Cruising catamaran rigs

An article from AES.

It is common for light weight racing catamarans to 'fly a hull' and use 100% of the available righting moment and at the same time reducing hull drag by removing a hull from the water. The overall "effect" is well known and the righting moment is generally the displacement multiplied by half the beam from hull centreline to hull centreline.The situation changes radically for cruising catamarans. Instead of a displacement of some 3 to 4 tonnes, a similar sized vessel may have a displacement of 12 to 15 tonnes. A four to five fold increase in displacement is not uncommon. The vessels are normally bridgedeck which affords a large volume for the nice things in life and each hull can contain various chest freezers and washing machines for live aboard comfort. This is in contrast to a racing catamaran with trampoline netting between two mostly empty hulls containing a few sail bags.So it follows that the righting moment of cruising multihull is some four times larger than a similar sized racing version and that the mast and rigging might become four times larger and more expensive. Not only this, but it takes considerably higher windstrength to fly a hull and that it would be outrageously dangerous to fly a hull in such a vessel at those windspeeds. So, this raises a number of interesting issues not the least of which is the refusal of owners to spend so much money on an enormous rig and the risk of capsize. We understand that in France it is common for rigs to be designed to withstand 60% of the righting moment. Given a factor of safety of 2.75x it would follow that if one were to fly a hull this is exceeding the working load (effectivley 166% of the working load). It can be shown this means that yacht flys a hull with only a small factor of safety of 1.0x at hull fly. Thus the rig should be close to falling down, but not quite... Hmm, anyway this is what we had heard. This is something of a compromise position which may work best for acatamaran which is relatively light in respect of cruising catamarans. NZ designers take a number of stances. One stance is to design for a wind range and whether this be 20% of the righting moment or 60% of the righting moment is irrelevant. Thus the rig is reefed at say 25 knots apparent windspeed, regardless of the displacement of the vessel. Whilst the windspeed varies between designers and projects this is in fact the same approach which is taken on multi-masted monohulls such as the huge sailing yacht Phocea (ex-Club Med). The argument is simple enough that where the righting moment is effectively infinite, then we might as well ignore righting moment; eventually at some windspeed the sail will shred and there is simply no point in carrying around enormously heavy and expensive masts simply to be able to fly a hull in (for example) 50 knots of wind. A further refinement of this is to put forward the proposition that it is most important that the vessel does not capsize. Therefore it is actually safer if the mast falls down prior to flying a hull. Thus rather than floating upside down offshore, the crew are left with a yacht floating the right way up albeit without a mast. For an offshore cruising catamaran of high displacement we think this approach has some merit and often engineer the rig accordingly under direction from the naval architect. Provided the crew reef the sails at the appropriate windspeeds the rig will never be in danger. However if full sail is left up the rig will fail prior to lift off. There may be a situation where due to mechanical failure full sail is left up unintentionally. Insurance companies of course take a dim view of rigs which are designed to fall down, and in general the wide variety of factors of safety and rig configurations make it hard to access which vessels represent a poor risk or indeed which are seaworthy at all.. The end result is that it is important to have good communication between owner, crew, designer, mast builder and insurer if the vessel is to be operated in a safe window with an insurance safety net to cover events which truly can not be foreseen. As if these issues of windspeed and righting moment were not enough, there is a similarly wide range of options for the actual rig configuration. This ranges from simple three stay rigs, either with or without multiple diamonds to keep them in column, to rigs which have really long swept spreaders in the style of the America's Cup yacht KZ1 finally through to rigs with no spreaders or diamonds at all but instead have multiple sidestays and forestays. This last class of rig was to our knowledge used most famously on the english cat APRICOT, fifteen or more years ago. Recently the racing 60's have moved back toward such spreaderless rigs. The cruising yachts do tend to follow the racing ones though it has to be said that many of the cruising yachts do not have rotating wing masts, so the reasons given for copying the racing versions are sometimes misguided. Owner sof mono-hull vessels are equally susceptible to this. The long spreader mono-hull KZ1 style rigs put massive loads on their spreaders and the spreaders sections then become almost as heavy as the mast itself, which quickly reaches a point where it becomes self-defeating.The good news is that with wide staying bases these rigs are always lighter than their mono-hull equivalents when operating in the same load regimes and the greater number of rig options provides for great flexibility to create an optimal rig plan for almost any type of sail plan on almost any catamaran configuration.