You are right here.

The main line of though I'm following now is that I don't feel that there are big loads on the in te sideways direction. So no material is really needed here. Neither stiff nor much of it. But their should be enough to prevent the boom from buckling or breaking in case it hits the stays or something else.

If there are any real loadings on the boom I suspect it is almost exclusively in the vertical place and mostly in the rear part of the boom. To check this I want to know how the current boom flexes.

Lets stick to your T-shaped boom for now, to keep things simple.





In the diagram above I've draw the forces that I suspect are dominant, that cause the flexing.

The black line gives the level of bending stresses inside of the beam. Clearly around the mainsheet point the internal bending moments are the largest. These reduce down to zero towards the ends. The internal shear forces are not drawn, but these are rather small (and constant) in the front part of the boom. In the small rear part these are significantly higher but still constant. I suspect that a decently thick planck will be able to transmit these shear forces, but reall calculations need to be performed here.

Clear the bending loads are not constant along the boom and so it pays to give the boom different crossections at different points along the boom. The wood is most there to act as spacers between the strip of alu or stainless steel on the top and bottom. These two strips take by far the most of the bending loads. The wood middle part handles the shear stresses.

The drawing on the bottom is the top view. Clearly I made the boom thicker in the middle and narrower at the ends. Clearly a cosntant crossection boom will be weakest in the middle so I've compensated for this by moving some material from the end to the middle. The wood will be strong enough to take any loads in this place, because they won't be very high.

You can just cut the top (horizontal) and bottom (vertical) planck by the outline and then cut out smaller sections to save weight (circles). You can attach the top strip with the top planck to the bottom plank by just screwing through all these sections with woodscrews. Some timber glue may well help but is may not be necessary. Now you only need to fold the bottom strip along the bottom contour and secure that with screws as well and the boom is ready to take the fittings.

By the way, I forgot to draw the rear section taking the traveller rail. Just adjust the outline of the bottom planck to get the right angle for the rail. Here I would put a U-shape metal strip over the bottom and secure that side-ways to the plank by schrews. Fit the rail to this additional U-shaped section. This should transmit the loads better to the plank.

The cut-outs need to be well places to save weight and not degrade stiffness and strength to much. You can buy saw-bits that can be mounted to your drill or simply saw out hexagonal shapes with a suitable blade. Both will work. This way of building the boom will be both easy and fast. Most likely it will be cheap as well. You can test the setup using some old planks you got in your shed and a section of alu strips (hardware stores). You can then test this and determine how it performance and adjust the setup.

Remember, wood like Douglas Fir have only 28 % of the density of carbon laminate. Meaning you can use 3.6 times as much (in volume) as you can in carbon for the same overall weight. This makes the boom alot more stable under load as here volume is just as important as material stiffness. Yield stress of these wood types are still at 55 % of that of aluminium. But aluminium is again 5 times more dense meaning that the wood per USED VOLUME can carry more load then aluminium ! This makes wood a very attractive material to make the main body of the boom from. Here you need volume for stability BUT you DON'T to want to accept much weight here.

A strip of stainless steel is 10 times as stiff per volume as the same volume Aluminium and about 7 times as stiff as a normal carbon laminate of the same volume. By the way stainless only weights 3 times as much as alu and only 4 times as much as carbon laminate. So stainless steel is what you want for the strips (like a on a dolphin striker strap), but it is expensive. Alu is dirt cheap. Carbon is hardly more interesting then alu (in the way of stiffness per volume (only 30 % more) and very much more expensive. So it will be either stainless or alu. Added advantage is that both metals are easier to work with. Just bend them around the slightly curved contour and screw them to the planck. You can finish the boom and sail away minutes later. No sticky fingers, curing time or chemical lungs.

Both the bottom contour and side contours are slight curved.

Now I want to see any constant carbon crossection even beat this simple wood/metal setup both in stiffness, strength AND weight. More is possible if we spend some more time optimizing the design.


Wouter