Multiple props, same over-rev'ing problem

[EricJRW]

Since my gut is telling me this is not a prop problem (I've been through 3), I'm posting here in the Evinrude forum. If you deem this to be a prop problem, then please move it to the proper propeller forum (I know there is only one, I just wanted to say "proper propeller).

Overview: After an hour of running or so (might be 2nd or 3rd trip), on various props, the motor suddenly will over-rev. Only backing down to about 3000 RPM will prevent this from occurring.

Anything above 3000 RPM and the over-rev'ing will occur again. At 3000 RPM, the boat is only going 4 or 5 MPH. If I was going to make a car analogy, I would say it sounds like the clutch is slipping. But outboards don't have a clutch that than can slip, or do they?

Motor: 1993 Evinrude 2-stroke 50HP Outboard, Model E50BEETB, Electric Start

Prop: 13 x 11 Aluminum (no idea of make)

Boat: 1993 Lowe 18'9" Aluminum Pontoon Boat

Details:

Bought the boat last June. On it was a pretty beat up prop, but while cleaning out some stowage, I found another prop. Here's the chronological order of my mishaps.

1. The fairly dinged prop was on the boat. One blade also had a "nibble" out of it. Used this prop for several trips (3 ~ 4 hrs each) without incident.
2. Found a prop, in better shape, though it had obviously been filed down a bit. Plan was to swap props in the winter and have dinged prop repaired.
3. Installed found prop which game me failure #1 on second or third trip (thought maybe this is why the prop was off the boat).
4. Installed repaired prop, and it seemed to work fine (1 or 2 trips). In the mean time I had the found prop "re-hub"ed, thinking bad (rubber) bushing.
5. Next trip out the repaired prop gave me the same over-rev problem (incident #2). I have the rehub with me, so I install it. Not 30 minutes later, I get the same over-rev problem with the re-hub'd prop (incident #3, same day as #2). So, unless the dinged prop has the same bushing problem (due to age?) as the found prop, AND the repair work was not done right (I don't think so, the propeller shop I used has been around since 1971), logic would indicate the problem is with the motor, not the prop.

Any ideas what could be causing this? Was I wrong the suspect the prop and is the problem actually with the motor? It's the only logical answer, but since it seems to come only after sustained running, though I have never dropped the boat back in the water after an incident (I meant to this week, but the weather was bad), I don't know what could be the source.

Thank you in advance for your help.

Sincerely,

Eric

PS. Sorry for the length, I'm trying to be as detailed as possible.

 

[ezeke]

Re: Multiple props, same over-rev'ing problem

Possibly ventilation, sometimes caused by something ahead of the propeller moving air into the propeller or could be caused by the motor being mounted too high.

Ventilation can be followed by massive cavitation because the engine tends to over-rev, leading to low pressure on the backs of the propeller blades.

 

[EricJRW]

Re: Multiple props, same over-rev'ing problem

Interesting idea...

I just don't understand how I could be going along at 4 or 5 MPH, with all sounding normal, at 3000 RPM, bump the throttle a hair (I don't even hear RPM change) and then suddenly over-rev.

The odd thing is when I swapped props I was able to run fine for about 30 minutes or so, then (with no trim changes) the problem came back. I have tried several trim positions (I'm new to boating and assume this will have some affect on ventilation) to no effect (the boat has power tilt and trim). The anti-ventilation plate is well below water, but who knows.

Thank you for your reply and I will certainly give this some thought.

I am open to any suggestions.

Eric

 

[ezeke]

Re: Multiple props, same over-rev'ing problem

When the boat is on plane, the A/V plate should be just above and parallel to the water.

You do have the thrust bushing and prop nut spacer in place before and after the propeller?

 

[evinruder13]

Re: Multiple props, same over-rev'ing problem

Possibly ventilation, sometimes caused by something ahead of the propeller moving air into the propeller or could be caused by the motor being mounted too high.

Ventilation can be followed by massive cavitation because the engine tends to over-rev, leading to low pressure on the backs of the propeller blades.

i had similiar problem,i had the motor sitting to high,so when i came on plane it revved high.so i lowered it and havent had a problem since.try that first

 

[EricJRW]

Re: Multiple props, same over-rev'ing problem

i had similiar problem,i had the motor sitting to high,so when i came on plane it revved high.so i lowered it and havent had a problem since.try that first

Well it certainly is easy enough, and inexpensive enough, to try. I just don't understand why it popped up now (I mean the boat is 17 years old, or course it's new to me).

