I can think of a couple of reasons why this would not be a good idea, but one of my fabricated offshore motor mounts has a cracked weld and needs repair. I would hate to have to pull the one engine to fix this, as it is not going to take more than 5 minutes to fix in the boat. Anyone have any horror stories as to why I should not do this?

depends on what you want to believe... first i would stop drill the crack. then if you believe you can effectively weld that piece from one side only, just clean it effectively and have at it.

disconnect the batteries, alts and electronics ( assuming arc or tig welding) and try not to light anything on fire with the sparks and you should be fine.

Make sure you disconnect every single wire and connection, and connect the ground clamp as close as possible to where the welding will occur. The welding won't hurt the starter, but typically there are other devices hooked up at the same terminal points.

If you don't completely disconnect all the wiring, including grounds to delicate electronics (ignition modules, distributor pick-ups, stereo, etc) you are running a high risk to damaging them, especially if you use an older style stick or MIG welder. Welding creates huge currents. Just disconnecting the battery is not enough.

Yea, what he said, and pull your compass out too if you use it with any accuracy. Lay some wet canvas down under the area to catch any weld spatter (if you use arc).

keep a fire extinguisher handy as well as a garden hose.

Well, I got the job done this evening and all is well! I wasn't taking any chances and wraped the fuel lines with tin foil, and put a large disposable aluminum turkey pan between the mount and the hull to shield the sparks! Sure was easier than pulling the engine! Thanks for all the advice.

DR