Jim is a retired software/electrical engineer who enjoys the outdoors. He likes to challenge himself with creative projects at home.
I built two cedar strip canoes and took each of them on week-long wilderness adventures. One of the things I fear is having my canoe sink or scuttle if I ever capsize. I have never capsized, in one of these wood canoes at least, but I don’t know exactly how well these canoes will float. I should try intentionally capsizing someday to find out, but I’ve never really had a time when I wanted to do that. They are wood and should float, but I read once that a wood/fiberglass canoe will float but not very high on the surface when it is filled with water, unless it has some floatation attached or built in. So, as an extra measure of caution I decided to add flotation chambers to the bow and stern.
What I Wanted to Avoid: A Waterlogged Canoe
Calculating the Buoyancy a Canoe Needs
Where to start? How large a flotation chamber does a small vessel like a canoe need? There are formulas to calculate that.
I found this information from Transport Canada on "Construction Standards for Small Vessels: Hull Design Requirements And Calculation Of Recommended Maximum Capacities For Vessels Not More Than 6 Metres In Length"
The swamped weight (Ws) of the vessel and permanently installed fittings, excluding the engine and engine related equipment, shall be determined as follows:
Ws = ∑ Whk + Kd . Wd + 0.69 . Wf
Ws = swamped weight in kilograms of vessel and fittings other than engine and engine related equipment
∑ Whk = Wh1k1 + Wh2k2 + Wh3k3...
Wh1, Wh2, Wh3... = the dry weight in kilograms of various materials used in hull construction
k1, k2, k3... = a conversion factor applied to the weight of each piece of hull material (Wh), to convert the dry material (h) to an equivalent weight when submerged in fresh water as determined by Table 4-3
Kd = a conversion factor applied to the weight of deck and superstructure, to convert the dry material weight to an equivalent weight when submerged in fresh water as determined by Table 4-3 in this document.
Wd = weight of deck and superstructure in kilograms
Wf = weight in kilograms of permanent fittings not included in Wd
The amount of buoyancy required (Wfl) for canoes shall be determined by the following formula:
Wfl = Ws + 0.85 . We + 0.55 . Wd + 0.1 . Wl
Ws = swamped weight of the canoe in kilograms
We = dry installed weight of the engine
Wd = dry weight of the battery in case of an electric motor
Wl = the maximum gross load in kilograms, less the weight of the installed engine and the battery.
The volume of buoyancy material (Vb) required in cubic metres shall be determined as follows:
Vb = Wfl / (1000 - 1.05 × Wb)
Wb = weight in kilograms of 1 m³ of buoyancy material used
Wfl = as calculated in 220.127.116.11.3 or 18.104.22.168.4 of this document.
Estimating the Volume of the Flotation Chamber My Canoe Needs
So I got numbers to plug into the formula above.
Canoe weight 55lbs (~25Kg)
- 9.1 kg of fiber glass laminate with Specific Gravity = 1.5, k factor .33
- 15.9 kg of cedar with Specific Gravity = 0.48, k factor -1.08
- 0 kg for deck
- 0 kg for fittings
Swamped weight = Ws = 9.1 kg (0.33 k fact.) + 15.9 kg (-1.08 k fact.) + 0 + 0 = (-14.2)
**slightly positive due to wood
Buoyancy Required = Wfl = (-14.2) + .85 ( 0 )+ .55(0) + .1(136)
(-14.2) + 13.6 = (-0.6) therefore almost neutral
**assume 2 – 150lb people (136kg) but weight in water is approx 1/9 actual body weight depending on density ;)
Note 0 kg for engine, 0 kg for battery
This swamped canoe with two people in it has almost NEUTRAL BUOYANCY, so the canoe won’t really help you stay afloat if you are sitting in it filled with water. Therefore assume it needs a buoyancy of 136kg (The weight of two people partially submerged, thus the 0.1 factor, wearing PFDs of course. Just an estimate!)
Assume Wfl = 136 (0.1) = 13.6
Then the volume required is: Wfl/1000 – 1.05(0) = 13.6/1000 = 0.0136 cu. m.
NOTE: original calculations in this article were incorrect because the lbs to kg conversion was inverted. They have been corrected.
**note: air is 0 weight
VOLUME REQUIRED = 0.0136 cubic meters or about 830 cubic inches
Volume of my chambers is about (3 X 15 X 12) / 2 cu. In. = 270 cu. In. each.
Of course this is a crude estimate but this should provide about 2/3 the buoyancy needed for 2 people. Barely adequate, by my estimates but, better than it was without the chambers. But then, I was also slightly on the conservative side when estimating the volume of my chambers. The buoyancy estimate should at least be in the ball park.
The chamber is just a sealed compartment with some volume of air. I’ve read that at least a gallon of volume on each end is suggested. I didn’t want to waste this space so I decided to put hatch covers in each compartment. Some of these type of chambers I’ve seen were made using strips of cedar that match the canoe. I decided to make mine from plywood because it seemed quicker and easier to work with.
