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Here's what happens: When I throttle up from idle the engine responds nicely, the boat goes on plane then it slips out of gear and revs like crazy while the boat slows to a crawl. When I throttle back I can feel it clunk back into gear. Here's the weird part: this seems only to happen in rough water. In smooth water I can nurse the throttle and get on a plane and hold it there going 27 to 28 mph (according to the GPS). What is wrong? Do I need a new lower unit? The boat is a '77 13' Whaler.

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I just had the same type of thing except my engine would slip in reverse. According to what my mechanic told me, it could be the clutch dog is wearing out or there is water in your gearcase. He said that both of those can cause the slipping to happen. He also said that if it slips out in forward gear, get it looked at immediately. Each time it slips out it will make the situation worse. When I pulled the plug on my gearcase, there was quite a bit of metal so I opted to replace the whole gearcase to avoid future issues. I got mine through Sterndrive Engineering. 3 Year warranty and a good price. You might be able to get away with just replacing the clutch dog if you caught it early enough.

-chris

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John Houlihan, i have a similiar motor on my classic and do all of my own mechanical work. Have you recently removed your lower unit?? If so, when you reinstalled it, you may not have adjusted you shift linkage properly.

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Thanks guys for the advise. Last year I had the impeller replaced so the lower unit was off. This may be the problem. I guess I have to haul the boat and check the gear case. How much will a replacement gear case cost and.... can a simple-minded mechanic like me do the job or do I need the professional at $100/hr to do the work?

I still don't know why it works in calm water but not in rough water. Also, I can rev the engine in gear at full throttle tied up to the dock and she stays in gear.

JTH

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John,

It probably works fine in calm water because the lower unit not getting jerked around in smooth water like it is in rough water. In my opinion and from what you're telling me, all you need done is the shift linkage needs to be adjusted.

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Uneven load on the prop in rough water, calm water has an even load on it. Are you sure that it's slipping out of gear, and not a spun hub on the prop, or maybe a worn prop ? I only ask because, in the past I've done that, and I thought that I was slipping out of gear too.

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A worn prop? Do you mean the rubber bushing that goes on the shaft? I didn't think of that. It could be. We don't have smooth water here on Long Island Sound every day so this will limit my boat use. I need to figure this out. I've spent a small fortune so far on this motor and hate to throw good money after bad so to speak. A new motor is out of the question.

JTH

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John,

Do you have a spare prop you can try?
Or borrow one?

If not, you can have a prop shop check out your prop and fix it if necessary.

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Joe, you are on to something. I had my son-in-law on the boat (he has some boating experience) and thinks the problem is indeed the prop. He thinks it's cavitating, then the engine over revs and pops out of gear. Could be.

I have to find someone who knows the correct size of prop for this motor on this boat. Perhaps you know what the prop diameter and pitch should be. Of course I have to find out what's on there bow. Is there a way to measure this?

JTH

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The size might be stamped on it.

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I'll look. Thanks for the tip. This site is great.

JTH

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I have four props and they all have the size on them. Good luck.

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I think that he's ventilating a worn prop.

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MW does "ventilating a worn prop" mean that it's cavitating?

JTH

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We have better prop experts here at W/C than myself but, I'll take a stab at it- "Cavitation" is generally a long term effect caused by uneven low pressure on the foreward side of the prop that causes the prop to degrade further by 'Bubbles" (they act as an abrasive over time and will wear the metal down), This can lead to "Ventilating".

More common is "Ventilation", or a "Ventilating" prop, it occurs when the prop is so worn that the "Cupping" or worn prop ears pull in exhaust gas, or air around the prop, blowing an air pocket around the prop, This usually happens "Quickly" when trying to get up on plane, the extra "load" on the prop causes it to spin an air bubble around itself, the RPM's quickly sky rocket and the hull just drop's down as if you chopped power, or went to "N". Rough water can do this more frequently on a worn prop as well (prop get's closer to the surface due to bouncing, and will pull in air). If you pull back on the throttle, or to "N" after the "Ventilation", the prop will "Bite" again as if nothing just happened (or it feel's like it popped out of gear). It (prop) will perform o.k. for a while then do it again. It does NOT take much to wear down a prop, they often look like they should perform o.k. but, are actually worn out.

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WOW! This sounds exactly like what's happening to my motor. Just as you describe. The prop looks ok though. It was painted so any surface defomations, pitting etc. were disguised. I'll bet what you describe is what is going on. I'll replace the prop with a new one and get back to you and all here on W/C.

I thank all who responded and helped.

John Houlihan

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GOOD NEWS! The problem turned out to be the transducer for the fish finder. I had mounted it before I had the boat in the water and evidently put it too close to the prop. That caused disturbance in the water in front of the prop and thus 'ventilation' as MW discussed. I just got back from a run on the sound and got her up to 32mph according to my GPS. Not bad for a 25 year old motor. The water was rough and it had no p roblem getting on and holding a plane. The problem was staying onboard. I had to hold on for dear life. In fact, I broke the seat back bracket. But the good news is there seems not to be any problem with the motor.

Thanks again for all your help.

JTH