My guess is just the hollow space around the build up which is usually rectangular in section but often oval in shape on the exterior. Having no idea the actual weight of my main mast, I'd guess (again, wildly) from the section that a 40 to 50' spar averages about 2 board feet of Sitka spruce (about 2.5 lbs/bd.ft.) per lineal foot of finished spar. That would make my spar about 200 lbs without the rigging. That alone with the metal fittings/winches, likely adds 30 - 50 lbs.
For years my family rolled the main mast to and from our boat at haul/launch, as I stored it outdoors alongside the boat.
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Some wooden spar photos I've taken:
The ancient boats, even if built new like GODSPEED launched in our harbor years ago, are often solid.
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There seems to be no maximum length on some of these old boats. In fact some have had extensions scarfed on to increase sail area. Easy enough to do with wood and the right skills.
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If spruce spars are cared for they don't seem to have a life span.
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At the yard in my harbor, they not only build the spars, they fabricate the fittings.
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