Richard K. Larson - MIRROR 70369 ("Tweedledum")


Between May '06 and June '08, working mainly during summers, I built an International Mirror dinghy, a very successful British design dating from the 1960's (over 70K hulls built!). Mirrors are about 11" feet long and 4.5" wide; they weigh around 125 lbs full up, and most are wooden (hull and sail plan).


RKL and 'Dum on Schroon Lake, July '08

The boat comes as a kit purchased from a manufacturer licensed by the International Sailing Federation (ISAF). I bought mine from Lorne Bellamy of Mirror Sailing Development (MSD), who assembles the kits in his garage workshop. In June '05 I drove to Lorne's house in Branford, Ontario to pick up the kit for Mirror 70369.

Lorne and his workshop, a treasure trove of boat parts and exotic hardwoods from all over the world:


Kit instructions estimate 120-150 hours required to build a Mirror - not something for an afternoon or a couple of weekends. I surely ended up taking at least twice that amount of time, and it was quite a ride!

Building Steps

In the course of building Tweedledum, I:

We launched Tweedledum on Schroon Lake in northern NY in July '08:





In Aug '08 my daughter & I completed sea trials, sailing 'Dum around Robins Island on Long Island's Great Peconic Bay:



Building a Mirror has been a unique experience - always engaging, occasionally frustrating, at times all-consuming, but imminently satisfying. I highly recommend it.

Building Notes

Barrier coating 1: should you? MSD recommends that, unless you're building a racing hull, you should give all wooden pieces at least 2 thin coats of epoxy before beginning. I agree. Barrier coating not only prolongs hull life, it protects the pieces from the scratches, dents, marks, stains, etc. that occur so easily during building. It also keeps spars, battens, transom risers, bilge pieces, etc. from warping. On the other hand, epoxy is toxic, so you must be careful about ventilation, wear neoprene gloves when applying it, wear a dustmask & eye protection when sanding it, etc. etc.

Barrier coating 2: procedures. One strategy is to apply 2 coats consecutively, the 2nd before the 1st fully cures. This gets the job done quickly, but it also yields a rough surface that will need sanding before varnish is applied. My experience: it's lots easier to sand panels when they're flat on sawhorses than when they're installed in the boat. My recommendation: don't rush. Apply a single coat of epoxy to sanded wood; when it's cured, scrub off the blush with a Scotch Brite pad & water, sand smooth & repeat twice. Yes, it takes longer. But when you reach the varnishing stage, you'll be working with a smooth surface needing minimal prep. Two more suggestions:(i) sand all printing & kit numbering off the plywood panels before you coat them. (Trust me on this.), (ii) after rolling on each coat of epoxy, tip it with an inexpensive chip brush. On the first coat, watch for bubbles ("outgassing") and continue to tip as they pop up. The smoother you can make the coat when you apply it, the less sanding you'll do later. Fully cured epoxy is very hard and a *nuisance* to sand.

Barrier coating 3: materials. West System epoxy is highly convenient & yields excellent results. I also bought West System 7" foam rolls, which I cut in half and used with a 3" roller. These are economical & work well. For a roller pan, I used cheap plastic liners from Home Depot that were divided in half. It was very easy to remove hardened epoxy and reuse them. [1]. I also ended up purchasing most of the various West System fillers, including the graphite, which I used on the interior of the centreboard case. [2]. Finally, I purchased a good dust mask and several pairs of safety glasses. The mask had a battery-powered blower system that kept the face cool.[3]

Trimming plywood. A boat-builder friend recommended using an an Exacto knife (or the equivalent) to do all cutting & trimming of plywood panels This was an excellent suggestion; a hand knife is slower than a saw or router, but the control & quality of the cut are much better.

Color coordination. My kit contained dark red meranti floor battens and the thwart & thwart risers were mahogany. I stained the centerboard trunk pieces to match the darker colors. The effect was good, but if I had it to do over again, I would have requested dark woods in the relevant kit pieces. Whenever you bump the edge, the lighter color shows through.

Fillets. MSD advised that using fillets in the cockpit would yield stronger seams than fiberglass tape. I followed this advice. Doing fillets is more complicated than glass tape, but the results are (I think) aesthetically nicer. I did put tape over the fillets around the centreboard case for abrasion resistance.

Inspection port holes. I cut the inspection port holes on 70369 using a keyhole saw, which left a somewhat ragged edge. On 26688 I used a circle cutting attachment with my rotary tool, and got a much better result. See the building notes for Mirror 26688 for pix.

Transom holes. MSD and others in the Ontario Mirror Dinghy Assn. advised enlarging the transom holes to improve drainage & facilitate carrying. I did this too using a hole saw.

Side tank bulkheads. The optional side tank bulkheads included in my kit could not be made to fit the hull properly. I discarded them and cut new ones from 5mm luan plywood.

