Bending Plywood and grain direction

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Cyclone
Posts: 62
Joined: Sun Jan 08, 2012 7:26 pm
Location: Barrie, Ontario

Bending Plywood and grain direction

Post by Cyclone » Sat Jul 01, 2017 11:14 am

I found it interesting that in the Glen L book "Boatbuilding with Plywood" there is a reference table for Fir plywood that shows a smaller minimum bend radius for bending the panel crosswise compared to bending the panel lengthwise. I was bending 1/4" thick mahogany marine plywood when cold molding and I always cut my strips so that the length of the strip was in alignment with the length of the plywood sheet (length of strip ran with the grain direction of the exterior veneer layers on the pane)l. In the case of the plywood I was using, I thought that the minimum bend radius would be in this direction and I was concerned that if the strips were cut with the exterior veneers cross grain to the bend that there would be a tenancy for the grain to open up on the outside bend radius. When the strips needed to be more than 4 feet long then I needed to cut them in the direction of the 8 foot long plywood sheet anyway. Looking at pictures of Douglas Fir plywood, the grain direction looks more random which might be part of the reason why bending crosswise is shown with a smaller radius than bending lengthwise. I think that the tighter bends that I had to do cold molding the 1/4" think mahogany marine plywood would have been easier had I been using Douglas Fir Marine Plywood. Perhaps Douglas Fir Marine plywood is more flexible than the plywood I used. Looking at the pictures in the Glen L book "Boatbuilding with Plywood" it would appears that Douglas Fir plywood was used.

I would be interested to hear of others experiences and opinions on bending plywood of different types, minimum bending radii, and grain direction.

Thanks

PeterG
Posts: 572
Joined: Mon Dec 03, 2007 7:08 am
Location: Connecticut

Re: Bending Plywood and grain direction

Post by PeterG » Sat Jul 01, 2017 5:57 pm

Plywood sheets are made with an odd number of plys so the face plys have the grain running parallel with the long dimension of the sheet. The grain direction of the plys of course alternates lengthwise and crosswise. Each ply resists bends across the grain better than a bend with the grain. You end up with one less crosswise ply than lengthwise in a sheet so the sheet is less stiff in the crosswise direction and able to be curved to a tighter radius. More plys in a sheet also means stiffer too.
Hope that makes sense?
Different wood species and manufacturing standards have big effects. Fir and okoume are known to be more flexible than meranti, so I would recommend stiffer plywood for framing and flat plywood parts, with the more bendy stuff for planking.
Sounds like you did it right by cutting your planking strips with the grain running lengthwise,
Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.

Cyclone
Posts: 62
Joined: Sun Jan 08, 2012 7:26 pm
Location: Barrie, Ontario

Re: Bending Plywood and grain direction

Post by Cyclone » Thu Jul 06, 2017 5:50 pm

Thanks for your reply Peter. Yes it makes sense that the plywood would be stiffer and more resistant to bending when bent in the direction that has the greater total thickness of ply layers (grain direction layers) running in the same direction. The difference between the minimum bend radius for bending the panel lengthwise compared to bending the panel crosswise that is given in the table for douglas fur plywood is greater that I expected. For example, for 1/4" thick material it shows the lengthwise minimum bend radius as 5 feet as compared to crosswise for the same thickness which is shown as 2 feet. That's approximately a 2:1 ratio. The plans for my boat call out 1/4" thick Douglas Fir Marine Plywood for the cold molding. I had a supply of 1/4" thick Mahogany Marine Plywood and so that was what I used. I found it difficult to make the required bends with the plywood I used and I had to steam many of the the strips in the areas of the hull with the greatest curvature. If I had to do it over again I would like to try the Douglas Fir Plywood per plans. I think it would have made things easier and reduced the time for the cold molding. Although the information I have found says that the plywood can make tighter bends crosswise that lengthwise, I think that the strips are best cut from the plywood lengthwise.

PeterG
Posts: 572
Joined: Mon Dec 03, 2007 7:08 am
Location: Connecticut

Re: Bending Plywood and grain direction

Post by PeterG » Thu Jul 06, 2017 8:22 pm

My only hesitation with douglas fir plywood has been with the grain checking on the outside surface of the bend. And, most of the suppliers near me have it priced the same as bs1088 okoume or meranti marine plywoods. I am using bruynzeel hechtout brand bs1088 okoume ply for my boat. While a bit pricey compared to other marine plywoods, it was locally available for same cost as ordering cheaper stuff from elsewhere and shipping it. It is excellent to work with, I've been scarf jointing it for planking on my Malahini.
As far as bends for cold molding, I will default to the mahogany or okoume, just my preference. One trick you could try for tighter bends where you seem to be fighting the ply, use narrower strips. They will flex a little more easily and helps a lot for areas with lots of shape like the bow which can have compound curvature (in two directions).
Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.

Cyclone
Posts: 62
Joined: Sun Jan 08, 2012 7:26 pm
Location: Barrie, Ontario

Re: Bending Plywood and grain direction

Post by Cyclone » Fri Jul 07, 2017 1:45 pm

The Glen L "Bluefin" I am building has definitely has a great deal of compound curvature up front and the stem is "S" shaped when viewed in profile which contributes further to compound curves. The plans stress that the cold molded bottom is intended to be covered in 2 layers of fiberglass cloth which would hide any grain checking that occurred if I had used Douglass Fir marine plywood. I did keep the strips narrow in the bow and that did help along with steaming.

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