Foam Canoe
Be the First on your Block!
After the first canoe trip I became interested in canoes, they’re kind of magical vehicles. And this of course led to me becoming interested in building one – a great father-son project, right? It’s not as crazy as it might sound, there are several books about building a cedar strip canoe out there. But preliminary research showed that while these boats were unquestionably beautiful, they weren’t necessarily lightweight, which becomes a big deal when you (or preferably your son) are carrying it around in the wilderness.
The canoes you can rent these days in the BWCA are usually Kevlar and weigh around 42-45 lbs, much better than the old aluminum Grumman beasts that tip the scale around 75-80 lbs. Cedar strip canoes come in all over the place but generally the lightest seem to be in the 55-ish lb range – hard to gauge due to variables in design, size, and objectives. So I started down the rabbit hole.
Believe it or not there are people who create spreadsheets calculating the per square foot weight of composites, so if you know the rough area of the design and the weights of the various components you can fiddle around and start to ballpark your finished weight. John Winters has a nice overview here.
Some of the pieces are less negotiable than others, IIRC I used 6 oz fiberglass on the outside and 4 oz. on the inside which is kinda the minimum I felt comfortable with given how rough the Boundary Waters can be on canoes. So then the main variables left are your core material and the seats and gunwales.
Core Material
People do all sorts of things to make boats lighter. Smaller certainly helps, of course a 16 foot design is going to be lighter than an 18 foot design. You can save some weight on the wood components by reducing the sizes of some of the parts and by using lighter weight species than more dense ones. But ultimately the biggest way to move the weight needle is the core material, so I started to research that component.
Western Red Cedar is the usual choice for strip canoes, so then the challenge becomes how thin can it be without sacrificing structural integrity. I think the normal strip is 3/8″ thick but people take them down to 1/4″ to save weight. The density of WRC is 21 lbs/cu. ft.. and its a good fit for this application. Basswood is slightly lighter at 20 lbs/cu ft. Pine for example is around 35 lbs/cu. ft.
So of course having build lots of model airplanes and knowing a little bit about composite construction I knew that balsa wood has been used for cores, so I started to wonder about that. Balsa weighs in at 8-16 lbs/cu ft., so it looks like an interesting option. At this time I was corresponding with the guy who sells the Kipewa plans, and since he’s built a number of canoes I asked his opinion.
And he said, well, if you really want it light you might try foam, it’s 5 lbs/cu ft. And this got me going. Foam is used all the time in aircraft and marine composites, and that would definitely make a difference in the weight, right? He mentioned a marine foam called Coracell, so off I went to track that down.
Test Panels
As things progressed I was getting further and further into unchartered waters so I decided to do some test panels to get a feel for the different materials and a rough idea of the real world weights I was dealing with.
Design
Of course when picturing a canoe most people think of the traditional 2 person design with the high rounded ends, and that’s where I started, but there are many variations on the theme. One of the things you learn about quickly in the BWCA is wind and it’s effects on a canoe’s progress. The wind tends to come up as the day goes on, so if you’re paddling towards the end of the day you can find yourself in very windy conditions, and regardless of your compass heading it is never at your back. This experience makes you appreciate the comments regarding lower profiles on wilderness canoe designs, particularly on the bow and stern. So while not as pretty, I started to come around on the look of the modern designs.
And as computers have made their way into yacht design they have trickled down into small boat design, which has led to the advent of the “asymmetrical” canoe. Traditionally the front and back half of a canoe are the same, but computers tell us this is not the optimal design, and that if you move the center of the canoe back about a foot, ease of paddling is much improved. The mathematics are able to confer other benefits as well such as improved speed and secondary stability. Plus, these profiles look really sexy. So after enough digging the geek in me won and I settled on a John Winters design named the Kipewa.
Plans & Forms
Strip canoes are built upside down on a series of forms – you can see the location of the numbered forms in the picture above. The forms are placed 12″ apart on a support, known as a “Strongback”. So the next step becomes creating the strongback and the forms.
The strongback is just a narrow table the length of the boat. There are lots of ways to to this, I opted to build two simple boxes that joined in the middle.
The forms are outlined on the plans, but you can also get them on a CNC file. Initially I figured I would lay these out and cut them by hand, but I quickly realized that over the number of forms there was a lot of room for small errors. I also recognized that, like a guitar, there are very few – actually zero – flat reference surfaces on this structure. So having the forms CNC’d would kill two birds: 1) they would be perfect (and perfectly symmetrical) and 2) they would have perfect center line and other reference keys marked on them. I poked around and was able to find a guy nearby who had a side business doing CNC work. Fortunately he was a computer guy by day so was able to translate everything correctly. It was one of those things that wasn’t cheap but proved to be well worth it. The forms were perfect and when I started to mount them it was incredibly helpful to have center lines and horizontal reference lines that were actually vertical and horizontal, it made this stage go pretty quickly.
