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Sheathing Part 3 — Sheathing the W17's Outer Beam

It could be that sheathing the curved outer beams is a job that you look at and wonder just how best to approach it. If so, here are some suggestions that should melt away your concerns.

(This approach first assumes that you have pre-sheathed the side webs internally before assembly and then added a 6oz UNI to the inside of the top and underside.)

As the substantial wood structure can well take the compression, only a basic sheathing will be required on the exterior topside, with a 45/45 bias cloth on the vertical sides and a UNI for the underside.

Outer Beam

First, radius the side corners and sand all surfaces with a sanding board. A flat one is fine for most of the surface, but a crescent shaped one with a radius of 11.5" (292 mm) will work better for the underside. I cut an old 24 x 3 sanding belt into two (down the middle) and then made the crescent block 1.5" wide, of a length to snugly fill the sanding belt.

Before doing any glass work, vacuum all the surfaces off and wipe over with a damp rag. Also vacuum up any sawdust around, as you don't want anything sticking to the glass while you're cutting or pre-fitting it.
The best approach for sheathing this complex shape, is to cover the top and sides with one cloth—but laid at 45 degrees to the beam and you'll then be surprised how easily this can be laid to the curved shape. Leave enough hanging down the sides, to later tuck under 1 to 1.5".

Outer Beam

Here's how it will look with a regular 6oz/200g boatcloth, laid at 45 degrees to the beam. You will note that if you pull this cloth lengthwise, its width will rapidly reduce and this is a useful thing to realize, to take up any slack later one.
But initially, you want to avoid pulling it at all.


Outer Beam
Outer Beam

Once in position, mark the cloth both ends with the center of the beam. Then fold up first one side over the top and then the other side, so that you have a loose sausage. Then fold it lengthwise and gently put it aside until you've coated the sides and top with epoxy, including the lower radiused corner also. Because you'll need to turn the piece, I find it better to apply the resin before the cloth in this case, as then the cloth stays in place better.

Now lay the cloth 'sausage' on the top of the beam, with the marks roughly at the center of each beam end. Try to avoid stretching the sausage lengthwise. Immediately, drop the sides down and make sure you have enough to tuck under at least 25mm (1"). If not, just lift the cloth and gently stretch it sideways. Assuming you always work with thin plastic gloves as I do, initially use your hands to smooth the top and once that's wrinkle free, work your hands over the top radius and straight down the sides. This action will add tension along the length and you'll find it will soon fit like a glove.

Outer Beam
Outer Beam

Then, add a little more resin with a brush until all the white areas on the top and near side are transparent. Cloths like the standard boatcloth, are one of the easiest cloths to work with as they have small holes where the fibers cross and this vents out any trapped air. As you'll find out soon enough, a more tightly woven cloth that you cannot see light through, is a much more difficult beast to tame.
Now lay a couple of pieces of wax paper just the width of the beam, on the top surface and smooth this over with a plastic spreader (commonly called a squeegee). Make sure this spreader has a clean straight edge—pass a plane along it to be sure there's no old resin on it that will scratch up the cloth.
Now, holding the ends, turn the beam over 180 degrees and immediately fold the extra cloth over on to the beam underside (that is now uppermost). Add extra resin here to keep it in place and work with a small (3") squeegee to keep it flat and free of air.
It's now important to trim off the excess cloth at both ends, as its weight will tend to pull the cloth away from the edge and you need to avoid that.
WARNING: Never attempt to pull cloth over a square corner—it just does not work! It will lift the cloth away just before the corner and when you later sand, you'll cut right through the cloth. So you either round the corner BEFORE you lay the cloth or you cut it tight to the corner, so that the cloth stays flat to the surface.
To get the cloth to fit snugly around the outer tip of the beam, use the flat of your gloved hand or squeegee to ease the cloth upwards towards the edge. This will automatically make use of the bias weave to tighten its grip around the end.

Outer Beam

After checking that all surfaces are transparent and free of air bubbles, make a final pass with the squeegee to remove any excess resin and then leave to cure. The underside will be done later.

Once cured, peel off the wax paper and pass a grinder lightly over the beam underside, so that only about 1" of the cloth is left on each side edge and there's no unbonded white cloth left. Then sand the surface and again use the crescent sander to keep things as fair as possible. Vacuum all the dust up and wipe-over in readiness for sheathing.

Outer Beam

Now cut a strip of UNI cloth (6–9oz is fine) that's slightly wider than the beam. Before rolling this up to apply resin, pull away any partial strands of the UNI fibers, until you get a continuous full length fiber. This will give you a straight edge to work with when laying it finally in position on the wet resin.

Then roll aside the cloth to one end and apply resin. You will quickly find that most UNI cloth is much tighter woven than boatcloth and is therefore harder to wet out. So applying resin BEFORE the cloth is really needed in this case. Using the flat of your gloved hands, pull the cloth straight and lay it into the resin. Then apply another light coat on the outside and leave it for a few minutes to soak through.

Outer Beam

It's very possible that you'll continue to see small grey spots on this UNI and that's an indication of an air bubble that cannot escape. I find that dabbing that spot with the rounded end of the brush handle will often push the air out, as well as repeated passes with the squeegee. Do not pull the cloth too tight at the curved part, or you risk to lift it from the surface.
There is no need (and not recommended either) to try and wrap the cloth around the radius to the sides. They are already sheathed with the preferred bias cloth after all.

Outer Beam

Once all is cured, cut off the excess and carefully sand. Another warning here—be VERY careful to wear gloves (preferably leather ones) when sanding cured glass like this. It's all too easy to drive those stiff stray fibres into your flesh and they are not so easy to get out!

While some may wish to add another epoxy coat immediately, I recommend to wait until the beams are all assembled and then start the final fairing process at that time. It's likely to be the lighter solution.

Once these beams are done you're well on your way, as they are probably the most complex apart of this boat.

But remember while doing ANY boatbuilding, to have fun. You'll be surprised when it's all over, how fondly you'll look back on your work and even miss doing it! So enjoy it while it's still with you, even if it seems long sometimes.

See also: Sheathing Flat Panels & Sheathing Narrow Hulls

Sheathing Tip:
Try to use the natural heat of the day (or raise interior heat), so that you start to apply resin for sheathing at the peak of the heat. It flows better on a warm surface.
But immediately you start, turn down the heat and move away from sunlight or lamps that are too warm.
Ideally, you want the temperature to be falling from the moment it's applied, so that there's contraction and not expansion of any air in the application. This will avoid blisters and make a neater job. For example, I often work on a raised bench, but after sheathing a unit, transfer it to the floor where it's cooler.


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