How to Calculate Buoyancy?

[otterbfishin]

Can one of you tell me how to calculate the buoyancy of a hull to establish its bottom size for carrying capacity?
Thanks,
otter

[Nick C]

The formula is on this web page: http://squid.law.cornell.edu:9000/cgi-bin/get-cfr.cgi?TITLE=33&PART=183&SECTION=53&TYPE=TEXT

I is a convaluted method that gives you a somewhat meaningless number for reference, but the Coastguard likes it'

[otterbfishin]

Thanks Nick, but I really was looking forward to a more simple caluclation that would perhaps relate square inches to pounds or some such. I'm not a really mathematical guy. I would hope that I could plug in a coefficient that would solve my weight to surface area of hull. For example, if my motor weighed 275 lbs, how much hull ould I need to support it at a 6 inch draft?
Is there anybody out there who can solve this??
Much in your debt,
otter

[seafox]

the more mesurement you tak the more acurate you will be but say you mesure your hull beam in 5 places at the waterline will be close enough

figure out how many square feet your water plane area is at the water line. is your boat straight vertical sides or dows the side flare out if the former then the bottom is the same size as the water line size and things are easer to figure.
say you figer your boat has 72 square feet devide that by 12 to determ cubic feet of displacement for each adition inch your boat sinks into the water in this case 6 cubic feet and times that by 62 pound per cubic foot to equell 372 pounds of weight your boat will carry for each aditional inch she sinks in the water.

for another example take a canoe 18 feet long and 3 feet wide at the center and we say it weights 150 pounds by its self. each quarter of the canoe looking at it from above is a rough triangle 1.5 feet by 9 feet = 13.5 feet devided by 2 becase a triangle only has half the area of a similar rectangle so adding the area of the 4 quarters together you get 27 square feet and devided by 12 gives you 2 and 3/12th cubic feet also known as 2.25 cubic feet or 139 pounds of displacement per inch of imersion so by its self the canoe will sit about an inch deep not counting the keel when I climb in weighting 300 pounds she will sink another 2.5 inches. if I sit in the middle of the boat load my x wife in and the doat sinks another 2.5 inches and we have 2 inches of freeboard left on the 8 inch high sides
dang we in trouble when 100 pounds of dogs join us couse they don't want to sit still
then the wife left me and the canoe sunk by the stern because the dogs jumped on my lap
jeffery
ps what size of boat are you trying to calculate?

[htom]

No simple way that I can think of. I'm sure it's done, so there is probably a complex way.

Given the total displacement of the hull, you could calculate the change in trim caused by the weight hanging off the end, and how that would change the fore and aft draft. You could then calculate how much displacement to add in the quarters ... this isn't going to be simple. I'm not even sure it's complex (detailed), it's probably complicated (the parts interact with each other in unexpected ways).

[Don Z.]

Originally posted by otterbfishin:
For example, if my motor weighed 275 lbs, how much hull ould I need to support it at a 6 inch draft?
Is there anybody out there who can solve this??
Much in your debt,
otterUmmm well,

with salt water at 64 pounds per cubic foot, your 275 pound motor would need a tiny bit less than 4.3 cubic feet...

so, with a six inch draft, you would have a box two feet wide by a little more than eight and a half feet long... and of course, it wouldn't be only six inches tall, that's only the part in the water... you'd need more for freeboard. Then again, the wood itself weighs, so you'd have to include that too.

but the motor isn't the only thing in the boat, as there would be fuel, and a fishing pole, and of course, you (OK, the last item is an assumption, but I feel pretty safe on the other two...) And the box we spoke of is just that, a box... so you'd be changing the shape to make it look more like a boat and less like a coffin.

The coast guard stuff would give you an idea of weight capacity and reserve bouyancy, but I'm not sure that's what you are asking. Instead of determining how much an existing boat can carry, it sounds like you're asking how big of a boat you should build to carry certain things. mmd can correct me if I'm wrong, but it's less a "surface area" question and more of a volume question. Once you figure out the total weight you want to float (including reserve bouyancy), you can divide that by 64ish. That tells you how many cubic feet of it would require to float it. Then, you have to build your vessel such that the immersed part displaces that many cubic feet. I used the box example because it's easier to figure the volume of a cube...Perhaps with more info someone can give a better answer...

[ 12-28-2004, 08:45 PM: Message edited by: Don Z. ]

[Nick C]

I thought you were looking for a legal definition. The legal definition is pretty much what everyone is getting around to in the replies. The thing is the legal definition is not the the safe limit. In fact it is most likely a long shot over the safe limit.

In a typical power boat when the hull gets swamped to it tips over due to the weight of the motor. This is the basis for the bouyancy "safety" factor. In reality the real load and passenger limit is less if you want to practice real safety.

This can vary regardless of what the boat rates for the safety plate that the state or Coast Guard wants. For instance if you have a dory it will handle more passengers or weight to one side of the boat than a boat with straighter up and down sides (dories have sides that slope outwards more than typical boats).

The real safety factor could easily be tested by loading the boat with gear and people in shallow water and having people move around to see what happens.

Consider a fishing boat, if you have three people in the boat and bring a large salmon up to the boat what will happen when two people race toward the same side and end of the boat. If that swamps the boat then it is a small lake boat and everyone should wear life jackets. I would not take a boat like that in the ocean or on a large body of water , except maybe if I stayed near shore and near my launch point.

The point is the safety factor is more up to you than some esoteric measurement. The Coast Guard measurement is more about a plate with a few numbers on it that they can base a ticket on so they can make it stick in court.