Prop hub slippage?

[DickR]

I'm into my second or third season on a replacement prop for my 17' Mercruiser Alpha One I/O boat. The prop is a Michigan Match 3-blade aluminum, and since I put it on, including a few trips out in the boat this spring, it has been fine, a proper match for the engine/boat.

Yesterday evening, near the end of a nice after-dinner cruise at around 3000 rpm, the engine suddenly surged in speed while boat speed dropped. I immediately dethrottled and went to neutral. The engine was running just fine. I had heard no thump or anything else to suggest a collision with anything. I put it back into gear, and the boat moved into headway speed. On advancing the throttle to come back onto plane, the engine speed again surged. I returned to the dock at little more than headway speed.

Visual examination of the prop did not show anything unusual, such as evidence of any collision with something.

I am not aware of anything in the drive train that is like a car's clutch and that could exhibit slippage under load. It is my understanding that in the hub of the prop there is some structure that is designed to slip if the prop does hit something, to protect the outdrive from damage.

So, help me diagnose my problem. Am I about to get a new prop to solve the problem, or could there be something else wrong so that a new prop would be a waste of money?

I do still have the old prop, which I replaced because the hub was worn and exhibited too much play but which otherwise performed adequately. I could put that one back on again to see if the boat at least gets up on plane and runs normally. If that is the result, I guess a failed prop is the answer. Your thoughts?

[Blue Thunder]

Dick, since it's an Alpha One, it's probably an older boat, I assume? If it's not the prop hub, which you can easily determine by swapping the prop, it's likely the splined coupler which is mounted on the flywheel and mates with the very end of the input shaft of the out drive. Unfortunately, you have to pull the engine to replace it. If you google Mercruiser Alpha One engine coupler you will get numerous results.

Edit: If it's the coupler, it should look something like this but there are other styles depending on whether its a 4, 6, or 8 cylinder. I had one go on my 76 Penn Yan with the inline 6 cylinder. I had to pull the engine. Once out, it was a five minute job with 6 bolts.
http://www.amazon.com/MERCRUISER-ENGINE-COUPLER-GLM-Number/dp/B004AQU83Y/ref=sr_1_13?ie=UTF8&qid=1401886457&sr=8-13&keywords=mercruiser+coupler


BT

[Slickcraft]

There is a type of clutch in the prop hub designed to slip under high torque like hitting a rock. After a lot of normal use it may slip like described, usually you can still limp along at headway speed. Normally the prop can be rebuilt, takes a while. Pick-up a used prop to use while it is being rebuilt and then keep that used prop as a spare.

[PENIVISA]

Had this problem in past and found it was the center core or hub of the prop
It can be fixed at a prop repair shop for less than half the cost of replacement.

[Woodsy]

Dick....

There are 2 possibilities... a cheap one and an expensive one.

The cheap one..... Most Mercury propellers have what they call a "Torque Flow" hub. Its a replaceable piece of plastic that you insert into a propeller. Its a pretty cheap fix, probably under $50 if you just take the prop off and do it yourself.

The expensive fix was mentioned earlier. Attached to the flywheel of the motor is a "Drive Coupler". The splined input shaft of your outdrive inserts into this. Its essentially a rubber hub with a female spline. Unfortunately, you have to pull the motor to replace it and coupler isn't cheap either.

Woodsy

[Dave R]

You can test for hub slip by scribing line that mate up on the prop body and the prop nut. Re-create the problem, then stop the engine, trim up the drive and see if the marks are still alaigned. If they are, replace the coupler, if they are not re-hub the prop.

Couples fail for two reasons:

1. Improper alignment. The engine and drive need to be perfectly aligned to avoid stress on the rubber part of the coupler. Misaligment makes the rubber overheat and fail,. You can immediately diagnose this problem because of the smell. Alignment needs to be checked once a year.

