Birdsmouth... click here to read or make an observation about this  article

By Roy McBride - Cape Town, South Africa -

... and Other Wooden Masts and Spars

click to enlarge

Examples from:
CKD Boats
Cape Town
South Africa

(click images to enlarge)

From time to time we are asked make masts, spinnker poles and the like. Our largest spar set to date was for an Italian Schooner that hit trouble when en route to Cape Town along Africas East Coast. The yacht was some 70 feet long and in a heavy Carvel Planked timber construction, so quite a large boat.

The Main mast (aft on a schooner) was all of eighteen meters long. The Fore Mast was fourteen and a half meters long. Both of these masts were quite new and the Oregon Pine (Douglas fir) timber and workmanship used to make them was very much top draw. It’s a pity both broke in some heavy weather when voyaging from Mombassa, Kenya, to Durban, then on to Cape Town, South Africa.

As mentioned the timber and labour to construct with was the best but they used the wrong glue. How can this be? Well, glues come in various types; waterproof may be just that but when its not suitable for boiling it may and most probably will fail under severe heat.

The name WBP in a glue stands for ‘Waterproof & Boil Proof’, WP stands for ‘Water Proof’ only. Now we know we are never going to actually Boil our timbers but given the Tropics and its humidity, added to that the heat that can exist, its not so far off such high temperatures on a mid summers day on the coast in Central Africa.

What happened with those masts was the glues used were just WP and a common type used world wide. It’s a white powder base and is mixed with clean water, making a nice paste that is easy to apply and wipe clean after clamping. What should have been used is the same glue that is used in both marine and WBP grade plys, Phenolic or Recourcenol glue. Its easy to identify. It has a very dark red, near black glue line and its either mixed in two parts with a white powder or a similar dark red liquid. The mix ratios are varied and some types can be ordered with an extended pot life; very usefull when building the larger masts.

Depending on the size of the masts being glued up, the choice of which to use can be important as some have a thicker mix consistency making application by brush less easy. You will also need to ensure that which ever type is being used has a reasonably long pot life. Once started with the mix process you have to be fully committed to complete the glue application and clamping of all the various mast partners in the time available!

The masts for the Italian Schooner were fine from the breakage down and into the boat, so in that case we just scarphed new timbers onto them and advised the boat's owner to steel strap what was then left of the old masts inside the boat.The masts were a simple rectangular box section,the glue was doing all the work, well should have originally. We rebated the new timbers to form a proper rebated joint. This way the glue line was near 50% larger than originally built, with that and the correct Phenolic Glues, the boat was ready to sail off on its voyage. The last we heard was they were safely arrived into Trinidad and Tobago, West Indies.

That type of construction is very easy to do and can be successfully attempted by most, even in their own back yard. A simple Skill Saw can do all the rebates and that is what we ourselves used on the Italian Masts. Another way of doing Masts, especially round ones is a more complex (seemingly?) ‘BirdsMouth’ construction, named from the angle joint that resembles a Birds Beak when open.

Drawn in a circle, typically a 152mm round (6”) mast will have eight staves 60mm x 22mm (2 3/8” x 7/8”)

click to enlarge

Birdsmouth Spars:

When the diameter of the mast or spar is known, it is is drawn out to look like eight parts, or Staves as they are called. Drawn in a circle, typically a 152mm round (6”) mast will have eight staves 60mm x 22mm (2 3/8” x 7/8”) with the Birdsmouth joint cut along just one long length of each stave. They are very easy to form into a round form. To make this more simple, all our Kits and Mast builds also have CNC cut building jigs,set up on a long bench or set on 220 liter drums. They allow the eight staves to be glued up one by one and then dropped into the jig, one at a time. We also taper our masts to allow a little weight saving and to make the masthead look a little sleeker.

Mast Tapers:

On this mast we decided a taper down to 125mm ( 5”) diameter would be fine,this was for the top 1500mm ( 5 feet) to do this,simply plane off a taper along one flat edge of the top of each stave,in this case 10mm (3/8”) was all it took,remember to do the taper after the birds mouth section has been cut.

