plywood mast

[jelfiser]

just a question :
may be an homebuilt plywood-epoxy or plywood- pvc -epoxy have better performances than one in alluminum ?
does anyone know tested techiques of building it ?

[kerosene]

doesn't sound right. layers of wood would seem more sensible as all the fiber would go along the mast. Plywood has the 90 dgeree zig zag in terms of fiber direction and the fibers that are perpendicular to the structure don't really do much to add strenght to the mast,.

[PAR]

You can build a plywood mast, but it will be quite heavy for it's dimensions compared to other hollow mast building methods and materials. Mast construction relies on fiber orientation (vertical) which provides compression strength. Plywood's compressive strength isn't as good as solid lumber (half of the veneers are running the wrong way) so to get the same strength, you need a lot more ply compared to solid lumber.

A cold molded mast, which is sort of home made plywood, will make an excellent mast construction method. It's labor intensive, but would produce a light, strong stick. The reason is simply because the fibers can be oriented in the direction you need it, plus provide some cross grain strength.

PVC doesn't have much of a chance, except in very small lengths. There are several reasons for the discounting of PVC, but the biggest reason is it doesn't accept glue and must be welded (either heat or chemically) which adds considerable difficulty. It's also very brittle, too flexible, etc., etc., etc. the list is lengthy. Some have tried foam filled and 'glass sheathed PVC for small masts, gaffs and booms, but the sheathing sheers pretty quickly, though the foam does stiffen the tubing substantially.

[tom28571]

There have been a number of masts and booms that were made with plywood as a partial or even complete component. One was a box section with spruce fore and aft edges and 3mm ply on the sides. Built this way as a mast or a boom, ply can work well. I have built a 40' wingmast (15" by 5") for a racing tri with 3mm shell and an interior spar. The only solid wood was for the nose and gluing surfaces at the interior joins.

So, it can be done but the spar must be designed to use the best properties of plywood and avoid the negative. No doubt Par's suggestion to laminate wood at an angle, say 20 degrees off vertical, would be superior to normal plywood in any application, but far more effort and expense. An aluminum wingmast of the above dimensions would have been much heavier than the 3mm ply and prohibitively expensive. How could such a mast be built?

PVC has few applications on a boat and none of them structural.

For a home builder, solid or hollow birdsmouth wood or aluminum can hardly be beat.

[Crag Cay]

I presume by 'PVC - epoxy' you are talking about PVC cores like Divinycell. They are certainly used structurally in boats.

I know Derek Kelsall was a champion of building masts in E glass - Foam sandwich: He would lay one glass surface down on a flat mold and immediately bond the foam core to it. Whilst still very green he would then fold it into a mast profile with the preformed face becoming the inner surface of the mast wall. He would then laminate the outer skin once the shape was fixed. Essentially he was using the first stage as a former that was incorporated into the mast, but where the outer face was the principal load bearer.

[jelfiser]

thank you to all for usefull informations.. i was in particular thinking to the wood sandwich composite ( pvc between two layer of plywood) ... I read that some class 40 of julien marin or minitransat went very good. see:http://www.julienmarin.com/_gb_newslire.asp?num=20
That material is very to carbon sandwich for weight and less expensive
So someone think that an hard chine chine wood sanwich mast may be built ??

[Raggi_Thor]

You can buy special made plywood from Denmark for keels and rudders where all the grain is longitudinal, approx 20(?) degrees variation.
I got some samples a while ago and it looks good, used by several boatbuilders in Denmark and Sweden, www.plywood.dk