FAQ Birdsmouth masts – butt joins OK? Scarfs are recommended.

Birdsmouth masts are a really neat method of making a hollow round or elliptical mast.

The mast is made of staves which have a 90 degree notch cut out of the stave’s side with a router or using a table saw.

For a round mast, identical staves are made up then held together with cable ties, plastic tape or hose clamps while the glue sets up.

Here are some online calculators if you want to work out a basic size.

http://www.duckworksmagazine.com/10/howto/birdsmouth/

For an elliptical mast two opposite staves are longer than the others.

If you want to taper … just taper the width of the staves the same percentages you want to taper the mast diameter at each height. And it all works out.

Another thing to keep in mind is that the traditional thickness of a wooden mast is 20 percent of the mast diameter. This makes a durable mast.

For racing boats it is possible to go down to 15% with some risks for durability. But below that is experiment and hope territory.

While the birdsmouth mast is an elegant solution for a round mast it is not the lightest mast for a given stiffness.  We have found we can actually produce much lighter masts using rectangular box sections (I always wondered why Francis Herreshoff used rectangular masts on his racing design).  It is also possible for the masts to be built with much thinner walls than previously though this adds some additional labour and materials.

There have been a few questions around the net about whether you need to scarf the staves or whether it is OK to just butt join them.

Some say it is OK to just butt join the staves. I disagree.

I think the instinct to scarf is a good one.

Scarfs allow the stave to carry much the same tensile loads as a solid piece of timber. Butt joining (and end grain gluing) is weak by comparison. I suspect the statements that it is possible to use butts don’t hold water.

If a mast is stronger than it needs to be – ie heavier than it needs to be – then you may get away with butts. If the mast is about right for the job then butt joining staves may weaken it so it can’t sustain the loads.

If the mast is made up of the normal 8 staves then at the level of a particular butt join you have given away quite a good proportion of strength in tension. 12%. I know the mast is never purely in tension – but other loading situations are worse.

For example hen a mast is loaded in bending one side will be in tension, the other in compression – roughly speaking you will have 4 staves with each type of load.

So the mast may be losing 25% of it’s capability to sustain bending loads with this very rough scenario.

Starting to think in pukka engineering terms – the stave that does the most work is the one furthest from the neutral axis. So if the butted stave is in that position maybe you could be losing much more than 25 percent.

I suspect the suggestion that scarfs are not necessary would come from one of three areas and none is a sustainable argument.

1/ that a stayed mast is never in tension and is more likely to fail in a compression mode. This is not true of unstayed masts or the top part of the mast in a fractional rig where bending is the dominant mode. It may also not be the case when the mast is knocked out of column by some mishap – big wave, broken rigging, misapplied runners, spinnaker pole loads etc etc etc

2/ that the butted stave is supported by the staves on either side much like a buttstrap or buttblock works when joining ply or planking respectively. A properly applied buttstrap (length is 20 times the width of the ply or plank it is joining) does give much the same strength as the original piece. However you have two pieces of timber carrying the load of one. With birdsmouth there is no extra material added to carry the load across the butt – the staves to either side are being asked to carry their own loads as well as the load of the butted stave. So they are more highly loaded and may fail.

3/ That the “glue will do the work”. Do the experiment of getting a small piece of timber – a cedar strip is perfect. Cut it into two lengths. Cut a Scarf in one 6:1 scarf (some use 8 or 10 to 1 for spar work – but theres a few pages of argument there). Cut and butt joing the other. Let the glue cure and try to break them. Butted one breaks easily right at the butt. Scarfed one is hard to break and may not actually break in the scarf join.

Butt joining staves in a mast doesn’t sound like good engineering to me. So like you, I will be doing standard scarfs on my birdsmouth work unless someone can come up with a good argument as to why it is OK to do it another way.

Also see the Birdsmouth mast and tabernacle we designed for the 18ft Fenwick Williams Catboat – it contains some information about detail design of this mast type.