Sailboat Compression Post Repair and Replacement

Pre-Surgery Planning

Before you start tackling a project like this for yourself, please read this all the way through. It isn't because I want you to do what I did, or because I somehow want to show off my techniques, or anything like that--it's because I did it wrong, and I don't want you to make the same mistakes.

I had three clues as to what kind of repair I would have to do. The rust color in the crack at the foot told me that somewhere in there was rusting steel and water intrusion somewhere underneath the mast. The port side of the deck was visibly higher than the starboard side, which could have been because of improper block replacement (all the halyard blocks were attached to the deck, not the mast) or something more nefarious. Finally, the bolt on the head door to lock it closed against the compression post didn't work. It was out of alignment which meant, to the greatest despair a boat owner can know, the compression post was bent. And you could see it, too.

No one knew what was wrong. I consulted multiple friends in the industry, and all of them said the same two things: it needed work, and they were too busy. So I decided to tackle the project myself (in my hard-headed attitude that typically gets me in trouble and drains my wallet). I had the mast craned off at a boatyard, I had them replace the standing rigging while I motored back to my marina to get my hands dirty.

Very, very dirty.

Recommended Tools and Gear

Removing the Tumor

After the mast was removed, the head door bolt fit back into place. Clearly the post was under too much pressure with the weight of the mast on top. Using a chisel, I scraped away the ancient sealant used to cover the mast foot and discovered four inconspicuous screws. Two of them sheared in half, and the third one had completely punched out of the shoe, so it wasn't even attached at all. And the fourth screw was wedged in so tight (not surprising, given that the bulk of the entire load was directed through that one screw), I had to use the dremel to drill new lines for the screwdriver.

With the mast shoe removed, the cracked gelcoat underneath finally saw the light of day for the first time in probably 40 years. Using the dremel and the wood chisel, I chipped away the damaged gelcoat. Even at this point, I knew all of it would have to be replaced, so it didn't hurt too much to rip the skin off my boat. And a frightening discovery awaited me underneath it all.

The steel compression plate had corroded so much that it flaked away with just a light scraping. It wasn't secured in any way (maybe it was at some point), so it just kind of fell off out once the gelcoat was gone. It was like
finding a tumor in your child, and probably almost just as satisfying to remove it. But that was only part of it.

The deck was warped, and the gelcoat was still cracked. Even though I removed part of the problem, the deck would've rotted. A few pilot holes confirmed my fears--the core was wet. So, in the interest of cosmetics and new deck blocks, I blocked out a square chunk of deck to replace.

I used a circular saw (carefully measured so as to only cut through the upper fiberglass and the core, and not the lower layer of fiberglass (that served as my cabin ceiling, after all)) to make the big cuts, and then a hammer and chisel to remove the rest. And a grinder. And a dremel. And a router for the edges, to make it a little finer than the circular saw would allow.

So now, at this point, I had removed the mast shoe (still good, though it needed a little touching up), the rusted compression plate (not good), and cut out a nice big square of deck, down to the lower layer of fiberglass (definitely not good). It was a very disheartening sight.

Engineering a Solution

The compression post had to go. It was bent, and would only exacerbate the problem if used any more. So I pulled it straight out from the top (removing four screws that secured it to the bulkhead first), and when I laid it out against a straight plank on my deck, it was clearly bent. Well, I certainly didn't want to use wood again.

Using the advice of one of my boat neighbors who saw me working, I visited the Millerick Bros. Marine Construction a little hike up north. I worked with them, and we engineered a suitable replacement post made of marine grade aluminum--much better than a pliable wood thing, if you ask me. The bottom of the post had a small plate welded on to distribute the pressure over a wooden foot on the keel; instead of the post pressing directly on the ceiling (and the pressure plate above), the pressure plate would be welded directly on top of the post, and would distribute the pressure from below. This whole post would then through-bolted on the deck, through an accessory plate (I used this one from Garhauer Marine), through a piece of PVC plastic, and then finally through the mast shoe itself. The whole thing would be one, rigid, immovable unit. (It should be mentioned that the plastic was there to separate dissimilar metals, since the Garhauer plate was stainless steel, and the mast shoe was aluminum--dissimilar metals should never touch in a saltwater environment due to electrolysis).

