The right strategy for getting the most performance out of a catamaran when sailing downwind will depend very much on the speed potential built in to the one on which you are sailing.
A bit of background here: any boat that sails slowly (irrespective of how many hulls it has) needs to set lots of sail area and sailing deep downwind if it is to achieve a decent speed made good (VMG) downwind ie. typically between 160 and 170 degrees (or 10 to 20 degrees of the dead downwind line if you prefer).
Whether you are aboard a monohull or multihull, sailing dead downwind doesn’t usually work well in terms of VMG – mainly because in that condition apparent wind speed (AWS) is limited to true wind speed (TWS) less your boat speed over the ground (SOG) so it is impossible to generate any apparent wind – as you go faster you simply sail away from it.
Another problem is that sails tend to shadow one another on this course so it’s hard, for example, to get much help from a foresail that is in the wind shadow of the mainsail.
As it happens, this dead downwind course will also mean that that you are running a permanent risk of causing an accidental gybe – particularly in any kind of seaway, although in a catamaran at least you won’t have the cyclic rolling problem that conventional single-hulled boats suffer from if they sail too deep downwind.
So assuming your catamaran is a bread-and-butter cruiser, without much of a performance capability, you’ll be doing much as you would do in an equivalent monohull, which is setting either a conventional symmetrical spinnaker or perhaps an asymmetric one.
The symmetric one has the advantage that when you gybe there’s not much to do as the flow is simply reversed across the sail on the other gybe. On the other hand to make the most of that sail you’d really need to set a conventional spinnaker pole – as seen on single-hulled boats of limited speed capability – and that means swapping the pole to the other side as you go through the gybe.
In reality you don’t see many spinnaker poles on catamarans that are flying symmetrical spinnakers. When gybing, most people simply haul the ‘new’ tack down to the ‘new’ windward bow and allow the ‘old’ tack to be drawn back by the ‘new’ sheet on the ‘new’ leeward side.
In practice this arrangement works less well than you might hope, mainly because if the tack of the sail is to be able to function as a clew on the other gybe, then it needs to be cut quite high. On the other hand when that corner of the sail is working as a tack it would work better if it was lower – and so able to be drawn right out to the windward bow.
By Nigel Irens @http://www.yachtingworld.com/