Building the Sawfish 12 – Part One

by Jim Brown - Sweetwater, Tennessee - USA

A Foam Kayak Designed by Rowerwet (Josh Withe)

Builder: Jim Brown – Sweetwater, TN

Part One – Part Two

Part One – Forming Foam

You may remember in my recent chronicle regarding the building of a Dave Gentry Chuckanut 12 skin-on-frame kayak, (12 feet in length, with a 28″ beam), that I had decided to next build something completely different. After considering several options, I elected to go with Josh Withe’s Sawfish 12, which can be found on the “Instructables” website. “Sawfish” happened to be exactly the same length and beam as the Chuckanut 12, which would lend itself to a reasonable comparison.

My first step after reading the instructions was to print out a copy which, because of the Instructables format with lots of ads, ended up being 65 pages! However, I wanted to have a copy available readily at hand in the shop during my first foray into this unfamiliar method of construction.

The instructions included a complete list of materials and tools required for this endeavor, so I hurried off to Home Depot, Lowes and Harbor Freight with list in hand. I used the pink Foamular foam sheets from Home Depot, which Rowerwet recommends over others available.

That mean looking spiked roller did not come from Home Depot, but from Amazon (Silverline 221199 Wall Paper Removal Perforator, about $20). It is not a medieval weapon, but produces a perforated surface on the foam, such that when glue is applied and eventually dries. The little columns of dried glue become like tiny nails which hold the foam surfaces together. And, those Bamboo Skewers came from Walmart ($0.55 for a pack of 100). The skewers keep the foam parts aligned while the Gorilla Glue (GG) cures.

The first building step includes a clever device Rowerwet calls a “butterfly scarph”, used to glue the 4x8x2″ foam panels together end-to-end. Plastic packing tape is used over the joint, then one panel is folded back over the other panel. The ends of the panels are perforated with that mean roller tool and Gorilla Glue is applied to the panel ends and sprayed lightly with water. The top panel is folded back down, and weight is placed over the joint while it cures. This is technically a “butt joint” but Rowerwet says that since it acts like a scarph, in the sense that there is no stiff area in the joint that would be produced by a butt block in a wooden butt joint, and the glued area remains as flexible as the rest of the panel as a scarph joint would, he calls it a scarph. The joint works like a champ! In practice, I had cut one of the 4×8 panels into two 4×4 panels, so I had enough over-head room to fold that panel over the other, and also because it suited an idea I had come up with.

The original Instructables had a foam sheet layout probably aimed at building the boat as inexpensively as possible. My idea involved laying out all the parts for the boat on just two 4x8x2″ foam sheets, as shown below:

This yielded six 3-1/4″ wide ribs which were all one piece, and which could be stacked up and fitted together with the ends of the ribs overlapping for strength, rather than a bunch of odd-shaped pieces which had to be pieced together, with the gaps filled with canned foam. The ends of the bottom piece were blocked up 2″ at each end to provide rocker for the kayak, which held it’s shape as the GG (Gorilla Glue) cured on each of the three layers. Rowerwet suggested cutting the foam with a drywall saw, but I found that using my trusty-rusty old B&D saber saw with a “Clean Wood” blade (a blade having no offset teeth, and ground smooth on both sides) did a much smoother and neater job, minimizing later finishing work. The result was this form with 3 layers of ribs on each side of the bottom panel. The middle ribs on each side were offset 1/2″ outboard to provide for “tumblehome” shape in the final product. With each respective layer being 1″ longer than the one below, a nicely sloped bow profile resulted.


I found out several weeks later, while reading a newer Instructable by Rowerwet about building a 17′ double seater Sawfish, that he had come up with this same basic layout idea. Great minds, etc, etc…..

I also found that I preferred different tools for shaping the foam than Rowerwet recommended – the primary ones being an Oscillating Saw with a half-moon shaped wood saw blade ($25 on sale at Harbor Freight for the variable speed model with a small attachment kit), and my trusty DeWalt 5″ Orbital Sander, with 60 grit sandpaper. You just have to be very careful not to produce low spots in the hull shape, which will later require filling. The instructions do not show any suggested contours for the hull shape, so you are on your own to just eyeball the shape until it looks right to you.

As the boat took shape, I realized that the 4×4 piece of 2″ foam I had previously cut off would be just right for the forward and rear decks, and would be stiff enough to negate the need for wooden internal supports suggested for the 1″ thick foam specified. And also, that I would have enough 2″ foam scraps to add a layer to the gunnel areas as well – making a four-layer cake instead of three. It turned out that Rowerwet had also used four layers on the 17′ double.

