Through Hull Replacement, Part 6: Drilling Holes through the Backing Plates

Drilling a hole through the new backing plate for the waste outlet through-hull

Having installed the new backing plates for the new bronze through-hulls, it was now time to drill the appropriately sized holes through them. This was one of the easier steps of this multi-step project. It was, though, one that called for some caution. When performing this operation, it is of course imperative that you select the correct hole saw. It is also imperative not only that you perfectly center the guide bit of the hole saw, but also that you cut the hole at a 90 degree angle to the backing plate. The latter task is complicated by the fact that you must cut the hole from the exterior of the hull. In other words, you must determine, as well as possible, the orientation of the backing plate on the interior of the hull without seeing it from the exterior. How I approached these various issues on Oystercatcher, my Ericson 25, is the subject of this posting.

Two of the three new Groco brand bronze through-hulls

You'll recall from the previous posting that each backing plate consisted of a 3/4 inch, ring-shaped piece of epoxy-coated mahogany, filled with 20 layers of 10 ounce cloth, and topped by two layers of 12 ounce biaxial cloth. When viewed from the exterior of the hull, all that was visible were the 20 layers of cloth.

The 20 layers of 10 ounce cloth oozing epoxy from each hole

In the picture below you see the way that each backing plate looked on the interior of the hull. Starting on the exterior, I needed to drill each hole through the 20 layers of cloth and the 2 layers of biaxial. The mahogany rings would remain unscathed, as they were greater in diameter than the holes themselves.

As I said in the introduction to this posting, it was important for me to select the correct hole saw for the job. I started with the waste outlet through-hull. You'll recall that the ID (inside dimension) of this through-hull was 1.25 inches. What I needed to focus on was the OD (outside dimension). Based on my homemade gauge (that I'd earlier made with variously sized hole saws), a 1-5/8 inch hole saw would create a hole that would provide a snug fit for this through-hull. I did not, however, want a snug fit. I wanted there to be a little bit of extra space for the sealant I would apply during the installation process.

Therefore, I moved up to a 1-3/4 inch saw. This one was just right.

Next, I grabbed my center-hole punch and centered it as well as I could on the surface of the cloth. I should mention that I allowed about two months to pass between the installation of the new backing plates and the drilling of these holes. I had plenty of other things to do, and I figured the more time the epoxy had to sit and cure the better. If I had been pressed for time, I probably would have waited for only about one week.

Before I put the hole saw to the hull, I decided to use the hole saw gauge one more time. Never hurts to measure twice, cut once, right?

If you click on the picture below (so as to enlarge it), you'll see the pilot hole and the initial cut marks made by the hole saw when I lightly pressed the teeth of the saw into the cloth. I stopped and took this picture at this point, because I was worried that I had not place the pilot bit at the absolute center of the hole. The teeth of the hole saw were beginning to bite into the hole in an uneven fashion.

The problem was that I had was slightly off center when I used the center-hole punch. I had tried to eyeball it. I should have measured. To remedy my error. I drilled out the pilot hole with a regular bit that was larger than the pilot bit on the hole saw.

This larger pilot hole would give me some leeway to make fine adjustments with the hole saw.

When it looked like the hole saw was perfectly centered and the teeth were evenly distributed around the original hole, I started the cut.

I stopped once or twice to check the orientation of the cut relative to the original hole. All looked good.

I should point out that I was assisted in this task two family members. One stood forward of the hole, the other one, aft. They checked to make sure that I was perpendicular to the hull. In other words, they made sure that I drilled the hole at a 90 degree angle rather than some whoppy-jawed 80 degree angle or something like that. I myself looked down on the drill from above to make sure that I did not stray forward or aft as I made the cut.

When I removed the hole saw, I was pleased with the sight that I beheld. It looked like a nice, clean cut.

What I removed from the hole saw was a 20 layer plug of epoxy-saturated cloth that was fully cured and thus rock hard.

The new bronze through-hull fit well in the newly-cut hole. This was one indication that I had cut this hole correctly.

