Oak Deckbeams

[JGPierce]

My Haven 12 1/2 calls for white oak deckbeams with a crown of 1 1/2" over 5 feet. I have cut the beams on a bandsaw leaving a little extra beyond the marked lines. I was wondering what the best way is to get a nice even curve which is the same on each beam? Should I clamp them all together and use a longboard on the upper convex curve? What about on the lower concave curve?

Thanks.

John

[Ken Hutchins]

See my godzilla.

http://www.woodenboat.com/forum/show...light=godzilla

[JGPierce]

Ken,

You're a genius!



I haven't installed the deckbeams yet and was hoping someone had some ingenious way to ensure the crown stays smooth and the same on each beam. What I am planning on doing is clamping them together and then hand planing and sanding them. The hard part will be the lower concave areas. Any thoughts?

[TimothyB]

Hey folks.. haven't been here for a LONG time (you wouldn't believe how life can hit you) but I've been trolling about here lately and I thought I would reference an article I saw some time ago on this very thing.

I'm sorry that I can't come up with the reference (EDIT: I did! See below), but the gist of it is this (and me being a Math guy, I remember it pretty well). According to the article, to do deck beams absolutely right, you don't want to cut them all out and bend them with the same arc of curve. It turns out that the proper shape for a deck beam is an arc section of an ellipse.. but circular sections are almost as good. They will be different at every station, and a real simple way to loft them up is to do this.

(By the way, if you loft deck beams this way, you shouldn't have to do much in the way of planing, if any at all, since this lofting process gives you the true sections)

I'm sure everyone is familiar with an angle tool.. the one that we use in carpentry to fetch angles from work and transfer them to material.. with a wing nut clamp to hold the angle. This is a BIG version of that tool, with a spot for a pencil right on the inside of where the two bits meet. Basically, make a BIG version of that tool with two battens and bolts, clamps, whatever you like and make sure a pencil can be seated right where the two battens meet. The battens must each be as long plus a bit of the widest distance between frames at the sheer, from the outside (assuming that is where the deck beams will end).

So, you get to your lofting floor and mark two marks at the distance where the ends of the deck beam will lie. Most likely this will be the half width at that station * 2, plus the width of the frame stock. Snap a chalkline or draw a line between those marks. Find the center however you like and make a mark above that center which is the height of the deckbeam at that station. This way, we're drawing an outside line.

Now put two big nails in the two end marks, and place your angle tool down over the nails so it peaks above it. Adjust the angle so that the height mark is exactly centered in the pencil hole and then tighten it up. Put in a pencil and then lightly "swoop" the whole assembly from one side to the other.

Presto, a perfect section. Every deck beam will be slightly different, but this is taken care of by the process, and in the end you will have reasonably curved deck beams, with no flat spots.

If you're dubious about it, try it out small scale and see how it works

PS: I just did a bit of hunting, and some time ago I posted a whole bunch of stuff on this. That thread also has more references to this method by none other than our illustrious Mr. McColgin He also references the article "Andy Davis's article in the WoodenBoat March/April 2002 #165.":

http://www.woodenboat.com/forum/show...ckbeam+ellipse