We have a mast in build now - its going on a 30 foot wood/ply/epoxy catarmaran. It will be Gaff rigged and we will next make up the Gaff. Wood is heavy you are thinking? Some are but if you can source the correct timbers they are actually lighter than Alloy, much to my own surprise when I first calculated the masts weight. The client was very happy. He said the wooden mast is lighter than the alloy option, which was to be around 67 kgs. Our timber spar in Clear Oregon comes in at about 44 kgs. The choice of timber used is important. Using a lower grade American Spruce or Pine would have given us a wall thickness or some 30mm or 1 ¼”; using the better and lighter Oregon Pine Clears we got away with a wall thickness of just 22mm,or 7/8”

click to enlarge

a set of mast staves for a nine meter mast,152mm in diameter

This is important to understand: as total end costs can easily become quite close between two similar timbers that have different cost structures. The 22mm will easily come out of 25mm but the 30mm needs to come out of 38mm timber stocks, so already we are buying in more timber but wasting it on the machine shop floor when it is later planed to thickness. So the final costs will not be as far off as you may have thought. Added to this the mast with the thicker timbers will be noticeably heavier. That is a penalty most will not want to carry.

Traditionally, a round wooden spar can also be made from just four sections of timber as wide as the spars diameter and to what ever the design thickness may be. This becomes a square box but has some 45 degree internal blocks glued in each corner prior to making up the box.Once the box and internal blocks are assembled and the glue is set,the corners can be planed right away, making a round section eventually. This build also uses eight parts and will make for a fine spar, if not a little time consuming by the time the internal blocks have been attatched and the whole thing glued and planed to its round shape. Which takes more skill, as when the eight part Birdsmouth section is being shaped, you have eight smaller flats to plane down to, rather than the four of the more traditional construction.

Spindle work on a 60mm x 22mm mast stave

click to enlarge

Either way make up a half section and full size jig template from a scrap of ply, this will allow you to check you are not planning to an undersize or misformed shape section as you go.

Gluing up:

click to enlarge

CNC cut mast building jigs,all part of what we supply with our mast kits,the one closest is smaller than the rest to allow for the mast head taper.

Our masts ,kits and those being built by ourselves to completion all have neat CNC cut MDF jigs. Each jig is exactly the right section and has a base that just slots together with the vertical jig. You will need a bench or stands nearly as long as the spar itself. On the 9 meter long (29.5 feet) mast we are building now, we have used empty 220 liter (44 Uk gallons) epoxy drums, plus some smaller ones that when stood on top of each other came to exactly the same hight as the larger drums.

Over the drums we then laid a pair of long 152 x 25mm planks screwed together at their ends, then we spaced out the jigs. This is not over important, excepting if the mast has a taper at the mast head as this one does. The smaller jig needs to be right at the mast head end as it will only fit in that position.

The jigs need to be both straight and level, with no twisting.

click to enlarge

The jigs need to be both straight and level, with no twisting. This is important, as once the eight staves are glued up and assembled in your jig, they will take its exact shape. It is easy to glue up a spar with a kink at this stage otherwise.

The easy way to set the jigs up is to screw down the mast head one to your bench or plank first but before doing this,pencil a line along part where the lower Stave will lay ,this is from side to side and using a set square.

Note: due to the mast mast taper, the mast head jig must have its pencil line drawn in the same position as the other jigs. This will then be lower than the stave face at the mast head but in line with all the others.

Fit the masthead top jig first; square it off to the building jig. Now screw it in position, then fit the lower jig at the other end. You now need a long line or string to line the jigs that fit in between. Nylon Fishing Line is perfect for this job. Screw a spare screw about midway on the lower face of the bottom jig; make sure it is exactly on the pencil line you have already made. Then a second screw on the upper face of the top jig. Make a loop at the end of the chosen line and pass it over one screw. Now take the other end of the line to the opposite end jig, wrap it around the screw and pull it nice and tight. Now make off the end of the line. This needs to be secure. Possibly a little masking tape around the line will help secure it.

