Get out a piece of paper and draw a sketch of your transom, drill test holes with 1/8 inch drill bit taped for full depth but not thru outer skin, and hold onto whatever material comes out of hole with bit, and mark on diagram, is it dry? cool to touch (means it wet) ?, normal color, or black? Do this on 6 inch centerlines starting along the bottom and work up.
If you have localized the area where it is surrounded by good dry wood.
You can get the largest hole saw sold, usually about 5 inch, use a shorter bit, and saw out the inner skin only, pry it off.
Start in the middle of your sketch. Make a few of these
around the perimeter on the sketch, and do so that good skin remains in any direction for at least 5 inches if using a 5 inch hole saw.
Think of swiss cheese but not so many holes.
Now if the wood is normal color, get a real heat lamp and go at it as close as you can place it, may take many days, but the lamp will suck out all the moisture presuming the area is well exposed as I've directed, you are wasting your time drying out via little holes.
You will even see the water precipitate and run down the inner skin when teh lamp is on and the area heated good.
Keep the lamp as close as possible so that to the touch you can still place you hand on the wood, but barely. Be carefull not to set the gas lines or boat on fire. The wood when wet will never get that hot to burn, not the case when dry, so keep an eye on it.
When the wood stops leaching if it doesr, then you have good wood acceptable to inject the thin penetration epoxies sold to reinforce rotted but dry wood. The dryed wood may look good as is but has diminished structural strenght. Follow instruction sby manufacturer.
Alternately if the area is relatively small, cut out all the inner skin, chop out all wet wood and bevel the remaning good wood, make a matching new plug with plywood and epoxy the plug in place, bevel the edges of the skin holes 2 inches back, then add 3 layers of 1708 on the new marine plywood onto the bevels of inner skin, starting with smaller layer on first.
grind flush.
You just saved yourself a few grand, ought to pay for a full season of gas, at least.
If the areas are all spread out or inaccessible, forget about it.
If the condition of the wood is any worse than above, including black in color which is more like soil than wood, the core can't be saved irrespective of claims by such products.
And find the source of water and seal it off, every screw, thru hull, and bolt into core is a source, all have to be refitted, including rub rail.
Good luck.
