Interstellar dynamics of fiberglass over plywood!

I was hoping the rhetorical inclusion of interstellar would capture the interest of a wide variety of people. I am designing and building a 28 ft ish open deck catamaran using plywood with a hard Chine flat bottom hull. My curiosity is this: how much increase in structural Integrity is there from draping the hull in glass? My notion, as perverse as it may seem, is to Glass only that portion of the hull below a Mark 1 foot above design water line. The reason is that finishing all of that fiberglass above my mark seems like a whole lot of work. I have downloaded study plans from a couple of the Jedi Masters in plywood catamaran design and even printed off 50 pages of Robert Woods website and put them under my pillow, thinking I might gain something osmotically from the exercise. It seems that in an open bridge deck catamaran 6 mm ply is about right for the hulls. I would gladly pay the weight penalty by going up to 9 mm ply if I did not have to glass. If the glass is mainly for abrasion resistance and water repulsion then I'm that much further ahead. Plan a is to roll on a coat of epoxy to the bare wood with a light sanding afterwards to prep for paint. The outsides of the holes I would pay a bit of attention to but the insides I could care less really as far as the Finish goes because they are not seen as much and are largely shaded and as such all of that work to get that mirror s finish is time I would rather be sailing.
Looking at a roughly 90 degree intersection of two sheets of plywood, if it is wrapped in glass on the outside then in one particular moment of stress the glass would support the outside of the structure and make a bit of difference in strength. Counter to this is that such a stress should not be exhibited much if bulkheads and stringers are applied appropriately.
In the late 80s I did build a house all by myself, having never worked in construction before, learning plumbing and electrical and such as I went. I built the house to code and amazingly, it still stands! So this is not my first foray into foolishness! Darts and Laurels all appreciated!
As much satisfaction is gained by building one's own vessel, all the more so is gained by designing and building one's own vessel, even if the trade-off is performance not as Sterling as if it were designed by someone who had completed their Jedi training in the art.

 

The answers to your question[ the length/strength of what specific string fellow grasshopper?]are in the history of this forum,you have heaps of reading to do, it is fascinating stuff.
If you for some reason aren't engrossed in it ,don't pursue your project, it means your attention to detail and your urge to learn isn't high enough . The key to using ply is painting every join before it is placed in situ, end grain/the edges absorb water more readily, any space that pools/traps water easily is your problem area/s[ie exquisite detail]. You will be closely replicating a working design with all the loads and stresses already calibrated if your boat is going to be sound and practical. Replaceable rubbing strips can protect the ply. Only some epoxies will absorb into ply effectively, dodgy ply with voids is your biggest issue without glassing, like yourself I have a heapa reading to do so it's back to the catacombs for me. I suppose my member name and equally cumbersome intro to the group suggests I am well suited to respond, good luck and ..ohmmm ...
PS If you like ply buy a classic that needs a little work,[every boat ever regardless of material]][tap it very carefully all over] marvel its genius and enjoy the camaraderie of your helpful co owners.
PPS tri's are better.. ha

 

I have done a good bit of reading but didn't find anything that zeroed in specifically on my query as to what degree sheathing plywood with fiberglass is a structural exercise.
I have built several thingies out of Jeep BC exterior plywood with rather amazing results. I am willing to throw the big coins out for proper Okhume marine plywood and understand about epoxies getting into the Grain and such.
I have even considered painting the outside like some of the Bahamas/ Jamaican work sailors boats I've seen, with a brush and whatever bright colors are handy, n order to reduce the probability of getting sucked down the black hole of seeking perfection! Once you start down that road it's easy to never come back! It is like setting up a room for symmetry and before you know it everything must be symmetrical.

 

PS, Tris might be better, but Proas are yet better still! Haha

 

"The Elements of Boat Strength: For Builders, Designers, and Owners" by Gerr
and
"The Gougeon Brothers on Boat Construction: Wood and West System Materials", I believe, do answer most of the questions on the topic.

 

Much of the information you seek will be buried deep.

From Antagon's recent thread "twisting 8mm ply", posted by Yellowjacket
How Tough are They? https://epoxyworks.com/index.php/how-tough-are-they/

 

The Gougeon book is available free online now I believe. Pity I paid a fortune for it.

The problem usually isn't strength, it's stiffness. If the build is stiff enough it is plenty strong enough. The glass isn't there to make the boat stronger or even stiffer. Wood is terribly stiff for weight. The glass is there to stop water getting into the wood particularly due to impacts and abrasion.

