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Floatation compartments

Discussion in 'Strippers, Stitch-n-Glue, and Other Wood Composite' started by erbinsky, Jan 3, 2011.

  1. erbinsky

    erbinsky Canoeist/Builder

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    Hi all ; I'm building a 16' stitch and glue canoe . The plans I'm following show the floatation compartments being airtight sealed by a bulkhead and the deck. A fine idea if you can be sure it's 100% waterproof. I'm not sure my building skills can garauntee that. Would floatation foam give a similiar amount of displacement? I've never used it before , how does it work and how much would I need? Thanks...Jeff
     
  2. Todd Bradshaw

    Todd Bradshaw Sailmaker

    Sorry this is long, but if you really want to understand the issues at play the subject gets a bit complex.
    TB

    Water weighs around 62 pounds per cubic foot (salt water and fresh water are slightly different, but 62 lbs. will be close enough for our purposes). So for every cubic foot of water we displace, we gain 62 lbs. of flotation power - less whatever we used to displace that water. If our cubic foot was air, it's virtually weightless and we gain the entire 62 lbs. of flotation power. If it was foam and that foam block weighed say 4 lbs. then we gain 58 lbs. of flotation. If the cubic foot was a 40 lb. chunk of solid wood or other material, it would only yield 22 lbs. of flotation (along with a rather heavy boat).

    So if you fill the tank with foam, the amount of flotation it will yield will depend upon the density of that particular foam, with lighter, less-dense types of foam yielding slightly more flotation for a specific volume. Many foams (including some of the mix-and pour varieties) are classified by their density or weight for a given volume and you'll see some foams listed as "3 lb. foam", "4 Lb. foam", etc.

    The idea of pour-in-place foam that expands to fill the cavity is often a bit better on wooden boats that it really is in practice. If water does ever get in there, it's hard to get it out and the foam can contribute to rot from trapped moisture, so the inner walls of the cavity need to be really well sealed (epoxy). The foam also generates a lot of heat and pressure as it expands and can actually stress, or even blow out, seams if you accidentally add a bit too much. Listening to the seams on your new boat creak and crack as the foam goes off is not a fun experience (been there, done that).

    The final potential problem with poured foams is that they (like a lot of polyurethane stuff) aren't completely dimensionally stable and often shrink over time. There are hundreds of fiberglass canoes out there with what are called "suck lines" at their ends. Poured foam is the easiest and most common flotation foam used on glass boats and the scenario goes like this: They wall-off the end with a nice, glassed-in fiberglass bulkhead and pour the foam in through a small opening to fill the space behind the wall, which it does. Over time though, the foam often shrinks, pulling the sides of the hull in the stem area inward as it does. The bulkhead can't shrink to match it and soon you start to see the raised outline of the bulkhead coming through on the outsides of the hull. Structurally, it's generally still fine, but it can be pretty ugly when you start distorting the smooth shape of what was once a nice looking stem area. So, you're often better off making your fail-safe tank filler material out of other types of foam (peanuts or small sawn cubes in a sturdy bag, cut and stacked slabs of styrofoam, empty soda bottles, ping-pong balls, etc.) They may have small spaces between them that could flood and cost you a little bit of flotation power, but the result tends to be more stable and is often a better idea on wooden boats. It's also usually cheaper and you don't have the toxic fumes, heat and hull creaking of the pour-in foams to worry about.

    How much do you need? That's a big question and a lot of people don't understand the issue very well. It's a wooden boat and wood floats, right?.... Well, kind of yes, and kind of no. Most wood does float, but in addition to keeping the parts of the boat itself that don't float from sinking, it may or may not have enough reserve buoyancy to also keep the canoe's former occupants afloat or allow them to self-rescue.

    There was an alien on Star Trek once who referred to humans as "ugly bags of mostly water". In some ways, he was right. Water weighs essentially nothing in water, so when the average Joe jumps into the pond (or falls out of his canoe) the water in him is neutral and he will generally weigh somewhere in the 16-20 lb. range. Even if his on-land weight is 200 lbs. or more and he went totally limp, you could probably keep him at the surface with a piece of 20 lb. test fishing line - or - if he's awake, we could hand him a cube of styrofoam that's about 8.5 inches on all sides (a bit over 1/3 cubic foot in volume and displacing more than 20 lbs. of water) and he could stay afloat all day hanging onto the cube. Life jacket manufacturers know this and the foam in a typical life jacket displaces around 17-20 lbs of water.

    So designers will shoot for enough float tank volume to support the hull and keep it afloat (factoring in whatever additional buoyancy the hull materials themselves may provide - wood, quite a bit, fiberglass, virtually none) and add a minimum of around 1/3 cubic foot of extra flotation for each potential occupant. That way, the boat won't sink and as long as the former paddlers cling to the swamped hull, they will all stay on the surface.

    Here, however, is where things get tricky. You may only weigh 17 lbs. in the water, but that's only while you remain in the water. If you slither over the gunwales of your swamped canoe to try to start bailing it out - and raise part of your body out of the water in the process, that part quickly returns to it's dry-land weight. With your torso exposed above water, the weight you are exerting on the boat may instantly go from 17 lbs. to 117 lbs. or more - overpowering the reserve flotation built into the hull. Swanp a canoe in deep water, hang onto the gunwales and you can be there all day if you want. Swamp that same canoe in three feet of water, slither in, stand up and it will go straight to the bottom - because as soon as you stood up, your weight increased, big-time, overpowering the flotation offered by the hull and tanks.

