Bouyancy

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........

 

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?

 

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.

 

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.

 

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

 

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

 

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.

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.

 

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?

 

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.

 

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

 

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

 

Go to Backyard Boat Builder Standards for USCG guidelines for calculating carrying capacity of home built boats.

 

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.

 

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

 

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