Skin on frame scantlings

A part from an engineering degree how would one figure out scantlings for a skin on frame boat. I have built boyne style coracles and am familiar with skin on frame canoes/kayaks with steam bent ribs but I was wondering about working out the feasibility of larger craft the size of Tim Severn's Brendan or and Umiak both of which are over 30ft. Thanks so much.

 

"A part from an engineering degree ..." shouldn't you be telling us? Fie, fie! Tell me who you are so I can tell your professor that you have learned nothing in your classes. I'm not going to do your thesis for you.

 

The following webpage discusses this problem:
Membranes | Engineering Library https://engineeringlibrary.org/reference/membranes-air-force-stress-manual
Use a factor of safety of at least 3.0 .
Regards AlanX

 

.. and that safety factor (3.0) that, depending on what it is for, can be very high, to which variable should it be applied?

 

Stress! What else?

 

Thanks for all the helpful replies.
As for engineering professors I haven't taken any engineering courses at all, I'm a former EMT with a disabling line of duty injury trying to find a productive use of my time.
Thanks again.

 

I should clarify I have done a fair amount of design work but all of it has been seat of the pants and experiment based (good enough to land in a NASA tech brief) but building boats is really expensive for trial and error so I'm trying to learn the engineering side so I can at least get in the ball pack. If I can wrap my mind around the concepts in a reasonable way then I might be able to justify the expense of formal training.
Godspeed.

 

Direct stress, in plane shear stress, interlaminar shear stress, yield point, ultimaye strength, breaking point, ....

 

@BMP what is your overall objective here?

Are you planning on building this boat once it is designed?
Are you intending to recreate a voyage, like what Tim Severin did with the Brendan, or just use the boat for local coastal cruising, or....??

I remember reading his book in the late 70's about his voyage across the North Atlantic. Here is some info about him and Brendan -
Tim Severin's The Brendan Voyage https://www.ouririshheritage.org/content/archive/topics/traditional-boats-of-irelands-wild-atlantic-way-topics/tim-severins-the-brendan-voyage

What materials do you intend to use for the framework and the skin of your boat?

 

My over all goal is to design a skin on frame cruising multi-hull that is relatively easy and inexpensive to build using sustainable and non-toxic materials (I have a daughter with a genetic mutation, so all those prop 65 warnings that I used to chuckle at...), and is seaworthy and comfortable. My plan is to run the calculations for material and structural strength to see if it is even reasonable to proceed to the design phase. If it all checks out then design and build a smaller prototype for real world testing. If all goes well with that then design and built a full scale version and test it. I I make it that far then I might consider offering plans for sale and see where that leads, most likely back to school at some point in the process if it looks promising.

I have read the Brendan voyage several times, as well as watched the movie :

part 1 and:

part 2
as well as the Hands video on curragh building:

While the Brendan voyage was successful (with lots of materials testing) Tim Severin's China voyage is a cautionary tale on material selection and failure to take the time to test how they react long term in the marine enviroment (spoiler: the raft disintegrated).
I'm pretty sure a multi-hull SoF cruiser could be done with carbon fiber frame and kevlar composite skin(worked with both in the past and not really a big fan), but if there was a way to use sawn/pegged frames and canvas it would make it cheaper and much less hazardous to build.

I have wondered about this for years but now after being disabled do I have time to actually work on it.

 

What will be the source of the loads will you use for your analysis, including loads from the sailing rig and twisting loads due to waves?

Analyzing the membranes will probably be the simpler part of the analysis. Analyzing the structure including both local and global loads will be more complicated.

 

My initial thought is to design the structure for the maximum loads the rig and bouyancy can produce at the moment of capsize on the different points of sail. Then for the hulls to be point lifted at opposite ends, ie starboard bow and port stern. Those should give a good idea of what the structure is capable of. Then calculate the wave pressure on the skin (I was planning on useing 500 psf). Then using a safety factor of about 3.
Does that sound reasonable?

 

The goal would be coastal or offshore cruising if it seems like a safe idea. I have a very healthy respect for what the sea is capable of and have no desire to chance it with a cobbled together experiment. I plan on doing a lot of testing on lakes, rivers, and protected waters before venturing out on to the sea. Hopefully I can find someone with a wave boat (if that is what the wake boarding boats with ballast tanks are called) to do wave tests where if things go wrong a line can be thrown from shore rather than calling the Coast Guard.

 

Has been done before, including sailing over the Atlantic. https://octopusgarden.mx/our-history/

 

That is really interesting. I wonder what purpose the paper serves?