R-25 low rpm

capnruss · Posted: Wed Aug 20, 2008 6:24 pm Post subject: R-25 low rpm

I went out this past weekend and could only get 3200 rpm out of the 125 hp Yanmar and my top speed is only about 9 knots. The Yanmar book says that engine should go to 3800 rpm. I know the fuel has been in the tank for a while and it could be that the fuel filter is getting plugged. It could have things growing in there. If anyone has thoughts on this let me know.

mcc272 · Posted: Wed Aug 20, 2008 8:06 pm Post subject:

If you keep your boat in the water and it has not been used for a while, there is the potential for barnacle growth on the prop. A friend was on a boat up here in the Northeast that had a similar problem recently with lower than normal RPM and a top speed of 23kt compared to a normal 35kt. The boat was hauled and there were barnacles on the prop. After prop cleaning and relaunch, the boat turns normal RPM and hits 35kt again. Just one possible cause and there could be others, including a clogged fuel filter issue as you describe where the fuel pump cannot pull enough fuel through a partially clogged filter. Fuel filter problems generally surface when one seeks higher RPMs because the fuel flow is higher and the restriction in the filter matters a lot more. If it is a fuel filter problem, changing the filter should give you some normal running anddepending on what kind of problems you have in your take, normal running will last for a shorter or longer time. Fuel filter loading generally happens over time. If you left the dock and promptly tried higher RPM without success, barnacles strikes me as the more likely cause. If the tank crud was so severe as to bring RPM down that fast without ever noticing a decline in RPM, it would likely be sufficient to really clog the filter and take your RPMs down even further.

thataway · Posted: Wed Aug 20, 2008 9:13 pm Post subject:

Not really enough information here. Is the boat kept in the water? (Salt or fresh)--how old is the bottom paint. Is the boat regularly cleaned by a diver?
Has the prop been hit or bent--certainly there can be growth on the prop--or on the bottom running gear.

Unlikely that fuel less than a year old per say could be the problem, but a plugged filter could be (remember that there will be the large pore size Racor, and the smaller pore size filter on the engine). Also there could be an air leak in the fuel system, some plugging of the pickup tube, a bad anti siphon valve and several other problems with the fuel system. Is there a biocide added, other additatives?

First be sure that the bottom and running gear are clean. Next check the fuel filter--many folks put a vacuum guage on the filter--and that shows an increasing vacuum when the filter begins to plug up. Fuel should be used or polished. You may want to set up a polishing system/dual filters with an electric pump.

jlgray · Posted: Thu Aug 21, 2008 3:13 pm Post subject: Low speed, not low RPM

I also have the Yanmar 125 hp but our wide open throttle (WOT) is 3200 rpm and at that the boat will go 12-13 knots. We cruise at 2800 rpm and are quite satisfied with 9.5 to 10.5 knots. I am wondering if your problem is not with the engine but elsewhere like the others have said: your bottom, prop efficiency, etc. Also check your prop shaft will it is turning (safely though) and make sure that is turning with out excessive heat.

By the way, get one of those infra-red temperature measuring guns that sell at the automotive stores for under $50. It is great to measure various engine and transmission parts because excessive heat is often the first symptom of a looming problem.

captd · Posted: Thu Aug 21, 2008 7:06 pm Post subject:

Yanmar in Cocoa, suggested that it was over propped. The engine is most efficient at 3800.
captd

capnruss · Posted: Fri Aug 22, 2008 4:11 pm Post subject:

Yes the boat is kept in the water. The bottom is clean , the prop is not bent. I got in the water and cleaned the prop. So it's clean. It wasn't bad just a few small barnicles. No fuel additives have been used at least since I took delivery. I will check for excessive heat on the prop shaft. Thanks for all your help. I will let you know what happens next

thataway · Posted: Fri Aug 22, 2008 6:22 pm Post subject:

The Yanmar 125 hp is rated Max Continous RPM at 3800--max HP and torque at between 3400 and 3600. I think that you would not want to run the engine at max rating continously--better at max torque.

If you are only getting 3200 and 9 plus knots, then you ar lugging the engine--and for that application it is over propped--but often "trawlers" are over proped to give slightly better effeciency--and keep the engine at temp. This does not mean that the engine should be run at the WOT setting--which will be lugging the engine.

"just a few" barnacles will certainly affect the performance. Also how much gear, weight in fuilds you have in the boat will affect its performance. As I recollect, 12 to 13 knots was the max that these boats woudl do when new and light. Consider that it is a 7 to 8 knot boat--maybe less if you want the most effecient use.

captd · Posted: Fri Aug 22, 2008 10:12 pm Post subject:

Ps,
Yanmar (in Cocoa) also told me not to use any fuel additives. Also said it didn't hurt it a bit to run it wide open. Probably would have added, to use common sense, and vary the speed. I would be interested in other opinions. I have had three 8V71 n's Detroits, I know they like to run wide open.These are two stroke and naturally aspirated.
captd

thataway · Posted: Fri Aug 22, 2008 11:52 pm Post subject:

Most of us run additatives in our diesels--BioBar as anti fungal/bacterial. Pri D and or Startron. If you have engines which have a high back flow (thru lubing the injector) or run lots of fuel thru the engine, then it is probably not necessary.

85% of WOT--certainly--100% of WOT--especially if you are lugging the engine--porobably not a good idea. What I find interesting is the high percent of the Hinkley Picinc boats which have been repowered after only a few years.

But I am sort of "old school"