Mast Beam Project

Mast beam at time of purchase.

Mast beam at time of purchase.

When I purchased the boat in 2007, I knew that one day I’d need to deal with the mast beam. I could see the fiberglass tabbing around the V-birth door trim was cracked, and the trim above the door had sunk down 1/8th inch or more.

In the spring of 2012 when stepping the mast, and gingerly tightening the shrouds the mast beam collapsed slightly, bringing  the trim around it down another 1/8th inch. It was time to deal with this.


Mast beam at time of purchase.


The first step was to remove the old beam.  I removed the mast step and the long bolts that go through the mast step, cabin top and beam. After removing the many bolts that go through the beam and bulkhead, I used an oscillating multi-tool to carefully cut any fiberglass tabbing and wood support braces to free  the beam. This worked great.  The multi-tool is like a surgical instrument and makes clean accurate cuts with little mess or fuss. I had the beam out in less than an hour.



I decided that I’d laminate a new beam using epoxy and white oak. I went with white oak for  strength and rot resistance. Price is reasonable too.  I also decided that I’d build the new beam oversize in height to give it a bit more strength. The old beam measured 4 inches in height, my new beam would be 5 inches.  I knew there would be some fiddling with trim, and the v-berth door would overlap the beam, and I’d need to duck a bit more when passing under, but all these details seemed like a small price to pay for the added strength.  After the old beam was out, it was obvious that a number of plies has de-laminated.

I went wood shopping and bought a couple of 5/4 white oak slabs.   An afternoon at the table saw and planer turned the slabs into a stack of 3/8 thick, 4 inch wide by 5 feet long boards.  I calculated I’d need 13 plies (boards) to give me my 5 inches in height.

Next came the mold.  Using the old beam I traced the curve on my workbench as a start.  When laminating, you need to calculate springback, which is based on the number of plies v. distance of the curve.  “The Gougeon Brothers on Boat Construction” as a section on this.  With 13 plies and a 4 inch curve I’d have very little spring back so I added only about a millimeter to the ends of the curve.

I used some scraps of maple to screw stops to my bench that I could pull the plies to using clamps.


Testing to see how well I could pull the plies to the mold. Kind of a dry run w/o the epoxy.

Epoxy time.  White oak has a reputation of being difficult to glue so I took no chances.  Each ply was well sanded to 60 grit. I also stacked the plies in my car in the sun to warm them up before applying any epoxy.  Epoxy flows out nice on warm wood, and when the wood cools (moving it into the barn) it draws the epoxy into the pores of the wood.

Working quickly, I pulled a few pieces from the car and brushes some neat (no filler) epoxy to both sides then stacked them in the cool shade.  I use neat epoxy first and let it cure to the point that it feels like the sticky side of scotch tape; by doing this, you lessen the chances for squeeze out. Unlike many wood glues, epoxy does not like high clamping pressure, but unfortunately there’s no way to avoid it here.


Oak boards with straight epoxy, waiting to cure to a tacky state.

Once the boards were tacky to the touch, I mixed up a batch of epoxy mixed with colloidal silica, brushed it on each board and then carefully pulled the boards into shape starting from the center and working my way to the ends.  To keep the boards flat against the bench I clamped two short sections of 2×4 over the top.   I was pleased that the whole mess pulled into position easily.


Note the poly sheeting to keep the whole mess from sticking to the bench and the mold.

After 48 hours, I removed the clamps and knocked off the larger pools of epoxy with a chisel and sent the beam through the planner clean up the faces, and sanded the top and bottom with a random orbital sander.





The original beam was 4 inches in height, my new beam is 4 7/8 inches so I needed to cut the vertical posts on the bulkhead to allow fit.  After trimming the ends of the new beam to match the shape/width of the old beam, and giving all hidden sides a couple coats of epoxy to protect it from water,  I held the new beam to position and scribed marks on the support posts.  Using a sharp Japanese style pull saw made quick work of removing the top 7/8″ of the posts.

With the shortened posts, I was able to slide the beam into position.  We used a small jack and a 4×4 post to push up on the bottom of the mast beam, which restored the cabin top to the original curve.

With the beam in the proper position,  there were now small gaps between the support posts and the beam, so I cut back the posts again for exactly 1 centimeter of gap.   I could then cut 4 1cm thick pads out of some scrap oak to fill the gap.

I was also concerned that the fit between the beam and the cabin top would not be an exact fit.  What I did was apply a coating of wax to the cabin top and the bulkhead, mixed up some thickened epoxy, brushed it on to the top of the beam, and set it in place. With a bit of pressure from the jack,  a bit of the epoxy squirted out and was easily cleaned up.

With the beam in place, and the jack holding it tight, I drilled the 1/4″ holes for the 6 bolts,  bolting the beam to the bulkhead.  I left the jack in place overnight.



Next, I cut spacers to add to the top of each post that would serve to fill the gap. I shaped the pads a little so they wouldn’t look like I was fixing a problem. For the vertical posts, I cut 4′ long “braces” that are tapered to reinforce the posts.  I cut a shelf near the top of the post which would receive the stepped cut brace. This way, the wood would take the load and not just the glue/bolts. For now, the braces are epoxy glued only. I’ll add some mechanical fasteners in the spring.

For tabbing, I added tabbing back to the beam ends where the beam meets the cabin sides. The old beam had tabbing all across the top of the beam from the cabin roof to the face of the beam. The new beam fits flush to the top of the cabin, and I think it looks cleaner without the tabbing so I’m leaving it as-is for now. Structurally, I don’t see the need for the tabbing. I’ve primed the new fiberglass, leaving finish paint until the warmer weather of the spring.


It’s getting colder here in NH, and the remainder of this will wait until spring. I still need to add the fasteners to the braces, finish paint my tabbing, and varnish the new wood.