Dry fitted

Now take some scrap wood or ply. 9mm (3/8”) is perfect. You need three pieces the same size. Take two and slip them under the line where it touches the side edge of the vertical jig, the line tension will hold them in place, if it does not, the line is too loose and needs more tension.

Now take the other jigs, in our case we had seven jigs so we have five left over to space out, more or less evenly but this is not critical. With a square, line them up at 90 degrees to your building bench. Then taking the third block of wood or ply you prepared, use that as a spacer guide to meet up to the line. The reason we use such a system is that with the spacer block being used there is never any danger of pushing the line out as each jig is being placed, which will be the case without the spacer. Once each jig is in place, screw it to the bench before moving to the next jig.

Check all of the jigs are in line as close as you can as this accuracy will help later when shaping your spar. One last job is now to check the lower edge of the jig. This is where the first stave will lay when assembly takes place. Due to the mast head taper in this case, we can not take a line along the center, end to end, you can now use the pencil lines and the tight line already in place to check the lower face of the jig is straight. When this is done you can easily check the see the bottom of the jig is straight, adjusting with packing as required.

click to enlarge

Dry fitted as a trial run

You can now lower the eight mast staves into place, dry at this time and ready for a check to see everything fits as it should. On the mast we have just built, the staves were premade to a length of around 9.5mtrs (31 feet) by scarphing and gluing in lengths of suitable timbers until we had the correct length. In our case we could then lay in each full length as we were gluing up the mast proper. In fact this pre scarphing and gluing on the bench first is not really required. It will add to the time taken to build the mast too, due to the extra gluing process time taken on the bench. As the scarphs (150mm, or 6 inches long) are all staggered and held in place by the birdsmouth joint section it is quite easy to glue the scarphs at the same time as the mast itsself.


Lay in the lower stave, keeping the top of the stave level to the outer face of the mast head jig, then lay in two more; port and starboard sides; then two more. You now have a ‘U’ shaped box. Now fit in the top and bottom mast blocks, normaly shorter at the head. If a masthead cable conduit is to be installed, this is the time to glue up the plastic conduits. The internal masthead and lower wood blocks will need to have been glued up with suitable internal holes for the conduit to pass through.

Dry assembly ensures you understand the process.

click to enlarge

Note: when the mast is glued up and the conduit installed, always keep a screw through the conduit and into the inside of the mast, as if you (or anyone else) ever pulls the conduit part way out of the mast, it is highly unlikely you will ever get it back in again!!

With the mast in its ‘U’ shape, top and bottom blocks in place, you can now fit the next three mast staves, port and starboard as before. Then the last and top one. The joints should push up neat and tight. To check this out take some 50mm (2”) plastic wrapping tape and make up tight bands around the mast every 500mm (18”) or so will be fine. You now have a mast as what we call ‘Dry and in The Round’. This is how we would ship such a mast as a ‘Kit’.

Gluing Up:

click to enlarge

This is the mast stave scarphs being glued. The glue is a special Phenolic Resin. Note! NEVER GLUE UP MASTS WITH EPOXY!

This is obvilously a repeat of what you have just done. When taking the staves apart, lay them in a reverse sequence so that you will pick them up in the right order; numbers may help in this regard.

The Phenolic Resin glue we use has a ‘Pot Life’ of around 30 to 40 minutes, so you need to be quick when applying the glue. Use rubber or surgical gloves as when this glue gets on your hands its liable to stay on them for some days. It is important to get as close to a 100% glue cover as possible. This is why we glue up on two edges of the joint. This may not be really needed but it sure takes any doubt from our minds that we have enough glue in each joint.