2. Inadequate lubrication on the coupler splines. The stern drive input shaft is made of steel and has splines. These splines mate with the aluminum splines in the coupler. The two surfaces are never supposed to touch each other and rely on a thin film of high pressure grease to prevent metal to metal contact. When the grease wears out or gets squeezed out, the steel shaft splines hammer the aluminun coupler splines until they fail. If memory serves, the coupler needs to be greased every 50 hours under normal operation, or every 25 hours if you often operate at low speed. You can't over-grease a coupler.

[DickR]

Update on the prop slippage issue:

I found the records on the new prop and see that I put it into service just two years ago. I removed it and put the old prop back on. A quick spin, bringing the boat up on plane first with something less than full throttle and then again with full throttle. There is no repeat of the "slippage"/over-rev problem I had with the newer prop. I guess I'll get a new prop and get the slipping one fixed as a backup. I'm hesitant to use the original prop too long, as the rubber splines in the hub, into which the brass spline that mates with the shaft seats, are somewhat worn. I suppose that under load there is minimal movement between brass and rubber. Thoughts?

Dave R, thanks for the reminder about lubrication of the coupler. The original engine was from 1997, when I got the boat new. I replaced the engine just three years ago, due to a cracked block resulting from a screwup on my part the previous fall. Would the original coupler have been retained, or would a new coupler have been installed? Bottom line question: Can the coupler in there now be lubricated from inside the boat (with a fitting), or must the outdrive be pulled?

[Dave R]

Update on the prop slippage issue:

I found the records on the new prop and see that I put it into service just two years ago. I removed it and put the old prop back on. A quick spin, bringing the boat up on plane first with something less than full throttle and then again with full throttle. There is no repeat of the "slippage"/over-rev problem I had with the newer prop. I guess I'll get a new prop and get the slipping one fixed as a backup. I'm hesitant to use the original prop too long, as the rubber splines in the hub, into which the brass spline that mates with the shaft seats, are somewhat worn. I suppose that under load there is minimal movement between brass and rubber. Thoughts?

Dave R, thanks for the reminder about lubrication of the coupler. The original engine was from 1997, when I got the boat new. I replaced the engine just three years ago, due to a cracked block resulting from a screwup on my part the previous fall. Would the original coupler have been retained, or would a new coupler have been installed? Bottom line question: Can the coupler in there now be lubricated from inside the boat (with a fitting), or must the outdrive be pulled?

Couplers can last the life of the engine, so it's probably the old coupler. You can grease it from inside the boat without pulling the drive. The coupler has two grease fittings, you only need to grease one of them. The only reason it has two is that the coupler spins when with the crankshaft so you can't predict where it will stop when the engine is turned off. Putting two fittings on there increases the odds of seeing/feeling a fitting when you look for it. Merc. recommends teflon, (2-4-C) grease like you should use on prop splines, but any grease is better than no grease.

[DickR]

The warranty slip that came with the prop recommended having the prop repaired by a "qualified trained technician," a locator for whom could be found at nmpa.com. I spoke with one I found in NH, and he advised me to throw the thing away. He said that too often the process of replacing the hub internals results in a crack of the aluminum casting. His price was $100 total, which would be wasted if the attempt failed. The prop cost me only $134 with shipping, and the same site has the same price now.

The technician suggested I "upgrade" to a carbon fiber composite prop from Piranha. They don't have a rubber hub to fail, and a broken blade can be replaced by itself, by the owner. And he just happens to be a Piranha dealer! I can get a "close" size replacement for $165 + shipping. I'm mulling it over, and any advice, taken or not, would be appreciated. Part of me says that the chance of an exact replacement also failing is very low, and the other part says that so far my results with that mfg/model is 100% failure rate.

[Dave R]

The warranty slip that came with the prop recommended having the prop repaired by a "qualified trained technician," a locator for whom could be found at nmpa.com. I spoke with one I found in NH, and he advised me to throw the thing away. He said that too often the process of replacing the hub internals results in a crack of the aluminum casting. His price was $100 total, which would be wasted if the attempt failed. The prop cost me only $134 with shipping, and the same site has the same price now.

