It would be a storm jib
not trisail. The trisail is used in place of the mainsail
Depending on the size, the staysail could have a number of uses. On a reach, a decent sized staysail in concert with a Yankee Jib
, is a very good sail combination. You can get the drive of a single
overlapping headsail from a smaller jib
and staysail largely because of the venturi effect of the slot between the two sails
. On a sloop rig, the staysail would most commonly be used as a storm sail, however. The 'J' dimensions are usually too small for enough separation between the headstay and the staysail for optimum performance. If the boat was an IOR design, the 'J' may be sufficient. Since this one is on a furler
, would expect the owner was using it as double headsail rig with the option of furling
the sail and using it as a storm sail.
Most owners will install a staysail stay that can be removed and use an overlapping headsail on a furler
as their working sail combination. The staysail stay seriously hampers tacking with an overlapping headsail which is why it's usually stowable.