Max RPM for 3HM35F in a PSC34

[mondofromredondo]

Hi all,
I have a 3HM35F Yanmar with a 3 blade 16" prop. I'd like some feedback regarding the RPM people are running their engines at. Sometimes I'll run it at 2400 RPM for 4-5 hours if I'm in a hurry. Not sure if this is pushing the envelope. I've heard that running the engine at too low an rpm can cause carbon buildup. Anyone out there with a similar setup with some input?

Thanks
Keith
1988 PSC 34

[Norstar]

Keith,
We typically run our 1990 34 with the same engine and a 3 blade max prop at 2400 cruising speed. More importantly, you need to check to see if you can reach max rpm to determine if you are over/under pitched. Check the plate on front of engine to verify max rpm and see if you can hit that at the dock in neutral after engine is warmed up. Then take it out in flat water and check to see what you can do with throttle wide open. Assuming a clean bottom, you should be able to get close to max. If so, your prop pitch is good.

John

[mondofromredondo]

John,
Thanks for the input. I'll give that a try this weekend

Keith

[JohnRPollard]

If anything, 2400 seems a bit on the low side for a cruise RPM -- diesels like to be pushed hard.

We run a fixed, three blade prop too. On our 3GM30F, our typical cruise RPM is 2700-2800. We will bump up to 3K with a strong headwind or countercurrent. I don't know the max RPM for your engine, but it's probably in the 3400-3500 range like ours. Running at 80% or so should put you in the same band, i.e. 27-28K rpm.

Give it a try. If you're having trouble reaching those RPMs, your prop may be over-pitched.

[JohnRPollard]

Okay, as an update to my previous post, I grabbed the Yanmar Service Manual from our boat today. It's the one that covers the 1GM, 2GM, 3GM, and 3HM engines.

According to the engine specs, your 3HM actually turns at a little bit slower speed than our smaller 3GM. Here are the RPM specs:

3GM30F Continuous Duty Rating= 3400 RPM (24 HP)
3GM30F Peak (One-Hour) Rating = 3600 RPM (27HP)

3HM35F Continuous Duty Rating = 3200 RPM (30 HP)
3HM35F Peak (One-Hour) Rating = 3400 RPM (34 HP)

Even still, the 3HM35 engine is designed to run continuously at 3200 rpm. So I would suggest that you experiment by bumping your cruise RPM up to the 2600-2700+ rpm range. The Fuel Consumption tables show that there is very little difference in burn rate between 2200-2800 rpm, virtually none between 2400-2600.

[mondofromredondo]

John,
Thanks for doing the homework.
I I generally run about 3200 - 3300 rpm when I need to get somewhere quick so I seem to be right in there.

Thanks
Keith

[stevemac00]

John, on my 31 with MaxProp it seems I have to take an RPM that causes the least cavitation. That depends on wind direction, waves, etc. but usually around 2200-2400 which yields around 5kt. When I take it up to 3000 or higher the cavitation is terrible and I only get a slight increase in speed. Do you have that issue?

[JohnRPollard]

Originally Posted by stevemac00

John, on my 31 with MaxProp it seems I have to take an RPM that causes the least cavitation. That depends on wind direction, waves, etc. but usually around 2200-2400 which yields around 5kt. When I take it up to 3000 or higher the cavitation is terrible and I only get a slight increase in speed. Do you have that issue?

Stevemac,

Can't say that we have had any cavitation issues. We don't have a max-prop, though (wish we did!).

With a clean bottom and prop we get about 5.6-5.7 knots at 2700 rpms, and can push it up to 6.0 at 3000 rpms. If the bottom or even just the prop is fouled, there's a big drop off in performance. Ours is a 15" fixed 3-blade. I seem to recall Larry (Asylum) reporting as good or better performance from his fixed 2-bade.

Personally, I would want a cruise rpm that gets the engine up to about 80-85% of the continuous rating. Even at those rpms, we are burning only about 1/3 gph, and of course the alternator is cranking out more amps too.