You can use an allen wrench in a drill to remove the coring around the hole. That way the hole can be drilled the same diameter all the way thru. Use thickened epoxy to fill the voided area.

 

you just need to use a router on the inside to remove about 1/4" of the coring. West System epoxy to create a margin of protection for the balsa and you should be good. I may have a picture or two that I will look for.

 

Here is how Airmar thinks a thru hull should be installled. This is for below the waterline install, but the same care should be taken on the sides of the boat.There is a thread on here recently where one side of the boat hull was wet and somebody was looking at a major repair bill.

http://www.airmar.com/uploads/InstallGuide/17-128.pdf

 

We replaced all of our through hulls awhile ago because one or two were showing signs of UV damage making it difficult to get a good detailing of those fittings in the springtime. A router was used to remove balsa which was completly dry with no damage. To prevent UV damage to the new fittings, we sprayed them with Imron paint to match the hull color. This was done about 5 years ago and they still look brand new. If you are going to replace a sink fitting consider painting it to match the hull

 

View attachment 83330 View attachment 83332 View attachment 83333
We replaced all of our through hulls awhile ago because one or two were showing signs of UV damage making it difficult to get a good detailing of those fittings in the springtime. A router was used to remove balsa which was completly dry with no damage. To prevent UV damage to the new fittings, we sprayed them with Imron paint to match the hull color. This was done about 5 years ago and they still look brand new. If you are going to replace a sink fitting consider painting it to match the hull

Thanks for the replies....the allen wrench on a drill is very creative. How does the router work....what does the bit look like?

 

I was looking at Mega Dripless Thru Hulls for at least the sinks and AC lines. Those are the areas that alway leave a nasty drip line.

 

hard to bring myself to cutting into the hull! 18" above the water line is about 2" below the sink drain. Is the goal to get it as high as possible or as low as possible. Do I need to install a trap of any kind or just go straight out.
The rest of the drains are about 12" above the water line and have check valves in them.

 

No trap needed. Traps are used in residential to prevent sewer gases from reentering the home. That can not happen on a boat. Your 2" pitch should be fine or you could go a little more. No harm either way.

 

A P-trap prevents exhaust gases and critters from entering the boat. You should install one.

 

Check valves, or an "upward loop", are used to prevent seawater from being forced in and up in certain conditions. Just be sure you install the thru-hull low enough to have good flow. On many boats, the drain hose is already on the small side so you want to err on having gravity help you more.

 

Did the deed....used a Dremel to cut out the wood coring (first pic) backfilled with West Epoxy repair kit and finished product. Did not install a seal loop or trap yet....drains great. Used a 1.5" fitting. Inside the boat was a false wall which was another cut thru and covered with an escutcheon plate. No more cleaning out a greasy sump pit!

 

I keep a toilet puck in mine and added easy access...