Mast Orientation

[KMacDonald]

I am making a mast out of Sitka Spruce. It will be comprised of two 8/4 planks epoxied together. How should the mast be stepped? Glue line running for and aft or side to side? Does it even matter. The gaff will be 2 1/8 x 1 7/8" so I dont haft to glue up two pieces but should I use two pieces for greater dimensional stability?

I am thinking it should run for and aft. As the mast is flexed sidways by the sail, the center of the mast will not be flexed. One side of the mast will be in tension and the other side in compression and no forces in the middle for and aft.

[KMacDonald]

Do I need to be concerned with wood grain orientation?

[George Ray]

http://www.duckworksmagazine.com/04/s/columns/gilbert/014/index.htm

http://www.boatdesign.net/forums/showthread.php?t=8545

http://www.braskermasten.nl/html/wooden_masts.html

[Thorne]

I built one from two clear grain DF 2x4's, and have the glue line running fore and aft, as you want the mast to flex side to side more than fore and aft.

I oriented the grain this way, sure hope it was right! ;0 )
http://www.luckhardt.com/newmast1.jpg

[Bob Cleek]

Yes, "grain matters." A mast made up of two pieces of wood will frequently take a hook and you will not be able to make it straight. This is because no two pieces of wood are alike. One will always swell more or less than the other. Glue two long sticks together and the unequal stress of their differing movements will cause one to pull the other out of line.

Masts should be made of at least three pieces. In that way, two will always overpower the movement of the third "odd man out" and keep the stick straight.

On a solid spar that is not that long, it is less of a problem. However, after you have her all finished and in the boat, if you notice it isn't straight, at least you'll know why.

[donald branscom]

Yes, "grain matters." A mast made up of two pieces of wood will frequently take a hook and you will not be able to make it straight. This is because no two pieces of wood are alike. One will always swell more or less than the other. Glue two long sticks together and the unequal stress of their differing movements will cause one to pull the other out of line.

Masts should be made of at least three pieces. In that way, two will always overpower the movement of the third "odd man out" and keep the stick straight.

On a solid spar that is not that long, it is less of a problem. However, after you have her all finished and in the boat, if you notice it isn't straight, at least you'll know why.


BOB.....What if you rip the board in half and turn one side end for end and then glue it together?

[sv Lorelei]

Don:

I did exactly that on my skiff's gaff. within about a month it took a nice port side set of about an inch. So.....probably no, unless you get lucky and get two pieces of wood that don't have any proclivity to go elsewhere.

[Bob Cleek]

Don, I'd guess that would reduce the tendency quite a bit. Of course, there would still be a difference in moisture content between the sunny side and the dark side, or from a bit more damp getting into one side than the other, maybe from where the dew sets and so on. Then again, if you have a whole piece of wood, why would you want to rip it in half and glue it back together again? LOL

[KMacDonald]

What is the weight of Sitka Spruce?

Thanks in advance

[Thorne]

Less than the weight of Doug Fir and most other suitable woods.

I made my mast and used it unfinished to see if the weight was too much. It was, so out came the power planer and I thinned it down a bit to remove that excess weight aloft. Then I got out the CPES and varnish and had a good old time.

I picked two reasonably straight DF 2x4's, and they glued up straight when bookmatched/reversed as shown in the photo. Works great, stays straight, no worries...

[Bob Smalser]

I'd laminate three pieces instead of two for a small solid mast, and use even more lams if I were making a larger solid mast.

The thicker the lam, the more force it moves with seasonally, not only in its tendency to warp, bow, or take a set lengthwise, but across the glue joint. Orienting the lams (as Thorne's pic shows) so the ring cups of each lam are flipped instead of parallel to each other nicely mitigates lengthwise warp, but significantly increases the crossgrain stress on the glue joint because cupping forces are working against each other.

Wood can bow or twist longitudinally in drying and as moisture content changes in use for a number of reasons, to include even minor grain runout and reaction wood caused by the tree growing on a hill. Perfectly-straight sticks that remain that way aren't the norm in a millrun of logs, hence the timeless carpenter's adage of "crowns up" when setting floor framing. And this USDA graphic shows the direction wood shrinks and swells seasonally...and where the crossgrain stresses in a lam will be...according to the shape of those end grain ring cups:

http://pic20.picturetrail.com/VOL12/1104763/3075040/189555324.jpg

So more lams are generally better. You can have your cake and eat it too....lamming with flipped cups to prevent the lengthwise warp/bowing'set a single stick is subject to and to maximize the longevity of the glue joints by minimizing the crossgrain stresses on them.

[KMacDonald]

Bob,

With a two piece mast, glue joint running for and aft, there is almost no shear force on the glue joint. The further the glue joint is from the centerline the greater the shear. If using quartersawn planks, would this be the way to go?

[Woxbox]

If I remember it right, Sitka runs around 28 lbs per cubic foot, and is notable as the strongest wood per pound you'll find anywhere. It's also really nice stuff to work with, wheras fir is a pain by comparison.

[Bob Smalser]

Bob,

With a two piece mast, glue joint running for and aft, there is almost no shear force on the glue joint. The further the glue joint is from the centerline the greater the shear. If using quartersawn planks, would this be the way to go?


That's the way I'd do it. But the stresses on the glue joints from wind and sail pale in comparison to the potential thick lams working against each other seasonally.