Planking the hull - June 2010

On reflection, using a polytunnel as a workshop may not have been my brightest idea. The temperature inside yesterday was 43C - too hot for me and certainly too hot for working with epoxy. West System epoxy seems best around 20 - 25C - I don't use it at temps above 30C - while that sold by Fyne Boats has a longer pot life, hardens more slowly and is better kept at around 25C. The Fyne Boats epoxy is more viscous until the 50% addition of catalyst and is good for wetting out large areas, coating, etc. I tend to swap from one to the other depending on the size of the job and the temperature. By volume, the Fyne Boats product is better value for money although East Coat Fibreglass offers good prices for the West System stuff. I try to keep costs down by measuring out the amounts I need for each job by using a digital balance instead of measuring by volume using the pumps available; a lot of jobs are quite small and may only require, say, 4 or 5 gms, and pumps are useless for this. Also, avoid buying acetone (for clean ups) at chandlery prices; search out your local industrial chemist and buy it in quart bottles at a quarter the price.

The planking starts with scarphing together the full size sheets of 9mm marine ply - space and good trestles needed! I scarphed them 2 at a time using a router with and extended base plate - as shown - where the top a bottom pieces are simply for guidance. It's not a particularly quick method but it's precise and doesn't demand the sort a stable base needed for planing. Use a new router bit (say 20mm) and think about the direction of cut when approaching the feather edge (or it'll tear). While scarphing I screw the boards together and for gluing full sheets, I fasten them together with five 5mm bolts either side of the scarph, going right through both sheets and 12mm backing pads top and bottom (don't forget the polythene separating sheets!). The holes left are easily filled if they coincide with the positions of any of the strakes.

Note: If you’re using a router outdoors, don’t be surprised if you become popular with the local insect population. It seems that the router whine has harmonics which serve as a super mating signal – called a super releaser – and it’s the actual cutter the wasps and bees home in on (not you) but I’ve not seen any get caught by the blades yet.

The patterns for the strakes are made using cheap ply (I use 6mm - any lighter proves too wobbly at 15ft long!). I work with lengths of about 1 - 1.5m perhaps 50 wider than the strake and tend to start at the mid sections. Making sure to hold it flat along the mould (clamps, screws, - whatever - see pic.) I trace along the lower ribband line, then remove it and mark the strake width at say, 150mm intervals to get the upper line. Clamp to the mould again and repeat the process on either side and connect the shaped pieces by clamping and gluing pads of scrap ply in situ on the mould.
If you take care with the pattern, you can transfer the shape to the marine ply and cut it on the line so that not much further shaping is required. However, with longer and narrower strakes, there's a fair bit of play and I prefer to cut with, say, 5mm allowance all round.The trick is to get as many strakes from your 16 x 4 ft sheet as possible – I’ll detail this later. After ensuring the garboard is a good fit on the mould, I glue and fasten it with clamps and silicon bronze ringed nails fore and aft and wipe off any excess epoxy both outside and inside where you can reach. I just use my fingers for this. I ensure that the strakes won’t stick to the mould at any point by laying ordinary sellotape along the ribbands and on the exposed edges of the frames (epoxy sticks to masking tape).

For the second and subsequent strakes, I cut a gain fore and aft using the modified rebate plane - as shown – so that the strakes lie flush. Not everyone bothers with this at the transom but I think it looks neater. I then use a similarly modified plane to cut the lands so that the strakes are flush where they overlap. When testing for fit and for final gluing, use the same clamping method as for making the patterns (if the mid section isn't clamped flat against the mould, the ends won't line up; if the ends are forced into position, the mid section will lift away from the mould); this allows you to gently knock the strake up or down to adjust positioning. The final location is aided by making one or two pencil marks and the use of a number of clamped tabs on which the strake can rest for final clamping and fastening. For temporary fastenings along the land, while the epoxy is hardening, I like Iain Oughtred’s method of self tapping screws (3.5 x 16mm for this boat) every 100 – 150mm – just remember to loosened them off half a turn after a few hours or they may snap when you finally try to remove them! Iain Oughtred suggests not allowing the heads of these screws to mark the strakes but I screw them down hard; the indentation left by the head is easily filled and when sanded down looks just like the head of a rivet – especially if you use a darker filler. Whilst the epoxy is still tacky, I run a fillet along the overlap of the strakes. For both fillets and bonding, I use an approx. 50:50 mix (by volume) of microfibres and colloidal silica - the amount added to the resin just depends on how thick you need it - I tend towards 12% by weight for bonding to about 16-20% for fillets.

Note: When sanding the marine ply, the Okoume outer layers may cause skin irritation - especially in delicate skin areas. Moral: wash your hands before going for a pee! Same goes when sanding resin, teak, rosewood - even beech can cause a reaction.

And that’s it really – a boat is appearing. At this point I’m on the 7th strake and am beginning to wonder how to turn the whole damn thing over. I think I may need friends. Hmm!