Side tank battens. This side of the Atlantic, the recommended attachment of seat tank battens is not the classic longitudinal one, but rather transverse, surely a stronger, stiffer set-up. I went one further and braced the battens on their hull-side ends with small, triangular glue blocks. This was done to provide additional support at a point where considerable force is exerted by crew netherparts.
Bow View (tank interiors)
Portside View (tank interiors, with new side tank bulkhead)

Fitting problems with the foredeck. For reasons I don't understand, my foredeck did not fit properly, leaving a 1/4" gap with the fore transom on the port side. I filled the gap with West System 405, and was able to hide it with a wide fillet when I sealed the decks. The fillet is largely hidden from view by the bow shapes.

Fitting the side decks.. The instructions suggest that fitting the side deck panels is simply a matter of adjusting their length and squeezing the hull sides a bit. With my kit this wasn't true. The curvature of the outer edge of the side deck panels was greater than the curvature of the hull and I wasn't able to simply squeeze the latter to make things fit. To fit the side decks properly to the hull, I used the technique of "scribing".
-First, I placed the midpoint of the panel against the hull so that the gap between hull & panel at either end was equal in distance (about 1/4" on my boat). I also centered the panel between the fore & aft decks, making sure there was material to trim at either end.
-Next, I wrapped sufficient tape around a marking pen so that when the pen was held against the hull, pointing downward, its point just touched the outer edge of the side deck panel at either end. That is, the point just spanned the gap between hull and deck panel at its widest (1/4").
-Next, holding the pen in a fixed vertical orientation, I slid it along the inside of the hull, drawing a line on the side deck panel along its length. This "scribes" a line of curvature on the panel matching the curvature of the hull.
-Next, I rasped/sanded the outer edge of the panel to match the line, beveling a bit also to match the angle of the hull sides. (A Stanley 6" Surform Pocket Plane is perfect for this sort of task.)
-Finally, I trimmed the ends of the panels to fit them snugly between the fore and aft decks, and sealed all raw edges with epoxy.
Proceeding this way left a small overhang of side deck in the cockpit interior. I trimmed this off using a router & flush-trim bit. By these means, I achieved an accurate fitting of the side tanks, without a lot of forcing.

Fixing outer gunwales. In installing the outer gunwales I tried to countersink the fixing screws after the gunwales were attached. The wood splintered on one end with an ugly result. Better would be to drill & countersink the aft fixing screws holes in a drill press before attachment, and not to attach the forward fixing screws at all until after the forward gunwale ends were cut off & blended with the bowshapes. When countersinking the forward ends I would use a Forstner bit, which doesn't pull & yields a cleaner cut.

Bowshapes. The instructions describe the bowshapes as running "flush along the gunwales". But when the bow shapes are joined at the midpoint of the forward transom, their lateral extension does not reach the edge of the outer gunwales. I sanded down the outer gunwale to match the radius of the bowshape, but even so the edge of the bowshape goes back at a much sharper angle than the gunwale edge itself. So blending the two is tricky. Some builders proceed by removing considerable material from the outer gunwale; this was Neil Marriott's solution on Mirror 70148. But I worried about weakening the bow, especially since I had already planed material from the inner gunwale. I solved the problem by taking a little material from the gunwale and using filler to blend with the bowshape edge. The result looks fine aesthetically, and the bow stays strong. [1], [2], [3], [4], [5], [6], [7]

Heating issues in winter. Working with epoxy in the winter raises the question of heating. Following advice from MSD, I purchased a quartz radiant heater, which I mounted on a garage rafter facing down into my work area. I considered using a propane convection heater, but rejected the idea for three reasons: (i) to raise temperature, convection heaters must heat the entire volume of air in the work area. I am working in an old drafty garage with a high peaked roof. That's a lot of air to heat. (ii) Propane heaters consume oxygen and produce CO2. One must therefore be careful about ventilation. (iii) Gougeon Bros, makers of the WEST SYSTEM epoxy I use, warn that propane and butane heaters leave hydrocarbon residue, which can contaminate epoxy. By contrast radiant heaters heat only the objects they are directed at, not the air. They are electric, do not (directly) consume oxygen or produce CO2, and leave no hydrocarbon residue. QED.

Fairing the rudder. I didn't consult the class rules properly and so ended up overfairing the trailing edge of the rudder blade. ISAF rules allow only 25mm (as opposed to 50 mm on the centreboard). As a consequence, I had to shave 3mm from the trailing edge and refair a bit. Fortunately, MSD cuts the rudder blade to the absolute maximun width permisible under class rules (295mm) so there was material to spare.

Attaching the centerboard handles. Rather than screwing the handles to the centerboard (as per the instructions), I drilled holes through both handles and the centreboard and used 1.5" 6x32 SS bolts. This yielded a strong attachment. I also countersunk the holes and put in teak pegs to contrast with the light-colored handles.

