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SanDiegoChip 03-09-2010 04:22 PM

How to make a 1/2 through hole a 1 1/2 through hole?
We have a through hole and it is above the water line. It is for waste discharge. We want to replace it with a 1 inch through hole.
How would be the best way to make the new hole?
Remove original through hole.
It leaves a hole in the boat.
Now how to make it the right size for a 1 through hole?
Use a hole saw, but how to get it started? Need a drill bit in center but nothing to sink the drill bit into.
Maybe put a small piece of plywood over hole and fasten with screws then use hole saw?
Which side to drill from so it does not wreck the gelcoat?
Any better ideas please?

nickmerc 03-09-2010 04:40 PM

I have used your idea of putting a backing plate behind the hole I want to enlarge. It works pretty well. I would start the hold from the side you want to protect. After getting the drill bit started and through the backing plate run the hold saw in reverse until you get through the gel coat. This should keep the gel coat from chipping.

The other option (if you don't care how clean the hole looks) is to use a RotoZip or Dremel and free hand the hole enlargement. This will create lots of dust so I would tape a box over the hole on the inside of the hull to catch most of it.

buckeyesailor 03-09-2010 04:47 PM

would it be possible to offset the hole saw bit a little?
That would still give you your 1 1/2" hole and cover the 1/2" existing hole.....
just a thought.

poopdeckpappy 03-09-2010 04:53 PM


Maybe put a small piece of plywood over hole and fasten with screws then use hole saw?
I would try this


Which side to drill from so it does not wreck the gelcoat?

from outside in just enough to start the new hole, then remove the ply backer and finish it off

Gene T 03-09-2010 04:55 PM

I would probably just drive a plug in the hole. Just something to hold the bit while you get it started. Wood or plastic will work OK. If you have time you could use thickened epoxy but that is overkill. Depends on what you have handy.Drill from both sides, just a little from the inside and finish from the outside.

scraph 03-09-2010 05:10 PM

I second offsetting the hole center to cover the existing hole yet bite the centering bit into material. With 1 1/2" hole ... center at 3/4" and a pre-existing hole of 1/2" you should have a good 1/4" of wiggle room.

Also, I agree with running the hole saw in reverse for a thickness to get through gel coat (and even first layer of cloth would help). Also, applying tape (ie painters tape) over the surface and cutting the hole through it may help.

scottyt 03-09-2010 05:14 PM

buck eye is right, offest the hole so the guide bit has something to run in. or use the ply wood but dont put it behind, use a rubber cement to glue it to the out side then drill thru it. the rubber cement will peel away easily.

Liquorice 03-09-2010 05:26 PM

Screw your 1/2" and 1 1/2" hole saw onto the same mandril so the 1/2" is inside the 1 1/2".
Then use the 1/2" as the starting guide for the 1 1/2".
Works real easy!
sam :-)

scraph 03-09-2010 05:33 PM

That's not a bad idea either, Liquorice. Well done, sir.

sailingdog 03-09-2010 05:39 PM

A couple of questions first—

What do you mean the through-hull is for waste discharge? Is it a bilge pump through-hull or a black water tank pumpout through-hull?

Where on the boat is it located—the transom, the hull topsides?

As for drilling the hole... filling it and then drilling the new hole is one way. Another is to drill a hole through a piece of 3/8" plywood and then attach the plywood to the hull and use it as a "guide" for the hole saw.

Running the holesaw in reverse will allow you to cut the gelcoat without chipping it much.

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