Borealis – the project

As it became apparent that my first strip-built kayak, the Geyrfugl, was going to be more successful (and look better) than I had imagined, I developed the confidence to start to plan a kayak of my “own” design. I put the “own” in inverted commas because inevitably it would be evolved from some other design rather than pulled out of thin air.

Clearly, if I want another Greenland style kayak much like my existing North Shore Mistral (which is an excellent boat) there are plenty of such designs on the market. But the whole point of strip building (to me) is to make something that you can’t buy. Geyrfugl was a sea kayak for a paddler so small that commercial kayak designers in the early 2000’s just didn’t find a big enough market (this is not quite so true 15 years on). My second boat – a hybrid Cormorant, is also scaled to a smaller volume than most commercially built boats, but is only suited to short trips. For longer trips, I wanted a kayak of a particular style – one which can’t be bought off-the-shelf, at least in the UK.

Inspired by Rob Mack’s North Star design, I decided that there must be something to be said for the Aleut style baidarka. But even after reading George Dyson’s book, I didn’t feel that I wanted to build a skin-on-frame kayak, (yet). Strip-built boats are just so attractive that I wanted to build another one, and I appreciate the benefits of watertight cargo compartments and hatches. So I returned to sources and looked at the design of several original Aleut baidarkas in various museums, documented by David Zimmerly.

The Atka Island kayak (LM 2/14886), collected in 1934 and now in the Hearst Museum at the University of California at Berkeley, is the most modern of these Aleut designs, and seems to have been the starting point for a number of people’s recent designs, particularly skin on frame (wood or aluminium) designs. I read Rob Mack’s description of the sorts of changes he made to an Aleut design to come up with the North Star. Some of those ideas seemed to agree with mine, and others perhaps weren’t what I wanted. Rob rounded out the keel of his design because this seemed better suited to a recreational boat which would most often be used with very little load. I wanted a boat for multi-day camping trips and I’m prepared to do as the Aleuts did and carry some ballast to improve handling with no cargo, so I left the keel somewhat angled.

original surface sectionsThe Atka Island kayak is shown with a waterline based on a displacement of 176.5 lb. The first thing I did was rebase the table of offsets to that waterline (which is not parallel with the datum line used by Zimmerly). The offsets given in Zimmerly’s pdf file, when lofted, don’t give a fair shape – perhaps because these are measurements from a kayak which was not in perfect condition when measured. So the next thing was to redraw a kayak based on these offsets, but faired out to give smooth chines. Then I re-scaled these offsets to give a length overall of 5.5m (I am probably bigger than an Aleut and a displacement of ~80 kg would be a bit on the low side even for just me and the boat). I now had some rather widely spaced forms – not ideal for strip building, so I wrote a little program to fit splines through the existing offsets and give me a table of offsets at new form positions. This was not quite perfect, but the few odd anomalous points were easily enough fixed. Then I tweaked and fiddled with the hull, with the eventual result that I had a set of offsets giving a displacement of 98kg, perhaps still a little low, but I didn’t want a longer boat than 5.5m. In fact, I suspect that an Aleut paddler with a hull containing the results of his day’s hunting would be displacing more than that 176lb figure, and the boat would ride a little lower than Zimmerly’s postulated waterline. By adjusting the offsets so the hull rides 2 cm lower in the water, I got a displacement of 130kg, which should be OK for several days camping.

To keep the skin-on-frame boat light, and shed water, the original deck has a sharp peaked section, with just one stringer along the ridge. The strip-construction technique doesn’t need stringers, and I wanted a little more room under the foredeck, and perhaps a slightly flatter aft deck, at least just behind the cockpit. I also prefer a more rounded sheer both to make rolling easier, and because I find a sharp corner quite hard to build. So I replaced the deck with a half-elliptical section of about the same height as the original, except in front of the cockpit, where I raised it a little. This cants the cockpit back to ease entry, so a fairly small cockpit would be possible. At the time I started designing, I didn’t want a huge keyhole affair where a breaking wave could perhaps implode my spray deck – I had built the cockpit in my Cormorant 59cm long, and found it very comfortable. However, having had a bit of practice and coaching in various rescue techniques, and given the quality of modern spraydecks, I am now much more in favour of large cockpits – if nothing else they make it easier to apply modern whitewater style outfitting with ratchets on the backstrap.

Having sat and thought all summer about the design, I added to my collection of code by adding routines to calculate centres of buoyancy, lateral resistance and wind resistance. This confirmed my “eyeball” suspicion that the centre of lateral resistance was some way behind that of wind resistance of the hull alone. A paddler would move it back, but I still worried about getting a boat with lee helm. So I deepened the keel over the forward 2 metres, which somewhat increased the waterline length and reduced the prismatic coefficient, which should have the additional benefit of making the boat easier to paddle at higher speeds, possibly at the expense of making it slightly harder work at low speeds. Lee helm is to be avoided at all costs, even if I overcorrect and get weather helm – the latter can be corrected by adding skeg. By October 2002, I was getting confident enough to think it worth building a prototype, but 5.5m was too big for the biggest workshop space I can have heated over winter. Hence the initial reason for delaying project whilst I built a smaller boat for additional experience.

