72' Billboard Skinned Longboat/Umiak

20' x 100' - Reused Vinyl Tarp https://www.billboardvinyls.com/collections/all/products/20-x-100-reused-vinyl-tarp
Just how big can you go with skin on frame boats?
Products https://www.billboardvinyls.com/collections/all
Umiak - Wikipedia https://en.m.wikipedia.org/wiki/Umiak
Do you think it's feasible to build a skin on frame long ship type boat. Using Billboard vinyl tarps for skin and common 2x4 and other dimensional lumber for framing?
Ultimately the goal it's to build a 72' skin on frame boat cheaply, quickly and lightly to do some coastal cruising in the Gulf, the icw and if I'm feeling frisky the Bahamas and some island hopping.
Spent over a year living aboard a project sailboat and working on my dinghy's after having gone through several inflatable boats I began thinking of using Billboard tarps as they're much thicker and more durable than anything I've seen on a dinghy and possibly most RIB's.

 

Regardless of what you build the hull from, the cost of the hull as a percentage of the final cost of the boat is going to be fairly small.
Hence is it really worthwhile to try to economise on hull construction at the expense of probably reducing the re-sale value considerably (because of the 'unique' hull construction method) when the cost of all the outfitting will be the same as for a boat built from eg fibreglass?
Especially so for a 'large' 72' boat.

I do not doubt your assertion that billboard tarps are more durable than ordinary hypalon - but equally one could say that a fibreglass or plywood dinghy is also more durable than an inflatable dinghy, regardless of it's tube construction material.

Have you seen this website about Geodesic Aerolite boats?
Geodesic AiroLITE Boats - ultra lightweight SOF canoes and boats; plans, projects and tutorials https://www.gaboats.com/

 

I don't know what dinghy's and RIB's you have seen, but the material you linked is to light. It clearly says that the weight range is 10-15 oz/sq yard (340-510g/sqm). Normal inflatable boat PVC weight is 29.5 oz/sq yard (1000g/sqm) and it goes up to 44 oz/sq yard for heavy duty. Some small lightweight boats use 17-19 oz/sq yard material, and that's about the lowest you can get inflatable boat PVC.

Yes you can make a 72' SOF boat. It does not really make economic sense but it can be done.

 

I have made skin of frame small canoes/kayaks with that material. A 72' boat makes no sense unless you are trying to escape North Korea. Plywood would be cheaper and faster, not to mention safer.

 

I thank you all for the responses and constructive criticism, much appreciated.
@gonzo You've made SOF Kayaks using Billboards in particular?
What was your opinion or experience with the matter, how did they hold up?
How do you see the plywood and the tarp as coming out the same in materials is it due to the amount of reinforcing needed the fitout or is it that you have a store of super cheap good ply somewhere?
Thank you @Rumars I appreciate you saying it can be done no matter how nonsensically.
@bajansailor I was hoping if I could build the thing as light as I desire. Was hoping to outfit it no heavier than a Travel trailer or even floating tent since that is what it will be. With as much chain and the heaviest anchor I can build or scrounge of course. If going cheap and ultralight what would you think is best outfitting such a ship?
Geodesic boats are one of the things I was thinking of actually just marvelously scaled up which seems to put it in league with the old school Umiaks.

 

Charles, those geodesic boats are a wonderful invention - however you cannot just scale them up linearly to 72' long.
Like with most things relating to boats, you would probably have to scale to a cubic law - ie if you double something (eg length), you would have to increase the scantlings by 8 times. It might even be to the power of 4 rather than 3, which would mean increasing the scantlings x 16.
If it was viable to build 70' geodesic boats, other folk would have done it by now.

 

Umiaks were never as long as 72'. Further, they were covered in narwhal skin which is much thicker and tougher than a sign tarp. I used billboards, sheet vinyl, sheet PE and blue tarps. They all work, but are not supposed to be for a durable boat. Are you trying for a proof of concept about how a geodesic hull can float, or do you want a good boat?

 

A 72' SOF boat has nothing to do with the light framing you see on kayaks. The frame has to be strong and stiff enough by itself, the only work the skin can do is put some compression on the frame. This means close stringers and frames plus crossbeams.
It takes just little more wood to do a strip planked version, or some thin ply to close the gaps. Using a PVC skin will not be cheaper or faster.
If you want to do such a boat you should study curraghs and bamboo basket boats.
I repeat, billboard pvc is not suitable for a serious boat. It is much to thin and is not properly reinforced.