The technician suggested I "upgrade" to a carbon fiber composite prop from Piranha. They don't have a rubber hub to fail, and a broken blade can be replaced by itself, by the owner. And he just happens to be a Piranha dealer! I can get a "close" size replacement for $165 + shipping. I'm mulling it over, and any advice, taken or not, would be appreciated. Part of me says that the chance of an exact replacement also failing is very low, and the other part says that so far my results with that mfg/model is 100% failure rate.

I have not heard much good about those composite props. I'd get a Mercury Alpha 4 prop in 16". They are quite good and a nice upgrade over a three blade. If you are stuck on three blade, Mercury makes them too, and they are not much more than other brands. They also have a superior hub design compared to Michigan Wheel.

[Charlie T]

I agree you should just buy a new prop rather than hope a repair can be accomplished on the old one. With that said unless you are 100% sure you have the correct prop on your boat and when I say "correct" I mean correct for your particular boating use and style I'd ask one of the local boat yards for some recommendations. Most boat yards have props that they will loan you to try with the understanding that they will work with you to find the proper prop for you needs as long as you purchase from them. You will find the local people want your business and they can be somewhat price competitive if asked. The value added service that a local yard will offer is worth a lot more than the few % that you may save buying over the internet. It never hurts to be an established customer is an emergency repair should be needed on a boating weekend either.

Good luck

CT

[DickR]

Update: I did order a new prop, a 3-blade Mercury "Black Max," for about the same money as for the Michigan Match. I did get a slightly different D/P, changing from the previous 13.75 x 21P to 14.5 x 19P. It arrived three days earlier than expected. It does have the newer user-replacable hub internals.

With the new prop, the boat comes out of the hole a small but noticeable bit faster. At cruising engine speed, typically 3000 rpm for me, the boat speed is about the same. At WOT over flat water and just me in the boat, the tach started approaching 5000 rpm, with speed approaching mid-40s. Specs for that engine call for WOT at 44-4800 rpm, so it seems that my WOT speed would be a bit high. But I don't run at WOT. That just burns a lot more gas and beats on the engine unnecessarily, in my view.

[BroadHopper]

Sorry to be off-topic but this is about props. Anyone has or know of a used 22p 4 blade available? I will buy or trade a 24p 4 blade.

[Dave R]

Update: I did order a new prop, a 3-blade Mercury "Black Max," for about the same money as for the Michigan Match. I did get a slightly different D/P, changing from the previous 13.75 x 21P to 14.5 x 19P. It arrived three days earlier than expected. It does have the newer user-replacable hub internals.

With the new prop, the boat comes out of the hole a small but noticeable bit faster. At cruising engine speed, typically 3000 rpm for me, the boat speed is about the same. At WOT over flat water and just me in the boat, the tach started approaching 5000 rpm, with speed approaching mid-40s. Specs for that engine call for WOT at 44-4800 rpm, so it seems that my WOT speed would be a bit high. But I don't run at WOT. That just burns a lot more gas and beats on the engine unnecessarily, in my view.

5000 RPM won't hurt a thing and any time you have a heavy load, being "under-propped" a little bit like that will be a plus. I think you;d be surprised how little WOT operation really affects your MPG. Yeah, you are burning more gas, but you are also going faster. My boat MPG rating only drops 20% from best cruise to WOT. As far engine wear goes, I bet your engine will fail due to corrosion or freeze damage long before it wears out. That said, I don't run WOT much either, but I certainly feel no guilt when I do.

[Dave R]

When I recommended the 16" 4 blade, I was under the impression you had a 17" 3 blade on there. Not sure where I got that...

[4Fun]

Just to add a real word experience to a coupler failure symptom. My 3.0 mercruiser coupler failed and the boat made a really high pitched squeel and would not move. It also overheated fast as there was no waterpump running. I thought it had a bad internal bearing at the time.

After looking at the damaged parts it seems it would not slip for long, , before the boat is disabled. Mine would not even turn the gearcase in neutral within seconds of failing.