    So, while there isn't a set formula for how much extra flotation volume is best, if self-rescue-ability is your goal, the more tank volume you add, the better. The swamped boat will likely be easier to bail out and you won't need to stay quite as low in it as you bail. Float tanks take up interior space, so obviously you'll need to strike some sort of compromise. As to the difference between the weight and buoyancy of air-flotation vs. foam flotation, it's actually fairly minimal in the grand scheme of things and generally no more than five to ten percent or so at most. If you're just switching from air-filled tanks to foam-filled tanks, there probably is no real need to make the tanks any bigger just to make up for the weight of the foam.
     
  3. BLK

    BLK Enthusiastic about Wooden Canoes

    floatation

    Hello,

    I built a 16 ft. CLC Sassafras three years ago. This is also a stick and glue canoe. What design are you building?

    If you follow your plans/instructions you should not have a problem getting a complete seal on your bulkheads. In my case, each of the planks fit together nicely and the joints are sealed with epoxy. The ends are little harder to get everything together, but should seal up tight with fiberglass and epoxy.
     
  4. OP
    OP
    erbinsky

    erbinsky Canoeist/Builder

    This is also a 16' sassafras.I'm following the book " THE CANOE SHOP " by Chris Kulczycki. This is the second one of these i've built , on the first I glued a bunch of 1" pieces styrofoam together and cut them to shape. I also added a drain plug to each bulkhead. I had a cedar strip canoe that I repaired several years ago that had floatation foam in it that seemed to be holding up well. I'm building this one with the mindset that I'm going to keep it for myself and I think I'll stay with the styrofoam or packing peanuts and do my best to make it airtight.Thanks for the replies... I'm always open sugestions and advice from other builders...Jeff
     
  5. Tom Widney

    Tom Widney LOVES Wooden Canoes

    Thanks Todd,
    I learned a lot with your reply and thinking back while on dry land with this in mind it truely explains the experience of very rapid changes in stability in canoe rescue. At the time it just seems to be best to stay as low as possible if in a swamped boat. A lot of times in the large hay stacks in the South Western rivers you just want the darn thing to float until you get it ashore and then bail. Clip in floatation bags seem to work just fine for that type of big water with the ability to change up the configuration if necessary.
     
  6. Pantry3cow

    Pantry3cow Curious about Wooden Canoes

    I just read this post and find it interesting. Most canoes i’ve Been in do not have floatation compartments. The canoe I’m building (wee too) calls for one but my plan was not putting it in. What danger does this cause other than the ones pointed out in this post?
     
  7. Todd Bradshaw

    Todd Bradshaw Sailmaker

    Some materials (wooden rib and plank constructions, foam plastic sandwich constructions, etc.) already contain enough buoyancy in the hull material itself that they don't need flotation tanks. This does not mean, however, that just because the boat won't sink it has all the flotation you might want if you get into a situation where it has been swamped. Others (aluminum, fiberglass, Kevlar, other composites) have no flotation at all unless you install tanks or ethafoam bumpers (Sportspal canoes). They will sink if swamped without tanks. In any case, the danger in omitting the tanks on a plan which calls for them is that it may make it much more difficult to recover from a capsize, and depending on the particular design, there may not even be enough flotation power to keep the boat up at the surface. Minimum safety is usually thought of as a boat with enough flotation power to keep the canoe itself at the surface in addition to its occupants in the water and hanging onto it, even if they don't have life jackets on. Eliminate tanks at your own risk. I suppose there might even be some legal liability if someone else were to use your modified boat and have a problem, but I'm not a lawyer.
     
  8. Pantry3cow

    Pantry3cow Curious about Wooden Canoes

    Thanks Todd
     
  9. Pantry3cow

    Pantry3cow Curious about Wooden Canoes

    Todd I’m getting the gunwales ready for install and posted some questions I was curious as to your thoughts?
    Coat with resin or varnish?
    Screws or glued on with resin?
    If screwed which screws are best?
     
  10. Todd Bradshaw

    Todd Bradshaw Sailmaker

    I'm not a big fan of epoxy coated trim wood. There is just something about it that often tends to look fake to me. Also, epoxy is not paint or varnish and doesn't go on as smoothly as paint or varnish does, so in order to get a good looking job you would probably need to sand it smooth before varnishing over it to protect the resin from UV (which is critical). It is also the hardest finish to chip, but the hardest to neatly fix if you do get any chips in it. I just varnish my gunwales, seat frames, thwarts, decks, etc. old-style. Oil (Deks Olje, Watco, etc.) can be nice finish as well once you have built up enough of it, but it tends to need more maintenance.

    I've replaced enough gunwales over the years due to damage or weather that I don't have any desire to glue them on and later possibly be forced to cut them off. I just use screws - stainless or bronze (I avoid brass, which are prone to shearing the heads off, leaving you with a real mess to fix). For the last set of gunwales I did, I used square drive bronze #10 flat head wood screws when installing new gunwales on my Old Town Guide. They aren't original-style Old Town brass slot heads, which may offend some purists, but they're fantastic screws. I use a #10 Fuller tapered countersink/counterbore drill bit for them, which is drastically better than countersink bits that you are likely to find in most local stores. It cuts very cleanly and you can adjust it to countersink flush, recess the screw just a bit or recess them enough to install a wooden plugs in the holes after installing the screws if desired.

    Mad River uses some sort of stainless Phillips flathead screws which I think are classified for sheet metal as the shaft is all the same diameter, unlike wood screws which get thicker up toward the head. They work fine, though I like the look of brass or bronze hardware better on wooden canoes.
     
  11. Pantry3cow

    Pantry3cow Curious about Wooden Canoes

    Thanks again. Todd
     

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