Prop pitch

[Capn Tony]

Howdy folks
Have a question to pose to all but before I ask let me give you the background. Six years ago I went from 3 bladed props to 4 bladed 21” Michigans with a four cup. After a tweek or two I hit 50 rpms above rated 2850 (Cats 3116 300hp 3000 hrs). At the end of last season after tree years of being in the water diver scrubbing every year they are missing their mark; port 2685 stbd 2735 rpms. I attribute 50 rpms to bottom needing a good power wash and paint but the rest might be a pitch issue; engines run great. Do you start at the props or do you pull the injectors to do a compression test to assure it’s not the port engine? Taking an inch of pitch from the port prop should bring the rpms over rated 2800 especially with a new bottom this year. For every inch of pitch removed you pick approx. 150 rpms. Your opinions are greatly appreciated.
By the way the site has been too quiet too long LOL.

[lobsta1]

If it was me, I would start with a clean bottom. If the boat has been out of the water all winter then the hull has probably dropped 500 lbs. in water weight. Those two might give you back your RPMs. Are your props dinged? Your props might just need to be reconditioned.
Good luck
Al

[Capn Tony]

TY Al. I’ve been in the water for three years. I come out very third and this year is the third with a diver cleaning every year. I attribute at least 50 RPM to the bottom the rest of the lacking RPMs ??????

[retman]

Tony, We sold our 33FBC a few years ago and moved to a 2002 37 Duffy with a 3126 420HP Cat. Our Engines are similar layout except the aftercooler on ours is raw water cooled. I replaced it out of caution as there were no records. From what I have learned the air side can get clogged up. Just thought I would chime in on something you are probably aware of.
Bill

[Capn Tony]

Thanks Bill
Port aftercooler was new about five years ago STB removed and cleaned at that time. I am due for cleaning both.

[jannickz]

A good power wash and paint will definitely help, especially if there's significant growth. Aim to get the bottom as smooth as possible. Losing 50-ish RPMs after three years with four-blade props suggests a potential pitch issue.

[Capn Tony]

Thanks,
Hauling out today and will let everyone know the out come once back in.