 

So what is this "billboard PVC" ? Is it reinforced with anything ? No doubt you would need a lot of battens. or it will be dished noticeably, and if it stretches, start looking like seersucker !

 

@bajansailor @Rumars What would the scantlings and Frame members on a 72' SOF boat look like?
What do you think the 60ft long umiak referenced below was using as a frame?
"Hans Egede, a Norwegian-Danish Lutheran missionary to Greenland in 1721, stated that he had seen umiaks 18 m (60 ft) long."
A proof of concept might be it exactly. As a proof of concept how would you go about it @gonzo ?
So is a billboard alright if doubled?, Quintupled?
I once had a good serious beauty of a Motorsailor, right proper salty and haven't been on another her size I'd trade her for and I may just have her again. Having learned on experiences with her though I discovered somethings. One being I don't like marring old school great craftsmanship with my work in most cases structural and that I have more fun designing and trying out new boats ideas etc than repairing other peoples.
My thoughts were admittedly probably too light. I'm thinking 72' x 12' simple shape with a flat bottom probably made out of light ply possibly reinforced. With a long 2x or 4x serving as a center keel and possibly 2ft of ply up the sides and just above a theoretical waterline.
What's something it can be compared to? I know that up to a certain point you can minimize the pain of making it longer by keeping the beam narrow hence the 12'.
Thank you all greatly appreciated.

 

What did they use for the frames, the ones skinned with animal hides ?

 

This book has a lot of technical information on canoes, kayaks and umiaks. It is the only comprehensive study of traditional constructions and lines plans. https://www.amazon.com/Bark-Canoes-Boats-North-America/dp/1628737921 At 72' a billboard will not be right regardless of how many times you fold it. You could float it in a small lake or other protected body of water, but it will not be adequate for the use you propose. Go to a leather shop and get a scrap of full thickness cow leather. That will give you an idea of what an umiak is covered with. A proof of concept is a crude prototype build to prove if a method or material works. A flat bottom is not possible with skin on frame. Also, geodesic structures use small flat surfaces but never a large flat panel like the bottom you propose. You should first decide what you actually want to do. Your posts are all over the map, narrow the focus. As far as scantlings (the dimensions and shape of all the parts) read Dave Gerr's book. It is written with very little need of math, just simple algebra. https://www.amazon.com/Elements-Boat-Strength-Builders-Designers/dp/0070231591

 

@Mr Efficiency Billboard Pvc is 3ply, made up of two layers of UV protected pvc vinyl with a layer of nylon/denier inbetween supposedly coming to a "10 oz. - 14 oz. per sq yard / 12-15 mil" seems to vary wildly and not be consistent but having experience I would say the heavy end being 15 and occasionally 20oz being pretty rare and 7.5 to 10oz be common. The original Umiak frames where made of Driftwood and skinned with Walrus. Walrus skin apparently being up to 3 inches thick on a bulls shoulders and pretty damn thick everywhere else as well.
@gonzo I'll possibly look at those books if I can get them or an excerpt of the most relevant parts.
Triloboats: Avast, Ye Curvy Dogs! http://www.triloboats.com/modeltours.html
http://www.triloboats.com/BuildingtheT32x12.pdf
As for my aim if I could stretch out the 32x12 Triloboat into 72'x12' and keep everything the same except the ply I'd be happy. However I didn't think such simplicity possible and was surprised to discover that Umiak's traditionally have a flat partially timber framed bottom.
Umiak frame | AlaskaPhotoGraphics.com https://alaskaphotographics.photoshelter.com/image/I0000BCQBjAaHeQ4
Check out the framing and bottom on this one.
If I was using Ply at all I figured I'd do it strategically on the bottom, bow, stern and along the waterline though we start heading into the whole just skin it entirely of ply category.

 

So it is a calendared material, and designed not to stretch much. Remember it would be limited to taking up fairly simple, gently changing shapes, or you get creases,

 

I will formulate this very politely, but for the last time, billboard pvc is for advertising, small kayaks and dinghys. You need something designed for railcar or truck trailer covers, eventually aramid reinforced. Will cost you more then ply.

As gonzo said, you can't make a square box and skin it with fabric. You want a triloboat use plywood or other building methods.

Now back to SOF business. The biggest boats I know of are both 36' long, one leather bound and skinned monohull, one trunnel fasten tarred canvas/paper catamaran. Both made it over the Atlantic under sail. None were built out of big box 2x4's.

Presuming a displacement powerboat, and mechanical fasteners, I would say you need scantlings equivalent to carvel. As Mr. Efficiency said the PVC will not strech so you better learn how to sew and glue.