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Discussion in 'Serious Boat Building Questions' started by funbun, Apr 15, 2009.

  1. funbun

    funbun Well-Known Member

    How do you figure out the capacity of your boat? I want build a flat-bottomed, zero rocker, zero flare outrigger canoe for mostly running up and down the river with a trolling motor. I'm thinking 21 or 22 inch beam, 16 feet length, with an eight foot outrigger off the port side. The trolling motor will be the primary propulsion, and I'll paddle if I have to.

    I mostly plan to fish bridge pilings at night. I'd run up fish, bridge piling and go straight back to the boat dock, I won't be turning much at all. Deep water no need for shallow water. My UJ pirogue can do that. I want a narrow long boat to go fast with low power (34lbs thrust trolling motor).

    It sounds crazy, but I really want to build it. Any suggestions or pitfalls to watch out for?
  2. JEM

    JEM Well-Known Member

    to get capacity:

    -select a draft depth (inches/centimeter of the hull in the water). Let's say 4"
    -Figure out how many cubic feet of water it would displace at that draft. (Imagine if your hull was only 4" high. How much water, in cubic feet, could you put in the boat?)
    -multiply the cubic feet by 62.4. That will give you gross displacement.

    I sketched up what you proposed and at 3" draft, you have about 285 lbs. gross displacement. About 370 lbs at 4".

    For the outrigger, attach it so the bottom of the outrigger is about 2" higher than the bottom of the main hull while in the water.
  3. Kayak Jack

    Kayak Jack Well-Known Member

    Displacement. One gallon of water weighs 8 pounds. For every gallon of water your boat displaces, that is 8 pounds. One gallon = 231 cubic inches. Shape is irrelevant.

    Put another way, if your empty boat weighs, say, 40 pounds, it will displace 5 gallons of water. For every 8 pounds of paddler & cargo you add, another gallon (231 cu in) will be displaced.

    If you want to retain, say, 6 " of freeboard, draw a line around your boat at that level, and calculate volume from there on down. A boat with flat bottom and straight sides would be easy.

    Another way, is to draw the line around the boat, place the boat in water that is calm and fairly shallow, and start pouring in 5 gallon pails of water. When the boat sinks to the line, multiply the number of pails of water times forty pounds. That is your capacity to 6" of freeboard. Make sure the boat is not resting on the bottom. Probably want another person there to help stabilize the boat.

    When done, grab one gunnel and roll the boat over to dump most of the water. When it is upside down and as empty as you can get it, snap roll it right side up again. This will capture the least amount of water. If the two of you can lift the empty boat, lift it up and carry it to shore.
  4. Jimmy W

    Jimmy W Well-Known Member

    Jack, Why carry the boat back to shore? Wouldn't the proper geezer technique be to let it float while holding on to it to keep your balance?
  5. dangermouse01

    dangermouse01 Well-Known Member

    Wooden boats full of water dont float! :lol:

  6. Jimmy W

    Jimmy W Well-Known Member

    Bear is going to disagree with that.

    Jack gave directions for emptying out the water, then wanted us to pick up and toke the empty boat back to shore. :?
    By the time that I get thru dumping all those 5 gallon buckets of water into the boat then dumping it back out, I sho ain't going to feel like picking the boat up and carrying it back to shore.
  7. keith

    keith Well-Known Member

    boy oh boy, ask a simply question and hell i dont remember the question. try this, boat building 101,( L x W x D x 62.4 ) sample: 12-foot long x 2-foot wide x .5 (6") deep x 62.4 = 750 pounds or 4"(.33) = 500 pounds and leave the water in the creek. later keith
  8. rpecot

    rpecot Well-Known Member

    Well, not really. Buoyancy, or specifically the buoyant force, is equal to the weight of water that is displaced. Ok, wait, I googled it...
    A 40 pound block of steel will displace only a small amount of water and sink. Flatten it out and bend up the sides and it floats.

    You are right otherwise, it's displacement. Which is based on the geometry of the hull. Figure out the volume of your boat that will be below the water line and you can figure out the buoyant force.

