How to cut out transom skin

[warthog5]

Several tool's are possible.

Die Grinder with a cutoff wheel aka Wizwheel.

Circular saw set at a shallow depth

Plunge router with a temp. fence made from scrap lumber afixed to the transom with some drywall screw's.

The ultimate would be a Fein MultiMaster. This will be the next tool I buy.
http://www.amazon.com/gp/product/B00...lance&n=228013

Your not going to be able to do it with a jigsaw. It will jump all over the place.