Help diagnosing possible Tranny/shaft issue

[JEESails]

Hello All - Here's the background:
Just bought the boat. Picked it up from the broker's dock and motored about 30 mins to my marina. No odd sounds, shifted and ran smoothly in both forward and reverse. While docking (for the first time with this boat) I inadvertently shifted from reverse, thru neutral, into forward without reducing throttle. Not good, but didn't notice or hear anything unusual in the process - other than my beating heart when the boat lurched forward...
For the past 4 weeks, I've removed and replaced most of the wet exhaust system (heat exchanger, manifold, elbow, hoses, etc), so have not started the engine again since that docking episode. I got it all back together yesterday. Started the engine - no leaks, purrs like a kitten.
I've had this nagging concern in the back of my mind, so at low idle I shifted into reverse just to ease my mind. There was immediately this very loud banging that shook the whole boat violently. I quickly shifted back to neutral and it stopped - the engine purring away as usual. It's a Yanmar 3QM30F with the Kanzaki Transmission.
I suspect my little docking stunt damaged something in the transmission or shaft connections, but not sure where to begin with my process of elimination to find the problem. Does anyone know from this description what I should check first? Obviously, I'm now a little worried my next project will be having a new transmission installed!! Any help would be much appreciated.
Thanks!
John

 

[Glyn Judson]

Transmission woes.

John, Sorry to read of your gut wrenching experience, that's always a horrible feeling. If push gets to shove and you need to buy a new transmission, consider this that I just spotted on eBay 150603763012 after a quick search. If your transmission is like mine attached to my Yanmar 3GMF (almost identical to yours), it's more than likely a Kanzaki KM2A with a 2.62:1, exactly like the one on eBay. I know nothing about the cost of a new or rebuilt transmission but this might be an affordable option. By the way, you can pretty easily remove it yourself if need be. I had mine rebuilt some years ago and the R&R wasn't too much hassle at all. Hint: If you do remove it yourself with the boat in the water, be sure to put a hose clamp around the prop shaft when you leave the boat. That way the shaft can't slip back too far to expose the keyway slot to the area of the packing and slowly flood the boat. Don't ask. Good luck, Glyn Judson, E31 hull #55, Marin del Rey, CA

 

[Frank Langer]

Before you replace the tranny, I would have a knowledgeable person do some diagnostic/detective work to make sure that it really is a tranny problem. For example, if you have an older folding propeller, they make a loud bang sound when they first open up (though that doesn't explain the violent vibration you mention, but that could be a shaft out of alignment or loose engine mounts, etc.).

Good luck!
Frank

 

[footrope]

It would be good to clear up the propeller question. That is more likely to cause boat-shaking vibration than a transmission problem. While I don't have any personal experience with folding props, a skipper I used to sail with had a two-blade folding that had an incident where it opened unsymmetrically. Low rpm might prevent a worn or slightly damaged prop from opening properly.

With the motor off and transmission in neutral, try turning the prop shaft by hand if you can reach it. You might hear something or notice something. Give the shaft a good shake and see if it's still solid. Maybe you have a strut or cutless bearing issue.

Having a diver take a look at the underwater parts wouldn't hurt either. I hope it's not the transmission!

 

[JEESails]

Thanks!

Thanks for the suggestions! It's the KH18 model (I found a used one on Ebay too). It's a three blade fixed prop, so I can eliminate the folding prop possibility.
Sounds like I should start with the prop/shaft and inspect everything carefully to the tranny. If I don't find something obvious, I'll bring in a professional to help figure it out from there. I also found out (if the worst case scenario plays out) that MER Equipment in Seattle can adapt a Hurth unit to this engine - but it's spendy!!!

 

[rwthomas1]

Are you sure that nothing is fouling the prop? Separating the shaft from the transmission and then test running, shifting gears, etc. should help to diagnose it. Can the engine be started with it in gear? Wouldn't hurt to check shaft alignment either. RT