Thanks for the help.

As I said, I'm new to boating and there is much to learn.

 

[kbait]

Re: Multiple props, same over-rev'ing problem

That sure sounds like a spun hub. Can you go wide open? What kind of RPM's then. Perhaps you need a shorter prop if you're not reaching your motors max. Perhaps that could cause it to be trying to grab way too much water while not moving forward enough due to boat drag, and ventilating and winding out? If you can get a shorter prop to try (from your prop guy since '71 perhaps), check it out.. Can't hurt. Here's a link to this sites props for yours, w/great prices!: http://boatpropellers.iboats.com/Ev...r=1993&**********=272030863&*******=346624818

 

[kbait]

Re: Multiple props, same over-rev'ing problem

And definitely test that engine lowering theory.. THAT'd cause ventilation!

 

[bob johnson]

Re: Multiple props, same over-rev'ing problem

Interesting idea...

I just don't understand how I could be going along at 4 or 5 MPH, with all sounding normal, at 3000 RPM, bump the throttle a hair (I don't even hear RPM change) and then suddenly over-rev.

The odd thing is when I swapped props I was able to run fine for about 30 minutes or so, then (with no trim changes) the problem came back. I have tried several trim positions (I'm new to boating and assume this will have some affect on ventilation) to no effect (the boat has power tilt and trim). The anti-ventilation plate is well below water, but who knows.

Thank you for your reply and I will certainly give this some thought.

I am open to any suggestions.

Eric

is there a typo in there? 4 or 5 MPH at 3000 rpms???

sounds like a motor set up could be the culprit..

make sure the cav plate is close to being level with the bottom of the hull!!!!

not the water level!!!!

I doubt all three hubs are spun...BUT..are they all used?
the motor is kinda small for such a big heavy boat...., but you shouldnt get an over rev all of the sudden.

Id look at the motor trim height as well as the mounting hieght

good luck

bob

 

[EricJRW]

Re: Multiple props, same over-rev'ing problem

When the boat is on plane, the A/V plate should be just above and parallel to the water.

You do have the thrust bushing and prop nut spacer in place before and after the propeller?

Seems the A/V plate is just below the water... Though maybe I'm thinking while at rest and I never really noticed while under way. I will certainly double-check that.

But yes both bushing and spacer are in place.

 

[EricJRW]

Re: Multiple props, same over-rev'ing problem

is there a typo in there? 4 or 5 MPH at 3000 rpms???

sounds like a motor set up could be the culprit..

make sure the cav plate is close to being level with the bottom of the hull!!!!

not the water level!!!!

I doubt all three hubs are spun...BUT..are they all used?
the motor is kinda small for such a big heavy boat...., but you shouldnt get an over rev all of the sudden.

Id look at the motor trim height as well as the mounting hieght

good luck

bob

I should qualify that RPM with another thought of mine. When at full throttle the RPM seems high (I'm thinking 5500 ~ 6000) for what is listed in the manual as maximum (I seem to recall 4500 ~ 5000, but don't have it in front of me), but the motor is not screaming (like it does when it over-revs). Since the boat is 17 years old, I had this thought that maybe the tachometer is set, or designed, for a 4-stroke application when this is a 2-stroke motor, and the RPMs are actually half. Does this make sense?

Also, regarding the HP, 50HP is what's listed on the capacity plate. Full throttle will do about 20MPH (all speeds per my handheld GPS). Not fast, but for just tooling around, it seems fine (though we do get passed all the time).

I'm in Fort Worth and the boat is down on Lake Granbury. I'll take a look at the various relationships (water, hull, anti-ventilation plate, etc.) the next time I'm down (in about 2 weeks).

All props are used, though I did have one "rehub"ed. Even with that prop I had this problem.

I did wonder if that RPM is right, if I'm spinning the hubs out. Does not seem possible, but like I said, I'm learning. If the wrong prop (right prop never installed) is causing too much resistance with the water, could that put the hub under excessive load? Or maybe the prop is too small and just looses its "bite" with the water? Both seem possible, though maybe not logical (keeping in mind I lack any marine logic).

Thanks for all the continued help. I'm learning a lot.

PS. I never did a speed vs. RPM under normal conditions, other than knowing that at full throttle, about 5500~6000 RPM, the speed is 19~20 mph. I'm adding this to my list of things to do (and have exact numbers). I also plan to get under the console an look at the back of that tach.

