[indykoch]

You're right. Could be cavitating as well. I've never heard of cavitating on a deep water start, though.

Then I remembered an experience from last year. I ran over a water-logged 2 X 4 and just so happened to split it directly in the middle. The wood was basically wrapped around the lower unit, directly in front of the prop. That caused major cavitation and did exactly like what we're seeing here. I knew something was wrong right away - felt like the prop flew off!

I suppose a lot of weeds stuck in front of the prop could cause this same thing. Hopefully that's all it was and you don't experience it again. Here's an idea, have the girlfriend bring out a bunch of her 120 lb. friends instead of you bringing your 200 lb boys!!!

[abillmann]

I'm in Madison, WI. Great area for boating -- three large lakes, two of them urban, one rural, interconnected by a river. Lotsa weeds this time of year, though.

Could someone explain exactly what a cavitating prop does/looks like? Is that ultimately a prop problem, or a lower unit problem? There's no damage to the prop, nothing that looks burnt, or anything that looks scratched or worn. Again, the prop is 14.5 x 19p, brand new, still in mint condish. Is cavitation something that could be intermittent? Based on what conditions?

Prior to the new prop, I had a 14" x 21p that exhibited the same symptoms. I bought that prop used, and thought it was spun -- that's why I have the brand-spankin' new one. The 19 is definitely better than the 21 (as far as hole shot is concerned), but still...on Saturday night with four normal-sized adults, it was NOT going to plane.

Today, the boat ran perfect -- with four adults, there was no doubt it was coming up, and quickly got on plane. Ran beautifully. Previously, though, there was NO WAY it was going to happen.

I personally feel like there's some sort of intermittent slip going on in the lower unit, but I have no idea what it could be. There's certainly no chugging/grinding/grunting or anything that seems obviously wrong.

Could the hydraulic trim pistons (or whatever they're called) stick when the drive isn't all the way down?

[abillmann]

"Here's an idea, have the girlfriend bring out a bunch of her 120 lb. friends instead of you bringing your 200 lb boys!!!"

Ryan, man, you're preaching to the choir! Amen, brother.

[blockp]

More common is ventilation, than cavitation. Ventilation is when air gets sucked down to the prop and causes it to slip and as a result revs the motor higher than it was before. Cavitation is when there is so much backward pressure on the water due to the blades spinning that it creates a vacuum in front of the blade.

Unless you hear and feel grinding/clunking, there's no way for "slippage" in the merc's lower unit (it is a merc, right, not a volvo?). All splines and toothed gears in there. If there was any slippage, you would feel it and get that painful feeling in your wallet.

Only places for slip are the coupler, the prop hub and prob blade to water.

[abillmann]

Here's the best way I can describe it:

If you set your drive position to half-way between fully down and fully up (not the trailer position, but halfway between the normal full "up" and "down" positions) and hit the throttle, you'll get the same result -- lots of RPMs, lots of wake backwash, rising bow, and maybe a 50/50 chance of getting on plane.

[abillmann]

Only places for slip are the coupler, the prop hub and prob blade to water.

Thank you -- that is awesome information. (And it makes sense.) I think I can safely eliminate the last two.

So... is the coupler something that can slip intermittently? Or under heavier load? Can it be fine one day and slipping the next? Or is it a symptom that once it develops, is always present?

[blockp]

Originally Posted by abillmann

Prior to the new prop, I had a 14" x 21p that exhibited the same symptoms. I bought that prop used, and thought it was spun -- that's why I have the brand-spankin' new one. The 19 is definitely better than the 21 (as far as hole shot is concerned), but still...on Saturday night with four normal-sized adults, it was NOT going to plane.

Today, the boat ran perfect -- with four adults, there was no doubt it was coming up, and quickly got on plane. Ran beautifully. Previously, though, there was NO WAY it was going to happen.

And watching the tach, the rpm's are definitely increasing when it won't get on plane? That sure sounds like it's ventilating. Different shaped props will cause that on startup vs turning vs trimming up. It sounds like your blade doesn't have enough cupping in it, but everything I read about that prop seems to say it will work fine for your setup.

[abillmann]

Correct -- RPMs start to jump at the very apex of the load on the engine -- just at the moment you'd expect the boat to level off. With the 21p, it would shoot up to 8000RPM very quickly. With the 19p, it's more gradual, but within 5 seconds or so, it's clear that you have to back off the throttle or it will redline.

Would either ventilation or cavitation be caused by a combination of different weight conditions AND water conditions? (ie., seaweed)? Or would these things ALWAYS be present, regardless?

[blockp]

Originally Posted by abillmann

So... is the coupler something that can slip intermittently? Or under heavier load? Can it be fine one day and slipping the next? Or is it a symptom that once it develops, is always present?

Once it slips, it will get progressively worse. It's a lot like the rubber hub in the prop, a rubber part pressed into a metal part. It's purpose is to separate a the outdrive from the motor. This will save your motor in the event that the gearset or something in the outdrive locks up.

Here's a pic of one so you get a better idea.
http://cgi.ebay.com/ebaymotors/Mercr...QQcmdZViewItem

But I suppose, if it only slipped a little bit, not enough to start smoking, it might work ok until you really got on it. When it runs right, is the motor and engine bay cold? When it slips is it hot? or vice versa?

[abillmann]

When the problem happens, there's nothing obviously burning, smoking, smelling, etc. I wonder, though, if the drive coupler IS slipping, but I'm backing off the throttle soon enough to prevent it from showing any signs.

I'll hit it pretty hard over the next few days and see what happens. I guess I agree with the service techs... it's either a clump of seaweed in the wrong place, or whatever it is will get worse and reveal itself.