With the vessel weight of the Antares at 22,000 lbs., the expected mast compression loads are about 19,750 lbs. For this experiment, we design with a safety factor of 3 – in other words, we can hold 3 times the expected load amount. We use 60,000 lbs. as a figure for the mast and bulkhead design. The safety factor accounts for calculated loads, extreme conditions, variations in vessel weight, and material property variations.
The spar and rigging loads for catamarans are 1-1/2 times higher than for monohulls of similar sail area due to their high resistance to heeling. We engineer the mast bulkhead much like a gigantic “I-beam”where the bridgedeck and hull are the top and bottom flanges and the bulkhead acts as the web (link to download PDF of the mast load diagram).
The image to the left shows the computerized finite element structural analysis of the bulkhead – a value is assigned to the contributions of the deck and bridgedeck and a load is applied to the web, producing patterns of greater and lesser stress.
The small bulkhead in the area immediately below the mast (that runs fore and aft), prevents localized buckling from the loads. While the mast itself isn’t of significant weight, it is the mast compression and the stays pulling up that create the significant loads.