As our regular followers know, we are always passionate about sharing lessons learned in development and explaining evolving methodologies.
For Katana we engineered and prototyped moulds for making integrated main and jib booms over a short length of 14mm ID carbon tube. The booms were then cut out of foam sandwich such that the core was in the vertical plane. The skins captured the piece of tube which effectively replaced, and created a bulge in, the core around the intersection with the mast. The 14mm ID tube that formed part of the boom/yard moulding would then be bonded to the outside of the mast tube.
Even with well thought out moulds this would have been a relatively labour-intensive approach. The result had good structural efficiency in the boom section but required considerable reinforcement around the junction area, offsetting most of the gains. Windage was marginally higher but the deck sealing effect could be maximised as the booms could be cut to exactly follow the foot droop permitted in the rule.
So we revisited an old solution that seems to have been abandoned due to an irrational preference for the latest material over an objective analysis of suitability for the application.
The difference today is that CNC milling allows exact replicability without expensive fixtures.
Most importantly, the penalty for adding complexity is considerably less than for manual processes (including laminating carbon fibre).
After several iterations, our late stage prototypes use aluminium in the high stress junction area where it is desirable to react the forces on the boom and yard over the shortest possible vertical distance to keep the mainsail tack close to the deck.
Carbon tube is used for the boom and yard as this provides an excellent compromise between stiffness, windage, ease of assembly, and cost.
Being machine laminated, tube has good consistency and, being round, it allows efficient attachments and adjustment systems.
Previous similar blocks by other manufacturers did not incorporate angled main boom connections so the boom was usually either made from bent aluminium (heavy and flexible) or required an elbow somewhere along its length (structurally inefficient).
With the correct angle machined in, efficient straight booms (cylindrical or tapered) can be used.
FEA allowed us to take as much weight as possible out of the part and hard anodising ensures good resistance against corrosion. Different colours are also possible.
Stay tuned to see the parts at work on our Katana test boats.