Remember salt water isn't what rots timber, it's fresh water. Ever seen old movies where the crew are swabbing the decks of a wooden ship ? They are washing it in salt water to kill rot. So whatever you do you have to keep water out of your wood,otherwise small saving in build time and trouble with bite you many times over in future repairs.

The Gougeons pioneered epoxy saturated plywood boat building and many of their designs do not sheath they ply. People add glass as insurance. In the old days they used things like dynel (sp) which is lighter and cheaper than glass but adds no strength.

Build Gypsy if you want a chined 28' open deck cat, best boat in class...and endless build options. Cuddy cabin, open deck, demountable, chined, rounded, ply, foam, solid glass, strip plank. Every permutation is available most have been built, it's a great boat.

 

Its not easy to come back if your boat starts to rot away before you can even finish it...

 

Ha, well if you've seen my stuff you'd know I haven't gone down that path, I think I would enjoy the aesthetics more, but structural details, ones that could lead to something breaking with no back up bothers me. Stuff like filling the joins well, compatible glues ,paint , etc, separating metals and removing angles that trap water is attention to detail that I really appreciate.

I agree it is more fun asking on a forum than getting loaded down with complex equations, so other peoples immediate experience is perfect if they want to, great. And of course there is a very large plump of serious education reading this finding firstly the crummy terminology we're both using and secondly, you can't just fudge this stuff. Part of the very concept of a floaty thing is the potential for catastrophic consequences, each detail, yada yada, I could go on here but it has been said by others so well over so many forums, .it does feel good to say though, you should try it, more so exactly because I/ we only have my/ and your bit of experience and others, professional anecdotes to draw on,. It is a bit shallow I spose pontificating in an uneducated manner and gaining satisfaction from it...hmm, I like boats , I wont excuse myself just saying
But if you must build copy something free really good, exactly, then make your mark in altering[spoiling,.. ha,..err] making ideal. But spare thought to those that help and plans are a part of their living,plans are pretty cheap really, it had to be said ,,,but they are the crew you need to ingratiate yourself here with , . Thats for stupid questions i've posted and been tolerated,..phew.. ha what's that smell?

Anyway to answer your question I’d say it is really taking it up to your joinery skills and attention to detail /fill, if you aren't going to tape all your joins, and I think you would need extra triangles or quad in your joins, or/as bracing, for the bulkheads and perhaps single strip over the joins ie 2 over one., there would be a way.
if you are taping your joins then even a light glass over the top is just sensible to stop a tape from catching and flaring, still there's wooden rubbing strips could help..
There is something lovely about that attention to detail, micro managed timber instead of relying on fibreglass tape that blends with plastic so well, all oil based like the glue that holds your ply together, with potentially less weight done perfect , but weight isn't your worry, read up on micro cruisers here, I have been which makes your post relevant and helped me and also partly enabled/emboldened forthwith[ha]] to comment, and I know stuff, so thanks .

Only heavily glassed hulls say a 50/50 ratio would start thinking about using the glass for much more than protection and maybe a bit of stiffness, just keeping the glues in place really.

So your thinking building a Warram look alike with a bit of Greek traditional fishing boat thrown in,.? old trimarans look better and are better anyway, .overall .ha! cheers .

 

Thank you that was a very interesting read as well as short and perfectly on point. I appreciate something that goes directly to the question I ask.

 

Thank you for your input and it validates what I've been thinking. I was under the impression most of Warren's designs were V bottom?? Many of Woods' designs seem to be flat bottom or at least buildable in a flat bottom.

 

Yes I think they are, I was responding to what seemed like a preference to simplicity in building, I haven't really explored that style of build ,sharpie etc, a hard chine doesn't seem to spoil performance much at all, when sorted nicely I have read, pleased to have input you found useful. I'm fascinated in all stuff about boats pretty much, regards

 

Glass can also serve to prevent the plywood surface from checking.

 

^^^ That's a good point, although you'd hope if you used a good quality paint...

Wharrams are a religious experience not a science. I've owned a few sets of their plans and they are incredibly wasteful of materials and labor. They don't sail particularly well. I would not build one. Then again I would not build in ply.

Richard Woods offers V bottom, dory (flat bottom chined) rounded and even a few multi chine boats. Many of his boats can be built in foam, solid glass, ply and strip. Then on top of this many offer deck options cuddy, no cuddy, trampolines, solid decks. Some offer daggers or LAR keels. He is remarkably accommodating. And once you've built it you will have a boat that sails well and is well fit for purpose. It is no surprise he is now probably the most prolific seller of catamaran plans.