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Replacing Seacocks and Thru Hulls for the DIY Owner

Replacing Seacocks and Thru Hulls for the DIY Owner

Looming large on the list of tasks to be completed during our 10 day yard period in April, was the replacement of all nine seacocks and thru hulls. The thru hulls were not in bad condition, but the seacocks were showing some serious signs of age. While some were probably serviceable, others were showing signs of zinc depletion in the bronze. In addition, several of the backing blocks were saturated and beginning to rot around the base. To eliminate any further issue we decided to replace all backing blocks, thru hulls, and seacocks. As we discussed in a prior post (find it here), we decided, after much deliberation, to go with all Marelon fittings.

Current corroded seacocks
Original seacocks showing some age. These were the best looking of the bunch and probably serviceable.

Within a few hours of hauling out we were into our first step of the process, removing the old fittings. First we used a stepped thru hull wrench to unscrew the thru hulls from the seacocks. They came out relatively easy once the sealant was broken. The old seacocks were through-bolted so Skye backed off the nuts from inside while I attempted to hold the slotted bolts from rotating. Let’s just say this was an exhausting and frustrating process as, even with a penetrating fluid, many of the nuts were frozen. Nonetheless, Skye channeled her inner “beast mode” as I held the flathead screwdriver with the assistance of vice grips. In the end we got almost all of them out, enough that we were able to sneak the flange of the seacock out from under the frozen nuts. I cut off the remaining bolts from the inside with a grinder and drove them out through the hull. Removing all of the old fittings took us the better part of a day, and thus began the 10 day filth and fatigue of the haul out project. Don’t get me wrong, I enjoyed 99% of the project and with every long hard day I felt that much more connected to Polynya and all of her dirty hidden secrets. Yet despite the enjoyment, I was ready for a warm shower and a soft couch at the end of every day, usually asleep moments after finishing dinner.

With all of the bronze fittings removed I dove into the old backing blocks that were beginning to rot. I tried a variety of tools for chiseling, cutting, and prying the old blocks out. I found that the easiest method was also the dirtiest: grinding with a course flap disk. I set to work slowly grinding the blocks into a fine dust that distributed itself into every nook and cranny of the boat. Each of the nine blocks took 30-60 minutes, some easier to access than others. With the wood blocks removed I sanded the exposed hull to remove all adhesives and paint. In order for the new blocks to adhere to the hull, the surface would have to be free of debris.

Fairing the hull underneath the old backing blocks
Fairing the hull underneath the old backing blocks

Before installing the new backing blocks, we needed to fill the old holes in the hull from the through bolted seacocks. There were 18 holes in total, 2 per seacock. After cleaning up the holes inside and out, and clearing out old adhesive with a drill bit, I applied tape over the outboard end of the holes. This tape would serve as a barrier to hold epoxy in the holes while it cured. I made up some thickened epoxy and used a syringe to force it into each of the holes. Once the epoxy had cured, I removed the tape revealing a perfectly smooth patch, ready for bottom paint.

Tape to keep epoxy filler from running out
Tape to keep epoxy filler from running out

New backing blocks were made of solid 1” oak board. I chose a circular shape, large enough to accept the bolt pattern of the new flanged seacocks. Once the 9 donut shaped blocks were cut and sanded, I coated them heavily with epoxy in two coats. The oak will already be resistant to water damage, but the epoxy will greatly reduce the chance of permeability. Wanting a permanent installation I opted to epoxy the backing blocks into place, making them integral to the hull. Even after sanding, the hull had slight irregularities and was not perfectly flat. To overcome this I mixed a super thick epoxy by adding a structural filler until I had a peanut butter consistency. I spread this onto the backing blocks like cream cheese onto a bagel, nice and thick, and then set them into place. Miniature bar clamps through the center of the blocks held them to the hull while I created a fillet around the edge, finishing the seal between the block and the hull. Once the blocks had cured into place I painted a final coat of epoxy over the entire block to encapsulate it as part of the hull.

New backing blocks getting coat of epoxy
New backing blocks getting coat of epoxy
New backing blocks being clamped into place
New backing blocks being clamped into place

A few days later, when the blocks had plenty of time to cure, and Skye was available to help me, we began installing the new thru hulls and seacocks. During the final install of the thru hulls it is important that you do not spin the fitting as it will ruin the seal between the mushroom head and the hull. Rather, you must hold the thru hull still while threading the seacock down onto it. As such, it is important that you do a dry fit to ensure that the handle of the seacock is facing a suitable direction in its final position. To do this we screwed the seacock onto the thru hull and then allowed the thru hull to rotate with it as we oriented it into the desired position. Using a sharpie pen, we indexed the thru hull with a mark on the hull so that we would know how to orient the thru hull upon final installation. A few of the thru hulls also had to be cut to length so as not to bottom out in the seacock. Forespar requires a minimum of five threads penetration into the seacock, but if you leave too many, the seacock flange will not properly seat to the backing block. Once we were happy with the dry fit of each thru hull, we took a few moments to develop our game plan for the final installation.

For a great overview of the process, check out this video from Forespar.

Skye worked inside the boat and I stayed outside. I would thoroughly cover the thru hull with 5200, trying to lean towards overkill whenever in doubt, and insert it up into the hull using the indexing mark for proper alignment. Skye would then thread on the seacock while I held the thru hull in place with the stepped wrench to prevent it from spinning. As recommended by Forespar, we made them firmly hand tight, which, thanks to our dry fitting, left them in the desired orientation.

New seacocks fully installed
New seacocks fully installed

The final step was to bolt the seacocks into place using the three mounting holes on the flanged base. We used large stainless lag screws, sized to maximize penetration in the backing block without touching the fiberglass hull. Each screw was coated in 4200 before final installation to further protect the wood from moisture.

Skye tightening the lag screws
Skye tightening the lag screws

As launch approached a few days later I was buzzing with anticipation and anxious to see Polynya floating again. As she was lowered into the water I hopped aboard to check all of our thru hulls. Unfortunately, despite our meticulous efforts, one of the thru hulls was not properly sealed. My heart sank when I found a small trickle coming from the aft head discharge. Whether it was a lack of 5200, a crack in the thru hull (which we discovered after removing), or a poorly seated seacock we are not sure. Nonetheless, the yard crew lifted us back out of the water for the night so that we could try again. Skye and I sourced a new thru hull, probably the last 1.5” Marelon fitting in southern Maine, and began reinstalling. We added additional 5200 to be safe, lightly sanded the backing block to ensure a smooth contact against the seacock flange, and crossed our fingers. We launched again at 0700 the following morning, and were delighted to find nine dry fittings. As stressful as those 24 hours were, they were an excellent lesson in humility, patience, and perseverance. We made our way down the Royal River and back across the bay to Portland Harbor.

Over the next few days we monitored the seacocks for any signs of leaks and were frustrated to find two of them producing a small amount of moisture. We could not determine where it was coming from until I sat with my hand below the seacock for a few moments, feeling for a source. What I found was that the handle, when turned in a certain direction produced a drip occasionally. I was relieved on one hand that it was not coming from around the backing block, but frustrated that this brand new seacocks was producing any water. It is only occurring on two of the nine seacocks and after some research online it seems there may be a hidden adjustment to tighten the valve stem. As of yet I have not had the opportunity to get on the phone with Forespar and discuss a solution, but I certainly intend to. For now, there is no immediate issue as the amount of moisture created is much less than a typical stuffing box would produce. I would like, however, to find a solution and will be sure to update the blog when I learn more.