Getting Started
The first task was to start making strips. I forget how many I had to make, but it was a lot. As I recall the material was 1/2″ thick so I was ripping 3/8″ slices on the table saw and then running the edges against a matched set of “canoe” cove and bullnose router bits to get the requisite profile. These nest into each other as you build them up the sides of the canoe.
The next step is to make the stems. These are the wooden pieces that go on the bow and the stern and create the ends of the boat. They consist of two parts: the inner stems go on first and the strip for the canoe go over them. After things are trimmed and shaped the outer stems go on and cap everything off. The stems are laminated from thinner strips and glued up on the forms before everything goes on the strongback.
After the stems have dried the forms can be set up.
After the inner stems are fixed in place they need to be shaped to flow with the shape of the hull so the foam strips will lay flat on the sides of the stems as they are put in place. To the left you can see the start of this process. The two lines give me a center reference point and I’m sneaking up on it with spokeshaves.
After the stems are in place and shaped, it’s time to start putting the strips on. And it was at this point that I discovered a problem.
The less expensive foam that I had ordered at the last second was not as stiff as the Coracell, so it was not holding its shape between the forms when sanding. This meant that when sanding I would be hitting the high spots over the forms but would be depressing the material between the forms, which would lead to a very wavy canoe…hmmmm.
The easiest solution would have been to buy Coracell, but the material was pricey and I couldn’t return it so I needed another solution. The idea I ended up going with was to create an additional form between the existing forms so that there was support every 6″ instead of 12″. These forms would be eyeballed to be slightly small and then, as the strips were installed, I would add wedges so that the contour was smooth. The test is shown below, you can see the wedge on the middle form:
So – major crisis theoretically averted. Back to putting the strips on!
The first strip is at the high point of the gunwales in the middle of the boat, and with a wood strip you just let it sag on the ends to it’s natural shape. Since I was using very flexible foam, I used a wood guide strip to start the process. The pieces above (below in the pics) this strip are filled in later.
After this it’s time to start gluing up the strips.
The foam really shone in this stage. It was easy to cut, easy fix in place with pins, and it followed the contours nicely. Wood strips can be difficult to handle when trying to get them to conform to reverse curves, and people get very creative with hold down systems to keep them tight to the forms at the bow and stern, and happily I could just pin them down.
Now it’s kind of rinse and repeat as you build up the strips.
In the pics below you can see how the strips glue to the surface of the inner stem.
Once the strips get past the stems you’re in the area they call “the football” for obvious reasons (below left). At this point its advisable to do one side at a time, running the ends over the center line and then trimming them straight before filling in from the other side. Below right you can see this started with the strips overlapping the center line.
At this point you have to build up the sides at either end so you can get the right profile – so we’ll be adding strips below (above) the current waistline.
The next step is to fit and glue the outer stems. In order to do this you have to cut away the foam that’s covering the inner stems. Once again, the workability of the foam made this pretty straightforward.
Now the outer stems are glued on. Clamping presents interesting challenges on this project!
After the outer stems are epoxied in place they have to be shaped to flow with the contours of the boat. At this point you start to shape the foam as well so that everything flows smoothly. “Just remove anything that doesn’t look like a canoe”.
The next step is sanding and shaping in preparation for fiberglassing the outside of the hull.
Fiberglass
With the outside sanded we’re now ready to fiberglass the outer portion of the hull. This is easier than fiberglassing the inside so it’s great that it happens first and you can get a feel for the process on a friendlier surface. The real challenge here is managing the epoxy. This type of epoxy starts to set in 10-15 minutes, so you will mix and spread multiple batches as you wet out this large piece fiberglass. You have to balance the amount you’re mixing with the working time, simultaneously managing the current batch, thinking ahead to the next batch and also checking behind on the last batch and make sure things are “just right” which means saturated but not flooded and with minimal drips. It’s a two person job, so my son and I traded off mixing and spreading duties.
First you carefully drape the cloth on the hull, wearing gloves so you don’t transfer any oils to the cloth. You smooth it out, remove wrinkles and make sure everything looks good. At the ends you have to cut the cloth on the stem line and trim it so it will lay flat. Cutting fiberglass is an adventure but fortunately it all goes transparent when it’s saturated with epoxy so no one will be able to critique your cutting skills.
With the first coat the objective is to saturate the weave and get good, uniform penetration so you get a good bond to the substrate. Then later coats are added in order to fill the weave and give you a smooth surface
As it turns out I don’t have pics of the “before” or “during” of this process, so we’ll just jump to after the first coat:
After the first coat you have to trim the material that’s sticking up at the stems and add a reinforcing strip of ‘glass to that area (due to the amount of wear that you get there). After that you can start to add the additional coats.
When you re-coat epoxy they talk about chemical vs mechanical bonds. If you re-coat in 24 hours you get a chemical bond to the coat underneath, after that you need to sand the surface in order to create a mechanical bond. The chemical bond is desirable but not a huge deal. The way our schedule was working we decided to take a break after the first coat so had to lightly sand the hull when we started again.
