Regaining RPM

[cordialdevil]

This may be of some interest for anybody running with underperforming engines. Two years ago I purchased Black Gold, a 1995 38' Open, powered by twin turbocharged Yanmar 420D diesels, 2300 hrs. I have known Black Gold since new and for a number of years she achieved 2800 rpm (24 knots) WOT with the propellers set at the factory setting of 25x31". Now she was running at 2200 rpm WOT with the same props. She was also producing black smoke while struggling to get onto plane.
The boat was weighed on haulout to see if she had become waterlogged (always a possibility with a fibreglass boat), but she was dry. The propellers were measured as 24.6x31, close to the original factory setting of 25x31. Next, I called in the local Yanmar dealer and had all the engine tests run, but could find nothing. However, the mechanic recommended removing the intercoolers for service, which was done and this cleaning produced a 200 rpm jump in performance. Then the turbines were serviced, but no gain was seen there.
So now Black Gold was running at 2400 rpm WOT, but still lacked 400 rpm. Then I read that some 38 owners were running 25x28 size propellers. Having verified with Luhrs that 25x31 is the recommended sizing, I was reluctant to
make the change, but did so, restructuring to 24.6x28. Now she runs at 2700 rpm. I believe the missing 100 rpm will be retrieved by buying new props that are exactly 25" diameter.
Why the rpm degraded is still a mystery to me. Any ideas?

[dpdash]

bottom growth? Additional weight added to the boat? Turbo's not working efficiently? Not enough air to the engines, filters, intakes? Throttle cable issues? Synchroniser on/off? Lot's to check.

[cordialdevil]

As stated, the boat was weighed when hauled and found to be correct. Turbines and intercoolers serviced, engines inspected and tested for compression, temperature etc and declared to be at factory spec. Shafts replaced and shaft speed calibrated to instruments. Hull and props always clean. Out of gear, the engines ran up to 2900 rpm, factory spec. Since reworking the props, the black smoke has almost disappeared and the boat planes easily. The question is, why did the boat work to spec for years and then fall off, and why have some other 38 owners also switched to 25x28 prop size? Did they have a similar experience and, if so, what caused it?

[Dave K S]

More pitch or larger dia will decrease your RPM's. you will need to reduce your pitch to gain an additional 100 RPM's. You will gain approx. 200 RPM's by REDUCING the pitch or diameter by one inch and lose approx. 200 RPM's by INCREASING the pitch or diameter by one inch. I would not change the diameter as the larger diameter usually results in more efficiency. Remove .5 inches of pitch and you should gain back the 100 RPM's. I would make the engines turn 50 to 100 RPM's over max rated at WOT when fully loaded. Check out "boatdiesel.com" for info on correctly proping your boat without over loading the engines.

[irishrogue]

2300hrs+....arent the motors just getting tired??? post your question on boatdiesel.com. Didnt the Yanmar dealer have any diagnosis? if not, find another servicing dealer. If everything else you state is accurate and been checked, i have to believe the motors are in need of rebuild.....but i m no diesel expert....seems odd that motors would run to rated rpm with no load...maybe when overloaded it is enough to cause blowby and loss of compression as the rings/cylinders are worn....

[Dave K S]

If I'm not mistaken that engine should be able to reach a min. of 3300 RPM at WOT fully loaded. I believe the no load RPM should be 3600. If this is the case you probally have done a great deal of damage to this engine running it with a WOT of only 2900, fully loaded. This engine has been severely over loaded. These engines must be able to reach the stated WOT plus at least 50 RPM's under full load. If that engine had been proped correctly you could expect many more hours before a rebuild but not if it has been overloaded. You will probably need a rebuild soon if it has been run overloaded. Check the owners manual for WOT specs. I hope this is not the case. Good luck and I hope you can solve these issues without a rebuild. I have Yanmars they are great engines but you cannot run them or any other engines in an overloaded condition and expect them to last.

[cordialdevil]

Dave, you are mistaken about the WOT rpms. As stated, WOT is around 2800 for the Yanmar 420D, not 3300 rpm. The engines, as I also wrote, have had every check possible and are considered to be in great shape by the dealership. They purr, burn little oil, and pressure and temps are right where they need to be. I have gone further up the line to the distributor and he says that these engines are just getting broken in at 2300 hrs. If well maintained, he expects at least 5000 hrs before rebuild.
Bottom line, the pros have not been able to find fault with the engines (and they have tried). They are designed to cruise at 2400 rpm with 24 knots, which is close to what I am getting since I reconfigured the props. But why did I have to do that?
What surprised me was the 200 rpm gain when we cleaned out the intercoolers. I am wondering if there is some other part that could be cleaned out that might give her some more rpm back.

[Dave K S]

You did not say which model engine you have. I thought you had the 6LY2 version with 420 HP. You must have the 6CX 420 HP. Then your RPM's are correct and those motors should have many more hours left. I hope you find the reason for the RPM loss. Keep us posted. It may help someone else.

[Martuni]

I have a similar problem with my 2002 38' with Yanmar 6cxm engines (900 hours each). The starboard engine lost 300 rpm, port engine fine. I had the Yanmar dealer check everything from compression to pop testing. Everthing checked out great. Everyone now is telling me the only thing it could possibly be is the prop. I may have touched bottom at some point enough to change the pitch. The props are out getting serviced. I will let you know how it works out when I get them back.

I did not try cleaning out the intercoolers.

[CipViaky]

does anyone here know a good database where you can find information about 45 rpm records? Like there is a database for CDs?

[Momentai]

Sorry to offer late feedback. Been out all summer with surgery.

I have 1998 420 CTX and they are 2700 WOT and mine turns 2750-2760 respectively. But how we got there was part props, part alignment of engine, trans and shafts and also AFTERCOOLERS/Innercoolers. Aftercoolers/innercoolers, $3600 a side are the heart and soul of the CTX format. Flush them regularly with these new flush out chemicals and they should last. Mine were replaced at 3400 hours and I watch them like a hawk. Wish they made a larger/better air intake filter set up. They are way too small.
But they are iron horses as they are 420HP against 450cuin displacement so they are workhorses. I have 3800 hours and they are still going. ZERO oil burn and yes they smoke when cold. I can tell the weather at start up. Colder it is, even 10 degrees means additional warm up time before the blue disappears. When I punch it, the black pours out but disappears in an instant after we go through the turbo 1800 rpm range. Turbo probably could stand cleaning as well.
Now WOT RPM, do you have FlowScans or anything that gives you a burn rate at given RPM? We found that once we put the digital tachs with the FLowScan system, we now had accurate information. I do not know how you are arriving at the RPM readings but if they are the VDO tachs, that may be the biggest part of the issue right there.
I also do not know the specs on your year engine but Yanmar says over and over again, the 420hp CTX is their longest life diesel. Yes, it is old school and does smoke a bit like the old million mile 15,000 hours diesels use to. I am OK with that.