If you build your mast out of one stick of wood and hallow it out you are setting yourself up for a broken mast. All the grain fibers in the wood are running in the same direction and you will probably have grain run out whick will give you a weak spot some where in the mast. If you build the mast from 4 seperate sticks any grain run out  in each stick would be strenthend by the other three sticks. I would do bird mouth construction and run carbon fiber inside the mast. Then there would be 8 sticks plus the carbon fiber for the ultimate in strong masts.

Good points on the wood, but put the carbon fiber outside, not inside. The inside of the mast is the neutral stress axis. That's where the tension and compression are both zero. That's why you can hollow out a mast. With no stress to resist, nothing has to be there and there's no strength lost. So putting carbon fiber in the center of a hollow mast is a waste of time, materials and money. It's also a pointless weight gain.

If you're going to use CF, put it on the outside. Not only is that the place where the stresses are at maximum, but the wider the diameter the stiffer the CF tube (think of the hollow mast as a circular I-beam). You'll get a lot more bang for your CF buck if the CF is on the outside.

On the other hand, lots of NE Dory builders have successfully built wooden masts, so there's no need to get into CF mast designs unless you actually want to. Just follow the manual and keep Wood Ogre's advice about the grain and construction in mind if you're going to make a hollow one and you'll be fine.

Have fun,

Laszlo

Laszlo.Puting carbon fiber inside a mast is not a waste ! You are right that the carbon fiber on the outside is the best place for it from an engineering stand point. But if you want to maintain the wood mast look then put the carbon fiber on the inside. I do hallow wood. I do hallow wood with carbon inside. I do hallow wood with carbon kevlar outside . I do hallow wood with carbon inside and kevlar outside. I do hallow wood with carbon outside. I have flex tested all to falure so I know first hand what is the strongest.

I'd expand on Wood Ogre's statement, "fiber on the outside is best place from engineering viewpoint".  This is only my non-expert thinking...I welcome comment and criticism from all.  I've learned to always include this disclaimer when disagreeing with Laszlo.

The engineering view of the performance of the design includes both strength and stiffness.  And other stuff.  There's a tradeoff between the two.

If the requirement is for the stiffest possible mast, then the statement is correct.  (And you have to ask in this case what value the wood is adding...it is to some extent dead weight, a mold left in after buiding.)

If the requirement is rather for the STRONGEST possible mast, then the reverse is true...put the less stiff material (wood) on the outside, where the geometry will allow it to take more of its share of the stress.  The CF tube goes inside, so that the strain on it is reduced when the mast bends: CF masts tend to rupture suddenly, compared to wood masts which tend to bend but not break.  This arrangement reduces that tendency.

My thinking comes from puzzling over why a CF tube kit manufactuter has you put the CF tube on the INSIDE and two FG tubes (less stiff) on the outside.  It seemed crazy at first...why put the expensive stuff where it does the "least good"?   I convinced myself that the above thinking is the reason.

>>My thinking comes from puzzling over why a CF tube kit manufactuter has you put the CF tube on the INSIDE and two FG tubes (less stiff) on the outside.

To protect the CF from abrasion and impact, and sailors from flying CF needles if the mask breaks.

You guys bring up one of the main points of composite design - sharing the load. If the materials share the same load the weaker one will fail before the stronger, leaving the stronger to carry the full load. The weaker provides no help and the composite is pointless then.

To successfully divide the load, each material must be where the load it can handle best is the strongest. Wood handles compression better than CF. CF handles tension better than wood. So the wood needs to be in the area of maximum compression and the CF in the area of maximum tension. That means wood on the inside, CF on the outside.

If the CF is strong enough to take the entire load by itself, then technically it does not need to be part of a true composite construction. It can be covered with all sorts of stuff for other reasons - FG for abrasion and damage control, wood for appearances, etc. But in that case the builder is trading off weight and a potential loss of strength for other design goals. Careful design, such as wood veneer for appearances, will help balance the trade-off.

Have fun,

Laszlo

Thanks to everyone for all the thoughtful feedback. Sounds like a lot of ins and outs to consider. On a small boat like the dory, how would a light, hallow mast perform in contrast to a solid mast? I saw in the maunual a couple references to the advantages of keeping things light above, but I have little experience sailing. Does a light mast help a boat sail smoother with less rocking etc?

