Wood movement on CS17 centerboard.

[Scott Dufour]

I know I'm about to highlight my woodworking ignorance here, but I'm a little confused.

I had thought that if you glue up a bunch of wood with parallel grain, and then glue another board perpendicular to them along the edges (in effect hiding the endgrain of the main boards with an endcap), you'd be running into trouble when the wood moved due to moisture changes. The endcap wouldn't let the glued-up board swell across the grain and something will buckle or split.

Obviously that isn't the case, because the Graham's plans call for endcapping one end of the centerboard to make the lever thingy that you raise and lower the centerboard with. (I've know doubt you people who know something about boats also know what that lever-thingy's really called). I'm sure he's never had a problem with wood movement on it.

Can somebody set me straight on the endcapping-wood-movement issue?

[Barry Pyeatt]

Yup, you can have wood movement. In fine woodworking circles that is a "NO-NO". On encapsulated woods there is less movement as there is less opportunity for moisture to get to the wood. Also it depends a lot on what woods you are using. Some of them have a lot of movement, some typically not much at all so they are fine. Hardwoods typically more than softwoods. Oak more than Mahogany. etc. Generally woods recommended for building boats are pretty stable in small pieces.

[Scott Dufour]

Ah, thank you.

It is one of those cases of being long on theory but short on common sense. :roll:

[John Burritt]

My recollection is that Graham has you rotate the individual small pieces of wood before you glue them back together to minimize wood movement and distortion. Check your plans.

[Barry Pyeatt]

You would turn over every other piece of wood so that the grain alternates back and forth to prevent cupping and warping. What I think is the concern here is placing an additional piece of wood cross grain. The potential is there for splitting that piece if it is glued to the end grains of the other pieces. But in the sized we are talking about here the risk is minimal.

If it were a table top, for example, and we glued end caps on the top to cover up the end grain, the likelihood of splitting is great with the seasonal movements of the woods. Fortunately some of the prettiest woods for furniture building are too expensive or not appropriate for what we are doing with a centerboard.

[Scott Dufour]

That's exactly what I was worried about, Barry. Of course, I knew all along that the cheap Lowes 2x4s I was using would be best and that's why I used them. Yeah, that's it. :wink:

Here's another question: If the forces working on a centerboard are similar to the forces on a rudder, then why wouldn't I use the same construction technique for the rudder? (Graham calls for plywood in the rudder.)

Mind you, with both this question and my original, I know Graham's got it right, I'm just not sure why I've got it wrong.

[Charlie Jones]

A centerboard takes a tremendous side loading and it's a cantelevered loading. Using plywood, half of your grain is not doing anything- it's running the wrong way. The way the CB is designed, ALL the grain is running across the load so you get max strength from it.

By the way, the Gougeon's have a brochure out on building rudders, dagger boards and centerboards. They reccommend strongly AGAINST using plywood for just that reason.

[Scott Dufour]

Thanks Charlie. Does that mean that there are far fewer forces on the rudder and so plywood is safe for it?

[Ed Hawkins]

I think the other thing going on here in the plans is that you take slices of flatsawn wood and reassemble it with each piece rotated 90 degrees from how it was in the board. When put together this way, the panel is essentially a quarter sawn board. Look at the end of the board, most of what you get at the home center will show the annular rings making an arc from one edge to the other (flat sawn). Quarter sawn boards show the annular rings in a nearly straight line from face to face. Quartersawn boards expand and contract much less and cup and warp much less than flat sawn.

[Ed Hawkins]

I think the other thing going on here in the plans is that you take slices of flatsawn wood and reassemble it with each piece rotated 90 degrees from how it was in the board. When put together this way, the panel is essentially a quarter sawn board. Look at the end of the board, most of what you get at the home center will show the annular rings making an arc from one edge to the other (flat sawn). Quarter sawn boards show the annular rings in a nearly straight line from face to face. Quartersawn boards expand and contract much less and cup and warp much less than flat sawn.

[Scott Dufour]

That's another thing I hadn't considered: that quarter sawn is inherently more stable. Thanks.