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The ABC System  ….  Align, Bond & Cove.                           

This system was developed by the author in 2008, to enable a faster & lighter assembly for the new W17 trimaran and avoid most of the negatives of the Stitch & Glue method,  ie: the necessity of using ties or wires, together with the making of holes that later require to be filled if the wires or ties are pulled, which is preferred for a neater layup of the corner reinforcing tape.   

Before sharing this system,  the author chose to let 10 years go by to see how the system stood up over that period and to give opportunity for builders using the system to comment, prior to making this more generally available for builders of other hard chine craft.

No structural failures and only positive comments have been reported during this time.

The conclusion thus far is that the system works well for its intended use …. for hard chine designs where the bottom and sides come together somewhere close to a right angle …. say up to +/-25 degrees from 90.

For the W17, the side panels of plywood are first laid around a few bulkheads and clamped to the stem and transom.    To temporarily hold up a pre-scarphed full length side panel, a small wood peg is passed through the approximate center of its weight, into a mid height stringer.   This takes the weight of the panel so that is does not sag and deform, as the plywood may be as thin as 4 mm.    Once fitted, the panel can be slipped off the pin to apply epoxy to the mating surfaces and then be re-mounted back on the same pin with 100% assurance it’s in the same correct location.

The ends of the side panel may need to be raised or lowered fractionally to make sure the ply lays flat against the bulkheads.   If the longitudinal space between the bulkheads exceeds a certain dimension (related to the ply thickness – see here**), then a temporary bulkhead is added to assure the correct hull shape is achieved.

** I recommend the longl. spacing of bulkheads not be greater than 220 times the ply thickness, (or 330 times if the plywood is pre-sheathed one side with fiberglass).   On a W17, the x330 is applicable when using presheathed 4mm, or x220 if using 6mm unsheathed.  Either way, the max recommended bhd. spacing becomes 1320mm, so a temporary bulkhead is fitted in the cockpit area of a W17 as it’s over 2 meters long.

With such relatively high spacing, the vertical spacing of stringers becomes important and I recommend not exceeding about 30% of the above bulkhead spacing.   Of course, how close one goes to the recommended minimum spacings will depend on the final design duty of the boat, such as 'ocean-going' or not, and other such considerations.

  
Even with the above limit of spacing, you can still expect an unsupported free edge of plywood to be wavy when laid against the bulkheads, prior to its attachment to the bottom panel.     So the first step of the ABC System is to ALIGN.   This simply involves springing a batten around the top edge of the plywood about 50mm down from the top edge so as not to interfere with fitting the bottom panel later.     For the W17, this batten is about 30mm x 10mm, with the 10mm face against the plywood.  Make sure the
batten is a straight one before installing it.

Holding this temporarily with clamps, a few short tacks of hot glue are added between the batten and the plywood, with most tacks on the underside of the batten.   

With the plywood edge nicely stiffened, the upper edge of the side (boat being built upside down of course) can now be planed off to match the height of the bulkheads and to also be in a fair line from bow to stern.  Once the plywood edge is a fair curve, the plywood is then given a BEVEL to the outside and this also helps to make sure that the plane does not hit and chip the bulkheads, as it might if the plane were held flat.     The result, is that only the inside edge of the side plywood is at the right height and in a fair line.  It’s the only thing important at this point.   Beveling plywood joints like this is similar to the welding preparation of thick metal plates and assures that the joint will be connected in the strongest way, as filled epoxy will fill that gap like a weld and is stronger than the wood.  If the plywood were left flat, there’s a real possibility that the epoxy would be squeezed or pushed out, leaving the joint dry and weak.  (This is also explained in my boat manuals of course).

The bottom panel is then laid on over the top and roughly marked to match the outside of the side ply.   Then rough cut about 3mm outside this line.

The next step is to BOND.    The upper beveled edge of the side (and any permanent bulkheads) is lightly pasted with epoxy resin filled with a dense filler powder like WEST System #404.  At this stage only a very small amount of epoxy paste is needed  … just enough to securely hold the joint together at the central point of contact.   The bottom panel is then held down to the side panel edge with strips of strong duct tape - but not forced in any way.  Just a full but light contact.   If you pull down too hard, you risk to pull the side panel in towards the centerline and lose the fairness of the side panel.    The epoxy paste will fill any small gap.    A small ‘clean-up blade’ can be created from a thin hardwood ‘tongue-depressor’, sniped off at about 45 degrees.   This ‘tool’ will allow you clean away any excess filler and at the same time, make sure that the bevel is filled.  

