Repower an E34 with a new engine.

Presently have a 20 year old 23 hp Kubota/Universal that is getting tired. I'm down to 4.5 knots in a two foot chop and 3.5 knots in worse. I am thinking of replacing with a Yanmar 3YM30G (29 hp) but wondering about a 3JH4SE (40 hp). I have a three blade 15 inch MaxProp with new one inch shaft and a very robust stainless strut. Any thoughts?

Duncan said:

Presently have a 20 year old 23 hp Kubota/Universal that is getting tired. I'm down to 4.5 knots in a two foot chop and 3.5 knots in worse. I am thinking of replacing with a Yanmar 3YM30G (29 hp) but wondering about a 3JH4SE (40 hp). I have a three blade 15 inch MaxProp with new one inch shaft and a very robust stainless strut. Any thoughts?

Click to expand...

Can you attain max rated RPM of the engine in flat water? If not then the pitch of the Max Prop can likely be adjusted.

You could also have as fuel supply issue or your throttle linkage has slipped. Many paths you can take before pulling out 15-20k. Unless it has bad compression that engine likely has life in it..

Maine Sail said:

Can you attain max rated RPM of the engine in flat water? If not then the pitch of the Max Prop can likely be adjusted.

You could also have as fuel supply issue or your throttle linkage has slipped. Many paths you can take before pulling out 15-20k. Unless it has bad compression that engine likely has life in it..

Click to expand...

I'm votin' with Maine....
More info is needed to give back useful advice. I would guess that a compression check is needed.
Can you motor at 2900 rpm's in smooth water? My M25XP can. Since you have a fathering prop, first adjust the prop pitch. I used to have a feathering prop for a decade and found that it took about three lifts total to get the pitch dialed in properly. After that it was fine.
Irony: the yard did the initial setting with too little pitch and we'd wind up to max revs and go about 5 kts!
In smooth water we motor at about 7.2 kts (smooth bottom condition) with any prop, and over the years we have used the factory 2-blade fixed, a 2-blade feathering, and now trying a 3-blade fixed.
How many hours on your engine? I have about 2000 hrs on ours.
Since running at top revs and torque requires that the fuel go in clean, thru clean injectors, and out thru an unrestricted exhaust, have you checked all this stuff out?
Some owners have reported plugged exhaust lines, but being in fresh water I have never (knock wood) had that problem.
To me, it gets back to max revs. You should be cruising at about 2500 at about 6.5 kts for your hull.

Please let us know some more details.

Regards,
Loren

Our 1986 M25 is still going strong, 2400 engine hours. A friend replaced his M25XP last year after 5000 engine hours. You have a long way to go. I also bet on the prop as your issue.

Dumb questions. When is the last time you hauled her and cleaned the bottom and the prop? Have you had the injectors rebuilt recently? Have you checked your valve clearence recently? How about your lift pump? Any one of these or combined could reduce your power dramatically.

Have you done a compression test? If your engine is worn from age it will show up here.

If you do repower, why not an M25XPB?

Good points, Tim. My friend re-powered with just that engine.

Thank you for the help.

I will have a mechanic give it a thorough check asap. I can get 3400 rpm and I cruise at 2600 to 2800 in flat water at 5.6 to 5.8 knots. (I have a bracket for my 4 hp Yamaha outboard on the stern and can hit 4.1 knots in flat calm -- the hull is easily pushed.) At 2500 I do 5.5 knots. It is the waves that REALLY slow me down. I will check the fuel supply and linkage, but I have had the tank out, put in inspection plates and replaced all plumbing, fuel pump, etc. Also completely rewired engine, panel, and replaced harness last winter. Yes, and hauled, cleaned, new injectors, checked valve clearances and lift pump July 2010....thanks Tim. I have had the Max Prop for nine years and it took a number of tweeks to get the pitch right. I think I'm there on that one. The exhaust I checked last summer, there is no muffler to create back pressure. All your comments make me think that my problem is likely lost compression and, given the knots Loren gets, I think the 29 hp Yanmar would be ample. I will let you know what the mechanic says. Most to all of my summer cruising is done from Desolation Sound north in fjords among islands where motoring accounts for 60 to 80% of travel time. The wind is either on your nose or tail, or not at all. The engine only has 2100 hours, but the PO admitted to motor negligence
Duncan.

I would be very surprised if you do not have a muffler. Although I would not suspect that first. The first part to get blockage in an exhaust is the riser coming out of the engine.

Duncan, One thing you did not post is what happens to the engine RPM when the boat slows down. If there is an engine problem, the RPM will drop as the load increases, slowing the prop, and reducing the speed. I find it curious that that is not how you framed the question, and leading me to wonder if it is not a prop damage problem. If you have the same RPM under load the prop is turning the same speed and the boat will move through the water at the same relative speed. Unless of course you are pushing the boat at the maximum power the prop can generate at a given RPM, and that power is in deficit to the expected performance. Hence the prop is not then converting RPM into thrust. So if there is not an RPM drop the engine does not have the power to restore, the place to look is at the prop! It could not be fully unfolding, or the pitch adjustment changed, or have a sheered key way, be bent or fouled. If the RPM is not the issue look closely at the prop before you condemn the engine! Good luck Edd

exoduse35 said:

Duncan, One thing you did not post is what happens to the engine RPM when the boat slows down. If there is an engine problem, the RPM will drop as the load increases, slowing the prop, and reducing the speed. I find it curious that that is not how you framed the question, and leading me to wonder if it is not a prop damage problem. If you have the same RPM under load the prop is turning the same speed and the boat will move through the water at the same relative speed. Unless of course you are pushing the boat at the maximum power the prop can generate at a given RPM, and that power is in deficit to the expected performance. Hence the prop is not then converting RPM into thrust. So if there is not an RPM drop the engine does not have the power to restore, the place to look is at the prop! It could not be fully unfolding, or the pitch adjustment changed, or have a sheered key way, be bent or fouled. If the RPM is not the issue look closely at the prop before you condemn the engine! Good luck Edd

Click to expand...

Great post. Also consider the tranny although those usually die quite suddenly. Believe me, I know.

Thanks Tim, Just trying to give back for all the great info this forum has given me! Edd