    Sorry guys, it's the curse of the engineer. My wife has learned to accept it.
  9. Kayak Jack

    Kayak Jack Well-Known Member

    A 40 pound object, floating in water, is displacing exactly 40 pounds of water, no matter what material of which it is made. A sunken object, regardless of weight, is displacing only its own volume of water, that weight would be less than the weight of the sunken object.

    A pound of lead and a pound of feathers weigh the same.

    At the point when I said to carry the empty boat back to shore, it was empty and upside own. Rotate it and float it back.
  10. funbun

    funbun Well-Known Member

    Keith, Thanks a lot. Where does the 62.4 come from?
  11. Jimmy W

    Jimmy W Well-Known Member

    That is the weight in pounds of a cubic foot of water.
  12. keith

    keith Well-Known Member

    to do a pirogue, you have to do triangles. later keith
  13. dangermouse01

    dangermouse01 Well-Known Member

    Triangles??? Cubic feet??? What's next? X's multiplied by Y's divided by a square root of Z.
    If its more than cypering a couple of oughts (maybe a double ought) and a goesinta or two, it's getting pretty complicated. :lol:

  14. Jimmy W

    Jimmy W Well-Known Member

    You could roughly figure the pirogue as two triangles joined in the middle. Then volume would be width times 1/2 the length times the height.
  15. JEM

    JEM Well-Known Member

    That's if the boat was box shaped. 750 is not accurate. 500 pounds (gross) for a 6" draft of a boat that size is more on the money.
  16. Steve

    Steve Well-Known Member

    Good Lord. . . All that calcalackin' kin git a feller confoosed! Jes' load the sucker up till it sinks then take out what ya need to git in yerself and have a 1/2 inch or so o' side stickin' out th' water........... :lol:

    Really, though; with all the different formulae you guys are giving here, I figure I could just about figger out the square footage of a 16 oz beer bottle. . . What got me was I had to read Keith's formula three times before I understood how 4" equaled 500 lbs. . . I am a bit on the slow side today :oops:
  17. funbun

    funbun Well-Known Member

    I want to build an outrigger capable of displacing my weight (250lbs max) should I go overboard. I plan to fish at night, and there be monster catfish in them waters.
  18. gbinga

    gbinga Well-Known Member

    OK - like this ain't complicated enough.... have you considered a pontoon instead of an outrigger?

    Go with two really skinny pirogue type hulls, span the space between them with a deck.

    More working space for fishing, and the whole rig wouldn't be any wider than the canoe with two outriggers (which is, in effect, a trimaran) Not sure if I spelled trimaran right, but ya'll see what I mean.

    The hull form you would want would be really long and skinny so as to get the best speed out of the power plant you specified. The hulls might end up looking more like rowing shells than pirogues.

    They had a (rather large and complicated) version of this at the Olympic rowing center here in Gainesville GA. It was called a "wakeless launch". The idea was to be able to pace, coach, supervise the rowers and kayakers, have a reasonable amount of speed available, and do it all with absolute minimum wake.

    I don't see why a minimalist version of this idea wouldn't be a good lightweight fishing craft.

    I've had a lot of fun over the years in the little "basshunter" type pontoons. What I'm talking about here would be stable like a basshunter, but a lot longer and sleeker and faster.

  19. funbun

    funbun Well-Known Member

    Thanks, George. That is a nice idea: wakeless launch.

    My main parameters are:

    a boat that can be carried in the bead of my truck. (No trailers. Sorry, I don't trust those Fulton truck mounts any more, but that's another story.)

    a boat that can be built and stored in a small space

    a boat pontoon like stability

    cool looks, gotta have the cool looks

    The design I keep coming back to is Gary Dierking's Wa'apa. The thing I like is that it's a separating hull, nothing is longer than eight feet in length.

    But a wakeless motorboat adapted for fishing could work. With the platform I could even take a nap if needed. Could a separating hull work on a catamaran? It should because Dierking's design has both a separating primary hull and outrigger. Maybe even lash some outdoor floor decking somehow?
  20. gbinga

    gbinga Well-Known Member

    Someone has plans for a rowing catamaran... JEM, Duckworks...? I'll hunt for it later but I'm in a hurry right now. I know I am not the only one who thought of it.


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