what wood should I use

[Steve W]

Vertical grain douglas fir would be a good choice for the top and bottom with a lighter wood in the neutral axis,not the lightest softwood but strong for its weight and the thickness can be reduced accordingly compared to lighter weaker woods,i would go for the max depth also for stiffness. "Simpson on boatbuilding" would also be good to have on your bookshelf,it has good info on box beam construction.
Steve.

[ancient kayaker]

Just a few quick notes on box spars. The strength to resist vertical bending forces is in the horizontal upper and lower surfaces multiplied by the spacing between them, which also contributes to stiffness. The vertical side members don't contribute an awful lot to resisting vertical forces except keep the upper and lower members spaced apart, but they do most of the work when resisting a horizontal force. Technically I should use "moment" rather than force, moment being lever length x force. I'm over-simplifying for some folks but maybe not all ...

The strength of a solid beam is proportional to depth^2 x width in the vertical direction. The stiffness is proportional to depth^3 x width.
For a hollow beam just calculate using the outer dimensions and subtract the contribution of the missing core. You can do the same for an I-beam.This ignores some of the members of beams but it's close enough.

[kim s]

Well I am feeling a bit bad, cos i am SURE I posted a big thank you to everyone, but here I am and I cant see it. hmmmmm must have had a stupid moment (again )

Well I have taken on board all the comments and it has made sense to me (ish) so am going measure up and try and get the thickest beam possible and am going for the spruce or cedar.

I will laminate using Epoxy with some unistrand inbetween .

So once again THANKYOU everyone who has taken part.

Kim

[Boston]

best of luck
now that you have been through the whole process and decided on what to do
go check out my site at

www.denverwoodandmill.com

no its not shameless advertising but it will give an idea of what things cost in some parts of the world
just something to cringe over I guess

if you go to the species available page and click the contact link for whole sale pricing you actually go to a page with the whole sale pricing

now imagine how much less it costs form the mill itself

torture I know but its my way to acquire enough material to do my own build

best of luck with your project
B

[kim s]

Boston,

I like the cut of your jib sir, not advertising as far as am concerned due to the distance and it would cost a huge amount to ship the little bits that I need.

If only you where in UK. but then you probably would not get the woods to cut up. not much red cedar grows here especially near London

I might try and find a mill in UK now you have given me the idea. I am not sure that the amount I require will make a huge difference but still might be worth while.

Cheers

Kim