Loud knocking with black smoke - Yanmar 70

[dillybar]

So, yesterday we are 10 minutes into our weekend get-away when all of a sudden I hear a loud knocking below and dark black smoke starts billowing from the exhaust.
Ive got very little power and the motor is vibrating.
So I put it in neutral and shut it down to ponder my options. We are now in a river that is running 4 knots with the spring freshet so I need to do something
quickly. I decide to turn around and see if the motor will make progress up river and hopefully last back to the Marina. If not we will have to drop the hook and wait for a tow.
So I start the engine and try to put it in gear but it won't go into forward. Whatever has let go seems to involve the tranny as well. After trying a few times It goes into reverse and then I quickly jam it in to forward.
We're now are making 1 knot against the river back to our slip with black smoke pouring out the back and the boat feeling like it's going to shake apart while I contemplate a $15,000 re-power - bad start to the day.
An hour later we are back at the marina, whew!
Anybody want to hazard a guess?

[Faster]

First guess would be a failed injector.. allowing unatomized fuel to enter a cylinder.. this produces a loud, mechanical sounding knock and smoke from unburnt fuel. We've had this occur before, the offending injector was 'breaking over' (spraying) at 500 psi instead of the required 1200 or more. As a result it was more of a 'peeing' of fuel than a misted spray. Sounded frighteningly loud, esp as this was right off a fresh engine rebuild. A new injector cleared it up.

Hopefully it's as simple as that....

[dillybar]

As it turned out it was even simpler...
We got it back to Captains Cove and the good folks there had it up on the lift in less than 20 minutes. This allowed me to remove the 10+ feet of 1 1/2" nylon tow rope that had twisted around the prop!
In retrospect the symptoms made sense. The dense black smoke was due to the huge load placed on the motor from the wrap, the power loss ditto, the vibration - obvious, the tranny issue must have been due to the shaft being unable to unload after it was placed in neutral.
All this added up to me being sure I was suffering catastrophic engine / tranny failure until I called the Yanmar guys and he told me to look for a wrap first (guess it's not the first time they heard this story).
Added bonus is that Captains Cove would not let me pay them for the lift,. Great guys.

[TQA]

Count yourself lucky, there was a story some weeks ago on this forum about someone in a similar situation who had the P bracket tear out of the bottom of the boat leaving a substantial hole and as a consequence came close to sinking.

He also required a new prop shaft I think it certainly looked bent.

[deniseO30]

Sorry for your bad luck Happy for your good luck! Jus wonderin.. why'd didn't you raise some sails to save the day instead of chancing it with the motor?

[dillybar]

Denise, winds were calm, we were in 4+ knots of current going the wrong way and we were in a busy shipping channel 400 yrds across. Sailing was not an option. I did consider launching the dingy with it's 9.9 if the motor quit , and it would have moved us laterally to an anchor spot but not back where we needed to go against the current. When I found we could make headway with the motor (1 knot) we stayed with that.
Faster, the injector info I will tuck away on the hard drive for future use, thanks.

[deniseO30]

Tying a dink alongside with the engine running does a remarkable good job of moving a mother ship. I learned that last year helping a new club member move his boat from the mooring to dock. The outboard actually did "steer" the boat if I gave it time and allowed for the momentum of the sailboat.