Bouyancy

[Ted1785]

I need to find out how bouyant my boat is. I'm building a boat. Dont know the volume of air in the pontoons yet. I need someone to tell me the equation or someway to find out how much bouyancy I have for whatever volume of air.

[Willallison]

Ted, perhaps you could give us a little more info to work on. From what you say, I gather the boat is supported by a couple of pontoons - are they cylinders?

Basically any boat - regardless of shape, building material etc will displace the same amount of water that is equivalent to its mass. So a boat weighing 1000 lbs will displace 1000lbs of water. To work out the volume, you need the density of water. Salt water weighs 64lbs per cubic foot, fresh water weighs 62.4.

You need to work out how heavy your boat is (including fuel, provisions, crew etc) and from that you can work out how much bouyancy is required to float it.

[Ted1785]

The boats going to weigh about 200 tops with about 250 lbs between me and my gear onboard. And, I'll be just using it in freshwater lakes. And, the pontoons are going to be make out of just something that can hold air. Havent decided what to use or the size yet because I dont know how much bouyancy I need.

[duluthboats]

Ted,
Lets round your displacement to 500lbs. You might catch a big fish or take some extra beer with you. 500lbs of fresh water will fill:
15ft of a 10inch tube,
12ft of a 12 inch tube,
4.5ft of a 18 inch tube.
These are rough numbers but should be near enough for your needs. If you double the length the water line will be half way up your tube. If I have this wrong, I’m sure it will be corrected shortly, many who lurk here are better than I at numbers. ;)
Gary

[james_r]

I get 10.2' for the 12" tube. The others, rounded off, seem correct.

[duluthboats]

Thank you :D Checked myself, 10.19'
Gary

[Ted1785]

is that 10.2 or 10.19' on each pontoon or on one for 12". And, I'm 17 yrs. old, Theres no way I'm going to be drinking on a lake.
Ted

[duluthboats]

LOL!!! Your right of course, drinking and boating don’t mix. If you had 2, 12 inch diameter pontoons, 10.2 feet long, each pontoon would be half way under water. If the boat, gear, and captain added up to 500 pounds.
Gary

[Ted1785]

thanks Gary. How bad would I be off if I have 2(8'x12") pontoons? Thats what I want to make them. That way it fits into my truck. And how do you know how to figure all this. I would just like to know so if I change what I'm going to use for pontoons that I can figure it myself.

[james_r]

Time for a little math!

Using the original all-up weight of 500lbs in fresh water (62.4lbs/cu.ft.) your boat would displace 500/62.4=8.01cu.ft.

To calculate the volume of a cylinder, first you have to calculate the area of a circle of the same diameter (Pi*r^2) and then multiply that by the cylinder's length. In the case of a 12" cylinder first you divide by 2 to get the radius and then by 12 to convert to feet, which gives you a radius of 0.5'. The area in square feet is 3.1416*0.5^2=0.7854.

To get the length, divide 8.01 by 0.7854 and you get 10.2'. In other words, a 12" by 10.2' cylinder would float just below the surface if it weighed 500lbs. Two 10.2' cylinders would float halfway up. If you were to use two 8' long cylinders they would float deeper than halfway.

By increasing the diameter by just 2" to 14" your two pontoons would only need to be 7.5' in length each to float halfway. If you were to make them 8' long you'd be ahead of the game.

Use the following equation in Excel to make the calculations yourself: (B1/B2)/(PI()*(B3/24)^2) where B1 is the weight of the boat in lbs, B2 is 62.4 for fresh water or 64 for salt water and B3 is the outside diameter of the cylinder in inches.

You can also calculate the exact draft for any given diameter/length/ displacement combination but we'll have to get into a bit of trigonometry.

[duluthboats]

Yah, what James said. :) Thanks James, if I had to explain it, Ted would have never figured it out. I’m not sure why it hadn’t dawned on me to use Excel, good tip.

Ted, I would also agree with James that 2, 12”X 8’ tubes would be marginal. I would use 3, or 2, 14” tubes.
Gary

[Ted1785]

thanks for the help Gary and James, but I dont have excel....

[lockhughes]

wait a sec guys...
your calculations...
you're figuring on JUST ENOUGH bouyancy? D'you mean the craft will float, but be awash? Don't you think that'd make the newbie boaters anxious? How `bout a little freeboard eh?
Lock

[james_r]

Ted:

Not a problem, Excel is overkill for this situation, a $5 calculator can do it. If your calculator doesn't have Pi use 3.14159. If it doesn't square numbers, simply multiply the radius by itself. I only mentioned Excel because most computers will have a spreadsheet program of some kind which would allow you to quickly do a bunch of what-if scenarios. Or you can use pencil and paper, just like us BC (before calculator) guys used to do.

Lock:

Well, I thought Gary and I had been pretty clear about it. We're talking about using two tubes, not one, of the calculated length. As an example, one 14" by 7.49' cylinder weighing 500 lbs will be awash. Two of them, lashed together, will have a draft of 7".