Before doing anything else, I needed to take some measurements of the new bronze through-hull hardware relative to the thickness of the hull. When you purchase this hardware, you do not receive the bronze screws, nuts, and washers necessary for its installation. You must purchase these separately. Makes sense. Everyone's hull thickness varies, and there is no way that the manufacturer could make everyone happy.

I also needed to determine just how much of the bronze through-hull I needed to cut off, so that I could fit it properly into the bronze flange. These large through-hulls are quite long. The idea, of course, is that they must be able to accommodate hulls that are especially thick. If you don't have an especially thick hull, you must cut it to size.

In determining the amount that I should cut from the through-hull, I first wanted to determine how far the through-hull needed to be threaded into the flange. As you can see from the picture below, that distance was 1 inch. Next, I measured the thickness of the hull, together with the new backing plate. The total thickness was 1.5 inches. According to my calculations, I needed to remove approximately 3/4 inch to 7/8 inch from the through-hull for it to fit just right when passed through the hull and threaded into the flange.

I next needed to determine the length of the screws that I needed to order. To aid me, I used the screws that I initially ordered for this job. They were 1.5 inches in length. I had made the measurements back before I had installed the new backing plates. Consequently, I had mistakenly not accounted for the thickness of them. As it turned out, however, it was beneficial for me to have these undersized screws (along with the washers, lock washers, and nuts). Using all of this stuff as a guide, I was able to determine that I needed at least 1 inch worth of threads above the new backing plate. Since the thickness of the backing plate and hull were, together, 1.5 inches, I determined that the total length of the screws I needed was 2.5 inches. In case you're interested, the screws were 5/16 inch in width.

Clearly these original 1.5 inch screws would not have allowed for a backing plate.

When the time came for me to cut the new holes for the new 3/4 inch bronze through-hulls (one of which would serve as the raw water intake for the head and the other of which would serve as the galley sink drain), I ran into some problems in terms of access. My Makita drill had served me well in the cutting of the large hole for the waste outlet. The bunk of the trailer had not impeded my work. The bunk, however, did impede my work, when I turned my attention to the raw water intake hole. At first, I thought I could get away with using my Milwaukee Tools right angle attachment. The only problem was that it had a quick-change chuck, not a traditional, screw chuck. This meant that it could not accommodate my hole saw. Therefore, I needed to make a trip to the hardware store. There, I found a Milescraft brand right angle attachment with a traditional, screw chuck. I took the picture below shortly before I applied this new tool to the hull.

I started by drilling a larger-than-necessary pilot hole.

I did not take a picture of the steps I took to accomplish the task pictured below, but, with the help of two friends, I lowered the bunk a much as possible to make room for the right angle attachment.

This new hole that I would drill would be larger than the original one. Note the original through-hull in the picture below. It was 1/2 inch (ID). The new one was 3/4 inch.

My two friends who were helping me stood forward and aft as I drilled this hole to make sure I was keeping the drill at a 90 degree angle relative to the hull.

I stood over the drill looking down at it to keep to balanced fore and aft. In this picture, it looks as if it is not, but it is.

Just as was the case with the first hole, I paused once or twice while drilling this one to make sure that everything was lining up as I hoped it would.

In a short time I was looking at a brand new, well positioned hole in the hull.

The final hole, underneath the galley sink was much easier to approach. No need to use the right angle attachment.

One more hole, ready for some bronze. Never mind the patches of old bottom paint. I had not yet finished painting those areas where the jackstands had stood during the painting process.

It would seem that the next thing I needed to do at this stage of the game was to install the bronze flanges on the new backing plates. Before I could do this, however, I needed cut the new bronze through-hulls down to size (so I could accomplish a dry-fit of the hardware), and I needed to put some time and effort into the drilling of the bolt holes for the flanges. How I cut the new bronze through-hulls and how I approached the drilling of the new bolt holes are the subject of my next two postings.

This ends this posting on my drilling of holes through the new backing plates for the new through-hulls in Oystercatcher, my Ericson 25.