Merc Alpha 1 slipping at WOT on plane

[moorebc]

Guys,
Trying to diagnose a problem that has come up on my new to me toy. Boat is as described in signature, about 500 hrs on original equipment. About a week ago I had a spun hub on a 14.25 x 23 SS prop, pain in the butt but no big deal as I had a spare aluminum of same size in the boat. Since this has happened everything seems fine with one exception. At WOT, after on plane, something will begin slipping, RPM's will rise to over 5,000, and the boat will slow. Once the slip starts I have to return all the way to idle and re-accelerate to get a good "grip" again. None of this was happening prior to the spun hub on the old prop. I checked my shift linkage on the starboard shift linkage today and the plunger switch was not dead center in the V of the linkage when in forward, it was centered in reverse and neutral. Adjusted the remote linkage today out of the water but haven't had a chance to recheck it. What I'm thinking is that the issue has something to do with one of the following:
1. linkage out of adjustment, drive slipping at high rpm - should know if that was the case next saturday when we hit the lake again.
2. trim set too high - I'm leaning against this because the slip starts with throttle adjustment, not with trim adjustment. I can leave trim in a good cruise setting, adjust throttle to max while on plane, and I'll slip.
3. whale tail on outdrive. I put one of those stingray stealth tails on the drive in an attempt to lower plane speed right before all this happened. Could it be causing my prop to ventilate at high speed on plane operation?
4. something else?? - this is the one that really scares me, I can deal with the above but its tough to risk stranding your family on the lake because I thought I had the problem isolated and was wrong.

Any thoughts on the situation? Appreciate any help in advance and have loved reading the advise in other posts.

Thanks,
Ben

 

[dubs283]

Re: Merc Alpha 1 slipping at WOT on plane

the whale tail is really doing nothing for you, in fact seeing as how this is happening after you installed it it is very possible this is causing the issue - cavitation at higher rpm

is the prop you installed the same diameter and pitch as the old one??

is the new prop in new condition, good smooth paint, no dings, dents, etc...??

when and if the slippage continues, try trimming the drive "in" to get it to stop, if it does not, then the issue could be a bad hub on the "new" prop you just installed

your issue has nothing to do with the cable adjustment you did - in fact if you had to adjust the cable due to interrupt switch engagement at the wrong time, your cable could be worn out

 

[moorebc]

Re: Merc Alpha 1 slipping at WOT on plane

thanks for the comment.

The tail really didn't do much for me and I believe I will be returning it. New and old prop are both the same size and no visible damage to the hub, blades, or finish on either.

Once the slipping starts, it doesn't stop until I return to idle, adjusting trim won't allow it to regain grip.

thanks again for the response.

 

[TheBlizzard]

Re: Merc Alpha 1 slipping at WOT on plane

Yeah sounds to me like a cavitation issue; its surprising to me thought that you cannot get it to grip again until you slow all they way down. Makes me think that the prop you put on has something wrong with it, because if your drive was trimmed to high it would stop sucking air when you slowed down.

Remember just because a prop doesn't look messed doesn't exactly mean its perfect. I would go back to the basics and take anything off that isn't stock. Then maybe try to get ahold of another prop (to borrow even) and try that. If it still does it I would start looking at the drive itself (unlikely).

 

[moorebc]

Re: Merc Alpha 1 slipping at WOT on plane

I believe my next step will be to strip off the hydrofoil and see if I get the same issues. I'm also considering changing the 14.25 x 23 3-blade to a 14x21 4-blade Solas or 14x22 4-blade Apollo to see if that helps me much with getting a lower planing speed. I hardly ever run the boat above 3,000 rpm so if I lose a little on top end it shouldn't be a killer. Prior to these issues the boat would pull about 4800 at WOT.