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Capacity

funbun

Well-Known Member
Sep 11, 2007
214
1
Alabama
#1
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?
 

JEM

Well-Known Member
#2
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.
 

Kayak Jack

Well-Known Member
Aug 26, 2003
12,762
43
80
Okemos / East Lansing Michigan
#3
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.
 

Jimmy W

Well-Known Member
May 1, 2006
611
1
north georgia, USA
#6
dangermouse01 said:
Wooden boats full of water dont float! :lol:
DM
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.
 

keith

Well-Known Member
#7
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
 

rpecot

Well-Known Member
Nov 10, 2006
406
0
Katy, TX
#8
Kayak Jack said:
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.
Well, not really. Buoyancy, or specifically the buoyant force, is equal to the weight of water that is displaced. Ok, wait, I googled it...
Archimedes Principle said:
Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
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.
 

Kayak Jack

Well-Known Member
Aug 26, 2003
12,762
43
80
Okemos / East Lansing Michigan
#9
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.
 

Steve

Well-Known Member
#16
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:
 

funbun

Well-Known Member
Sep 11, 2007
214
1
Alabama
#17
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.
 

gbinga

Well-Known Member
Nov 7, 2008
736
2
Hoschton, GA
#18
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.

George
 

funbun

Well-Known Member
Sep 11, 2007
214
1
Alabama
#19
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?
 

gbinga

Well-Known Member
Nov 7, 2008
736
2
Hoschton, GA
#20
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.

gbinga