Glassing sharp seams. The building instructions note the difficulty of getting glass tape to lie snug & without bubbles on sharp seams (90 deg or higher). I tried 2 techniques on the aft chimes: (i) that recommended in the instructions, where you continually stiple with new resin; (ii) that suggested in Guy Wilkins' RACING THE MIRROR, where you put stretch-wrap over the seam and press it down. Both require continual attention and yielded about the same results on the chines. However, for the centerboard exit, where the angle is very sharp indeed, I could only get a tight seam with stretch-wrap.

Bilge keel position. Before attaching the side decks, I drilled small holes through the hull by the fore- & aftmost glue blocks on port & strbd sides, carefully noting the positions of all glue blocks wrt the holes so that when I attached the bilge pieces, I'd be screwing into blocks. When I did attach the bilge pieces, I found they weren't precisely parallel to the centerline: the front is 1/2" further from the centerline than the rear on both sides. Lorne Bellamy, of MSD, told me that the measurements on his 25 year old boat, which has been raced in the world championships, are identical to mine. So I guess that just how things work out.

Aluminum keel strips. The kit provides 6x1/2" stainless screws for attachment. In attaching the forward strip I found that the screw points poked through the centerline into the interior. In attaching the remainder of the keel strip, I substituted 6x3/8" screws for all those that might protrude into the cockpit space (where tender feet are located). The result: no protruding screw points. I recommend this substitution. Most of the substituted screws are located where there is little bending of the strip, so there is no tendency to pull-out. Furthermore the epoxy + filler holds everything secure once it's cured.

Fairing the hull. Without a doubt, this was the hardest, most time-consuming step for me in the whole building process. Fairing the chines & aft transom was straightforward (although tedious). The goal is simply to produce sharp edges, clean corners and smooth transitions into the unfaired areas. Fairing the forward transom was likewise clearcut (albeit slow): cover the whole thing with fairing filler; sand until it's smooth, flat, and makes a nice edge with the side panels. The problem came with the keel strip, which I wanted to fair to the hull, and not simply leave projecting on its surface. How to proceed? I'm not good at judging curves. In the end, the only systematic procedure I could devise was to lay a batten on the hull, perpendicular to the keel strip and fill the empty area beneath it. In other words I faired along a tangent from the hull to the keel strip. This was long & labor-intensive, but I simply couldn't see a better way. Whatever your procedure, I heartily recommend West System's 410 Microlight Fairing Filler (which sands very well) and a longboard sanding block, like this 17" one I got from Harbor Freight Tools, along with a roll of 60 grit aluminum oxide sandpaper. They give you even lines and help to ease a hard job. My ace boat-building friend Scott Sandell says "fairing is a process". Amen. For a while I felt like it was going to be my life's work.

Priming the hull. After fairing, I primed the hull using Zinsser Bullseye 1-2-3 Primer. This primer is thick, and when applied with a foam roller yields a matte/semi-gloss finish. It has excellent adhesion, cleans up with water, dries fully in one hour, and fills in lots of those annoying little pinholes that get left in the fairing compound. Furthermore, after you prime, and see all those heartbreaking flaws in your fairing, you can rough it up with sandpaper apply additional fairing compound over the top, and reprime.

Adjusting the fairing. Priming did reveal flaws in my fairing, especially aft of the centerboard slot where the hull is very flat. I sanded through the primer, using it to reveal hills & dips, applied a very small amount of additional of fairing filler where needed, sanded that, and reprimed. Finally, I sanded the hull smooth in prep for painting. Zinsser Bullseye is thick & gummy; dry-sanding it clogs any sandpaper - even 60 grit. So in the final stages I wet-sanded, starting with a 180 grit 3M sponge sanding pad and finishing with 320 wet-dry paper on a block. Putting a little liquid soap in the water helped to keep the sanding surface from clogging.

Spraying. It was an extravagence, no doubt, but in the end I decided to splurge and have the hull sprayed at a local body shop. I supplied the paint: Interlux Royal Blue two-part linear polyurethane, by reputation an extremely tough and durable coating. It's also quite lethal in aerosol form if inhaled; spraying must be done professionally. (Remember Bhopal, India? Same family of poisons.).
The result was great, but perfectionists beware: the super-shiny surface reveals every last fairing mistake that you make - every pinhole, every place where the fairing compound hills or dips a little bit. This amateur boatbuilder made plenty of such mistakes. But, as my in-laws are fond of repeating whenever they give me a gift, or do me a favor: "Good enough for who it's for." :-)

A Final Word of Thanks

A highlight of my Mirror building experience was the group of Mirror sailors & fellow boat builders I met and consulted with along the way, including Lorne Bellamy and John McCullough of MSD, Shawn Crane, Luke Dolman, Vince Juliano, Steve Richardson, Scott Sandell, and Mark Tomlinson. Thanks to one and all, and particulary to Lorne, John and Scott, for advice and encouragement, and for answering even the silliest inquiries with patience and good humor.
Progress on Mirror 26688.
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