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W17 Main Hull: Fairings, Beam Alignment & Trampolines

Additional thoughts and suggestions…

Tips on attaching the forward fairing

Fairing attachment

Once the pieces have been cut out and sheathed internally, they should be lightly stitched at their forward edge with copper wire or plastic ties.

The curved beam should then be prepared by bonding a small 6–8mm dowel just inside the upper and lower edges, spaced so that the 3mm ply will come flush with the upper and lower surface of the beam, when laid at the roughly 60° angle. Tape this dowel in place until the bonding has cured. This is to support the final fit of the fairing.

Then set up some short vertical strips of wood, like a barricade around the outer arm. This will prevent the fairing from slipping past the edge.

Fairing attachment

Now lay in some heavy epoxy 'bog' inside the joint of the fairing, over the wire or ties. Smooth this off with a 1" dia 'tongue depressor' and then place the fairing temporarily on the beam, using duct tape to make it fit well with the corners of the outer beam. Once the epoxy inside the front Vee has cured, you can remove it and finish the interior with a 50mm FG tape over the filler.

The fairing can then be attached permanently to the beam, provided there is a little spare material at the outer end, say 5–10 mm. Otherwise it will be preferable to wait until the outer arm is fitted to the ama deck, so that the fairing can be fitted to the ama deck pad at the same time, using adequate epoxy bog to fill all the crevices and give good contact. Once cured, fit the inner end partition. (see photo on The Waters Edge — Jan 2013)


How to line up and assemble the ama to the main beams

Hinge alignment

Although not absolutely essential, I think it's easier if the cockpit seats are installed first, with some temporary beams across the deck to hold them up. Of course, these ply sheets should first be sheathed on their underside and have their stiffeners added. (see manual)

Make sure you have a straight centerline marked on the main hull deck—at the bow, fore & aft cockpit bulkheads, and transom.

When ready for the beams, the main hull should be leveled side-to-side (athwartships). With the forward central main beam laid across the deck and its center on the centerline, position the after face over the aft edge of the deck plywood, both about 10 mm into the cockpit and then measure from the stem centerline back to the beam ends, and adjust until perfectly equal. The beam should now be 90° to the hull centerline. Mark the location at the hull side and move to the rear one. Position this parallel to the forward one, with its forward face 10 mm forward of the aft bulkhead, again over the fwd edge of the aft deck plywood.

Hinge alignment

Now lay a deep straight-edge over the tops of the beams—fore & aft. Measure how thick a small batten needs to be, to fit under the beam at the cockpit edge, in order for the beam tops to be perfectly in line with the straight edge. Typically 2–3 mm under the cockpit side of each beam. Leveling the beam tops like this is absolutely essential for the hinge system to work properly—see photo above. Bond those battens to the deck and then check the alignment again. Once all is in line, 90° to the centerline, equal length port & stb'd, with beam-top surfaces in the same plane; then drill the holes through the beams and main hull deck as per plans. Then mix up a good amount of lightweight filler and epoxy and set the beams in this, using the bolts to snug down the beams. Clean off the excess epoxy.

Then hang on the outer beams. The hinges and latches should already be installed, so that the outer beam tops all rise 21–22 mm above the main beam. The amas can then be brought close under the aka beam ends with the ama decks (sheer line) laying parallel to the main hull, but slightly lower. The bow of each ama should also be 35 mm closer to the centerline than the centerline at the stern—this is for the deck toe-in (see the manual for more explanation on this). With each ama inclined-in 10°, the ama stern should be positioned 280 mm aft of the after beam, with the bows almost in line with the main hull bow, an equal distance (P&S) below the main hull stem. The 10° ama deck inclination is measured with a spirit level and a 52mm wood block, as per plans. Then mark the ends of the outer aka beams to see where to grind off the surplus—typically, 2–3 mm off 1 or 2 corners, so that it mates level on the 8mm ama deck pads. Once you have a good fit and the ama deck is lying parallel to the main hull deck with all the other positioning (noted above) checked, you can drill down through the beam ends with a long 8mm drill and prepare to bond and bolt the akas to the amas with the 8mm pads in place. Again, see the manual for more details.


Ideas for attaching the trampolines

Trampoline attachment

There are almost as many ways to attach trampolines as there are trimarans, but not all work well. A loose net trampoline is certainly comfortable for the feet when sitting on the ama but is hard to walk on, it drags in the water and looks most untidy. At the other extreme, a raised, tight, hard, flat surface is easy to walk on but far less comfortable to sit near and the point attachments are highly loaded so tend to fail more readily.

So for the W17, I prefer something between these two. Black polypropylene mesh is cheap and strong and although it drains a little slower than an open net, it generally drains fast enough. The manual gives some dimensions but the first thing to decide is how and where to attach it.

Trampoline attachment

For the internal edge by the cockpit boxes, I suggest using a bolted-on aluminum extrusion for a bolt rope, formed by sewing the polypropylene mesh tightly around a dense 6–8mm rope. Another solution that is just as effective, but less easily removable, is to slip the bolt rope under a 19 mm × 3 mm alloy flat bar that is bolted to the lower corner of the cockpit box with 5mm (316") bolts every 100 mm (see sketch).

For the outer edge, I suggest creating a large (100+mm deep) pocket and then slipping in a fiberglass tube of about 16mm diameter. If the mesh is then cut away every 400 mm or so and the cut edge sealed with a hot knife, then there will be a place to pass a nylon strap around the tube and this can also pass around a D‑ring that is attached to the deck of the ama; either bolt down through the deck into the inside gunwale or down into the centerline deck stringer.

Trampoline attachment

The ends can either use similar pockets and tubes, or simply have eyelets installed through areas that have been tripled in mesh thickness, and then lash these points to eye straps securely screwed to the curved akas, into oversize holes that have been epoxy filled. Ideally, such holes should be pre-planned, so that these epoxy plugs can be poured downhand into the curved akas, before they are attached to the ama, as only then can gravity help.

More information on these 3 items can be found in the updated 2013 Build Manual — see January 2013 note on 'The Waters Edge'.


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