Screwing into fiberglass w/o cracking it

For your run-of-the-mill fiberglass boat with gelcoat, these drill sizes should suffice to make a nice pilot hole.

#4 use 3/32"
#6 use 1/8"
#8 use 9/64"
#10 use 11/64" (or 5/32" in a pinch)
#12 use 3/16"

To drill and put the screw in without cracking the gelcoat, here's what I do. YMMV.

1) Use a 1/16" drill and drill in REVERSE until you get through the gelcoat in into raw fiberglass or wood, then you can put the motor in forward and finish up.

2) Use a countersink bit to chamfer the hole to be a little larger diameter than the screw you are going to insert.

3) Use the appropriate size drill for your pilot hole and again, drill it in REVERSE until you get through the gelcoat.

4) If possible, use a hand screwdriver to insert the screw and GO SLOW and DO NOT OVER TIGHTEN.

Some optional ideas and methods....

-- You can use masking tape over the area you are drilling and it will further help prevent gelcoat cracking.

-- You can make a groove in the screw, on each side, from the point up to the screw's widest area which makes it look like a regular self-tapping screw. I have not used this method but I hear it works well to prevent cracking. I think it gives the debris a place to go and doesn't bind up as much.

-- You should probably use 3M 4200 (or silicone at the very least) on all screws if you want to ensure no water gets behind them.

(lifted from CarolinaSkiffOwner.com)