Overpropped or engine issues?

[mej]

This spring will be the third season we have owned a 1985 37' Hunter cutter. The engine is a Yanmar 4JHE diesel (40HP@3500rpm). We love the boat, but there is one issue I have been struggling with. I am really hoping that your feedback may help solve it.

When we bought the boat, it had a 15 x 15 three blade prop; the engine couldn't get past 2000 rpm. The surveyor indicated it was overpropped. Not that I would like to stay with the largest 3 blade possible (it's sometimes easy to find the mud bottom in the creek I stay in and a three blade seems to provide some advantage in getting out of trouble). I have since twice reduced the pitch. Last season I installed a new 15 x 10, non-cupped three blade. With this prop, I still am unable to exceed 2700 - 2800 rpm, even with a clean bottom and prop. I should say that, although I am unable to get to max.rpm, the boat cruises very nicely (5 - 5.5 knots) at 2000 - 2200 rpm. At 2700 I believe the boat is very nearly at hull speed.

I started being concerned there may be engine issues. I have done all the simple and obvious things, like running with engine compartment open, changing fuel filters and checking that the throttle linkage is adjusted properly. Also,I checked my tach with a calibrated tach (it's right on) and changed the exhaust elbow which also was clean. I have not yet had a tech come run any tests, trying to avoid that cost if possible. The engine does not run hot and only smokes slightly for a few minutes when first started.

I have been told that this engine is really a bit large for the boat, which may be true since I see that smaller diesels were also used in this model.

I have several questions at this point.
- From Yanmar performance curves, the engine running at 2200 rpm is
producing 28HP, (about 70% of the continuous rating of 40HP). Can I just
leave well enough alone, or will running this way be harmful to the engine
over time?
- Is it likely the boat is still overpropped or is an engine issue more
probable? Can any forum members with this engine give some feedback on
their prop specs and experience?

I really appreciate any advice you can give me.

 

[rardiH36]

Hi:

Your question seems to be more generic rather than specific to only the Hunter 37c. Might also post the same question on Ask All Sailors ... more people following the threads over there. And many have great experience with engine and propping topics.

Curious, does your engine reach the rated rpm in neutral?

I've got a 22hp engine on my 13800lb 1980 Cherubini 36. Reaches the 2650 rated rpm. Speed in calm conditions about 6.5 kts.

 

[Don12364]

What is your gearbox ratio?
My Yanmar 3JH2E with a KBW10 gearbox (2.83:1 reduction) will run at 3,600 RPM and pull a 3 blade 17.5" x 16" feathering prop. The engine is rated at 38BHP.
I would expect yours to either have a higher geared box or it is in some kind of trouble.

 

[kloudie1]

Right on with Donalex.. Usually an overpropped engine will smoke .. lots of black smoke.. because the governor will continue to try to add fuel and the prop will keep the engine from running fast enough to satisfy the governor.. so ya get a lot of unburned fuel and that shows as black smoke. No smoke is telling me that you aren't getting enough fuel .. which could be a restricted fuel line or weak lift pump... or the governor is out of adjustment and not adding more fuel to get to more RPM. I am assuming that the throttle lever is pushing the lever on the engine all the way to the stop..

 

[mej]

Yes it does reach 3600 in neutral. I was talking with a Yanmar rep at a boat show last fall who suggested I try this to make sure that the fuel pump linkage is functioning properly. I only tried it once being nervous about racing the engine without load.

 

[mej]

Thanks guys - this is a huge help. I think my box is the 2.17 ratio, but won't be able to check for sure until I can get to the boat (on the hard a couple of hours away). Knowing that you are pushing a much bigger prop, coupled with the info that an over propped engine would be a smoker, makes me believe your assessment of engine issues is right on. It looks like I may need to hire a tech to come take a look.
.

 

[anchorclanker]

As always. Injectors out for cleaning/service, check valve clearance, then compression while their out, check injection timing. If the engine has good compression and is in time, everything check, you should also check your tachometer against something with known accuracy. Once everything is done, prop the motor to reach full rated RPM at full power.

Running the engine at full power with too much prop is very hard on it. It should smoke but not always. Need to check everything out, its just to hard to guess at.

 

[Jim Legere]

Lots of good advice here. Find out what other H37C owners with the 4JHE use for props. Get your injectors checked. It is also possible that your exhaust elbow is coked up. It is quite common on the 2 & 3 cylinder Yanmars (don't know about the 4JHE) to have the exhaust elbow become so restricted that you lose significant power.