PDQ OWNERS FORUM

After taking out a smaller monohull cruiser and seeing its performance (7 knots max while motor) with a skinny 3 blade prop and 3gm motor, i wondered why my cat cant get up and go like that one did.

We have 2 fixed 3- blade props (unsure what size) right now that seem in fine shape.. however we can barely get 2500 rpms under full power and 5.5 to 6 knots max.
Im not complaining but i feel like i should be able to push the motors to 3000 at least and get 7 if i need to. Whats going on here? Clean bottoms and good exhaust flow.
Port motor does seem struggly with black smoke at 2400 rpms ±
What do yall have?
Also ours sits low in the water , so maybe just too much stuff onboard?

hi,

you don't mention what the max rpms are for you engines. you should be able to get fairly close to the max rpms with the boat loaded as normal. with that said, if i had to guess i'd say you're probably over propped, aka too much pitch. you'll want to talk to a prop shop. they'll want to know what is max rpms you can make on each engine and the pitch & diameter of the props. they have a formula that will tell you what diameter & pitch will get you to max rpms. i will warn you that this exercise is aimed at saving your engines from an early death, not to optimize speed. we went through this exercise when we bought our powercat....we actually LOST top speed but saved our engines from death by too much pitch.

good luck

Thanks for the reply!
The max rpms for a yanmar 2gm20f is 3600 under no load.
I havent checked what we can get yet in our situation but i know we can reach at least 3000 rpms in neutral. I havent pushed farther, it just sounds intense after 3000.

The boat used to have 2 max props 2 blade feathering 16" x 18 and 16" x 20. I dont know why the pitch was different.

There is a sticker under the panels that read "3000 rpms max"
Maybe this was when the 2 blades were installed?

Ill heed the warning of killing the engines and maybe opt for slight overpropping, although i feel thats whats going on already.

Again thanks for the input!
Theres a prop shop near us in Georgia that ive used before and they're very helpful

when we got our first diesel, a westerbeke, i went to the 1 day, everything you need to know class. the first thing the instructor said was 9 diesels die by under-revving for everyone that die's by over-revving. if yanmar is saying 3600 that should be your target. also, i'm not sure what at idle tells you. it's all about allowing the engine to reach it's max rpms under normal load conditions. if your engine is over propped it's not going to totally burn the fuel and the unburnt fuel is going to cake the valves and exhaust elbow and then bad things happen.

talk to your prop guy and see what he says.

good luck.

According the my notes: "Propellers - Michigan 16 x 13 (2 blade) per owner's manual."

We get 3200-3400 max rpm with our 2GM20 diesels on our chronically overloaded boat.

I occasionally run the diesels up to 3600 before shut down to blow out carbon.

The sell them here: https://www.deepblueyachtsupply.com/mic ... geNumber=1

Maybe other places as well; after seeing the price I decided to have my 2 blade props refinished in Miami Tri County propeller.

I had the same issue when I bought my boat in 2017: 2800 RPM max and the engines were smoking. The three blade props were 15inch with a pitch of 12. I took them to Coastal Prop and they re-pitched them to 9 pitch. That did the trick. I could then get the RPMS to 3600 if I wanted to, and there was no more smoking. I have since put on Max Props feathering props which greatly improves sailing performance. I still have the three blade props with the correct pitch for your boat. I would consider selling if you are interested, they are Michigans.