A minor flash of brilliance was when I decided to install a means for the old Playmate Cooler to fit into the foredeck. But, Rowerwet had shown a greater flash of brilliance when he recommended modifying a “Gamma 2” screw-in lid ($8 at Lowes) for a 5-gallon paint bucket as a waterproof and easily removable large access to the rear deck space.

Rowerwet strongly recommended strengthening the hull in the cockpit area with wooden plates, as he had experienced some hull flexing and tearing of his bedsheet fabric covering when paddling in waves, so I used some 6″ wide 1/4″ plywood pieces from the wood pile to reinforce the 56″ long cockpit.

Since I had a limited number of suitable clamps to use when gluing these pieces in with PL Premium 3X Construction Adhesive, I followed his advice and bought a 2 foot length of 4″ diameter PVC Core-Cell Pipe (about $8 at Lowes) to cut numerous slices on my cut-off saw, and make about a dozen clamps. I tried slices from 1″ to 1-1/2″ thick, but found that 1-1/4″ thick was about the right strength for me in this application. Note that you must have some plywood feet set between the PVC clamps and the foam sides, or you will punch holes in the foam, which would have to be be filled later.

I must say that at first, I really did not enjoy working with the Gorilla Glue. It cures by absorbing moisture from the surroundings. Therefore, in this cold, dry winter air, it was necessary to spray the joints with water. The GG foams up and expands, sometimes causing gaps between the parts being joined. Parts need to be pressed together tightly, using either clamps or weights (Rowerwet calls weights “gravity clamps”), or both. Early on, I had used too much GG by drizzling each part with glue, then spraying water and clamping. Later, I found that by drizzling the glue on only one part in a sort of sine-wave pattern, then spraying the other part with water, it seemed to work much better. Doing it again, I would smear the GG out with a putty knife to force the GG into the perforations, and to get a more even covering. By the time I was done, I was fairly comfortable with the GG.

I had enough 2″ foam scraps left to cut a couple of curved pieces to level the gunnel area out with the fore and aft decks and provide a little more freeboard, and probably enough beyond that to piece together a 2″x2″ keel strake to promote tracking ability as Rowerwet now recommends for all Sawfish.

Now things were really coming together. After my rough (some of my early attempts were really rough) shaping of the foam hull, I did some filling in of various cuts, gouges and gaps caused by the expanding GG, with a light weight DAP Fast’NFinal spackle, which was easily sanded smooth when dried. This would provide a good base for the fabric covering. I got to this point in about one month, and thought I would soon be done – but not so. I had at least that much more to do time to go.

Weighing the bare hull (no seats, access ports, coolers, etc.) resulted in a surprisingly low 17 pound weight! Things are looking up! Tune in for the next thrilling episode.


    • I didn’t know about the double when I built this one, but mama like to paddle her own kayak anyway. Also, it is easier for us older folk to cartop two lighter boats than one heavier one.

  1. Love the idea of the foam kayak, don’t love the idea of creating the foam shavings/dust that static cling to EVERYTHING. Is there a good method for dealing with that? Hmm… could that readiness to static cling be harnessed as a means of dust collection???

  2. I didn’t note that much static using the Hope Depot pink foam. If I had used the white styrofoam, I’m sure that would have been a problem. The Dave Lucas design uses the white styrofoam with fiberglass and epoxy coating. Josh Withe uses the more expensive pink (or blue or green) foam and cheaper cloth and Titebond 2 coating, which is probably cheaper overall.

  3. I live in Knoxville and am glad to hear of a fellow boatbuilder so close. I am a sailor an wondered if you had thought of eventually sailing this boat?

  4. I have been a sailor much of my life also, but have recently sold my Marc Barto wooden Melonseed and my refurbished Mike Storer PDRacer, so am down to two kayaks (Chuckanut 12 & Sawfish 12) and our Gheenoe with a 5 hp Mercury. Will soon be 84, so no longer have the energy to rig and sail a sailboat – but I still think about a simple sail rig and maybe an outrigger setup for one of the kayaks!

  5. Nice cuts on your ends, fantastic shape. I also liked your idea of extra hull support. Hope to start one in the spring as I have to do this outdoors (Michigan)

  6. Trapper Paul, you will be rewarded with a very nice, inexpensive and lightweight paddling machine. Best wishes on your project. I am now contemplating a Sawfish-type scaled up to a 16′ trimaran, also using foam floats, with minimal wood structure used to distribute the considerable loads involved.

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