Yanmar 39HP Diesel Low Revs Issue

[oceanshoretiga]

I've never tried such high revs but will look into it. thanks for the useful discussion.

My 3JH4SE is 39hp.

cheers, Jim

[oceanshoretiga]

Sorry to repost.

When I installed my Yanmar I was 'annoyed' at the large size of the specified exhaust system and had to add a new exhaust outlet to cope. I mention this because of other posts that have mentioned possible blockages in the exhaust system. Check the sizing from the manifold to make sure someone hasn't modified the exhaust hose and muffler/pong box system to smaller diameter. Sorry, I'm not onboard to measure but the installation manual will specify exhaust system dimensions.

My engine is driving a steel monohull of traditional design and weighs about 13tonnes. At 5 knots in calm water it consumes less than 1.5litres per hour.

jim

[peter57]

Originally Posted by oceanshoretiga

I've never tried such high revs but will look into it. thanks for the useful discussion.

My 3JH4SE is 39hp.

cheers, Jim

Hi Jim. The high Revs are in neutral for a short time I have my prop pitched to 2000 RPM 6 knots in smooth water.

[peter57]

Originally Posted by oceanshoretiga

Sorry to repost.

When I installed my Yanmar I was 'annoyed' at the large size of the specified exhaust system and had to add a new exhaust outlet to cope. I mention this because of other posts that have mentioned possible blockages in the exhaust system. Check the sizing from the manifold to make sure someone hasn't modified the exhaust hose and muffler/pong box system to smaller diameter. Sorry, I'm not onboard to measure but the installation manual will specify exhaust system dimensions.

My engine is driving a steel monohull of traditional design and weighs about 13tonnes. At 5 knots in calm water it consumes less than 1.5litres per hour.

jim

Hi Jim, The exhaust blockage they are talking about. is at the elbow straight of the end of engine manifold where the raw water enters the exhaust pointing at angle down and should be cleaned every 500 hour or so. worth looking into.

[oceanshoretiga]

Originally Posted by peter57

Hi Jim, The exhaust blockage they are talking about. is at the elbow straight of the end of engine manifold where the raw water enters the exhaust pointing at angle down and should be cleaned every 500 hour or so. worth looking into.

thanks, Peter. But, relevant to this thread, do you think a smaller exhaust (eg 50% of the Yanmar specified diameter) would make a difference. Personally, I think the larger diameter is probably not important but one problem I've had is the resonant noise at certain rpms. I think there is not enough water for the size of pipe to cut out the resonance. otherwise, it is well muffled.

cheers, Jim

[UFO]

Had a Mechanic in to confirm issue and seller has agreed to get my mechanic to fix issue - Once I hear more I'll let you know what the problem was

[peter57]

Originally Posted by oceanshoretiga

thanks, Peter. But, relevant to this thread, do you think a smaller exhaust (eg 50% of the Yanmar specified diameter) would make a difference. Personally, I think the larger diameter is probably not important but one problem I've had is the resonant noise at certain rpms. I think there is not enough water for the size of pipe to cut out the resonance. otherwise, it is well muffled.

cheers, Jim

I wouldn't like to say if it is okay to go smaller. My is 65mm id and a muffler after the Pong box going vertical about 650 mm long and 125 mm diameter with a huge sea cock on exit and is very quiet, you can run it hard for 24 hrs and still put my hand on any part of the exhaust. All about the same size as my old 4108 Perkins except the Yanmar is cooler, quieter, less fuel, no oil leaks and 100% easier to fuel bleed. Plus 100 cc smaller capacity.
Cheers, Peter.

[Mike Banks]

If your exhaust is less than the recommended diameter, that is a restriction.

If it is half the size of the recommended diameter, it is only allowing one quarter of the exhaust gas flow for which the exhaust system is designed.

[UFO]

FYI - The Counters were out - Digital Tachometer confirmed this - Had them re-calibrated and all is good

[Wotname]

Thanks for the update - that was a easy fix