I've got a BIG problem

[hokiepoq]

I have a 1986 VIP bowrider with a 1986 115hp in-line Mercury OB. I had just fixed up the boat to be workable by rebuilding the transom, replacing the steering. I took the kids out tubing on the boat and the engine began to rev up when I put it under load but the higher rpms were not transferring to the prop. When I backed off, I was fine. I was hoping I was losing a propeller so I went to a local Mercury place and bought a used propeller and had that put on. Put it in the water and the same thing happened. Took it back to the Mercury mechanic (Tucker Marine in Mathews, Va - GREAT guy) and we chatted for a while. He did some basic checks (play in the prop in different gears, checked the lower gear unit oil) and he said nothing appeared wrong. He said his next step would be to take off the lower unit. He said either (1) the connection between the drive shaft and crank shaft is going in which case he said "just throw the engine away" or (2) somethings gone wrong in the lower unit. So, I paid all of $2,000 for the boat, motor, and trailer and it has served me well for several years. I hate to part with it, but this seems to be a big problem. I plan to drop the lower unit - should I be able to tell if the drive shaft/crankshaft connection is the culprit by examining the top of the drive shaft? He also said that fixing a lower unit is DEFINITELY not a do-it-yourself job (even though SELOC makes it looks doable) - is that because of all the specialty tools involved? Looking at factory rebuilt lower units, assuming that's what it is, is depressing. Any thoughts anyone has?

 

[j_martin]

Re: I've got a BIG problem

I have a 1986 VIP bowrider with a 1986 115hp in-line Mercury OB. I had just fixed up the boat to be workable by rebuilding the transom, replacing the steering. I took the kids out tubing on the boat and the engine began to rev up when I put it under load but the higher rpms were not transferring to the prop. When I backed off, I was fine. I was hoping I was losing a propeller so I went to a local Mercury place and bought a used propeller and had that put on. Put it in the water and the same thing happened. Took it back to the Mercury mechanic (Tucker Marine in Mathews, Va - GREAT guy) and we chatted for a while. He did some basic checks (play in the prop in different gears, checked the lower gear unit oil) and he said nothing appeared wrong. He said his next step would be to take off the lower unit. He said either (1) the connection between the drive shaft and crank shaft is going in which case he said "just throw the engine away" or (2) somethings gone wrong in the lower unit. So, I paid all of $2,000 for the boat, motor, and trailer and it has served me well for several years. I hate to part with it, but this seems to be a big problem. I plan to drop the lower unit - should I be able to tell if the drive shaft/crankshaft connection is the culprit by examining the top of the drive shaft? He also said that fixing a lower unit is DEFINITELY not a do-it-yourself job (even though SELOC makes it looks doable) - is that because of all the specialty tools involved? Looking at factory rebuilt lower units, assuming that's what it is, is depressing. Any thoughts anyone has?

I can't imagine anything slipping in the lower unit and re-hooking without jamming it tight, breaking it loose, or sending something out the side. Certainly there'd be debris in the oil.

Usually if the spline on the top of the driveshaft fails, it pretty well shells out. I think you have 2 props with spun hubs.

Mark the hub and the prop nut (scratch well so you can see it), go make it misbehave, and then look at the marks. If they do not line up, you've spun a hub. If they are perfectly lined up, about the only other thing that could be is the spline. Both the driveshaft and the crankshaft would be ruined.

You might be able to hold the flywheel with a wrench and wrestle the prop backwards in forward gear, proving a spun hub. Be sure to take safety precautions if you do this. (disconnect and ground the spark plug wires.)

hope it helps
John

 

[hokiepoq]

Re: I've got a BIG problem

That is awesome advice. I will put the marks on it, go dunk it in the water in the next day or two and get it to "misbehave". Great idea - I hope that's it. The used prop that the Mercury guy sold me was just that, a used prop, and he did say that there was a chance that it was "spun" too - but it was the only thing he had on-hand that would fit my boat (I was on vacation and desperate to get it working again).

Thanks again.

 

[j_martin]

Re: I've got a BIG problem

The symptoms are classic. Check out the obvious first.

Then fire that mechanic.

hope it helps
John

 

[hokiepoq]

Re: I've got a BIG problem

Well, I marked the propeller shaft and the propeller so lines lined up. I put the boat in the water ran it so it "misbehaved" and took it to the dock. Didn't look like the lines had moved. Took it out again and ran it around again until it misbehaved (I thought maybe, by fluke, the lines happened to spin and land right back in the same spot). Well, the mark on the propeller shaft pretty well lined up with the mark on the propeller. Depressing. I may wrestle the prop backwards in forward gear as suggested, but I'm not optimistic. I guess the next step would be to drop the lower unit. It's a shame, because it runs fine at low load and the powerhead is fine and strong. Oh well, thanks for the advice and any other comments are welcome.

 

[achris]

Re: I've got a BIG problem

If you have a spun crank/driveshaft then when it is playing up you will loss water flow. Water pump is driven by driveshaft. Check that.

Chris.........

 

[hokiepoq]

Re: I've got a BIG problem

Another good piece of advice...I thought there might be a reason I left the boat looked up to my Explorer. I guess that's one way to narrow it down to see if it's that connection. Seems pretty important to know since that would prove to be the death of the engine by all accounts if that turns out to be it. I guess I'll get it in the water again in the next day or two.

Thanks for the advice.

