Birdsmouth revisited

A few years ago I used the birdsmouth technique to make the upper sections of the masts for my EC22 Skorpa. I learned a lot and I thought it would be fun to put some of that experience to work building the upper section of the mast for my Spindrift 10. I will use eight strips of wood about 7 feet long by 3/4 inch wide and about 1/4 thick. I am using some Alaskan Yellow Cedar. Here you can see the table saw set up to make the birdsmouth cut. The finger boards help hold the flimsy strips, and insure even cuts.

Below you can see my eight staves. One side has the notch or birdsmouth cut into it.

I screwed a series of blocks into my work table and used a string to make sure they were in line and at the same level. The first stave will be temporarily fastened to the blocks. This helps insure that the spar is glued up straight, and means you only have to wrestle seven glue covered staves into position.

Some builders use hose clamps or zip ties to hold the staves together until the glue sets but I like to use old bicycle inner tubes like big rubber bands. I had to add some lead weights to convince the mast to lay down on the blocks.

Below is an end on view of the small end of the mast. I shaped a solid plug to fit inside of the mast where a sheave will go and to seal the end of the mast.

The idea is that a hollow mast is almost as strong as a solid mast but much lighter because the wood in the center is not really doing much work. The thinner the wall the lighter but at some point you start to lose more strength than you save weight. The small size of the pieces makes the work a little finicky.

I have a short piece that was cut off from Skorpa's mizzen mast for comparison. Saving weight aloft is important because it makes the boat more stable and if it should capsize the light mast will make the boat less likely to turn turtle. That is completely upside down.

Now I use a plane and sanding board to turn the octagonal cylinder into a round cylinder. I forgot to tell you that after I cut the birdsmouth notches in the staves I tapered each stave so it was only about 1/2 inch wide at the upper end. This produced a tapered hollow wooden cylinder.

 

 

At the top I cut out an opening for a masthead sheave. The halyard will run through this so I can raise sail.

 

 

 

I shaped a small plug to seal the lower end of the spar so no water can get in.

This six foot wooden spar section fits together with two aluminum pieces to make a total mast length of nearly 17 feet. The aluminum is stiffer and is used at the base where that is needed. The hollow wooden spar is lighter than the aluminum and stiffness is not so important in the upper section.