Drilling Holes for Vents and Drains

When designing my ballast system, I tried very hard to figure out a design which would not require me to drill any holes in the boat.  I kept having nightmares of me screwing something up and having to take the boat in for some serious fiberglass repair.  Of course, there's my wife saying "You're going to do WHAT??" as well.  I came up with a couple of different possibilities which included emptying back out the bottom of the boat (similar to what others are doing with reversible pumps), and combining vent lines into a single thru hull.  All this would have worked but would have taken away from some important features I wanted in my design.  I wanted to be able to empty over the side of the boat so that it would be easy to tell if sacs are empty and water is no longer going through the pumps.  I also wanted to be able to tell when a sac was full.  If I was to combine vent lines into a single thru hull then I wouldn't be able to tell which sac was overflowing and I'd have to visually check each sac.  I didn't want to be visually checking each sac so I made up my mind and decided to take the risk of drilling the holes.

My design would require 3 new holes.  I would only require 3 holes because I would be reusing two stock 3/4" thru hulls (previously used for emptying) as the vents for the new Jumbo sacs in the lockers.  I would need to drill a 3/4" thru hull for the bow sac vent line located towards the bow on the port side and also would need to drill two 1" thru hulls to be used as a drain line for each new pump.  I chose to use a 1" thru hull for the drain line since I was using 1" line for filling.  I wanted to keep the time it takes to empty the sac close to the time it takes to fill it.  So I decided to keep the hose size the same.  That probably isn't necessary but it is what I went with.   I used 3/4" for the vent lines because I could reuse two of the existing thru hulls and I also felt that a single 3/4" vent would be able to keep up even with the amount of water that would be fed into each sac.  The two Johnson pumps will not produce enough flow or pressure to burst a fat sac with a 3/4" overflow/vent.

A lot of good information is out on the web regarding tips on how to drill a hole in a boat.  Read up as much as you can on this.  It also helped me to have some friends who have done this same thing before.   BigCatPt and Kris K from the CenturionCrew were a big help.

The first thing to do before anything is to make sure no hoses, wiring, etc is behind where you'll be drilling.  Next, use blue painters tape to mask off the area where you'll be drilling.  Then I  marked where my hole would be using a pencil.  I used the existing thru hulls as a point of reference.  Measure the distance between your existing thru hulls to make sure you have consistent spacing to your new hole.  I used a measuring tape and straight edge to transfer reference points from the existing thru hulls over to my new hole.  Be sure to adjust the measurements as needed if your new hole is bigger than the other thru hulls as mine is.  Remember, my stock thru hulls are 3/4" but I'm adding a 1" thru hull next to them.  Take the time to make sure it will look like it was done on purpose.

I was able to use a 1 3/8" hole saw for my 1" thru hulls and a 1" hole saw for my 3/4" thru hull.  It helps to drill a test hole into a piece of wood to verify that you will have a proper size hole before drilling into the boat.  Attach your hole saw to a power drill and put the drill in reverse.  Do not use the drill in forward as the teeth will want to grab and chip the gelcoat.  Take your time drilling in reverse.  It will be very slow.  Don't get in a hurry and apply too much pressure to the drill.  Let the drill do the work.

Once your hole is drilled, you want to round the corners of the edge of the hole to avoid cracking later on.  A dremmel tool with a sanding drum works well for this.  Take it easy and slow doing this as well.  Then be sure to cut any carpet away from the open of the hole so that it will not get caught in the threads of the thru hull when you screw the backing nut on.

Next, clean up the area and then place some 3M 5200 marine sealant on the back edge of the thru hull.  Gently press the thru hull through the hole.  Don't use too much sealant as it will just squish out and make a mess.  You just need enough to make a water tight seal.  Then thread the backing nut onto the thru hull.   If your thru hull has notches in the design like mine did then it helps to have someone hold the position on the outside of the boat while you tighten the backing nut from the inside of the boat.  Once that sealant sets up you won't be able to rotate the thru hull.

Here's a picture of one of my new 1" thru hull on the port side.  My new 1" thru hull matches the stock 3/4" thru hulls (notice the notches in the design I talked about, make sure those are straight)


I drilled the 3/4" vent hole for the bow sac under the rub rail near the front of the windshield.  This location allowed me to access the vent hole from within the storage compartment under the glove box/observers seat.  Here's a picture showing the location.



If you were nervous about drilling the holes, you'll be more confident after drilling the first one and realize it isn't that big of a deal as long as you're careful.  After drilling 3 of them in my boat I still don't like to do it but I am more confident now.

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