Most of what Laszlo says is correct, if you use enough carbon fiber then the wood isnt even need.  The reason for putting carbon fiber inside a wood mast is to make it more rigid. Flex is not good in a wood mast.  Over time as a mast flexes it loses strength, eventually it will brake from fatique. Wood that is compression loses its ability to be tensioned. so usually a wood mast would brake from putting wood that is in compression in tension.   A compression brake will be a clean brake a tension brake will be a long splintery brake. Putting carbon fiber inside a mast incresses it rigidity and somewhat increases its breaking load. This allows you to use a thinner wood mast that is very rigid and light (and stiff) which keeps the center of gravity lower. A hallow wood mast (bird mouth construction) will be about half the weight of solid wood mast. (same wood species). One of my hallow woods will get between 450,000 and 500,000 flex cycles before braking. The hallow wood with carbon fiber inside is over one million flex cycles and has not broken . The thinner the wood thickness of the mast construction  the more it is able to twist. To eliminate twisting carbon kevlar or fiberglass is put on the outside to increase hoop strenth. Carbon fiber alone is not so good because it does not take abuse from impact or abrasion. There is no need to put anything on the outside of the wood mast if the right amount of carbon fiber is on the inside.

>>>A woodworking friend of mine suggested that I consider making the mast hollow by splitting it down the middle, routting out material from the center and then glueing back together before shaping. >>>>

I haven't had a chance to absorb all of the feedback above, but from a purely technical point of view, there's nothing wrong with this scheme.  The sloop-rigged Northeaster Dory's mast is not highly loaded. so strength concerns are academic.  You'd just want to be sure to leave a 12-inch solid section at the hounds (jib and shroud attachment points) to handle the point-loading there.  

If I were to make a hollow mast for the Northeaster Dory, I'd follow the prescribed shape for the solid spar, and give it 1/2-inch thick side walls (again with solid blocking at the hounds), but use four separate bits of wood rather than milling out a single piece.  This would be a bit stronger and innoculate against warping.

The actual improvement in performance will be small, bordering on imperceptible.  The mast would be lighter, which would improve stability, but the Northeaster Dory isn't noted for stability problems in any case.  

A perfectly rational case can be made for building a hollow mast on the grounds that it's a fun bit of carpentry.  And since we build these boats for fun, why not?

How about the mast for the lug rig? I noticed it's squarish and has a hole on top of it? I'm leaning towards a regular mast this go around and then maybe delve into hallow later or use these ideas to make another SUP paddle.

Thanks,  Bill

I'm a little late to the party, and you may already have ripped your mast in half and routed out both halves, but I think that's the wrong way to get a hollow mast. Why not try the "bird's mouth" method:

https://www.google.com/search?q=bird%27s+mouth+mast&client=firefox-a&hs=sxI&rls=org.mozilla:en-US:official&channel=np&tbm=isch&tbo=u&source=univ&sa=X&ei=CQPWUo_wFeLgsAS-0IDYAg&ved=0CCoQsAQ&biw=1024&bih=614

Jim Norman

>>>>>How about the mast for the lug rig?

In a mast this small, the only reason to make it hollow would be to do it for fun.  And I LOVE mast-making, so that's a perfectly good reason to do it.  But no performance advantages convey.  

(Well, if you made mast, boom, and yard entirely out of carbon fiber, THAT would be such a huge weight-savings as to be noticeable under sail.  But that's out of reach for nearly all of us...)

I think you'd want to increase the sectional size of the Northeaster Dory's lug mast if you made it hollow.  The solid version is pretty small in section.

>>>(Well, if you made mast, boom, and yard entirely out of carbon fiber, THAT would be such a huge weight-savings as to be noticeable under sail.  But that's out of reach for nearly all of us...)

A 15-ft long 2-inch diameter 5-lb mast could be made for about $250.00 plus the cost of the epoxy (about $60.00). So $310.00 plus incidentals. Putting away a dollar a day for a year would cover it. Make it $2.00 a day and you cover the boom and yard, too. I think that that is actually within reach for nearly all of us.

Laszlo