Bert from the Boot Bouw School recommends that a given mast can be built hollow for a weight saving of around 50%. Being of a mathematical bent, I wanted to know how this all works out. So let’s try it.
Given a hollow mast of diameter 100mm, we know from the suggestions from an earlier post that the pieces that make it up need to be 20mm x 10mm. We can draw this up in CAD or use a pencil and paper, and work out how the amount of wood will be once we have shaved the mast down to a round shape.
Doing the calculations and adding it all up, we get that around 41% of the cross section is hollow. So a hollow mast will be 41% lighter than a solid mast of the same size. In theory it will be even lighter, as we end up taking more of the wood off that we expected, so the outside diameter is a bit smaller.
The problem that arises is that it is suggested to increase the radius of a given solid mast by 10% before we make it hollow. So the solid mast that we could be using would have size 91mm instead of 100, or be 10/11 of the diameter. Which gives a reduction in cross section area (i.e. weight) of 18%. So we have around 59% of the weight for a hollow mast, or 82% for a solid mast of appropriately reduced diameter. So our saving by moving from a solid mast to a hollow mast in on 22 from 82, i.e. around 27%.
Plus of course our spar is not completely hollow: we have solid ends, plus areas of sideways compression in the mast that need to have a solid core to withstand the pressure. So the weight saved is even less; maybe we get a 20% reduction. This is not bad, but not a huge difference. All things count when we are racing boats. And it is a noticeable difference when picking up the boom (solid) and the gaff (hollow) that are of comparable dimensions and then feeling the gaff almost float away in your hand.
But best of all: building it was fun!