Ted estimates that him and his gear will weigh 250 lbs and that the craft will weigh another 250 lbs. Personally I think that a couple of PVC pipes or aluminum tubes (or whatever other suitable material) of the required diameter lashed together with a few 2 by 4s will weigh less than that.

[Ted1785]

The 250 for the boat is just a rough estimate. it could be 50 one way or the other. But for one alum. pontoons are to expensive to make and most anything else looks like ****.

[Ted1785]

one thought......those old propane bottles(for a normal grill, their about the size of a 5 gallon bucket), would they work for pontoons. If I welded like 8-10 together. That way I would have a welding surface for mounting the deck........

[james_r]

Yikes! First a word about safety. You probably already know this but for those who don't and may be tempted to try to cut into an old propane tank - empty propane tanks still contain propane vapour and WILL BLOW UP RATHER VIOLENTLY if you try to cut into them with a metal saw or torch. So unless you're 150% sure you got every last little bit of propane out of those tanks don't do it.

A steel tube will cerainly be heavier than other materials so you'll have to consider that in your calculations. Have you thought about trying to find something suitable in a scrap yard?

[Ted1785]

for one, I'm not going to cut into a propane tank. I mean take the fitting out of the top and let them air out first and then weld the ends together. The ends they sit on and the top guard parts together. I've had a propane tank explode on my before I know how to handle them. and if I use propane tanks, I know it would be heavier, more of a 2 man job to load and unload but still, would it be enough bouyancy to float roughly 500 lbs??? And, about a scrap yard. Theres no such thing around my area. Only old car junk yards with nothing but cars and engines.

[james_r]

Even if you let everything out and remove the fitting you'll still have propane vapour at atmospheric pressure in there. That's still enough propane to blow up a small house (not that you'd be doing this indoors). Even if you don't cut the tanks, the heat from welding may be enough to ignite the propane. Remember that flammable liquids burn, whereas flammable vapours explode.

To calculate the volume of a propane tank measure its circumfrence and height and work out its volume from there. The weight of the tank is stamped on the collar.

[lockhughes]

Ted -

I am so totally impressed by this:

http://www.rqriley.com/frp-foam.html

Shape the volume you need/want, outta foam, coat with FRP or carbon (or cotton-reinforced - eg cheap cloth material!), go boating.

Lock

[Ted1785]

About the weight thing. I'm building pontoons right now, would that foam support about 500 lbs tops? Anyway I might try it if I knew how to.

Ted

[lockhughes]

Originally posted by Ted1785
About the weight thing. I'm building pontoons right now, would that foam support about 500 lbs tops?

Remember - it's the volume of the hulls that provide the flotation, not the foam itself. The foam just stiffens the structure. The weight of the foam will not be much, but you would plan a slightly larger volume of hull to compensate for it not being empty.

Anyway I might try it if I knew how to.
Ted
I say again:
Shape the volume you need/want, outta foam, coat with FRP or carbon (or cotton-reinforced - eg cheap cloth material!), go boating.

I'd just make sure any nicks/dings are sealed promptly. Haven't seen a "closed-cell" foam yet that doesn't take up water if given the chance. Polyester resin would be cheaper for the shell, but epoxy would be not too much more $$$ for the hulls you're looking at, and the epoxy'd be stiffer/stronger. Paint, to protect the epoxy from sunlight.

[fredjer]

I also have some questions about buoyancy, but my boat is already built. I've built a drift boat that is now for sale, and I'm trying to figure out how much weight I can safely claim it will carry.

I'd feel comfortable carrying two people, but what I feel comfortable with might not necessarily be legal. I could float it and see how much it would hold before it started to feel spooky, but that might not hold up very well if I sold the boat and somebody drowned.

Is there a web site with a capacity calculator, where I could plug in my boat's dimensions and find out how much it would safely hold? Is there a better way than that?

[fredjer]

james_r wrote:

Even if you let everything out and remove the fitting you'll still have propane vapour at atmospheric pressure in there. That's still enough propane to blow up a small house (not that you'd be doing this indoors). Even if you don't cut the tanks, the heat from welding may be enough to ignite the propane. Remember that flammable liquids burn, whereas flammable vapours explode.

I agree 100%. The only way to work on propane tanks safely is to purge the tank of any remnant of propane. Filling it with water is one way to make sure all the propane is out, but then you have to drain the tank and wait for it to dry. Other ways to purge the tank would be to drop chunks of dry ice into the tank (the CO2 displaces the propane), to run an air hose down to the bottom of the tank and let it blow about 10 times longer than you think is necessary, or go to a compressed gasses dealer (welding suppliers often have other gases) and ask them to purge the tank with nitrogen.

[lockhughes]

Originally posted by fredjer
I also have some questions about buoyancy, but my boat is already built. I've built a drift boat that is now for sale, and I'm trying to figure out how much weight I can safely claim it will carry.

Howdy Fredjer...

Funny, I just read a tread on this today... some human-powered-boats folks suggesting two options.