- One piece of Project Panels Birch Plywood (Common: 1/4 in. x 2 ft. x 4 ft.; Actual: 0.195 in. x 23.75 in. x 47.75 in.) - Home Depot - $12.42
- Two 4” Seadog Polypropylene Screw-out Deck Plates – Black - Duckworks BBS - $6.92
- #8 x 3/4 in. Brass Phillips Flat-Head Drive Wood Screw (100-Piece) – Home Depot - $7.98/100
- scrap 6-oz. fiberglass cloth (left over from canoe build projects)
- Scrap 8-oz. carbon fiber cloth fabric plain weave 3K - 20"W 17.00 – left over from kayak build projects
- Polyurethane sealant
- Helmsman spar urethane spray
- Double-sided tape
- ½” foam weather stripping
- US Composites medium epoxy resin/hardener (left over from canoe build projects)
Making a Template
I first made a template of the panels that would seal off the bow and stern from stiff cardboard. I rough cut an approximate shape of each panel and pressed them in, gradually cutting away pieces until the fit where I wanted. Then I traced the shapes onto the plywood and cut them out, slightly oversized, with a jig saw. Those were then fitted into each end by removing material where needed and beveling the edges. Once I was satisfied with the fit I traced the hatch covers onto the panels and cut the holes.
Fiberglassing the Panels for Extra Strength
The panels were then covered with epoxy then fiberglass cloth applied to the back and carbon fiber cloth to the front. I made sure to coat the edges of the panels and holes thoroughly with resin. Once the epoxy had cured I trimmed away cloth at the edges. See my articles on building a canoe and fiberglassing a canoe.
Preparing the Hull Surface
Next I sanded away the varnish on the canoe where the panels would be set, then placed the panels and traced along their edges onto the sanded canoe surface. I removed the panels and applied a strip of double sided tape and ½” weather foam stripping about ¼” away from the marked line on the inside of where the chamber would eventually be. I figured this would provide a lip to help hold the panel in place while I sealed the edges and also help capture the sealant. I replaced the panels and held them in place with a small piece of wood taped to the floor of the canoe.
Sealing the Compartment
To seal the edges I used a mixture of epoxy, sawdust, Aerosil-Cabosil (silica), and water putty powder to form a thick paste. Just epoxy thickened with wood sanding dust would work, I just had this stuff available. I added a little graphite powder to make it black so it matched the carbon fiber cloth. I pressed the mixture into the edge of the panels where they met the canoe surface and did my best to make a smooth fillet. I had to apply another batch once the first one had hardened and was sanded to smooth out the joints and be sure the compartment was sealed.
Applying Fiberglass Cloth
I cut 3-inch wide strips of spare 4-ounce fiber glass cloth and applied them with resin/hardener epoxy mixture to the joint between the panel and the hull. Since the cloth does not easily bend around the curve of the hull and stay flat, I used four overlapping pieces. The area under the deck was the most difficult and I found it easier to apply the clothe while sitting under the canoe as it rested upside down on sawhorses. I applied one coat of epoxy to fill the weave then another once it started to cure and firm up. My biggest concern was that the compartment be water tight.
Once the epoxy had cured thoroughly, I sanded the surface to feather in the edges of the cloth, and varnished the panel and previously sanded portions of the hull with Helmsman semi-gloss spar urethane.
The epoxy is sensitive to UV light. After much exposure it can break down and become weakened. It needs to be covered with an exterior spar varnish that contains UV blockers. I used several coats, probably about eight, of Helmsman Spar Urethane spray semi-gloss.
Installing the Hatches
Next I placed the hatch covers in the opening and marked the location of the screw holes. On the reverse side of the cover rim I put a generous bead of polyurethane roofing sealant. I used this because I had a tube left over from when I installed my sauna chimney. I figured it would be good because it is weather proof. It's sticky and messy to work with, a better choice would probably have been some silicone sealant.
I kept the hatch covers screwed into their frames to prevent the frame from twisting when I screwed it to the panel with #10 - 1/2 brass screws. I was afraid the opening may become distorted so much that I could not screw in the covers. A bit of the sealant pushed out, letting me know the seal would be good. I wiped off the excess but because this stuff is so sticky that was not clean.
This was not the best work I've done in terms of appearance, but, it is functional, sturdy, water-proof, and provides a small amount of dry storage.
Jimmar on June 20, 2018:
Jake, it looks like you are correct. Oops. I'll have to change this.
Jake Bruce on June 20, 2018:
Hey Jim! I used your calculations to help me design my own flotation chambers. Thank you so much for putting in the work to get this out to the public. Excellent stuff!
I did, however, notice a mistake (I think). You state that the canoe weights 55lbs, but relate that to 121kg, when in fact 55lbs equates to 25kg.
I was curious to how that could've happened. And it looks like if you convert 55 kilograms to lbs, you get 121 lbs. Maybe this was the source of error?
Let me know if I'm plain wrong, haha!
jimmar (author) from Michigan on August 28, 2015:
Thanks for reading Larry.
Larry Rankin from Oklahoma on August 28, 2015:
Excellent, thorough explanation of this technology.
jimmar (author) from Michigan on August 28, 2015:
You could use the roofing sealant or even an exterior silicone.
DebMartin on August 28, 2015:
Wow. A lot of work. I'm impressed. My canoe (kevlar) has a small chamber in the front and back. I'd like to reseal both chambers as the canoe is at least 20 years old, probably older. Any recommendations for sealer? Will the polyurethane roofing sealant work or do you think something different for kevlar?