Once cured, the fairing batten is cut free from the hot glue with a sharp chisel and the outer edge of the bottom panel cleaned off flush with the side panel.      Finally the corner is rounded to a radius not exceeding that of the thickness of the bottom plywood .. 6mm in this case.      After filling any voids, the corner is covered with a 50mm wide tape of biaxial or bi-directional fiberglass.

Once the tape is cured, the edges of the tape are faired into the plywood surface, before the whole hull is covered with a boat cloth  … at least up to 50mm above the waterline.

When the hull is turned over, the inside corner is filled with epoxy filler and a COVING created with a plastic or thin wood scraper, with a 40mm diameter tip*.     Once cured and sanded, a 75mm 12 oz tape is laid inside to complete the chine join, making a very clean interior with no ugly framing.    For a stronger joint, overlapping two tapes at the throat of the coving is recommended.

* Useful to make tips of various radius .... ranging from a minimum of 3mm radius right up to 35mm radius for either very high strength joints, or those that are more open.

Bulkheads have the same coving but generally only need to be glass taped if they are serving double-duty, say for the attachment of main beams or some folding system.

For the W17, the amas are built using the same ABC System and here are some pics showing the stiffening-fairing batten at the plywood edge, held in place with hot glue. 

In both cases, the hulls turned out very fair with minimal interior framing and show a worthwhile saving in time, material, clutter & weight.

To stiffen the shell, I try to avoid wood frames** unless required for some other structural reason, preferring to stiffen the plywood internally with what I call ‘Tension Strip Framing’ (or TSF System), using vertical or near vertical UNI tapes as shown here in this sketch.   

** Wood frames, while occasionally necessary, not only clutter the interior making it harder to maintain, but also add hard spots.  Using this TSF system permits a little more flex on impact and where appropriate, also saves  work, expense and weight.

Numerous hulls have since been built using this system and I’ve received no negatives to date.

It’s interesting to compare the interior structure chosen by designers of other somewhat similar hulls.  Some still use the rugged ‘old-style’ structure with wood chines and internal framing but from my perhaps biased viewpoint, I find this adds unnecessary internal clutter, weight and added work, protruding into cockpit areas affecting sitting comfort, as well as making future maintenance around the extra parts more difficult.  

Compare such cockpits with internal gunwale rails, exposed framing and sometimes bulky folding-system boxes with this one, having the gunwale external and no frames or other obstructions in the cockpit, and you will better appreciate the advantages.  'Perhaps' the older system will prove more durable, but with more difficult maintenance, I really doubt it.    With exposed interior gunwales and framing, stretching out in the cockpit would certainly not be so comfortable as it is here on the W17, with a variety of positions that allow one to sail for hours without getting stiff.

Some designs, (the Strike being just one), still use 'the tried and true' interior framing system (see Construction Method #1) but I now consider this method, heavy, space consuming and hard-to-maintain compared to cleaner, modern ones.   By comparison, for the Seaclipper amas the designer choose to add numerous vertical frames to bridge the paneling from the gunwale to the chines, but still uses wood chines at the knuckle with the bottom.   The bulkheads are also shown wood blocked to the side panels, adding weight, more pieces and more corners to collect and hold moisture.

While neither approach is the way I'd personally choose to frame such boats for minimum work and weight, one cannot argue with the strength and the Seaclipper approach is certainly original.

But many years ago now, the Gougeon Brothers tested and proved that plywood could be joined together with only fillets of epoxy compound and if of adequate size, be as strong as the plywood itself.

The ABC construction system uses that knowledge and experience to advantage.

The ama does have to be built strong enough to be fully submerged, which could mean the equivalent of about 70 lbs loaded on to the sides for each foot of length.

For that reason, bulkhead spacing needs to be closer than for the main hull and the W17 uses a mid-depth longitudinal stringer of mahogany with more plywood bulkheads to achieve the required strength, but still with less work and lower maintenance than with older methods using wood chines and bulkhead blocking.

So for hulls of similar format, perhaps consider the 'Simple as ABC System' next time ;-)     

                                                                                                                                                   Mike 2018

Your comments are welcome and can only help to make better boats in the future, either with plywood or other competitive panel materials that occasionally come on the market.


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