I have a short shaft, 1987 Evinrude, 4hp 2-Cycle Outboard Motor, Model Number #E4RCUD, Serial Number #B 1373608. I am using this outboard on a very light weight 13ft. aluminum boat. The motor & prop have never hit any underwater obstacles or been run thru the mud or sandy bottom.
My outboard has a 1987 (vintage) OEM Factory 3-Bladed Plastic Prop is a (P/n #0319774) which has a 7 1/2 Inch Outside Diameter and a 7 Inch Pitch. At engine speeds up to 1/2 Throttle, the propeller bites in to the water nicely and powers the boat right along at trolling speeds. Anything above 1/2 Throttle the engine RPMs rev up suddenly and the boat seams to slow down. If I lower the engine RPM back down below 1/2 Throttle, the propeller bites in again and seams to power the boat right along at trolling speeds. With the outboard secured to the boat’s transom, the cavitation plate appears to be located 3 to 4 inches below the surface of the water.
I took the Prop off the outboard, and carefully removed the Rubber Prop Clutch Ring from the interior of the Propeller.
I then visually inspected the interior and exterior surfaces of the Rubber Prop Clutch Ring. There appears to be some visible signs of rubbing marks (or slippage) on the splines located on the exterior diameter of the Clutch Ring. The splines located on the inside diameter of the Clutch Ring might also have some minor visible signs of rubbing, but it is difficult to see the interior surface of the Clutch Ring very well. I also visually inspected the Prop Clutch Hub (P/n 0398316) and there is are no signs of rubbing or ware marks on the splines located on the exterior diameter of the Clutch Hub.
From Evinrude, I just purchased a brand new Rubber Prop Clutch Ring (P/n #0310331) and pressed it into the interior of my original Prop. The new Clutch Ring fit very tightly into my original Prop and took some serious effort to press it down into the splines located inside the Plastic Prop.
I then reinstalled the Prop onto my outboard and took it for a test ride. Again at engine speeds up to 1/2 Throttle, the Propeller seams to bite in nicely and powers the boat right along at trolling speeds. I slowly increased the Throttle and made it to about ¾ Throttle and then the engine RPMs rev’d up suddenly and the boat seamed to slow down again. I lowered the engine RPM back down below 1/2 Throttle and the propeller bit back in again and powered the boat right along at trolling speeds.
I then took the Prop off the outboard and turned the Engine’s Output Shaft by hand and I could feel the engine’s pistons generating compression in the cylinders. I’m not sure if there is a coupling located between the base of the Motor and the drive shaft but there doesn’t appear to be any slippage.
I’m at a bit of a loss as to how to proceed. Any advice you can provide would be hugely appreciated.
Measure the transom height on your boat to see if it is near 20" from the keel straight up to the top lip of the transom.
If the measurement is about 20" or so, then you have a short shaft engine on a long shaft boat, which will not work. The boat transom and motor should both measure 15" to be correct.
Another possibility is worn splines on the driveshaft where it enters the crankshaft. That can happen when the splines are not greased regularly then wear out - they should be greased during the annual water pump inspections each time the gearcase is removed.
Worn teeth on the gears could be slipping, but that is pretty rare.
Thank you for your responce and suggestions.
I went ahead and measured from the top lip of the transom to the base of the keel and it measures 15 3/4 Inches.
I then measured my engine from the surface of the mounting bracket that sitts on the top lip of the transom and down to the surface of the Cavitation Plate and it measures 17 Inches.
So it seams as though my engine shaft is the correct length for the boat I am trying to use it on.
Do you have any other thoughts as to what might be going on with my set up?
I am tempted to make a larger cavitation plate/planning fin that would extend out over the Prop to help prevent sucking air down into the prop.
Thanks again for all your help
2 weeks ago - last edited 2 weeks ago
Is the worn driveshaft splines easy to fix?
Worn driveshaft splines almost always cause worn crankshaft splines, too.
There is also the possibility of a worn rubber clutch ring and/or broken drive pin inside the prop.