Re-decking a composite boat

This page is my (Steve Whitby's) experience in redecking a late Minisprint Mk2 composite, formerly home-completed by Tom Moore around 1979-80. Its worth noting before I start that I had planned to use an all-glass Minisprint Mk2 as a basis for a composite conversion until Tom kindly offered me his hull for nothing. Starting from an all glass boat would be very little different I believe (just a bit tricky at the transom and around the centreboard case and mast tube where the hull and deck are bonded together without a very obvious way of cutting the bond). 

 

The old deck had become very brittle (or just plain soft), the bulkheads were glass-taped in place and the adhesion had become pretty weak; all of the old material (decks, bulkheads, frames) were removed with a paint scraper and a mallet. Tom had used a handful of screws all of which came out without a fight. The inwales were bonded in with epoxy and these were still sound and well-stuck so they stayed (except around the bow which had de-laminated). The hull was mounted on blocks on heavy trestles to try to keep it in shape through the work - this was okay but should have been more rigid as the blocks moved a few times necessitating a lot of careful realigning to get everything true and plumb.

 

 

 

 

 

 New frames were made in 4mm Okoume plywood, to a revised design with an aft tank and drain tubes - the thinking being that the stern of fully open-transomed boats sinks too easily if you need to go aft to sort the rudder or mainsheet. I also chose a slightly lower level for the cockpit floor - taking measurements of my all-glass Minisprint Mk2 and gambling a little! I was keen to keep the floor low to ease getting under the boom and to provide legroom on a tack. The end result worked well and drains successfully though not if I'm sat right forward (which would only be in light airs... so no excuse for shipping water!). Deck layout design has few contraints in the Minisail class rules.

I made the forward end of the side tanks narrower for the same reasons of legroom - I toyed with the idea of making them open-sided down to the cockpit floor, but I wanted to have the strength to mount a sliding seat carriage inboard from the gunwale (to keep it out of the water when heeled) - the side tanks helped to provide structure.

I decided to make the main forward bulkhead as far forward as the rules allow since in light airs this would be a help.

 

 

I cut lightening holes after fitting bulkheads - likewise the holes for hatches later on - though the large (non-circular) holes were cut before fitting as clearly that is way easier.

 

 

 

 

 

 The mast tube in glass boats has a mild steel plate at the bottom, eventually this rusts and the resulting expansion damages the hull. I cut all this out and rebuilt it in glass epoxy. It wasn't difficult (except I think I used too much of everything!). I took the opportunity to move the mast forward 2". This was a decision that I am very happy with, the mast tube in minisprint Mk2 boats is conical (the deck mold fitting inside it at the top making it effectively parallel sided again) - I kept the conical shape and made an elongated hole at deck level (see later) allowing the mast to rake aft by 6mm (apologies for mixing my units - bad habit!). My calculations made it that 2" further forward and 6mm rake aft at deck level kept the centre of effort the same but increased mast rake (which I hoped will improve upwind performance). The elongated hole at deck level allows the mast rake to reduce downwind.... The tube is big enough to take a standard Laser mast, thus giving options on the rig.

The mast tube needed to be extended (as I had cut the bottom off to get it out); I wrapped a suitable tube (drain pipe) in paper and peel ply before laminating with glass and epoxy, soaking the tube in water once the epoxy was dry meant that the drain pipe could come out. Stupidly I left the peel ply in when I glassed the tube into the hull, so there it will be for all time! Once glassed in I added a fillet of epoxy and colloidal silica (I used colloidal silica in all my epoxy bonding, its very hard to sand, but otherwise easier than using a mix of balloons, fibre or whatever (I'm not saying that I recommend this approach - just that I use it!).

 

 

Getting the curve of the foredeck to be fair was very tricky, Tom had warned me of the issues he had faced with fitting plywood onto a compound curve (though he had done a very good job and I wanted to mirror his design). A good deal of trial and error was required, using mahogany stringers as the main support. The foredeck is the maximum height allowed by the rules. Its worth noting that the Minisprint Mk2 has much a deeper hull than the other glassfibre models, its also wider, both forward and at maximum beam...

