grain orientation in mast?

[Billy Bones]

I am about to make a mast for an unstayed rig. The mast is 3" dia and 11' long, SA 80SF. I have some 2x6's in clear douglas fir and will have to glue them together.

I am wondering if there is a significantly larger load in a specific direction on these rigs (lug), ie fore-n-aft as in sheeting strain, or side to side as in wind load. If the stresses are generally greater in one direction, I'd glue the slabs together with the grain running accordingly. If the stresses are generally uniform around the mast, I thought I would rip 4 1.5" square strips and glue them with the grain running radially.

Also, my impression is that the grain in the yard should run vertically (parallel to the sail) is this generally correct?

Thanks,

Billy

[Bob Cleek]

Ya, you got it right. Keep the mast grain running fore and aft. "Radial" glue up would be nice. Be sure to glue up at least 3 pieces parrallel. Problem is, with only two, one will inevitably shrink and swell more than the other and cause a bend in the spar. With three or more, the other two will counteract the stronger one. However, for the size of spar you are building, I'd think springing for a solid chunk of wood would really be worth it in terms of time and effort, and, of course, you won't ever worry about delamination. The cost of the adhesive alone should make up the difference in cost.

[Tom Lathrop]

Have to disagree with Sir Bob on this one. If your wood baulk is from a large tree, and it certainly should be, an even number of laminations will resist natural bending better. For two laminations, split the wood and rotate one piece 180 degrees and glue up. This will put all natural bends in opposition and tend to cancel them out. Three pieces will always leave any residual stress in one of the pieces unopposed. This is also a good method of gluing up centerboards and rudders to prevent warping later, that is, ajacent laminations should be rotated as they are cut and marked for assembly later.

[Tonyr]

Billy,

That's a big sail for such a small mast, assuming that you loose around 2 ft below the partner, making an effective 9 ft available. What are your yard and boom dimensions? I have a 13 ft mast, and with an 8'6" boom can only manage to hoist a lug of around 60 sq ft. Perhaps yours is one of those very tall ones like the Golant range from Cornwall? Of course, I could have screwed up with my calculations, too!

Regards, Tony.

[Will]

Bill I've got a 9 ft. 6 in. mast I glued up from a pair of 2 by4 s that's worked well for years . I made it out of SPF from the lumberyard and picked the lightest , clearest ones . The two halves of the mast are clearly not even the same species , but I've had no warping . I thought it might be a problem ,so I coated with WEST and varnish and it's never absorbed enough water to matter apparently .I don't think your fir would give any problem ,but I recommend sealing it carefully . I don't think the number of laminations is a significant issue in a layup with all the grain going in the same direction ,and with a similar moisture content . The issue of the grain orientation in cross section is interesting ,I wonder how much difference it makes ?

[Billy Bones]

TonyR, the rig is actually 79SF which includes a cute 14SF jib. I add it in because its force still works on the mast, to some degree anyway. The yard is 9'+ and the boom is 8'+ and it has a high peak.

Thanks for the input so far guys. Keep it coming. I look forward to hearing anyone's experiences and/or warnings.

My hunch is that the fore-n-aft stress is more significant, yet the first thing designers add when rigging a boat are shrouds which add support athwartship, which suggests that is the direction of significant stress. Perhaps the stresses are a funcion of load and ballast working against the wind on a reach....aw heck, just thinking out loud...and confusing myself.

[Wayne Jeffers]

In my limited experience, I have to agree with Tom Lathrop.

If I wanted to build up a solid mast with a 3" diameter, I would get a 2 X 10 as clear as I could find with the center of the tree running down the middle of the board. This is easier than you might think, especially in longer lengths. I would then rip a 3" slice off each side and throw the middle piece away. I'm left with 2 pieces that are basically vertical grain. From the way they were originally oriented in the board, I would "fold" the pieces back on each other side-to-side and glue, setting up the bending tendencies cancelling each other, as Tom says.

Wayne

[Bob Cleek]

Yep, you guys are right. My point about at least three pieces wasn't meant to distinguish between odd and even numbers, just more than two for the reasons stated. I agree that if you are ripping your stock out of the same hunk of wood, reversing the sides will accomplish the same result, so long as they are really matched side by side. My observation really applied to larger spars where the size precludes cutting parts out of the same plank. On the other hand, for the size of the spar he is talking about, if you are cutting out of a 2X12, why not just buy a 4x4 and plane it down?

[Tom Lathrop]

Right Bob, if the lumber stock is good enough, there is no need to laminate. Finding a really good chunk of lumber at the average yard these days is not easy though.

[Will]

Bob in cross section the spar you were thinking of would be divided into 3 radially ,with the grain of each piece running as close as possible toward the center of the spar ? I can see how the staves could be ripped to produce a square exterior blank that could be clamped up with regular clamps . Could even leave a center void for lightening .

[ 02-19-2002, 10:47 PM: Message edited by: Will ]