'93 115hp Mercury - while planing, slips and revs up

[convergent]

I went out to test a boat that I'm thinking of buying this weekend. It is a '93 Glasstream Fish and Ski with a 115hp Mercury. Within the last few days, the seller replaced the prop with a 4 blade prop he said he paid $200 for new. The old one had some damage and was original, and another potential buyer had complained about it (I saw it... had a couple of small chunks missing in a couple of the blades... not horrible). When we went out to test, with the trim all the way down as he applied power it got about half way to planing off and there was an abrupt rev up of the engine and our forward push abruptly stopped. It felt like if slipped out of gear or something, but he was able to slow the throttle back a little, adjust the trim up, and then it planed off and ran fine. Every time we tried to take off with the trim all the way down, it did this slipping thing. He said it had never done it before and wasn't sure why it was happening but thought the prop was different, and there were 4 people in the boat which is about a full load for this boat. From my prior experience with boats, I was under the impression that with the trim all the way down, it should be fine to go full throttle to quickly plane off. Any idea what could cause this problem? Is it possible that he incorrectly installed the propeller? We are now discussing having his mechanic look at it, but just curious what folks thought here. Thanks.

 

[Barnacle_Bill]

Re: '93 115hp Mercury - while planing, slips and revs up

Sounds like it has a slipped hub. This can be fixed by a prop shop.

 

[convergent]

Re: '93 115hp Mercury - while planing, slips and revs up

Since he just put on a new prop, is the "slipped hub" likely due to a defective prop, improper installation of the new prop, or something coincidental?

 

[Chris1956]

Re: '93 115hp Mercury - while planing, slips and revs up

I never saw a prop with a bad hub slip while accelerating and then plane off and run properly, at least, I never saw one do this multiple times.

Is it possible the boat has a keel or some other structure that causes it to cavitate or ventilate, when trimmed all the way down?

 

[convergent]

Re: '93 115hp Mercury - while planing, slips and revs up

I never saw a prop with a bad hub slip while accelerating and then plane off and run properly, at least, I never saw one do this multiple times.

Is it possible the boat has a keel or some other structure that causes it to cavitate or ventilate, when trimmed all the way down?

The boat is a "fish and ski" which has a hull that is very flat and most similar to a bass boat. Would that cause what you are describing? I was thinking prop since he said he had never experienced the problem before and he had just replaced the prop. The only other thing that might have been unusual for him was that we had four people in the boat and the max capacity is 4 people/500 pounds. Because the boat doesn't have an open bow, we were all in the back half of the boat and he thought that may have affected it.

I agree with you that a slipping hub thing wouldn't seem like something that would just change with trimming, but I have no experience to base it on. It was pulling pretty solidly and then abruptly stopped pulling and the engine revved... and it did that for several seconds before he adjusted the throttle and trim and it stopped doing it. It happened twice when he was trying to accelerate hard with the trim down. We never really did the "floor it" test at all and I don't recall being on a boat in the past that you couldn't just let it rip on take off under full power until you planed off.

 

[convergent]

Re: '93 115hp Mercury - while planing, slips and revs up

OK, a little more information on this from the seller. I talked to him a bit about the suggestions here. It turns out that I was wrong about him changing the prop type. The new one is the same size, pitch, and number of blades as original. But after discussing with him whether he assembled it correctly, he went back and checked and found that the friction plate from the old prop was still on there... so the old and new friction plates were stacked. He believes this was the problem and is going to check it out tomorrow on the water. Does this make sense as a cause for the symptom we were getting?

 

[Chris1956]

Re: '93 115hp Mercury - while planing, slips and revs up

When you say friction plate, do you mean thrust washer? If so, a second thrustwasher will push the hub of the prop out of the exhaust tube of the gearcase. This allows exhaust to ventilate around the prop hub and onto the blades. This could cause ventilation, and would likely only ventilate at low boat speed, and clear up at higher boat speed. I guess it sorta fits....

 

[ufm82]

Re: '93 115hp Mercury - while planing, slips and revs up

Yes, the two washers will cause that. As stated above the exhaust is going out through that excessive gap and allowing the prop to cavitate. Once the gap is closed properly you shouldn't have an issue. However, if you still have a problem, you could very well be overpropped and the prop is losing "bite" and cavitating. So as Chris said, it does fit the problem.
On some boats some ventilation is desired. Many SS props have holes drilled in the hub by the base of the blades. Some have plugs that you change to adjust the size of the holes. This allows the prop to "slip" when launching to help the engine spin up. Once the boat is moving the exhaust goes out the hub normally instead of going through the ports and the prop performs normally.
Install the prop properly (no pun intended) and see what happens.

UFM82

 

[convergent]

Re: '93 115hp Mercury - while planing, slips and revs up

Install the prop properly (no pun intended) and see what happens.

UFM82

Thanks guys... you have been a great help in getting me closer to buying this boat. He took the extra washer out and test the boat today... said it is running great now. He felt like an idiot for the extra washer, so I didn't give him too hard of a time about it. I've done stuff like that many times. I'm going to be testing the boat again tomorrow and if all goes well I'll probably be buying it.

This boat has been garage kept since new and barely uses... looks great and hopefully will be running great tomorrow. I also talked to the mechanic that has done maintenance on it for him... new oil pump and a few other things in April, and he said the engine was great... good compression and all.

Thanks again for all your great insightful help. I'm sure I'll be a regular in this forum now as this will be my first outboard boat. Its also been about 20 years since I've owned any boat, and this place looks like the place to get help.

 

[convergent]

Re: '93 115hp Mercury - while planing, slips and revs up

As a follow-up... the final test went great and I bought the boat. I've been out several times with it and its running great. No slippage at all any longer with the prop.