scantlings for a boom

[Murray Campbell]

hello

i'm replacing an old pole boom...will probably have to laminate a spar and plane it round or perhaps even go for a hollow box spar

the old one is 20' long and tapers from 3.75 to 3.5 inches diameter..it has developed a big sag along its length...i don't want to end up with the same problem so i'm thinking of increasing the diameter a bit...do you think a laminated spar would be stiffer?

thanks for any thoughts...

[Jack Heinlen]

Spars move for reasons: inherent structure, poor storage, poor stringing.

An elegant spar would be a hollow staved one, otherwise know around these parts as a bird's mouth. Such will be much more resistant to such movement, and economical to make, relatively.

Are you sure you couldn't hang the spar differently this winter and take some of the bow out?

[Murray Campbell]

i think you hit the nail on the head with the storage comment...the boat's fairly new to me but i'm sure the boom has hung from the topping lift for 25 years

perhaps i'll be making a boom crutch too, to give the new one some more support...

[paladin]

Murray, Tana Mari's spars are laminated from what is called around here "Coastal Douglas Fir" and we cut it and made 4 laminations from plane sawn wood and laminated them back together after switching the grain pattern on two of the laminations...then......the bird's mouth technique. The spar is actually lighter and stiffer than a comparable aluminum one......