I think that leaving the batteries connected or any of the wiring connected is in the category of "hard to defend if it screws up". There is so much current available from a welder and so little control over where it goes, that I would be tempted to disconnect the grounding wire from the trim tab before welding. What if all that current wants to go thru the wire to get back to the ground clamp? I assume that you have two ground leads so that you can clamp both hull and tab, or that you clamp the two pieces together during welding so that one clamp will do.

By the way, I can't weld. I am a NASA-certified welding inspector, so I don't need to, I just criticise. It's a great job.... :-)

I am planning to take a welding course this summer at the local community college if the dates work out. So I'll be able to criticise myself.

Always error on the side of of caution, especially when it comes out of your pocket.
Mark

Think we are all agreed-- disconnect the batts!! Joes idea shortest run possible is something to keep in mind also, clamp the grond to the parent metal you are welding, thus bypassing the electronics ect. Doug when you leard to weld, DON' BREATH THE FUMES!!! I AM SURE THIS IS COMMON KNOWLEDGE NOW!!! Back in the 60's-70's we had NO WARNING!! THE FLUX ON WELDING RODS IS MADE FROM MAGNNESIUM AND YOU WILL GET MAGNNESIUM POISIONING-NOW CALLED WELDING ROD SICKNESS( TYPE IT IN YOUR COMPUTER-WELDING ROD SICKNESS-READ IT AND THE SIMPTOMS--I HAVE IT!!!) AM SEEING A NEOSURGEON NOW- STARTS OUT WITH-HAND TREMORS-IT IS UNFORTUNATLY THE EARLY STAGES OF PARKINSONS!! YOUNG WELDERS NOW KNOW ABOUT IT AND VENTILATE-IF YOU ARE AN OLD WELDER CHECK IT OUT!!!!!- NO CURE! MILLION DOLLAR LAWSUIT IN TAMPA! IT TAKES ME 2 HANDS TO SIGN MY NAME!!! IT SHOWS UP MOSTLY IN YOUR PREDOMINANT HAND-AFTER ALL THESE YEARS LEARN TO USE YOUR OTHER!!! ANY OLD WELDERS WANTING MORE INFO CONTACT ME OR IF YOU HAVE INFO SEND IT TO ME, THANKS ALL-SORRYDOG