Taking it off the Forms
After ever the outside has set it’s time to take it off the forms. This is pretty exciting, the first time you see it right side up and looking like a canoe.
The next step is to set it back up on the strongback and sand the inside. The inside contours are tougher to sand than the outside, but the foam makes it pretty straightforward.
Once the inside is smooth and vacuumed out it’s time to lay in the fiberglass. Again, this is more of a process than the outside but its surprising how you can coax out most of the wrinkles. At the stems things bunch up and you have to make some cuts, and fiberglass is really tricky to cut, but this is also not too bad. And fortunately it all goes clear when the epoxy hits it so no one will ever know about your sketchy fiberglass cutting.
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This is a little trickier due to the inside curves and the stems. Gravity also plays a part, pulling the epoxy down into the bottom of the boat so you have to pull and spread it up the sides, being careful not to pull the fiberglass off in the process. It’s a little hairy the first time but fairly forgiving.
At this point I sanded the outside of the hull. The weave is filled but the epoxy has sort of a pebble-y surface. So you sand it down to smooth it out, being careful not to hit the fiberglass.
Fortunately you start to see the weave before you get into it!
Epoxy isn’t UV resistant and will degrade in sunlight, so when everything is done the whole boat will get several coats of UV blocking marine varnish which will bring the finish back up.
Seats
While the above was going on I started the seats. Canoe seats are pretty simple but I fell in love with a design by Martin Step on the Green Valley Boat Works site (right) so lets turn this bad boy into a work of art.
These aren’t super complicated but they are much more involved than regular seats. He provides great directions so off we go to the land of mortises and tenons!
Gunwales
Okay – now we need something to attach the seats to. The gunwales are the pieces of wood along the top edges (or “sheer line”) of the hull, these provide strength, protection, and a way to mount the seats. Mine are from Ash, and since the boat is 16′ long I have to scarf two pieces for length. We have inner and outer gunwales so I’ll need 4 pieces, 2 for each. Let’s start on the inners.
Typically you trim the sheer line a little high, glue on the inside and the outside gunwales on the sheer line and then trim the whole thing flush. However, since I have a foam core I thought that might look odd, and I was also worried about durability and permeability, so I decided to cap the hull with 1/4″ ash strips. This made the operation a bit more involved…..
So first I had to establish the sheer line with the gunwale and then drop it down 1/4″ to allow for the cap strip (below left).
With that in place I could trim the hull with a flush router bit, leapfrogging the clamps and repositioning them to clean up (below right).
Then I glued the gunwale on 1/4″ high – you need every clamp and then some. Even with as many as I’ve accumulated over the years, I had to borrow some from neighbors.
Both inner gunwales on – you can see the slight offset to the edge of the hull for the cap strip.
Gluing in the cap strip. This is then trimmed flush top and side, and then the outside gunwales go on.
At this point I started to fit the seats. You have to figure out how to get them level side to side and fore & aft, so I snuck up on it with dummy seats, some sticks, and a lot of measurements. It was a little tricky but not as bad as I anticipated.
After this I glued in the cap strip, trimmed it flush and then proceed to install the outer gunwales.
Home Stretch
So now we’re getting close, basically down to the finishing work. Trimming, shaping, sanding. installation and varnishing.
Oh, and caning the seats…
Caning the !@#$%^&* seats
One of the very cool things about these seats is the caning pattern, and having never caned anything before I had the enthusiasm of the ignorant. I decided to go with natural cane; there is a plastic version but I try to stay away from plastic when I can.
I ordered everything from a caning supply house and watched a bunch of videos. Didn’t look difficult but definitely takes some patience, not a fast process and you have to keep track of where you are in the weave.
Since these seats aren’t square you have to figure out where the “corners” are for the weave. So as I was doing this I got a BIG surprise…somehow when I did the layout for the holes I missed two! So I had two fewer holes than what the plans and the weaving pattern called for…this was such a rookie mistake I was kind of in shock for a minute. I thought about making new seats but I didn’t have a lot of time because our trip was two weeks away. I also thought about making very simple seats that I could do quickly to get us through the trip. But after some panicking I decided to see what would happen if I did it with two fewer holes. To save time I used string and to my astonishment it seemed to work!
With the canoe seat gods smiling down upon me I set to work.
The cane needs to be soaked to be pliable, and it has natural defects so you have to manage those by breaking them off and introducing new pieces at the holes. The pegs hold the tension and enable you to tie off the pieces.
Without question this was the most difficult part of the project!
Sprint to the Finish
I was up against the clock at this point, but fortunately it was now pretty straightforward. Assembly and varnish should do it. I used gloss varnish on the brightwork (nautical term for woodwork), matt on the inside and satin on the outside.
Just add water
With the bolt-holes plugged and some final touch-up varnishing we’re done!
A few beauty shots…
And we're off!
Great trip and the canoe performed beautifully.
Postcard from the BWCA!