They gave me all the pieces to the new post before the welded them, and insisted I do a "mock up" to make sure it's all good. And it was very good that they recommended that, because I made a curious discovery.

The center of the post was offset from the center of the mast to starboard by 5/16 of an inch. I don't know what sort of naval architecture principle that adheres to, but it seemed to explain why the old post bulged out to the right. Well, I figured the new post should be the same as the old one, since maybe there was a specific reason for the offset (maybe in the keel?), but more importantly, if I were to make the center of the post 5/16" to port, then I would've had to remove part of the bulkhead, cabin sole, and the holding tank. I was already hurting on time (remember, my rig was getting worked on and I was paying for storage every day at the boatyard, but on top of that, I had a looming deployment coming up). So I quite wrongly chose to keep it the same as before.

The New Compression Post

They welded that thing together, and goddamn, I was a happy man picking that up. The first step was to epoxy in a piece of marine grade ply to the keel, which is where the post would rest. I sanded and painted a piece, put it in, slid the new post on, saw it wasn't even, removed it all, sanded again, put it all in again, saw it was still tilted, and repeated this whole process over and over again until the piece of wood was shaped such that the post rested completely straight up and down on top of it.

But how the heck do you know when it's straight up and down? It's especially difficult on a moving, roving boat, that's for sure. But I came up with a genius way to approximate the post's position to a degree good enough for my non-industrial purposes. For roll (left and right), I couldn't use the bulkhead because that warped and wasn't straight anymore. So instead, I cut a dowel so that it just fit snugly in between the post and the non-supporting bulkhead on the other side of the companionway. 

Then I ran the dowel up and down the post, and if it was straight up and down, it would fit; if it wasn't, then it would either jam or there would be a gap. And thus, I finagled the post until the distance between the dowel and the post remained the same as I ran it up and down between the post and the opposite bulkhead.

As a way to crosscheck this, I ran a bungee from one handrail to the other on top, and made it so that the distance between the bungee and the mast shoe was the same throughout.

How about pitch (forward and back)? I used the same principle, but instead I used a compass (the kind with two points for drawing circles) and used the door to the head. The door was secured to the bulkhead for the captain's quarters, well away from the warped compression post, so I felt safe relying on that too.

As for rotation (twisting the post left or right), that was pretty easy on deck, using the compression plate and the lines on the non-skid and handrails. And for physical forward and back placement, I tied a line between the chainplates and made sure that line passed through the center of the compression post.

Once that bottom marine ply was good, I epoxied it in place. Then I placed the post on top, verified it was straight, and all was well.

Stitching the Wound

I used West System Epoxy for this project. More specifically, I used the West System 105 Resin, the West System 206 Hardener (for my environment; see this guide to determine which hardener to use), colloidal silica as my additive, and I also bought the calibrated pumps (be sure to get the appropriate size pump for your container).

To prepare the surface, I sanded it, vacuumed it, wiped it down with acetone (if you use acetone, be sure not to let it touch gelcoat), and made sure it was nice, clean, and dry for application. Then I mixed the resin, hardener, and silica (adding silica until it had a mayonnaise-like consistency), mixed it for sixty seconds, then got to work.

First, I coated the surface with the epoxy mixture, then applied a layer of fiberglass, and then applied another coating of epoxy, basically making a fiberglass sandwich. I tried a few different types of fiberglass, but I am convinced that the only kind to use is woven roving, but that might be because I was working in a unsuitable environment (lots of wind... so much wind).

