by John O'Neill



What follows is another birdsmouth article. Sorry! I know you already have several - but bear with me here. I was the guy who built the mast in Dave Burdecki's article. I also wrote a pair of articles for Boatbuilder Magazine on birdsmouth (May/June and July/Aug 2001).

What is lacking in your other birdsmouth articles is practical advice. I try to break it down to bare essentials here, while providing some tips and tricks builders may not think of until too late. It's (brutally!) long, but I think you will find it easy to read and packed with practical advice.

I didn't think any pictures or drawings of birdsmouth were needed; there are plenty of examples in other articles. But I did send a pic of my birdsmouth vase.

John O'Neill

A 12-stave, elliptical birdsmouth "spar" - of an entirely different nature: A vase. It's birch and the staves had to be pre-bent to take that "taper." It's also a rather dramatic demonstration that stave width equates to "spar" circumference (and diameter if it's round).
Editor's note: See also articles by Dave Burdecki, David Farless, Gaétan Jetté, and Frank Hagen


Three truths:

  • Building a birdsmouth mast is easy (granted minimal tablesaw skills).
  • A birdsmouth mast makes for a remarkably efficient use of a scarce and expensive resource - wood (meaning you have to buy considerably less of it).
  • Birdsmouth is harder to describe than to do (most of what follows is in the form of tips and jig descriptions, most of which you won't need to bother with).

And a statement:

Birdsmouth is demanding - but only of the very same thing you've been using all along in building your craft anyway: Forethought.

Let's get to it:

I've built several birdsmouth masts, a birdsmouth vase (18" high, hourglass profile, elliptical), and am presently building birdsmouth dining room table legs. (I've also built a number of solid wood spars - but that was before I discovered birdsmouth.)

One problem for those who haven't tackled the technique is simply picturing the staves and being able to tell which dimension is what, where the birdsmouth goes, where to take the taper, etc.

It is confusing. But no more . . .

Do this: Picture in your head a drawing of a birdsmouth spar viewed end-on. Now cut the ring and straighten it out (in your head) into a flat, horizontal board. The birdsmouth faces are now all aligned (in your head) the same way, either facing left or right (your choice).

The width of the stave is the horizontal dimension. The width of the staves determines mast diameter. The width of the stave is where the taper is cut (because it is width that determines diameter), off the square face opposite the birdsmouth cut.

The wall-thickness (or just "thickness") is the vertical dimension, and is just what it says. The longer the thickness dimension, the thicker the wall of the finished spar.

Keep that picture in your head when you go to buy your stock: a flat "board" with all the birdsmouths facing the same way that when curled into a circle will form a spar - because that's what you're buying. That's exactly what you're buying - stock that will be curled lengthwise into a circle after you cut it.

You may want to add one more thing to your imaginary picture - grain. If the stock you buy is plain-sawn then bending that board lengthwise into a circle will result in grain running around your mast (just like it does in a tree), and vice versa for quarter-sawn stock.

However, you can construct a vertical-grained spar (a quarter-sawn mast?) from plain-sawn stock (or vice versa) by cutting the width of the staves from the thickness of the stock (in other words, in the picture in your head, individually flip all the staves 90 degrees). This necessarily means that the thickness of your stock must at least equal the width of your staves. If it isn't, either give up on that idea or add more staves. (Cut using this method, 1" stock can yield a 3.75" diameter, 12-stave stick, or as much as 5" diameter using 16 staves.)

Note: Be aware that using any number of staves other than 8 (which uses a symmetrical birdsmouth, 45 degrees to each face) means that once the birdsmouth is cut the longitudinal orientation of each stave is fixed (so you can't swap a stave end-for-end).

I am not qualified to comment on which is better in a spar, plain-sawn or quarter; maybe somebody out there is (I'm guessing it doesn't make much difference, if any). My preference is quarter-sawn. It hand-planes easier after glue-up and I like the looks of the finished article better (the glue lines tend to get hidden by all that pretty grain rising vertically up the mast).

Now, with that imaginary picture in your head, let's talk dimensions. They are only as complicated as you want to make them.

The rule-of-thumb is 10 percent oversize in diameter (of an equivalent solid wood spar), and 15 percent of diameter wall thickness. Previous articles can give you more exact figures for more exacting needs and specifications, but for general purposes you won't go wrong using those numbers. (I made my Bolger Cartopper's 12-stave birdsmouth mast the same diameter as the solid wood version, at 20 percent wall thickness, and haven't had a problem in four years sailing it, often in big San Francisco Bay breezes. However I wouldn't do it that way again; I'd stick with the rule-of-thumb.)

Example: If the specified spar is 4" diameter and solid, make your hollow version 4.4" diameter with a .66" wall.

(Does the thicker mast bother you? Do visions of mast-induced eddies breaking the smooth lee-side flow over your perfectly cut, perfectly trimmed sails and the resulting potential speed decrease of 0.05 knots (or whatever) have you shuddering to your core? If so, I have a suggestion: Either reach for you wallet and buy a carbon stick, or make your birdsmouth mast oval or elliptical - it can be done.)

There is no need to get fancy figuring stave width unless you want to. Simply figure the mast's circumference (diameter times 3.1416), divide by the number of staves you want to use, round up to the nearest demarcation you're comfortable with, say 1/16" or so, or even .1" (it's for a pleasure craft, not the space shuttle) - and your end result will be plenty close enough.

At some point you will probably have to cut some scarfs because the staves are so small that they will break at knots of any size. That's good. It'll keep your stick clear of significant defects.

What's also good is that scarfing such small section pieces is a breeze. Slice both pieces to be scarfed together at the same shallow angle (10:1 is more than adequate), at the same time, fit together and glue up. If you use epoxy you don't even need (nor do you want) to clamp, just find a way to hold the staves in contact with each other while keeping them straight and aligned. You can even scarf staves that already have the birdsmouth cut into them. Arrange the scarfs so they are staggered up and down the length of the finished mast.

Cut all your staves on the same saw setup so they are as identical as possible (to make setting up and cutting your birdsmouth easier). Cut extra lengths from scrap to use in making sure your birdsmouth cut setup is right.

Cut the birdsmouth with router or tablesaw. It will probably be easier if you haven't introduced taper yet. Take your time getting the set-up right. Cut the birdsmouth to a feather edge with the sides.

On the tablesaw you can make two cuts with a standard blade to get the birdsmouth, or one cut with a dado blade. You'll need lots of feather boards to support and hold down the staves if using a dado blade, and a slow feed rate. To set the correct angle on a wobble-type dado blade find the angle with a standard blade (measure as precisely as you can from the face of the blade) then switch to the dado blade.

Remember, stave width determines mast diameter, so introduce taper from the face opposite the birdsmouth. Find the diameter on the plans at various points along the tapered area and figure what stave width should be at those points (again, diameter times 3.1416 divided by number of staves, and rounded _up_).

Taper can be introduced on the table saw, with a hand plane or even a belt sander. On the table saw you'll need a jig to do it right. With hand tools you could just eyeball it, measuring as you go. Line up all the staves, birdsmouth side down, and sand or plane away. Important: Keep the taper face square! (My technique is to fashion a kind of shooting-board, making two identical pieces of plywood that mimic stave taper, clamping them to either side of the laid-out staves and cutting or sanding the staves - either all at once or in multiples - down to the plywood.)

It's easier on a table saw to cut the taper straight. The rule of thumb (if curved is specified) is to halve the taper. In other words, start the taper at the same point along the mast, but halve the dimension change at the top. (If from full size to masthead the diameter decreases by 2", make the decrease 1" instead).

It is possible to cut curved taper on a tablesaw (although I've never tried it). You'll need a convex taper jig to turn the concave taper of the stave into a straight line, and some way of securing the stave to the jig against the curve. Curved taper is parabolic - the taper starts out easy but increases fairly dramatically towards the end - so the last few feet or so may prove difficult to secure to the jig, especially if you only have 8 staves. Spars with 12, or even better 16, staves will prove much, _much_ easier to taper using this method because each individual stave will have significantly less taper curve they need to be bent to in the first place, and significantly less width in the second place (so they'll bend easier to boot).

You may find that the birdsmouth cut itself limits the amount of taper, because you'll only be able to take off so much width before you're cutting into the birdsmouth (giving your stave a terminal case of split ends!). The thicker your wall the more this becomes a potential issue, because the thicker the wall the wider, and thus deeper, the birdsmouth cut. However, with a 15 percent wall thickness this shouldn't be a problem except in cases of extreme taper.

Note: Introducing spar taper solely from stave width results in a constant wall thickness end to end in the spar. Obviously this results in more strength - and weight - for its diameter in the tapered sections than in the straight sections. There is no way around this short of automated machinery - unless! - (and I've never done this) you don't fully taper the spar by tapering the staves, and instead introduce a portion of the taper by planing off or sanding the finished article. (For most applications you won't be able to taper using this method alone because there won't be enough wall thickness to play with.) But it strikes me you could calculate how much wall thickness you want at various points along the taper, taper the staves' width to a lessor amount than you otherwise would, and introduce the rest of the taper after glue-up, thus taking some taper out of the wall thickness. The mast would bend somewhat easier and more evenly at the ends, which can be a good thing, and would weigh somewhat less. Is it worth the extra effort and minimal weight savings (albeit at the mast-head, where it counts? I don't know; it's your stick!

After tapering the staves, test fit your spar.

Now, before you do anything else, MAKE A PLUG. Let me repeat that - MAKE A PLUG!

Make it long enough to extend from the step through the partner and well past all the lower fittings, cleats and whatnot. (Yes it's extra weight, but it's down low where it doesn't count).

If this seems like a lot of trouble, consider: You've just become an expert in making long, hollow cylinders, so follow through on that and make the plug birdsmouth style, this time sized to fit inside your mast. It'll be a breeze.

Make it from any old solid wood you have laying around. If you made an 8 stave spar the interior will be octagonal, so you can make an octagonal plug to fit. (The distance across the interior octagon divided by 2.4 will give you a stave width that should fit when cleaned up.) If your spar has 12 or more staves the interior will be close enough to round that you'll want a round plug.

Taper the plug staves but don't taper the plug itself. My suggestion is to take the taper, this time, from the birdsmouth face. Just use the table saw or a hand plane and cut a long, straight taper in the end of each stave. When you glue-up the plug (do it before you glue-up the mast) leave the tapered staves sticking straight up. Inside the mast they'll serve to soften the transition from plug to no-plug so bending stresses won't concentrate at one point.

Test fit the glued-up plug (don't forget to epoxy seal the interior of the plug) in your test-fit mast and make double sure that all the mast staves still fit together tightly along the length of the plug. If you're using epoxy for glue-up the plug does not need to be a tight fit.

I HIGHLY recommend epoxy for all glueing. It's slippery when applied (a very good thing in this case), slow setting (another very good thing in this case), it seals the interior face (still another very good thing), it doesn't need tight clamping (a very good thing in this case) and it fills gaps at full strength (a very, very, VERY, VERY GOOD THING!).

Glue-up can be a one-person job. Lay all the staves out next to one another and paint clear epoxy on three faces (skipping the exterior face), and the exterior of the plug. Don't skimp.

Turn all the staves birdsmouth side up. Moderately thicken the epoxy with silica or wood flour - you won't be sanding the stuff - and paint it on the birdsmouth faces and the exterior of the plug. Don't skimp. If you have large mistakes, gouges or whatnot, thicken the epoxy some more and trowel it on the mistakes. If your plug seemed way too loose a fit, trowel the thickened epoxy on the exterior of the plug and even on the interior of the staves where the plug will lay.

Lay all your staves on their sides just like in the imaginary picture in your head, all birdsmouths facing the same way.

Place the plug on the staves where it's going to go and roll the staves up around it. Make sure the plug is positioned properly and put a hose clamp around the base to hold everything in place. No need to crank down on the clamp.

Now, using waxed twine make a hand-tight spiral wrap up the mast. The staves will more or less fall in place as your go. The twine is your clamp.

Epoxy does not want tight clamping. Repeat - epoxy does not want tight clamping. You just want to securely hold everything in place, nothing more. Two or at most three revolutions of twine per foot is all you need.

Make double-sure the staves are fully engaged with each other in way of the plug. You may have to resort to temporarily hose-clamping that area to squeeze out excess epoxy between the plug and the interior wall.

Sight along it to make sure the spar is straight. If you want the aft face of a tapered mast to describe a straight line just lay that face down on a flat floor; the staves will automatically align themselves into place. Take your time. The slippery epoxy makes this job easy, and the slow set-up makes it relaxed.

You can plane and sand down the excess wood or build yourself a "lathe" and use inside-out belt-sander belts to sand it down. (If you're building more than one spar at a time the lathe might be the way to go.)

To do that fashion a secure bench (I use a fir timber I have laying around, set up on saw horses) and use a variable speed drill to turn the whole spar. I've used tied-down lawn mower wheels at each end to support the mast, with lag bolts screwed into the spar ends as axles (with the head sawn off the lower "axle" to chuck my drill to). Plug the bottom few inches of the mast solid to hold that lag bolt (however, be sure to read the last paragraph below). Put a hose clamp around the masthead to keep the very narrow staves at that end from splitting out when drilling and screwing in the lag bolt. Fashion "bearings" to support the span of the spar by drilling through 2x scrap with a hole saw, then sawing through the hole so to make top and bottom halves, which are securely clamped to your bench around the spar. Wax the bearing faces for lubrication. You can offset the bearing halves as the size of the spar diminishes as a way of keeping slop to a minimum. Use two inside-out belt-sander belts at once, holding them opposite each other, to equalize stress on the spinning spar as you sand it.

Historical fact: Back in the bad-old days hollow spars were glued-up using non-waterproof glue and they held up just fine, even on sea-going boats. The lesson is that you don't need to epoxy-coat the outside of your spar. Varnish or paint will do. (Keep it chucked in your "lathe" and the job will go easy!)

Nor do I recommend fiberglass for the exterior, or anywhere on it for that matter. If you are concerned about the strength of the spar then make it fatter in the first place; it's a much more efficient use of weight for a spar that's wood to begin with. If it's wear you're concerned with, say from a sprit-boom, then leather the boom.

However, and this is a big "however," don't seal off the inside of the spar from outside air. Go ahead and plug the top solid. But no matter how you plug the base make sure to leave a fat hole through it so that the moisture that will inevitably find its way in, can find a way out.