral and rev full throttle briefly just to check tach. If you can reach 3,000+ rpm then you can have some faith that the engine and tach are ok and you are probably over pitched ... if still stuck at around 2,000 while in neutral, either your tach is malfunctioning (not uncommon) or your motor/rev limiter/fuel delivery has problems or both. Excessive smoke is a sign of too much pitch. The tach on my Beta 14 is not very accurate ... it will wander +- 250 rpm then sort of settle down.

https://www.yanmar.com/marine/product/engines/1gm10/

"Is there a method for measuring pitch?"
.... many rigid props have pitch stamped on the face of the hub, ie not necessarily visible if mounted on the shaft.

"Have others had issues with switching from two to three blades?"

... I fitted a three bladed feathering prop on a new Beta 14 in my CD 27. The engine redline is 3,600 rpm. I can only manage 3,200- so the prop is theoretically over pitched. After some conversations with the mfg, Featherstream, I decided to leave well enough alone. I typically cruise at 2,000 -2,200 and motor sail at 1,600 - 1,800 where a little extra "traction" is probably a good thing. There is no smoke or even smoke residue on the transom.

good luck

[hdoliner]

Thanks Fred, much appreciated!
Harlan

[John Stone]

... I fitted a three bladed feathering prop on a new Beta 14 in my CD 27. The engine redline is 3,600 rpm. I can only manage 3,200- so the prop is theoretically over pitched. After some conversations with the mfg, Featherstream, I decided to leave well enough alone. I typically cruise at 2,000 -2,200 and motor sail at 1,600 - 1,800 where a little extra "traction" is probably a good thing. There is no smoke or even smoke residue on the transom.

Fred, that's about the Rapam I had after I installed the Beta. Farron Peffer at Beta wore me out over the importance of getting the RPM correct. He was insistant being over pitched would damage the engine and shorten its life span over the long haul.

I contacted FlexOFold and was informed that one inch less in diameter equates to change of about 250 RPM and one inch change in pitch to about 125-150 RPM. FlexOFold recommended we go from a 16X12R to a 15X11R. I discussed this with Farron at Beta and he agreed. His comment is that FlexOFold is a very competent propeller manufacturer with a solid reputation. He reminded me we should strive to get within 50 RPM but in a perfect world being a little over is better than under.

With the new pitch the RPM is with 25 RPM.

Here the link to the entire post about getting the RPM correct.
https://farreachvoyages.net/2021/07/19/ ... ler-right/