an idea for making a hull.. would it work? and how well?

hey everyone.. i was thinking of ideas on how to build a small fiberglass, or fiberglass like pontoon for a fishing boat... and the idea i come up with im curious if it would work for a larger design pontoon boat, or perhaps for making a solid shell of a hull to be used as a monohull or large catamaran?....

someone may have, and probably had this idea before.. but the idea is to make a canvas tube for the pontoon in the shape you want the pontoon to be... then inside that canvas bag, place and fill a weather balloon to inflate the canvas to the size you want... while inflated you could brush a resin into the canvas, (you can double, triple, or add as many layers of canvas as needed).. and once soaked with a resin such as epoxy it would harden and retain shape.. at which point the weather balloon can be removed and inserted into another shell to inflate

or.. you could use the inflated shape as a mould to apply fiberglass to.. layering sheets of fiberglass over this and sealing it in place until you have the hull

now im sure this would be done before, but would it actually be possible to do this on a large scale if i had some way of ensuring the canvas would take the proper shape once inflated?... obviously it would want to bulge in areas you may want it.. but perhaps with reinforcements where its needed, and other ways to guide shape would this be a viable solution for a one-off composite structure?... and for how large of a hull would this be suitable for if you used it as a fiberglass mould?.. you could always add composite structures to the inside (if its an open monohull and not a pontoon) to reinforce it further...

anyway, id like to hear any feedback anyone may have on this... im thinking im going to try it small scale... no more than a 14-16 foot hull and see what my results are... remember, even if it doesnt work there is still invaluable data to learn from it

 

Jason, this is not going to be a cylindrical pontoon. It will be larger diameter in the middle and smaller diameter at each end. Even if you try to use restriction hoops in the middle... Try it with a small child baloon and some fabric on a small scale. If you manage to make it cylindrical in shape, I would be very interested how you did it. Regards.

 

A couple of weeks ago someone posted this film clip from over 50 years ago: http://www.britishpathe.com/video/do-it-yourself-boat/query/boat

The principle is broadly similar to using an inflatable mould, and seemed to work OK back then.

 

well.. simple really, canvas doesnt stretch, and the latex in a balloon is very stretchy, the air inside the balloon will be forced to take on the dimensions of the canvas structure... it may be better instead of having 1 large ballon, but two smaller ones placed 1/3 and 2/3 of the structure, to fill the ends better...

for a small pontoon boat people ask me why not leave the balloons in and have inflatable pontoons but i guess its really no suprise why, considering the likeliness of a puncture

 

jeremy, you must have posted that message as soon as i posted mine.. i just looked at the video and its very interesting.. it would appear that was constructed from cardboard, had the fabric stretched over and then resin brushed into it... seems it can take a lot of pressure from that thin fabric since its being used as a speed boat

so i guess a hard canvas would probably be stronger... hmm, i should look into this.. see if i can actually shape canvas well enough to make a hull, though it would end up needing multiple pieces sewn together at different sizes to give it a 3D shape.. might be better to use cheap 1/4 inch plywood and make a a mould like they did on that video if i were to make a shaped monohull...

the balloon idea would probably work really well for pontoons since you dont have to give any real 3D shape to the canvas

 

You can make it cylindrical shape no problem, the long skinny balloons used to make balloon sculptures at amusement parks and parties is an example of a constant section balloon.

The real problem I see is the hulls would be round in cross section. If that is what you want than it would work, but most hulls are not perfectly round for other reasons. So it would limit your design options.

That said, it would be possible to make a simple frame out of inexpensive plywood to place inside your canvas "form", and than insert inflatable chambers to get say round bilges or compound curves along the length, and to tension to the canvas bag "form" over the frame. This is what they do on some brands of folding kayaks.

BTW, canvas does stretch, just not nearly as much as a latex balloon bladder. You have to consider stretch in the design if you are trying to get a precise shape.

 

i was planning on them being round in cross section for the pontoons...

im really not sure how this idea would work for a single hull.. probably be easier to make a wireframe out of bent and welded rebar to stretch the canvas over i guess if i was going to do something bigger... a rebar wireframe wouldnt be all that expensive on something large