PDA

View Full Version : Mixing ply and solid planking



BrianY
01-12-2005, 03:38 PM
John Gardner suggests using plywood planking with a solid wood sheer strake for Amesbury Skiff in one of his books (forgot which one - it's the one with all the dorys and skiffs in it ;) ) The first two ply strakes are to be lapped and epoxy glued in the usual way, but what about the top (solid) strake? Is it wise to glue a solid plank to a ply plank because of the movement issues with solid wood? If not, I guess the way to do it would be rivets or clench nails, right?

Thad Van Gilder
01-12-2005, 03:43 PM
I have done various boats (garveys, skiffs, a semi dory, and will soome do a pram) with ply bottoms and cedar sides.

I have always either planked the sides with a dory lap or like normal clinker planking.

I bed the planks to the plywood with dolphinite.

I seems to hold up well...

-Thad

TimothyB
01-13-2005, 03:26 PM
Well... if you used a dimensionally stable softwood like cedar or pine (decent pine) then I'd bet you wouldn't have any issue with movement, given the board would be mostly dry most of the time. Mr. Gardner knows of what he speaks, and gluing it up would be the easiest and quickest way to go, not to mention it avoids the possibility that moisture will get into the plywood sides through condensate from fasteners.

But, if you were really worried, no problem attaching it mechanically, however I wouldn't nail it.. I'd screw or even lightly bolt it in so when it inevitably starts to get somewhat punky from all that banging around, I could easily remove it and replace it. I suppose you could rivet it too.

Which is another advantage to not using glue.

Always a compromise smile.gif

Thad Van Gilder
01-13-2005, 04:23 PM
In my experience, atlantic white cedar is not dimensionably stable, and moves a lot in wet/dry cycling.

-Thad

Lazy Jack
01-13-2005, 05:10 PM
The sheer strake of my gunning dory is pine the rest of the planking is 3/8 A/C fir. The sheer is lapped over the plywood binder strake, glued with epoxy and riveted with copper rivits. It has cycled through four seasons outdoors in Vermont with no sign of trouble. I survey the boat at the beginning of every season and the only thing I have done since launching is to add a fresh coat of hardware store grade alkyd enamel and an additional coat of varnish. That was two years ago.

http://www.imagestation.com/picture/sraid154/pd3bc24e17bc77f83ffec9773a3c2989e/f581cca8.jpg

[ 01-13-2005, 05:13 PM: Message edited by: Lazy Jack ]

Gary E
01-13-2005, 05:11 PM
Whutz the dif?

Some screw a plywood plank right to the solid wood keel and that's ok... Other screw a solid plank to the same place.. Tell me one is wrong? Which one and why?

Why not the same deal up at the shear?
How about at the chine?..izdat ok?

More and more reasons to not build anything from dead trees... cept maybe a fire..

TimothyB
01-14-2005, 09:17 AM
Originally posted by Thad Van Gilder:
In my experience, atlantic white cedar is not dimensionably stable, and moves a lot in wet/dry cycling.

-ThadLets see what the US Forestry folks say about it:
<code>
Atlantic White Cedar
--------------------
Type of shrinkage 0% Mca 6% MCb 20% MCb
Tangential__________5.4____4_____1.8
Radial______________2.9____2.3___1.0
Volumetric__________8.8____7.0___2.9
</code>

As compared to something known stable, and has a comment in general characteristics "it has very low shrinkage".. Western Red Cedar:

<code>
Western Red Cedar
-----------------
Type of shrinkage 0% MCb 6% MCc 20% MCc
Tangential_________5.0____4.0____1.7
Radial_____________2.4____1.9____0.8
Volumetric_________6.8____5.4____2.3
</code>
And for reference:

<code>
Northern White Cedar
--------------------
Type of shrinkage 0% MCa 6% MCb 20% MCb
Tangential__________4.9____3.9____1.6
Radial______________2.2____1.8____0.7
Volumetric__________7.2____5.8____2.4
</code>
Well, it looks very close to me.. in fact Tangential movement at 6% MCc is identical. It's possible that low quality, low ring count A. WHite Cedar has bad properties, but I'd venture to say that given this study, and given good stock, it should be pretty durn stable. It does have somewhat higher 'Volumetric' movement, at lower moisture contents, so some care should be taken, but if you seal the wood the movement should be reduced significantly.

Personally, I would go with Northern White Cedar given it's lightness and it being more stable than Atlantic White. I have a bunch of it out back, in fact, waiting to be put to use. It is amazingly light, too. But if I had a bunch of A. White, I'd use it for the same stuff. I'd just give it some additional sealing.

Thad Van Gilder
01-14-2005, 10:42 AM
I didn't say cedar was bad!!!! Christ, that's just about all I plank with, save a few varnished sheerplanks
!!!!

All I know Is when you pull a white cedar planked hull out for a while to work on, the planking shrinks a lot more than h. mahoghany or teak or doug fir or longleaf.

If it shrinks and swells a lot in wet / dry cycling, that would make it dimensionally unstable, right?

I know that if you epoxy ( or any other glue) a chunk of cedar to a square of ply, and put it through 50 wet dry cycles, the glue brakes due to the shrinkage and swelling rate of the cedar. do this with quarter sawn fir, or longleaf, and it doesn't happen, as they shrink and swell less.

-Thad

[ 01-14-2005, 10:49 AM: Message edited by: Thad Van Gilder ]

TimothyB
01-14-2005, 11:17 AM
Hey Thad! I wasn't criticizing you smile.gif

No, I just checked out the numbers, and it seemed that they say that A. White Cedar does move some, but not all that much more than Western Red, which is reputed to have very low movement.

So, I would assume that if you used tight ringed A. White you wouldn't see a lot of movement.

I'm going to guess, though, that most A. White around these days is NOT tight growth, so probably would shrink and swell alot.

Anyhoo, I was just on a numbers kick this morning. smile.gif Plus I've been posting up a storm.. maybe I should get a snack!

--T

Thad Van Gilder
01-14-2005, 01:21 PM
I gues the numbers do point to that. I dunno. Maybe those number do match what I observe...I'm no engineer... crap, I'm doing my mastre's in philosophy!!!!you can't get much further from engineering than that!!!!

-Thad