Plywood for frames?

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Beagleboy
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Plywood for frames?

Post by Beagleboy » Mon Jan 31, 2011 6:13 pm

This has probably been discussed before but if so I can't find it. Why couldn't plywood be used for frames? I've always thought that plywood is stronger and more stable than solid wood. I have seen some plans that use plywood but not Glen-L's. What am I missing?

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TiredParent
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Re: Plywood for frames?

Post by TiredParent » Mon Jan 31, 2011 7:57 pm

Some of the plans do incorporate plywood into the frames, but (usually) not as the sole frame element. Plywood's edge won't accept screws that fasten the battens and chines to the frame elements.

slug
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Re: Plywood for frames?

Post by slug » Tue Feb 01, 2011 5:47 am

Beagle; Check this recent and sometimes longwinded discussion on ply frames.
viewtopic.php?f=15&t=11689

Hope this helps, Doug

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Stuart
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Re: Plywood for frames?

Post by Stuart » Tue Feb 01, 2011 6:04 am

While I am no expert, what you say about plywood is not incorrect. As Tiredparent has stated, screwing to plywood on edge is difficult. While it will glue nicely, this only demonstrates the excellent wicking properties of the plywood edge where in the hull it will be constantly wetted. In addition, getting a good wood in ply is not always available. If you did use longer screws and had some kind of technique, you might still find voids or some inconsistancy inside the ply core. While it is possible to get teak in ply these can be expensive and in unregistered grades. The screws tend to split the plys depending on the hardness of the wood.
That being said, while I would have been advised to stick to the plans in my build I did fabricate some plywood frames because I had a very strong arch for the cabin top. I laminated 1/8" vener on top and 1/2 of solid wood on the bottom so that it looks like solid mahogany, but it ain't. The arch is very strong so the curvature of the group of woods will keep a significant amount of downward force under control. The problem is getting the screws to hold and I am relying on the glue joint to keep the top on because the screws would pull out very easily. But as I said, the arch is key and the top planks are attached to a big thick piece of mahogany stringer on each side just up from a bigger thicker piece of Ipe below it. There are stringers of Saple running the length in any event so I am fairly comfortable. Plywood is very strong. Perhaps if you bent the wood on its side and formed a stressed curve then the screws would hold better but plywood still tends to pull and wick at the edges and the first lamination is often very thin. I would tend to worry over the long term in high stress frames below the water line but... :idea:


Stuart

Beagleboy
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Re: Plywood for frames?

Post by Beagleboy » Tue Feb 01, 2011 3:44 pm

Thanks to everyone for the info. I knew there had to be a good reason. Plywood would just be too easy!

Rob Myran
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Re: Plywood for frames?

Post by Rob Myran » Wed Feb 02, 2011 7:14 pm

If one did want to use plywood for frames they could be sistered with some solid wood for screwing the hull to the frames. It seem like extra work though, unless there is a real reason to make lighter frames with plywood.
Another fine mess I've gotten myself into!

Oyster
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Re: Plywood for frames?

Post by Oyster » Wed Feb 02, 2011 7:24 pm

Rob Myran wrote:If one did want to use plywood for frames they could be sistered with some solid wood for screwing the hull to the frames. It seem like extra work though, unless there is a real reason to make lighter frames with plywood.
There are occasions where the method of using hardwood sandwiched with plywood works to your advantage. If a frame is shapely, then the grain can be an issue if you cut the frame out of a solid piece of timber. When doing so some of the grains can have a big run out and will crack over time.


So gluing thin plywood with glues deals with this issue. Overhead deck beams with a big camber and large spans can be done this way and save material and when using certain quality softwoods can also save a lot of weight, even while the labor is increased. Of course laminated strips flatways also eliminated a lot of these issues too.

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