 

[achris]

Re: I've got a BIG problem

Just be aware that you may not lose water flow immediately, as the block will drain down first. Give it a minute or so....

I hate to see one of those old 6's die, so here's hoping it's not that....

Chris.......

 

[hokiepoq]

Re: I've got a BIG problem

Ok, I've had the boat in the water a number of times trying to identify exactly what the problem is. I'm coming to the conclusion that I MAY not have a problem after all but I'd like for y'all to see if my thinking makes sense. As mentioned earlier, I said that the motor revved up and the power wasn't transfered to the propeller under load. I can only confirm that this happened once and that's when I was towing both my 15 year old water skiing and my 13 year old tubing at the same time. I lost power to the prop while the engine continued to run fine. I switched props and thought I heard the engine rev while not getting up to speed but not I'm not so sure. Like I said, I took it out a couple of times and couldn't say that I was really losing power. Well, I took two boys tubing today and the motor seemed to be working fine except it sounded like it revved without powering up like I thought it should, at least right away. (I know this next part is going to sound stupid) I thought maybe that I have the motor trimmed to high and the prop is too far out of the water for a moment and spins too freely. Duh, that stopped all the mysterious revving, at least today. My question is is that the time I KNOW I lost power, could that have been the prop being "spun" temporarily? The load was really high and I'm thinking (hoping?) that it was a temporary thing. Can that happen? I sure hope so <keeping fingers crossed>

 

[hkeiner]

Re: I've got a BIG problem

My shop manual contains the following comments about the consequences of mounting a motor higher on the transom. Likely your problem is that your motor is mounted too high. I have never heard of a prop being temporarily "spun". Once it is spun, I think that it can only get worse as further spinning wears on the hub/prop connection.

IMPORTANT: Increasing the height of outboard generally will provide the following: 1) Less steering torque, 2) more top speed, 3) greater boat stability, but, 4) will cause more prop ?break loose? which may be particularly noticeable when planing off or with heavy load.​

 

[achris]

Re: I've got a BIG problem

Sounds like ventilation.... Air being introduced in to the propeller...

Chris..............

 

[Silvertip]

Re: I've got a BIG problem

Since you rebuilt the transom, you had to have the engine off the boat and when reinstalling it, did you get it mounted too high? The AV plate should be flush with the bottom of the boat to 1-1/2 inch above. You may be experiencing ventilation (the prop is sucking air) because it is mounted too high. DO NOT drop the lower unit at this point. You have not done nearly enough diagnosis. Drive shaft splines do not generally slip on an intermittent basis and if they did, would make a fair bit of noise.

 

[hokiepoq]

Re: I've got a BIG problem

Thanks for the comments. I'm pretty embarrassed but relieved at the same time that the engine may be fine. I'm not familiar with ventilation problems with props. When I put the engine back on, I believe it was mounted in the same holes in the outside skin that it was in before. Can a motor trimmed to high cause ventilation problems? I'm exclusively a trailer boat guy so I'm thinking that maybe I just didn't lower enough when I put it in the water. I know that was the problem last evening. I pulled two kids at the same time on a tube and then separately for a while. It had the "revving" early on but after I trimmed the engine down, it did fine. I sure hope that's the problem - way cheaper than other potential problems!

 

[j_martin]

Re: I've got a BIG problem

The reason you have power trim on that engine is because trim could be different for different situations, and even during the same power run. It's a dynamic thing.

BTW, if you're losing bite on a strait pull, you're also probably not getting enough cooling water, and could damage the engine by overheating.

Normally you start out with the engine trimmed fully down, or to a pre-installed stop pin if that would be too far for the boat. Then trim it up/out when you get on plane. Trimming raises the bow, making the boat ride drier and lighter, and also usually either increases speed or lessens the power needed to hold the speed you desire. Too much trim and the prop may lose grip on the water, boat may porpoise, cooling can fail, etc. Learning the feel for it and how to operate it as needed will greatly increase the overall performance of your boat and your enjoyment of it.

On my boat, set up on the ragged edge of destruction, the trim angle gauge and the water pressure gauge are critical instruments when flying it.

hope it helps
John

 

[hokiepoq]

Re: I've got a BIG problem

What a relief! Thanks, Silvertip, for telling me not to take off the lower unit and the others to suggest that the engine would have other symptoms is the lower unit or driveshaft splines were spent. I was out over the weekend and took people tubing and waterskiing and the engine ran great! Lots of power once I had it trimmed it right. Another "tower or power" continues on. Did have one odd thing happen - after I shut off the engine, I heard a clicking and "woosh" that kind of sounded like a little steam engine from the fuel tank. Not sure what that is. It eventually faded and went away. Any ideas? I'm really excited that my boat is not trashed and thanks for the advice!

 

[j_martin]

Re: I've got a BIG problem

Get a maintainance manual and read it. Get involved in the care and feeding of yer engine and it'll give you long and trouble free service. Hang around sites like this and gain understanding of other aspects of operation, like the trim issue.

You could have trashed your motor just by operating it improperly. Most high performance fishing boats from the 80's and 90's, before the "idiot proof" motors that just shut down if something was wrong, could be driven in a way that wrecked the motor in a coupla minutes.

I just did a transom replacement on a high performance bass boat. I noticed that now that the transom isn't flexing, the motor trim is more touchy. I even had to change the mounting height a little to get the range of operation I needed.

Fun out,
John