Easy ways are often the best. This is just 50mm packing tapes but plenty of it.

click to enlarge


As mentioned you can us the packaging tape. This is all we ever use to date on this type of spar - only that for the gluing stage we apply every 150mm (6”) to ensure we have maximum pressure but a better system is based around using large hose clamps, the type we see on Auto Hoses on Radiators and the like, but you will need a great deal of them and at some cost too. Avery neat system is to use a plastic or steel strapping or banding tool. You may be able to hire one of these. If you do it needs to be one that does not have the tool sitting ‘Under’ the strapping, as you will never remove the tool if this is the case.

click to enlarge

A simple and low cost way to clamp. Large hose clamps work well too.

Plane to Shape:

This will be the next day. Remove whatever clamping system has been used. You will now have a nice stiff mast or spar. You can remove it from the building jigs as they will now be in your way for this next stage of shaping. Each flat section will have a small corner standing up; use this as a guide before you plane this away. Take a small block of wood, half the width of the stave flat showing, then with a sharp pencil and walking along the mast apply a pencil line the full length of the mast,do this on each of the eight flats. This is your secondary planing line, which in a perfect world will not be planed off ,just sanded away. You will need to use a straight edge at the mast head to pencil in the line.

This is a number 6 Record steel plane. It’s a good size for the job but a longer one would be even better.

click to enlarge

On this mast we wanted to see what was taken off when planer and sanding was complete so we collected it all - 7 ½ kgs would you imagine! The wooden mast was then at least 22 kgs lighter than the Alloy alternative, our mast was cheaper as well.

This is where you will become fitter; after the short session with the power planer, its back to traditional hand planning. This is a number 6 Record steel plane. It’s a good size for the job but a longer one would be even better.

With luck you will have access to an electric power planer. If not you're in for a keep fit session, well, you are in for that already! In truth you will do about one third by hand plane anyway. It is good to collect those shavings so that you can work out what the finished spar will weigh.


click to enlarge

A simple way of making a sander mould; this is a half section of a 100mm plastic plumber pipe with a Teak grab rail screwed from the inside.

Masts and spars up to 100mm (4”) can use a home made sanding jig from a piece of Plastic Plumber Pipe. Split in half, its just the right shape and you can buy various diameter pipe sizes to suit your application. On this 152mm (6 1/16 ”) mast we made our own sanding jig from two pieces of Long Grain Bendable 5mm (1/4”) Plywood (in the background in photo above). This is how: Wrap the mast at its thickest diameter with thin plastic sheet, a dustbin bag is fine, to protect it from excess glue, then take the bendable plywood, pre-glued with the same phenolic glue you used for the mast. This is important as this form of glue sets hard and will allow no stretch or flexing back to the ply's previous flat form.

Glued and clamped, the 40 grit floor sander paper is on the inside to give the correct clearance.

click to enlarge

Wrap the half section of plastic covered mast with some 40 grit floor paper. This is dry and used only as a spacer to ensure the correct size is arrived at. Then clamp the plys onto the mast. We used four large ‘G‘ clamps. Some stiffeners will work well here on the two long edges. When the glue has dried by the following day, you have the exact shape to sand with, after first applying on a double handed handle. Then contact glue on the 40 grit floor paper you used as a spacer the day before.

click to enlarge

Happy Sanding!!

A shaper jig is not always required but its nice to check your progress with.

click to enlarge

click to enlarge

This is where we are today. Mike our local guy has lots to keep himself busy with,so he is a happy guy right now.This will take a number of days,after which we will install the mast sheave boxes for the boats forestay,this is a single sheave at the front of the masthead and a double at the lower end and the rear of the mast.

A Cape Cutter 19 with one of our Birds Mouth Masts.

click to enlarge

We then apply six coats of Perfection, International Paints clear twin pack varnish and Bill, the mast owner, will hire a truck to take his new mast across the Cape Flats and onto False Bay where he will step his mast and launch his new 30ft Catamaran.

To be continued,with assembly pictures and final stepping of mast in about four weeks time False Bay YC, Simonstown.

Yours in wood and Kits,
Roy Mc Bride - Founder -
email -
Cape Town, South Africa

More articles about Birdsmouth Masts: