Workshop: Building Birdsmouth Masts & Spars
Link to Calculator
Mast Making on My Blog
Two masts for Shellback dinghies, one after glue up the other after 5-years of hard use in the builder's boat
Above are "staves" cut on one edge with a "birdsmouth" dado and on the other side, tapered with a square edge. When the staves are fit together, the resultant shape is an octagon with corners sticking out that you see above. The spar is rounded directly from this glued-up shape.
How do you determine the number and dimension of the staves for a Birdsmouth spar?
How do you cut the staves in the shop and glue them up into the right shape?
How do you make the mast rounded and finished?
Let's See...
Hot off the Press! These staves have been planed and ripped from the spruce stock. Thickness is based on a 15-20% wall thickness, depending on the boat and the wood type. Width is determined an online calculator developed by Gaetan Jette. Staves are ripped 'proud' and planed to final thickness: accuracy is important.
The "birdsmouths" are actually just dadoes cut at exact 45-degree angle on the table saw or the router table. We use both means depending on the size of the stave. Once the stave is 1/2" or above, we always use the saw. Below we are cutting staves 3/16" thick by 1/2" wide for ultralight kayak paddles. This tiny stave works well on the router table using a birdsmouth router bit.
Cutting the tapers on the staves is usually done after the birdsmouth is cut...
Staves are tapered so that the mast diameter can taper. Tapers are planed on a jig as pictures or by simply clamping the staves together (four at a time will do) and planing the taper by power plane and then hand plane. We can do this in about 20 minutes for a small boat spar.
After a dry-fit, staves are glued up with a structural epoxy out of a tube or brushed on. Stave #1 and 5 get a piece of tape on the square edge so that the spar is glued up as two halves. It is very important to stay organized by labeling the spars with the order they go in during the dry-fit and it helps to have the grain in each stave running the same direction!
The two halves come apart after the first, big glue up....or you can glue the staves and filler pieces together in one glue up.
Blocking is place in way of the mast partner to reinforce the area (this is where masts usually break), as well as at the masthead, and a plug at the base. The blocks can also be place where hardware may be mounted to the mast.
All masts and spars are glued up on a simple jig to keep the spar straight while the epoxy cures. Hose clamps are used to clamp the staves together along the length of the spar. You can see a hose clamp in the mast photograph at the top of the page.
A different approach...
The staves in this spar are all flush on the outside, the corners meeting perfectly to form an octagon that may be rounded or kept octagonal. The staves on the right are "assymetrical" where as the staves for the masts above were symmetrical. This is a shaft in the ultralight kayak paddles we are developing. The walls were kept quite thin for lightness at the risk of losing gluing surface and stiffness. The ultimate experiment for strength in the glue joint for anything is to break it. So we purposely broke our prototype...
That was too much fun, so we decided to take the hammer to one of the mast cut offs...
We couldn't be happier with the results. Why? A good glue joint is stronger than the wood around it, so the wood should break, not the glue joint. That is what we found and we test all our spars this way with equal results. If we found a problem, this is where it would show up and the mast would be thrown out and redone.
Making Laminated Spars
Typically, we make booms, yards, sprits, and similar spars solid by laminated two halves from the same spruce board, keeping the grain of the wood in one half opposite and closely symmetrical to the grain in the other half. The result is a spar that glues up straight and stays true over time.
The spars above are from a Goat Island Skiff. Below is an oval, Shellback Dinghy yard done birdsmouth: it is sensationally lightweight....you can see the glue lines between staves.
Our sail rigs come with sails laced and rigging is available. Spars are always delivered with 5 coats of spar varnish. That will last a long time. The yard above and mast below have been used very hard in my Shellback dinghy for the past 5 years with only one maintenance coat of varnish.
We do wooden cleats and other customized rigging. All these products can be made into kit-form, as can the rigging kits.
Duckworks has good resources on Birdsmouth Theory and Application by Jette
Determining the Number and Dimension of Staves for a Mast or Spar
- The number of staves (N) depends on how large is the diameter of the spar. As a rule-of-thumb, all masts for small boats should be 8-stave. For sticks such as boomkins, sprits, and the like, feel free to try 6-sided! Less wood, less work.
- To determine the stave thickness (H) for your mast you need the spar plan with the designed diameters for your mast. Stave thickness is generally 15-20% the max outside diameter (OD).
- Use the Bird's Mouth Spar Size Calculator to input N and H and O.D. You will get the right width of the stave, L.
Use 0.15 if you mast is a small mast for a mizzen or small sail below about 50 Sq Ft or you are in an ultra-lightweight raceboat that won't be overly loaded at anytime with people or ballast
Use 0.20 if the mast is unstayed and supporting a rig with more then 50 Sq Ft and the boat is bound to take some load at some point in time, like gear for camping or three guys hiking out in a 15 knot wind having a great time.
Use 0.15 if you are using a timber like Douglas Fir and for Spruce use more like 0.20. For pine or a non-typical spar grade timber, I would go no less than 20%. Less than 15% wall thickness is risky: in a birdsmouth spar the faying surface is greatly reduced so strength can be compromised.
For pure octagonal spars, like shown for paddle shaft above, you will need the router bit from Lee Valley and a different forumula:
Keep the spar octagonal or round it.
Link to calculators for pure octagonal spars
Bird's Mouth Spar Size Calculator generously provided by Gaétan Jetté of Sherbrooke, Canada.