What type of drill bit to use?

[hman]

Ok, I've gotten all kinds of advice from neighbors, family and friends and all different so I thought I'd take it to the guys who know....

I'm going to drill several large holes (1/2 inch) in my transom to mount my swim ladder. I have a difference of opinion on which bit is best to drill through, wood or metal....They all say "don't use THAT kind, it wiill splinter and crack fiberglass". For small holes to mount cleats and winches, I've always used metal bits with no problem, but I reallly don't want to screw up my transom!

 

[Joe11688]

I just use regular drill bits myself. It's a good idea to countersink the holes just a tiny bit to keep the Gelcoat from cracking. It will give the caulking a place to go when you tighten the nuts down which will give it a nice tight seal. My friend Wayne uses centering bits for cutting accurate holes. He has a set of them and he just picks the size that fits the hole. I've drilled a lot of holes in my boat and I wouldn't use a wood bit or a high speed bit. So really, what do you have left to use? Those centering bits sound like an ideal thing to use for accurate holes.
Nobody wants to screw up their transom, I agree. One of my friends drilled some holes for a motor bracket that he was installing on his O'Day 26. When he got through, the outboard skeg didn't go down low enough, so he had to remove the bracket and re-drill the holes all over again. When he got through, his transom looked like it got attacked by a woodpecker. S&(t happens. Measure twice and drill once. As long as the holes are accurate, it should be OK.
Joe

 

[JRacer]

See my post here http://forums.sbo.sailboatowners.com/showthread.php?t=132577

for recommended way to drill through the boat. In the absence of the ability to remove the core between the "hulls" around the primary hole and replace it with epoxy, I suggest over-borinig a hole, filling it with epoxy and then going back and drilling the correct sized hole. The post above explains why.

 

[hman]

Thanks!

I'll see if I can find some of those self-centering bits. Sounds like the best way to go. My transom is solid fiberglass, there is no core that I can see. The motor mount has a block backing plate that's about 4" thick. My swim ladder came with a backing plate to put in as well, so I don't think I'll over size the holes and fill in, just seal it with marine caulking and butyl tape..

 

[RichBone]

I think they're called brad point bits.

 

[Joe11688]

I think they're called brad point bits.

Do they also call them Forstner bits Rich?
I have a few of those myself that I bought many years ago. Of course my son lost a couple of them.

 

[Maine Sail]

You want either brad point bits, started in reverse, or a 1/2" hole saw, also started in reverse but only AFTER drilling the pilot hole.

A while ago while installing a battery monitor on a customers boat I remembered to take some pics. Measuring for it took far longer than the act of drilling..

The only reason for tape was to mark my hole center and I get holes just as clean without tape:

I then pre-drilled a 1/4" pilot hole, the same size as my hole saws arbor drill, using a brad point drill bit starting out in reverse to get through the gelcoat:

Started in reverse with the brad point and once through the gelcoat I switch to forward:

Then I busted out one of my trusty Lennox hole saws and drilled through about half way in reverse before switching to forward:

Hole done, no chips and perfectly clean:

Just for you guys I saved the plug to show how easy this is:

The right tools and the right technique make for easy work..

 

[Joe11688]

Those are great tips Phil! Thank you very much.