Once I had paddled this new boat, which is really quite low volume, but a very comfortable fit, I went back to the baidarka design. More additions to my code produced an overall volume figure of almost 500 litres, and I had already been worrying that the boat was a lot deeper (and therefore a lot higher in the water) than other baidarka-inspired designs I had looked at. British waters are windy, and I didn’t want something that bobbed about on the waves at the slightest whim of the breeze. Another consideration was that I had thought of building a stitch-and-glue hull as my prototype, and at 5.5m, the boat was fractionally too long to get out of two plywood sheets. Making it a few centimetres shorter was going to be a lot more practical than scarfing on a whole extra sheet.

Borealis surface sectionsUnfolding the panels for stitch and glue showed the chines were still not as fair as would be needed for this type of construction. So I printed the cross-sections full sized, redrew the chines and measured off new offsets. This made almost no change to the hydrostatics, but should make fitting ply panels together a whole lot easier. It will probably be something of an aesthetic improvement, even if I build it as a stripper.

In 2004, having had a season of paddling the Cormorant, I had a fair idea of what I wanted in terms of stability and fit, and I could compare the sections of the proposed Borealis design with those of the Cormorant. It was apparent that Borealis as originally drawn had a lot more freeboard than would be comfortable to paddle, and a lot more knee room than was required. The overall volume was also very high, even for an expedition boat. Over several iterations, this resulted in a revised design scaled to 5.4m long, beam almost unchanged and depth reduced to 84% of my earlier version. The lower volume design is still 375 litres, with a beam of 55 cm (51 cm at waterline) and a displacement of 120 kg at the waterline as now drawn, with enough freeboard to load it a lot more than that. I now believe this is a boat I can build and paddle, as an expedition single.

“Borealis” is my code name for the project – not the name of the boat, which has yet to be decided. Early thinking was to build a stitch and glue prototype (the original rationale for scaling down to 5.4m), followed by a strip-built boat with whatever adjustment seemed necessary. You can download my offsets as revised to 2004-02-27 in plain text (Unix newlines) and see my study drawings on this page. The rather odd form spacing is the result of starting off with a neat 0.3m spacing on the 5.5m design and scaling it down to 5.4m.

plan and elevation (click for enlargement - 27k png)

Having revised and refined for over 2 years, the vital statistics were:

Length overall 5.4m Waterline length 5.28m
Beam 55.1 cm Waterline beam 51.1 cm
Seawater displacement 121 Kg overall volume 375 litres
freeboard 7.4 cm draught 11.8 cm
Prismatic Coeff. 0.56 Block Coeff. 0.39

Having paddled both a commercial fibreglass kayak (4.9 m x 55 cm) and my new hybrid Cormorant (4.66 m x 52 cm) in quite rough conditions for many years, and Geyrfugl (a boat
designed for a much lighter paddler, with a 49 cm beam) in a variety of conditions (though all more sheltered), I am confirmed in my opinion that I want a boat with a reserve of secondary stability and a low windage for longer trips. The extra length and flared sections fore and aft should meet these criteria with the current depth and a beam of 55 cm.

My original drawings showed the boat with two aft bulkheads giving a main hatch and a day hatch. For some time, I didn’t find much benefit from having a day hatch in my fibreglass boat, and didn’t build one into either of my wooden boats, so I was considering not having one in Borealis. A clean, and fairly low deck aft of the cockpit (a “rolling recess”) would make the boat easier to roll, nad would rake the cockpit more for easier entry and exit. With a keyhole cockpit, entry/exit are easier anyway, and too low an aft cockpit rim is asking for water ingress. With the Nordkapp, the original design had its aft bulkhead very far back, giving a huge volume to the cockpit (a real disadvantage in a rescue situation). So in that boat I added a new bulkhead (making the cockpit much easier to empty) and dayhatch. This is persuading me that dividing the space aft into two compartments, whilst adding weight, does have real benefits. The idea that a knee tube would be a better solution for keeping small things handy still works, though not as effectively as with a small “Ocean-style” cockpit (59×47 cm). The study drawing still reflects my earlier ideas.

An idea I toyed with, that would benefit from the dayhatch, was to build the boat in three-sections as a take-apart. However, my current thoughts are that this will have too much of a weight penalty for use in the UK, and I can’t see it being that much help travelling either, as even if any one section is small enough to take on a plane, how does one travel with all three ? It would also make it problematical to implement a drop-skeg. So this is an idea that I have abandoned.

Having read Norm Sanders’ article in New South Wales Sea Kayaker, I planned to add a mast step and some deck reinforcement to support his design for a downwind sail rig.