One idea was to scale model the hull, make a mold of the model hull, fill the empty mold with water to it's designed waterline (fully loaded eh?), then carefully measure the volume of water in the mold. Quoting from the thread:

Then pour the water into a measuring cup/container and measure it --accuracy counts, because your error will be multiplied considerably!
The actual displacement of your full-size hull will be the CUBE of the relative size of the full-size hull to the model. Example: if your model is 2 ft long, and the full-size boat is to be 18 feet long, the full size boat is 9x the length, but will have 729x the volume (9 CUBED) of the model.
The bigger the model, the better the accuracy of this method (less
multiplication of any error).
[end of quote]

The second suggestion was to use a bit of math... again a quote:

the way i do it draws from basic logic and middle school geometry

break it into a simple pattern of basic shapes. for example, for your design, one could use two trapizoids and 2 triangles to represent the vertical profile. since it is the same all the way down, just multiply by the height for volume. i suggested earlier a method of easily constructing the curved hulls without freehanding or taking much time, if you refer back to it you will see how simple it is to use the above in conjunction with horizontal sectioning to get a fairly accurate #. you also will always know to which side the error will be on. if this explaination is unclear let me know. i am of the keep it simple stupid mentality and although you can use basic calculus to figure
the volume quite easily, this method is much more easy to visualize for me.
[end of quote]

As I understand it, and I'm no expert, ok?, is that once you know the volume of the water displaced, then you know the weight of said water, and that's the weight of the boat et al that you're floating.

Don't know of any programs, but many (most!) here are a lot more knowledgeable about this stuff than I!

Cheers

Lock

[fredjer]

Hi, Lock,

I'd like to be able to fill either a model or the hull itself "to its designed waterline," but (blushing with shame) I don't know where the waterline is or should be. Since I'm a gambler by nature, I designed this boat myself, primarily by eye, and I'm not sure how deep it should sit.

I'd be happy if it would float in eight inches of water, with two guys in it. Maybe I could just calculate the volume of the boat up to the 8" mark, translate that to the weight of an equal volume of water, subtract the weight of the hull (140 lb), and see what I'm left with.

Fred

[james_r]

Go to Backyard Boat Builder Standards (http://www.uscg.mil/d8/mso/louisville/WebStuff/comdtpubp16761_3b.pdf) for USCG guidelines for calculating carrying capacity of home built boats.

[james_r]

To find the waterline, since the boat is already built, take it to the water on a calm day. Stick a couple of your friends in it, equal to the weight that you would normally have onboard and make sure they're seated in the proper spots. Then simply mark the waterline in a few spots with a pencil.

[fredjer]

Thanks to Lock and James_R!

I followed your guidance and links and found that my little boat is going to hold more than I thought it would.

Lock included a thread that said, to "break it into a simple pattern of basic shapes." As I looked at my boat, I realized that the middle portion was nearly square, so I calculated the volume of that section. The bow and stern were a tapering triangle and a trapezoid, respectively, so I calculated the volumes of those sections. To keep my calculations simpler and more conservative, I ignored the fact that the boat gets wider as it goes up.

Empty, my boat will float in 2-1/2 inches of water. With one 200-pound guy in it, it'll sink to 4-1/2 inches. With two guys in it, it'll take just a smidgen over six inches of water.

The USGS guidelines James_R referred me to were also very useful. Using the same calculations I performed above, those guidelines show that I could safely carry a third person if I wanted to.

Thanks to both of you. I feel a lot better now, and the calculations weren't nearly as hard as I thought they'd be.

Fred

[lockhughes]

Originally posted by Ted1785
About the weight thing. I'm building pontoons right now, would that foam support about 500 lbs tops? Anyway I might try it if I knew how to.
Ted

Ted - I'm thinking of building two little pontoon hulls for myself.

Here's my plan.

Use software to develop the pontoon shape I think I need. The right software will loft it for me. I think that's the right expression -gives me "templates" to cut out to form the hull sections? Basically tubes, pointed bow and stern.

Cut the pieces I need out of cheap cotton cloth. Have a friend sew these pieces up into "bags" (for want of a better word) that are shaped like the hulls I want.

Mix up low-density expanding foam, and pour enough into the bags through a hole, to fill the bags completely when the foam is fully expanded.

Figure out next, how to finish the hulls by covering them with more cloth and resin. Cloth might be more cotton, or glass or carbon fibre, or a combination, depending on strength/abrasion resistance needed at different parts of the hull. Resin will be, whatever doesn't eat the foam! Final coat resin, probably epoxy as a water barrier.

Then fill surface dimples/whatever as necessary, and paint.

I'm hoping others here will tell me this is a crazy idea, if it is.

Actually, what I'm *really* hoping they will say is, neat! (Do people still say "neat"?)

Anchors will be introduced into the decks of the bag, and probably supported outside the bags by cheap knock-together temporary jigs to hold `em in the right position before the foam is introduced and as it expands and sets.

I believe I could build slightly *lighter*, but not as safe or as maintenance free, or as easily!

Cheers

Lock

[Ted1785]

I think that buying a factory fabricated paddle boat would be a hell of a lot easier.. A lot cheaper also, plus a lot faster.