Here's a few photos and some text of the construction of Rambling Rose's new beam.

The rot was discovered when preparing the exterior of the boat for repainting. I knew that the cockpit was less than perfect, but hadn't allowed for the fact that problems in the cockpit could have been transferred to the beams onto which it is built. This photo was taken after I'd sawn off some of the cockpit to follow the rot along. It turned out that the rot was present in a similar place on the other side, so, out with the jigsaw and off with the cockpit:

Easily said now, but pretty traumatic at the time! In fact, I only took off enough cockpit to allow us to remove the beam. I expect I'll build a new one, but decided to leave my options open for the time being. With the front of the cockpit sawn off, it was an easy job to get the old beam out:

I was pretty nervous about building a beam on my own. It's a heavy thing to lug about, even with two people. I also worried about making it accurately enough to fit the existing mountings. Luckily Christiano turned up from Curacao on Tayo at just the right time, and agreed to help. Well more correctly, given our relative abilities, he did it and I helped him

Actually laminating the beam was the easy and quick bit; trimming the wood and gluing it took one day, the rest of the process took another four.
We took a bit of a chance with trimming the wood, and sawed the individual pieces to the correct sizes to make the beam exactly. With no surplus to remove to correct mistakes later we had to be accurate with the construction, but it meant we didn't have to remove a lot of wood with a hand plane. It also meant that we were able to saw off one or two of the knots that we didn't like. We spent a lot of time working out how to fit the planks together to minimise the problems caused by the remaining knots and the irregularities in the planks. Once this had been done we numbered all the joint faces and scored them with Stanley knives to help them to be wetted by the epoxy.
We were lent space indoors to do the work, and did the gluing on two large benches and a drill stand. As we glued each new plank in place the joint faces had one thin coat of pure epoxy rolled on to wet the surface of the wood, then they were glued with a fairly wet epoxy/microfibres mix. Each plank was nailed with galvanised nails to the plank below.
These photos were taken after we'd completed the gluing. The line visible above the beam was used to keep track of whether the beam was straight. There was a slight edge set in some of the planks, so we glued them with the sets opposing each other to give a straight result. It worked, too.
More difficult to deal with was a twist in many of the planks. When we had a couple of planks joined we put some G cramps onto the beam and used a Spanish windlass on their ends to twist the beam straight. As we added more of the twisted planks we had to wind the windlass tighter to counteract the increased twist. The windlass was left on overnight while the epoxy cured and when it was removed next morning we had a beam with no more than a couple of millimeters of twist in it.

The beam was cut to length, then planed and sanded. We took it down to the boat to check the fit, and to mark it for fastenings. At the same time we threw the old beam onto the boatyard's bonfire pile.
Back in the shed, we set it on trestles and sheathed it with surface tissue. This was done in two stages: first the top, one side and the bottom were covered with a single length of tissue. When the epoxy had cured the beam was turned over, the glass edges trimmed and the top, other side and bottom were given the same treatment. This put two layers of glass on the top and bottom, which we expect to need the most protection. It's not obvious from the picture, but there are blocks between the beam and trestles to allow room for the excess tissue to hang down free.

The holes for the screws to attach the beam hangings were drilled and counterbored.

The beam ends were sheathed when we had a quiet moment:

Then we wrapped tissue round the bolt and wet it out thoroughly:

To prevent water entry through the pivot bolt we laminated an epoxy glass tube to take the bolt. We made this on the bolts themselves. First we covered the bolts fairly loosely with several layers of kitchen wrap:

The pivot bolt holes were drilled out oversize and the bottoms covered with gaffer (duck) tape. The tubes were stood on end in the holes, the gaffer tape sealing the bottom ends and holding them in place. The tops were supported by strips of masking tape.
Then, when we were sure that all was properly lined up it was time to pour filler mix into the hole around the tube. We made the mix as thick as we thought could be poured without trapping air. Also, to minimise the risk of trapping air, we poured the mix in slowly from one side only.
Next morning, we trimmed the tubes and sanded the area.

The galvanising on the threads of the screws was so rough that it prevented the screws from holding properly, so we had to find a way of cleaning up the threads. After a lot of thought the best we could come up with was the arrangement below; not very high tech, but it worked:
We clamped a piece of bicycle chain in a vice between a couple of blocks of wood. The screw was screwed in and out once or twice between the piece of chain and one of the wood blocks.