Through Hull Replacement, Part 7: Cutting the Bronze to Size

Cutting one of the new 3/4 inch bronze through-hulls to size
Bronze through-hulls are manufactured with threaded shafts. These threaded shafts are designed to be screwed into bronze seacocks or into bronze flanges (onto which an in-line valve can then be screwed). As I described in some detail in the second part of this multi-part posting, I opted for the latter approach, i.e., the use of bronze flanges together with in-line valves. The picture below shows the Groco brand hardware that I purchased for the waste outlet of the marine head.
Groco brand bronze flange with in-line valve and full-flow threaded barb
Since the thickness of hulls vary from one boat to the next, the manufacturers of through-hulls tend to make their through-hulls with threaded shafts that are especially long. In this way, the manufacturers can please more people and have fewer complaints, since their through-hulls can accommodate as many hulls as possible. This, of course, shifts the burden to the individual consumer of custom-fitting the through-hulls to his own boat.
The simple and easy-to-understand diagram (provided in the promotional materials by Groco itself) allows you to see a little more clearly what I'm talking about when I say a new bronze through-hull often must be custom-cut by the consumer. If he does not cut the through-hull to a size appropriate for the thickness of his hull, then it is impossible for him to screw it firmly into place. In other words, it's impossible for him to make the flange and the through-hull snugly hug the hull. It's also worth mentioning that the consumer will also, of course, want to ensure that he does not cut the through-hull too short. If this occurs, then his through-hull will not have enough threads to seat itself in the flange.
It should be clear now that not only is it often necessary to cut new bronze through-hulls to size, but also it is necessary to exercise some discretion in making the cut. As in so many other areas related to sailboats and sailing, balance is the key. Too much and too little are not good. There is a happy medium.
As I said in my previous posting, I determined (from the combined thickness of the hull, the new backing plate, and the flange itself) that I needed to remove somewhere between 3/4 of an inch and 7/8 of an inch from the waste outlet through-hull. To make the cut in this through-hull, I began by firmly securing it in my bench vice. Then I grabbed my hack saw and proceeded to score the bronze with a few initial strokes of the saw. Pay no attention to the black line. I had earlier marked the through-hull in that area in my effort to determine the depth to which the through-hull could be fully threaded into the flange. That depth was 1 inch.
After I had made it about halfway through this piece of hardware, I handed off the task to a friend, who had volunteered to jump in.
There was no turning back. I hoped that my measurements and the cut that we had just made had been accurate.
The blade on the hacksaw was not brand, spanking new. Consequently, it had walked a little bit as we had made the cut. To level off the through-hull and remove the burrs, I pulled out the angle grinder.
In the close-up below you can see some of the burrs.
It was imperative that I not get too carried away with the angle grinder with regard to these burrs. If I damaged the threads, the whole thing would be ruined.
A more delicate approach was necessary. For this, I turned to the Dremel. Note also the old dental instrument. A local hardware stores sells these. I find them quite useful for many small-scale tasks. The Dremel, the dental instrument, and repeated attempts to screw the flange onto the through-hull eventually brought success.
After we had finished cleaning up the 1.25 inch waste outlet through-hull, we turned our attention to the 3/4 through-hulls. It just so happened that it was unnecessary to cut the 3/4 inch through-hull that would provide raw water to the marine head. The shaft of this through-hull was shorter than the one for the 1.25 inch through-hull. Fortunately, it wasn't so short that it wouldn't seat within the flange. I would have been in dire straights if that had been the case. As far as the 3/4 inch galley sink drain through-hull was concerned, it was necessary for me to remove about 1/8 of an inch from it.

To remove this 1/8 of an inch of material from the 3/4 inch through-hull, I began, just as I had begun with the 1.25 inch through-hull, by clamping it in the bench vice. The only difference was that I approached the clamping of this smaller through-hull in a different way. Instead of clamping the shaft, I clamped the head. Clamping the shaft, of course, was not an option, since the cut I would be making was so small. To protect the head, I place scrap pieces of mahogany on either side of it.
For this cut I decided it would be good for me to put a new blade in the hacksaw. This made a big difference.
The cut was far more precise than it had been on the 1.25 inch through-hull. There was also, of course, a lot less material to cut through. This helped.
After a quick clean-up with the angle grinder and the Dremel, this little through-hull was ready to go.
This ends this brief, yet pertinent posting on how I cut the new bronze through-hulls to size as part of my overall effort to replace the old bronze through-hulls with new ones on Oystercatcher, my Ericson 25.

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