ply epoxy strength

greetings all.

I am becoming obsessed with the idea of building a ply epoxied boat upright. I know its mad but bear with me. I just want some feedback on whether anyone thinks I might lose out strengthwise because I just cannot see it myself.
I know most of the strength of a ply epoxy sandwich comes from the skins but can't the ply in the bottom of the boat take more responsibility by being eg 1/2" rather than 3/8"?
Looney build sequence as follows :-

1.Set up a cradle/jig for the bottom panels.
2.Fit the bottom, fillet and tape the seams. Glass it.
3.Flip the bottom. Fillet and tape the seams. Glass it. Fair and paint leaving a strip along the chine for taping the side panels.
4.Flip it and set back on the jig.
5.Make up and fit internal bulkheads as former for the side panels.
6.Fit the side panels (screwed from the outside into the bulkheads)
7.Fillet, tape and glass everything inside.
8.Fillet and tape the side panels. Glass fair and paint.

I think this method would be no less strong than building it upside down and the only place I might lose out is possibly some areas of finish.
I know that people like to fair the bottom immaculate but was wondering how drastically important this was, as a boat that lives in the water gets anti fouled regularly and this gets done with the boat upright.

thanks

arthor

 

Hi arthor,
when you look at my gallery you´ll notice that we build right side up (all models).
Although not S&G we feel better this way (and save time). For a one man show it is a bit easier upside down though, but after all only a question of personal preference.

Richard

 

what a fantastic looking craft you have there, Richard. Is the glass to seal it or is it intended to be structural as well? I like the close up of the veneers. I have always pondered the possibilty of using strips of ply as a subsitute for veneer planks. If a top layer was at 45 degrees to the botom one then the veneers in the ply would effectively give you multidirectional/multilayered veneering which I reckon to be very strong even without the glass.
Is there a website that has some more pics and details of those boats so I can drool.

arthor

 

Hi mate, thank you for the compliments.

which one?
Leave a comment (at the Gallery) if you like a picture, please.
The glass has to provide abrasion resistence, nothing else. Of course at a weight of over 300gsm multidir. it adds some strength too. Ply strips instead of veneer is not the best of both worlds, it resists bending too much to be a handsome material! And you can never be absolutely sure about the quality of inner layers and glue. Not worth a risk. And price is another argument against.
Sorry, I do not unveil my anonimity here, so no additional pictures at present.

Regards
Richard

 

The strength of plywood/glass/epoxy is created by both glass and wood, depending on what part of the boat you are discussing. Panels suspended between edges get the vast majority of their strength from the wood. The joints (chines, corners, etc.) get all of their strength from the glass/epoxy matrix (and whatever fasteners are used).
Upside down or rightside up, every build has a best way for one man. If you haven't built much in the past, go with the plans and you will save the most time 99% of the time.

 

Arthor,

An upright ply/epoxy build is certainly just as feasible as an inverted build. The exterior glassing, though, will be quite a nuisance (having to hold glass cloth against an overhead surface while you wet it out - not fun). The build sequence you describe to alleviate this issue adds the problem of flipping a partially-completed hull twice. It does make sense, though, at least in my opinion....

The plywood itself plays a major structural role in such a hull; the skins are more for abrasion resistance and moisture control. When you get to the point where the skins are handling most of the structural loads, you're probably not looking at plywood anymore, but rather foam or balsa core.

If you're going to go to all the effort of a true cold-moulded hull, you may as well do it properly with individual veneers. I don't think you'd save much if anything, in either the cost or time department, by cutting up ply into strips and doing multiple cross-planked layers of these.

 

Well spotted about the flipping of the bottom twice. Doh!!!
Best way there then is to just build and finish the bottom upside down then flip it, put the stringers, bulkheads etc in. It will mean that the only part of the boat that will need turning will be the bottom. Flatter and lighter than the whole hull and it eliminates any totally upside down glassing.

Points taken about the ply. It was just a thought that floated into my head. Maybe I'm spending too much time reading and thinking about boats. Maybe not though.

Richard, noticed your thread about the icebreaker capable boat. What a project. Good luck.

thanks again gents.

arthor

 

No need to be obsessed Arthor. Lots of builders use the right side up method for building ply/epoxy boats. Flipping the boat twice is no problem for most boats and there is no need for temporary framing. It's especially useful for S&G construction. If the transom is left open until the last moment before flipping, you can get in and out easily without a lot of climbing. The hull can be made completely rigid prior to the first turnover and there will be no distortion caused by moving the boat.

Matt, most builders of Offshore Fisherman boats along the North Carolina coast build them cold molded with plywood as veneers. Not necessarily because it's better, but because the material is more reliable, predictable and last, but not least, good ply is more available than good solid wood veneer. These are big expensive boats, so money is not always an issue. I suppose if quality mahogany or cedar veneer were readily available at a competitive price and in large quantity, they might choose that but---

 

Hey Arthor - I am planning a 28footer using the technique you discuss.

I have asked the NA to put a radius on the bottom of the moulds, so I can mount them on small wheels. When joined together, the whole 'basket' will be able to rotate 90 degrees either way.

When building a boat, access to the interior is always difficult when laying up inside FG. By tipping the boat on its side, the epoxy will be almost flat, and avoid all those runs.

Once the inside is laid up, and major bulkheads installed, I will bolt mould extensions to the top of the 'basket', so the whole boat can be rolled upside down, and the outside of the hull can be finished off.

Rough picture attached

 

Thats a good and proven way rwatson.
Look here:

http://www.boatdesign.net/forums/fiberglass-composite-boat-building/scaffolding-help-28383.html

post#5

 

oh yes, I know its done a lot, but the pictures were very good. Thanks for that.

Arthor seemed to think he was heading out on a limb, but its been done for years, as you say.