prop proplem??? Please help!! 6000+RPM with overpitched prop???

[BigBassin78]

I have a perplexing issue with my prop. I am running a 78 evinrude 115 on a 78 hurst 16' fiberglass bass boat. It currently has a 21" pitch prop. My original operating manual for the engine specifies a 13 X 19 be used for this model. Here is why it has me scratching my head: boat has a tough time getting on plane with two people on the boat sitting where we are supposed to be sitting, so you think over propped right? Then why does my motor turn it 6000+ RPMs and go 52 mph with just me on the boat 49 with another body???

more specific characteristics: easily planes with just me, if person is moved to the front then planes quickly, planes better when gunning it, slowly going into full throttle can get stuck with bow in the air, no more than 2800-3000 rpm when when its having trouble getting on plane.

If I drop the pitch down to 19 where the manual says it should be. I would gain another 400 rpms or so, which will be well out of operating range for my motor, which is 5000-5500.

 

[Haffiman]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

If you are running an alu prop, not surprised.
You may need a SST prop and perhaps a double cupped that holds better on the water.
6000 rpm with 21" pitch and 'only' 52mph, you are far out in slippage.

 

[BigBassin78]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

Yes it is an aluminum prop and it has been ground on the leading edges it looks like. It has some damage to the leading edges as well. Is this contributing to my problem?

 

[Lyle29464]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

Tach could be off? Mid range carb problem. If that is all ok I would go with a smaller SST and just cool it with the r's

 

[Haffiman]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

By experience, no alu props above 19" or around 40 mph!
Try testing a 18 or 20 Raker if you are in for top speed.
The give a bit of bow-lift, which may reduce your trimming needs.
There are a lot of other brands around, but at least go for an SST cupped and throw the Alu as an emergency spare!

 

[boobie]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

Before I did anything I'd make sure my tach was accurate.

 

[ajgraz]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

Agreed with check tach accuracy, use an optical tach on the flywheel.

If your prop has been, ahem, "modified" at the leading edges, you may well be cavitating at high rpm and/or high load conditions.

 

[BigBassin78]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

Well the cavitation theory sounds good. I cant imagine why such a strong motor wont plane such a little boat and load much easier. About the tach being accurate, from the sound of the motor it is in very high RPMs at WOT. It may not be accurate down to the rpm but I would say it is pretty close. I have a multimeter that can test rpms and I get within a hundred rpms matching the tachometer.

Update on prop info. The part number stamped on it is 389512 which is an OMC OEM 12 3/4 X 21 Pitch Right turn Stainless Steal propeller. The leading edge if one is damaged pretty good, like about 1/2 inch indentation. The rest of the leading edges I can see grinding marks. Also it looks like it has after the fact vent holes in the prop.

New question: Should I try and get this prop rebuilt considering it is Stainless, or should I buy a new one that has a lower pitch.

 

[Lyle29464]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

You have several things going on at once. I would talk to a prop shop and see if they will do a loaner if you agree to buy a new one. you might try plugging the vents but I do not like to run a damaged SST. It can cause other problems.

 

[Haffiman]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

0389512 ia an 'old' type SST prop and originally did not have vent holes. (3 holes releasing exhaust just in front of blades).
If it came with the boat, the holes might have been drilled to get the rpm up at start and not bogging down the engine.
Send it to a prop shop and have it repaired and polished, probably costing a fraction of a new.
Leave the vent holes, they may be blocked easily by plugs later if needed.

 

[emdsapmgr]

Re: prop proplem??? Please help!! 6000+RPM with overpitched prop???

You've gotten some good recommendations. Make sure the arrow on the back of the tach is set to the number 6. That is the correct setting for your engine. A burr on the prop can cause excess ventilation-the problem you describe-especially if the engine is mounted fairly high on the transom already. You may want to have a propshot rework it. The non-stock vent holes usually function when just coming up on plane-to improve holeshot. When on plane, usually the flow of water past them will negate/prevent any ventilation through those holes. Those early black stainless props were great props in their day.