 

 

 

 

 

 

 

 I laminated the bow inwale in situ (using very thin ~2mm mahogany). Further reinforcement was added around the mast after this photo, partly to support deck fittings, partly because of an issue with plywood. I had started off with 4mm Okoume sourced from Fyne Boat Kits, this was timber I had in stock and it was very nice wood; I ordered more for the decking but it was pale and scattered with small knots, it really wasn't up to standard for decking in my view. Fyne Boats were very decent about it and offered to take it back, however they didn't have any of the good-looking okoume that I'd had from them previously.... what they did have was some delightful Sapele plywood (Sapele throughout, therefore heavier by about 20%) but only in 3mm (which is not class legal). I shopped around and the Fyne 3mm Sapele was going to cost me exactly half the price of a good marine ply from Robbins or similar. Since I had already spent more than the budget I went for the 3mm - yes its outside the class rules, but it doesn't save weight over Okoume board once the additional framing was in (and it was £150 cheaper).... The 4mm would have been preferable as 3mm is very much softer, though it did make the curving of sidedecks a lot easier to achieve... The crewdeck (cockpit floor) is 4mm by the way, and I wished I had glassed the underside as it feels a bit flexible despite the many stringers and half-bulkheads.

The two main bulkheads form a sealed compartment for and aft of the cockpit, screw-in drain bungs can be accessed via the hatches to allow draining (hopefully never needed!). This gives three independant buoyancy tanks (fore, aft and cockpit/side tanks), the fore and aft ones also have buoyancy bags inside to comply wiht class rules.

 

 

 

I attempted to fit the foredeck as Tom had done, by torturing the ply over a compound curve - I gave in though and cut a small dart aft of the mast. This was a good call as it was still tricky to get the deck pinned down. In the photo I had experimented with covering strip over the dart, but I changed this for an inlay as it looked better.

I used stainless pins through small ply scraps to pull the deck down; once the epoxy had cured I snapped off the ply scraps and pulled the pin out. Once the hole was filled they were not too visible... Tom had retained the hockeystick hull mould at the gunwale, but I cut them off as they make sense for me only when bonding on a glassfibe deck. In order to keep weight to a minimum I didn't fit a gunwale either (just a rubbing strake after decking was finished), so I had nowhere that I could use clamps around the deck edges - the pinning approach was essential and the plywood scraps avoided hollows (or hammer dents!) around the pins.

This photo shows the laminated centreboard capping, this is glued on top of the glassfibre with no mechanical fixings, in fact there are no screws, bolts or pins in the boat anywhere (except deck fittings of course). The cockpit floor is the second batch of okoume plywood that came from Fyne Boats - you can decide for yourself whether I was being a fussy customer by not wanting that for decking.... once the sapele decks were finished I was in no doubt that I was happy with my decision! 

 

 

 

 

  The side tanks were panelled after the cockpit floor with an epoxy fillet inside the tank (and a stringer under the floor just inboard from the tank side). The beefed up area under the side decks is to allow a sliding seat carriage to be mounted on 24" long tracks (to move fore and aft). At the time of writing I have sailed without the seat (and very much enjoyed it); I had previously tested the sliding-seat-on-tracks approach on my Monaco and it does work very well indeed though.

 

 

 

 

 

 

Once the side decks were on then I fitted a mahogany stringer to the inside edge - partly to protect the ply edge but mainly for comfort and a hand-hold. A spoke-shave comes in very hand for rounding the edge...

 

 

 

 

 

 

 

Woodwork done - its time for epoxy and then varnish. All of the internal surfaces having been epoxied already of course. The cockpit drain tubes can be clearly seen along with a hatch cover opening to allow the pintles to be through-bolted. All the holes for fittings were drilled before finishing; almost all are through-bolted.

 

Overall effort was about 120-150 hours. I had the benefit of a heated (and insulated) workshop othewise working with epoxy in March would not have been do-able. The back of the boat was near the uninsulated door and this gave me problems with sagging of coatings due to slow drying times...

The total spend was around £500 - mostly ply and epoxy - I already had a stash of old mahogany floorboards for stringers and gunwales, I also already had all the tools I needed. I thoughly enjoyed it and I'm happy with the result both in terms of its design and also the finish I managed to get. Its not perfect, but then I'm not a professional boatbuilder either.