 

[ezeke]

Re: Multiple props, same over-rev'ing problem

Your tachometer dial should be set for 6P.

This is a pontoon boat, correct?

 

[EricJRW]

Re: Multiple props, same over-rev'ing problem

Your tachometer dial should be set for 6P.

This is a pontoon boat, correct?

Yes, it is a pontoon boat.

While I have learned quite a bit on the mechanics of the boat, I don't know much about the instrumentation. I'll have to get under the console and see what I have. I have done some wiring maintenance for corrosion, but not much else.

Thanks for the 6P tip.

 

[BigB9000]

Re: Multiple props, same over-rev'ing problem

Similar problem on one of my boats, I always figured it was a spinning hub. Would only happen when under a good load, or the right speed.

Never had the money to fix it, so I would back of the throttle for a second, then give it a little more, and the thing would grab an go.

Finally fixed it by getting a new(er) engine.

 

[Hdog412]

Re: Multiple props, same over-rev'ing problem

sounds like a spun hub and the fact that all the props are used makes it impossible to rule out spun hub. I would invest in re-hubbing one prop and see if the problem continue.

 

[JB]

Re: Multiple props, same over-rev'ing problem

I would carefully inspect the bottom of the 'toon for something that is making bubbles on plane that are getting into the prop and ventilating it.

I also wonder if you have a suitable pitch for the load you are pushing.

 

[EricJRW]

Re: Multiple props, same over-rev'ing problem

sounds like a spun hub and the fact that all the props are used makes it impossible to rule out spun hub. I would invest in re-hubbing one prop and see if the problem continue.

I did have one of the props re-hub'ed. Sadly, it did not solve the problem. It ran well for about 30 mins, then acted as if it spun out.

A friend had a good idea to try to mark the inner and outer portions of the prop, run it, and then see if the alignment changes. I thought this a good idea to at least determine if I am spinning out.

If I'm not, ventilation is the next best bet.

 

[EricJRW]

Re: Multiple props, same over-rev'ing problem

I would carefully inspect the bottom of the 'toon for something that is making bubbles on plane that are getting into the prop and ventilating it.

I also wonder if you have a suitable pitch for the load you are pushing.

Good ideas...

As of now I don't know how to determine the proper pitch, but I'm assuming (I know the dangers of this) whomever put this boat together (I'm at least the third owner, previous owner knows less about boats than I do) did so correctly. But you make a good point and I should look into prop recommendations for this setup.

I'll add checking the bottom of the boat to the list.

 

[EricJRW]

Re: Multiple props, same over-rev'ing problem

Would you believe it took until LAST weekend for me to finally resolve this once and for all (I hope)? Boating got off to a late start this year, and a new home purchase did not help, but I finally feel as certain as I can that I know what the problem is: A spun hub!

How do I know? A friend came up with a brilliant idea. Mark the hub and prop and see if the alignment changes. With the prop on the boat, it's hard to get to the "hub", but then I realized the hub is locked to the shaft, and the shaft has a hole for the cotter pin. So all I needed was a mark on the prop to show the alignment to the cotter pin.

Unlike previous outings, the over-reving occurred within minutes of leaving the dock. A quick check of the mark and it's no longer aligned with the cotter pin.

So today, after a bit too much research, I ordered a new prop. I was tempted by the Hustler, but the free shipping on the Volvo Penta hooked me. Both feature a two-part "non-slipable" design, so I think this will be the solution. I also increased the pitch to 13", primarily to lower the RPM at WOT, but maybe to get an MPH or two as well.

All things being equal, I would have gone with the Hustler. I like the fact that the hub adapter was a bit more but the prop a bit less (compared to the Volvo), making adding or replacing a prop less expensive. But since I really only need one working prop, I decided to save some money and go with the Volvo.

I'm going to take another shot at getting one of 13x11 props re-hubed and use it as a spare, but I'm feeling more confident with the two-part non-slip hub design.

Thanks again for all the help.

Eric

 

[Faztbullet]

Re: Multiple props, same over-rev'ing problem

I would pull the lower unit and check the water passage above torque tab, this passage feeds water into the exhaust to cool the exhaust and prevent damage to the prop hub. I have seen this a a couple of V-4's that sat in the water all the time. Alos I replaced several units when the "lighting case" came out as hole was to small and caused spun hubs and if unit was on a Ficht seen several melted plastic and rubber hubs due to the higher exhaust temps they ran.