Hollow hardwood spars

Hi Mik,

I recall you mentioned somewhere that you had experimented with hollow (birdsmouth?) spars using hardwood. What was your experience?

At least in theory a thinner wall with stiffer but heavier material should be stiffer for the same weight and spar OD, though going much thinner than 15% wall might be dicey even with tougher timber.

Some of the medium density hardwoods such as Vic Ash seem to have better stiffness for weight ratios than any of the softwoods.

Vic Ash would seem to be about 20% heavier than usual spar timbers but 40-50% stiffer.

Oregon: 500kg/m^2 for Elastic Modulus of 10 - 11 GPa

Vic Ash/Tassie Oak 620 kg/m^2 for Elastic Modulus of 15 GPa

Ian

In theory it's diameter that really counts with wall thickness to match requirements. Technically, it's fairly simple as, a column's efficiency can be described as being equal to the square root of it's stiffness divided by it's density. Unfortunately, you can have too stiff a material, which generally makes it fragile, if unstayed or heavy if stayed. Weight is a RM killer, especially in small craft, so this is why light, exceptionally strong for it's weight species are employed in spars.

The net result of these conflicting elements of physics is, you want as narrow a mast section as you can possably tolerate for the use expectations, while offering the appropriate amount of flex for a free standing rig or stiffness for a stayed rig, all the while keeping it as light as possible and preferably not snapping in the first heavy gust it experiences.