Part II: A Hollow Mast, How Do You Do That?

By Shane Hall

 

The size of the mast determines the size of the parts. A lot of complicated geometry can be broken down into two simple equations using the outside diameter of the mast to determine the dimensions of the individual staves. 

 

Stave Thickness H = .2xO.D

Stave Width L= .4x O.D

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We begin by milling out eight rectangular staves the dimensions of which are based upon the maximum diameter of the mast. Throughout this process, a decimal to fraction conversion chart is extremely helpful to round numbers up to the nearest 1/16th of an inch. We rip out the staves on the table saw and run them through the thickness planer to ensure uniformity. We are still working with rectangular strips of wood.  Next we cut out the “birdsmouth” notch along the edges of each stave. In order to create a circle (desired shape of mast) or a 360 degree turn using eight staves, each notch needs to be a 45 degree angle.  360 divided by 8 equals 45. We cut the notches with two passes on the table saw.

 

We now have long, narrow, thin pieces of wood with notches along the length of one edge.  If we were making a perfect cylinder then we would go ahead and glue everything together. However, the mast for the Deblois St. Dory, like most spars, tapers in thickness: it is thicker in the middle and narrower towards the ends.

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We use the same formula as before (.2xOD) to calculate the taper along the length of each stave.

The tapers are marked on each stave and planed to the line by hand.  Note that the birdsmouth edge of the stave remains straight and only the square edge of the stave is tapered. The thickness of the staves remains constant.

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Stands made with semicircle notches are set up along a level surface.  The notches are sized to reflect the taper in the mast but are cut oversized to allow for the lips along the edges of the staves. The staves are dryfit together and ready for glue.

 

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