Once I had a bottom layer of glass, I sanded it down, cleaned it, made it nice and smooth, then fitted in a piece of 1/2" marine plywood for my new core. This part took a while, and because of the warped deck situation (the port side was higher than starboard), I ended up using two pieces of wood. Once again, lots of sanding, cleaning, and more sanding, until everything fit snugly. In order for the top layer of new glass to bond to the old layer, I kind of ground out a "V" at the edges, so that the two layers would overlap partially. My marine maintenance bible recommends a 1 to 12 ratio for this, such that if the top layer of fiberglass was 1/8" thick, then the bevel would go out 12/8" from the edge of the new core.

One more sanding, one more cleaning session, and then I liberally applied epoxy into the wound. Then I wedged the new core in, smearing the epoxy around the cracks, making sure there were no gaps anywhere. Then I put a layer of paper on top, and a few bags of rocks on top of that (sand would work better, but was unavailable for me). This is to keep positive pressure on it while it cures, ensuring it has a tighter bond to the rest of the hull.

Finally, no more hole in my boat!

After it cured, I removed the bags, the paper, and sanded it lightly. More cleaning, a layer of epoxy, a layer of fiberglass, a layer of epoxy, sanding, cleaning, epoxy... it was tedious, but the end product was rock solid.

Interestingly, I realized a crucial point that I failed to notice before. The previous design had the plate on top of the deck, which raised the mast a little bit. My new design had the plate below deck, which did not raise the mast. So to compensate for this, I made a "bump" as illustrated in MS Paint, such that the mast shoe had a few extra layers of fiberglass underneath it so it would be raised slightly as it was before. This was crucial, because my rig, although new, would be the same dimensions as before.

Okay, things were looking great at this point. I had a new core, with a few layers of fiberglass on top, a water tight deck once more, and a straight compression post! The last part was to really make it as even as possible.

But for this step, the mast shoe, not the deck, was most important. So I went below, drilled the holes up through the plate and deck, attached the halyard organizer, plastic separator, and through-bolted the mast shoe. I used a string between the hand rails (in line with the chainplates) to gauge which part was uneven and where to grind. I also used a plank and the hatch lid as a straight edge to assist in this step, and once everything was sanded, cleaned, straight, and level, I...

That’s actually a lie. It wasn't satisfyingly flat, and it never will be. None of the steps of this project were completed to 100% satisfaction. If I had the boat in a factory, out of the water, on the hard, with state-of-the-art equipment and every tool you could imagine, then I could do it to satisfaction. But I wasn’t in a factory, in a warehouse, or even on land. I was next to the ocean with only a few days to complete my project. So I had to settle with a technique that I used when flying: the TLAR method, or the “That Looks About Right,” method. So, I made it look about right, then moved on to painting.

Cosmetic Surgery (Not Really)

Because of my time constraints, I didn't put much into the cosmetics of this project. I put on a few coats of EasyPoxy Primer, made sure it looked "nice enough," then motored over to the boat yard for the mast to go back on. It was absolutely crazy, because I wasn't even ready yet. While they got the mast ready, I worked feverishly to finish my project.

Before I drilled anything down, I used a drill bit to bevel the holes in the deck very slightly. I put a bunch of Dolphinite in the new space, so as to give a last line of defense in case water managed to get down there. Then I slathered the bottom of the halyard organizer with it, stuck it on, put the plastic separator in, and placed the shoe on top. Then I covered my new stainless steel bolts with it (so the contact between stainless steel and aluminum would be minimized), slid them through, slathered the washers and nuts, and tightened it down. Marvelous.
Next up, I put the wire pipe in (the little pipe through which all the wires go), coated the bottom of the shoe with Dolphinite, and it was ready for the mast.

They put it back on, and one of the guys recommended I put some sort of protection around the post. He said it’ll support the mast no problem, but if you’re sailing around and get knocked down, and someone falls into it, giving it a strong side load right in the middle, it might not be good. That wasn’t an issue, because I was going to wrap the old post around it. It had to go in anyway, since the head door needs it and it’s aesthetically more pleasing. So I cut up the old post so that it would fit, and made a cool little cover for the aluminum using the original wood. The headliner was super simple too--a local fabric store had the correct color of headliner (it's typically the same for cars, too), and I bought a sheet of foam for insulation to go behind it. Then I used the old headliner pieces to kind of make a template, and after I stapled the headliner back up around the edges (with the foam insulation behind it), I took a step back and surveyed the scene before me.

The project was completed.

Final Thoughts

My initial quote from my friend’s “boat guy” was $3-5000, but that was just for taking the rig off and replacing the deck core. And that was also just labor. If you added in engineering a new post, installing it, all the work that I did except charging myself an hourly rate—it would be closer to a $10,000 project. The new rig itself was a couple thousand dollars, so if I had elected to give it to someone else, I would be looking at shelling out at least $15,000. And since I bought the boat for $20,000, that would have hurt a lot.

What did it cost me, then? The Millerick Brothers were the most expensive part at $320 for everything—consultation, supplies, labor. The Dolphinite was $50, the epoxy and fiberglass came to about $150, the marine plywood was $80. Add in all the miscellaneous parts and tools, and it probably came down to about $800 for me to do this project. Basically less than a tenth of what it would have cost me otherwise.

A few lessons learned: don’t skimp on tools. You might think you can save money by buying a tool and using it for everything (read: Dremel), but you’re just wasting your time and money. Get a nice router. Get a nice grinder. Get a good toolkit with ratchets and screwdrivers and a power drill with plenty drill bits. You’ll have to pay the price at once point, whether earlier or later. And the finished product is much higher quality with good tools than with shoddy ones. And you should strive for the best job that you can do, and that’s only possible if you have good tools.

Don’t rush yourself. I was forced to rush the job towards the end because of my military obligations, and it could have been a much high quality product if I had plenty of time. On the other hand, realize that unless you are in a factory working for a company with a budget of thousands of dollars, you can only do so much. You can strive for perfection, but you can’t expect it if you’re doing a DIY project like this.

And then finally talking vs doing. I talked about this project for a year. I even had the boom off for 4 months while trying to figure out what to do. But I just couldn’t do it. I don’t know why—I just had such a hard time actually committing to the work. I said I would just hire someone to do it, but never did. I said I had to research more about the boat and the issue at hand, but that is a never-ending road. I kept putting it off, putting it off, coming up with what I thought were good reasons to delay.

During this time, I read Sailing to the Reefs by Bernard Moitessier, and he describes preparing his first boat to go cruising with two other friends. One of them was all talk and all money, but he never actually got off his ass to do anything. He had the desire, but not the tenacity to do what he said he wanted to do. So Bernard left him behind.

After reading that passage, I felt like his friend. Like I was just talking about doing it, maybe inching closer, maybe throwing some money at it. But not actually doing it. After I read that, I called the boat yard and scheduled an appointment.

I wish, however, I had figured out a way to make the post completely centered. The 5/16 of an inch offset will keep bothering me as long as I own the boat. Eventually it'll warp like before. But I like to think that if it took forty years for a piece of teak to warp, it'll take at least that long for a hunk of aluminum.

Another thing--I wish I had rewired the mast. Now that I'm looking at installing an anemometer, I'm going to have to lift the pole again, and it would've just been simple if I had done it all at once.

But you know what? The fact that I tackled this serious project myself is what makes me so damn satisfied. Through careful planning and consideration, I designed this newer, cleaner, stronger version of my boat. It wasn't all just me--I couldn’t have done it without Jerry from two boats down lending me his tools, or without the services of the Millerick Brothers.

And you know what? Every time I look at that post, at the deck, at the mast even, I think to myself, "That looks about right."


  1. I admire what you have done here. I like the part where you say you are doing this to give back but I would assume by all the comments that this is working for you as well.
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  4. This post is so awesome. Your post is full of information. Thanks for sharing.

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