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Merc 25 HP Cavitation

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"I have a Mercury 25HP 1997 S

"I have a Mercury 25HP 1997 Serial No. 0G444580 on a 4m forward control alum runabout.

Out using the boat on the broadwater yesterday, suddenly I lost power and the engine revved out. Managed to limp home with engine just above idle, but every time I opened throttled it was as if the prop was cavitating.

Have inspected the lowerend and the prop looks undamaged, but the fin on the trim anode beneath the cavitation plate has broken off. The prop shaft spins freely in neutral, turns the drive shaft one way and rotates and clicks in the other direction in forward gear and turns the drive shaft in both directions in reverse. ( I think the ratchetting in one direction in forward gear is normal as Graham Lamb mentions it in one of his posts)

Is the loss of the trim anode fin enough to cause such gross loss of drive power? The fin snapped off about 1/4" from its base. I removed the trim anode and smoothed out the rough edge with a grinder and put it back on the lower unit and tested the boat on the water. But still the same with the engine revving out under power.

I am concerned that it is just not cavitation from loss of the anode fin, but could there be something amiss with the gear train in the lower unit, as if there is a slipping clutch.

Not sure where to go fom here, prop looks OK and drive train seems intact, could there be something else wrong?"
 
"Hi there,

Welcome to the f

"Hi there,

Welcome to the foremost forum on boating!


Not too sure if the trim tab would have such a profound effect, I'd look elsewhere.



How's the condition of the prop bush? Could be slipping under load?


Select reverse or forward gear, key out of ignition switch...

Mark a line with suitable pen through bush and prop. Without damaging the blades, try to twist the prop. If the line distorts, time for a new prop.



Hope this helps?"
 
"thanks BJ.

Could the prop

"thanks BJ.

Could the prop be spinning on the shaft under load?

Can't seem to get it to slip by turning by hand. Should I take the prop off the shaft and look?

Here is a picture of the prop with the modified trim fin
247478.jpg
Prop on 25 hp Mercury 1997
 
"Russell, the hub must be pres

"Russell, the hub must be pressed in correctly or you will just have more issues. It's not usually something you would do at home unless you had a very well kitted shop and a small press.

On our side of the pond most prop shops will install a new hub on this size of prop for about 35 or 40 bucks (which is almost the cost of the hub itself), so usually not something you would try on your own..."
 
"Propellor Repairs Australia q

"Propellor Repairs Australia quotes $12(labour) + $60 dollars for bush replacement, Just got to find out if they are open over the break. http://www.propellerrepairsaustralia.com.au/

Maybe I should carry a spare bushed prop for when this happens or just hope that it is unlikely to happen again.

Is there anypoint in going the track of the Turning Point Masterguard Hub Systen that is matched to their propellors Hustler, rascal etc lines? These hub systems look as though they are replceable by the user? http://www.turningpointpropellers.com/MasterGuard_Hub_Kit.php

Probably can fix up the anode trim fin at a later date and get back to enjoying my holiday.

Thanks everyone for helping with the diagnosis and advice.
R"
 
"Hi there,


Are you fasten

"Hi there,


Are you fastening the prop correctly? Looks like you have a lock nut there. Doesn't seem right from my experience. Shouldn't affect performance though.


You'd not get it to crack by hand, it takes 25 hp to get it to slip, if you can do it by hand... My utmost respect!!!
thumbs_up.gif



You'd need to make a rig to test prop-slip. A decent plank of about 2m long with two pegs on it. These pegs would lock against the prop blades, gently apply pressure to see if there's slip.


I'm not familiar with all the new age props and acc. My belief has always been that if it's THAT GOOD, the manufacturers would be marketing these goodies. I may be wrong though...



Regards!"
 
"Russell, I normally use Solas

"Russell, I normally use Solas props on my small Merc's (9.9 and 15 horse) that use their "removable/replaceable" hub system (three torsion bars that can be replaced while on the water if you hit something etc). The theory is, if you hit something hard the bars, which are a little plastic stick surronded by a spring, should give way before you severely damage the prop. I have not yet had any first hand experience of how well this works, but that's probably just a matter of time.

Plus I carry a spare quicksilver prop (standard kit for the motors) with the pressed in style hub just incase I end up doing severe damage to the prop.

So if you can get ahold of a "spare" even if it's a little chewed on the edges etc, it's well worth leaving it on-board. Nothing worse than being a few miles from the dock with a toasted propeller.

And Benjamin - that black nut on the prop (which is either plastic or nylon) is standard kit on the little Mercs. It merely snugs up to the prop hub, finger tight is fine, to keep the prop mated securely to the splined prop shaft. Unlike larger motors (or other mfg's), the little Mercs did not use either a cotter pin/castle nut, or one of those bend over washers to keep the prop on the shaft..."
 
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