As the names suggest, a symmetrical spinnaker is cut in symmetrical shape. An Asymmetric spinnaker (otherwise known as a gennaker or A-Sail) is cut more like a lightweight genoa and as such it has a permanent luff, tack and clew. Symmetrical spinnakers have alternating tacks/clews, dependent on which end is the windward corner when sailing.
Symmetrical spinnakers are better used for sailing deeper down wind as their shape lends itself to broad reaches and runs. In order to stabilize a symmetrical spinnaker (and to enable it to be trimmed higher on the wind than a dead run) symmetrical spinnakers are designed to be flown with the use of a pole which attaches to the leading edge of the mast and is adjustable on the mast end and at the tack end by use of running rigging. The running rigging used is known as (i) the pole up or boom hoist (ii) the pole down and (iii) the guy.
Once the pole has been set, the sail can be hoisted. The idea is that the guys attach to the spinnaker’s sheets so that at any one time there is one working sheet and one working gye on the opposite (pole) side of the boat. The gye runs through the end of the pole. The spinnaker is hoisted on a halyard and when set the pole is trimmed back and the working sheet is trimmed on. A symmetrical kite can be gybed by disconnecting the pole end from the working guy, dipping the pole under the forestay and connecting it to the new working guy. During this process the kite is flown downwind on both sheets. As you might imagine, timing and technique go a long way to making the gybing of a symmetrical kite seamless and without drama.
Asymmetric spinnakers are more triangular in shape and look much more like a large, lightweight genoa. They have one leading luff edge and a permanent tack and clew. The asymmetric spinnaker does not require a pole as such and therefore requires no pole up, pole down or guys to trim it. However, most asymmetrical kites have their tack set to a small pole at the bow known as a bowsprit.
The tackline is attached to the tack and is adjustable. The head is attached to a halyard and the sheets are attached to the one permanent clew. It’s important to run all lines outside everything and, if you are planning to do outside gybes, you must run both sheets outside and over the top of the bowsprit, forward of the sail. Unlike with the gybing of a symmetrical kite, the gybing of an asymmetrical kite is relatively simple with one sheet being released as you drive deep into the gybe and the other being pulled in. An outside gybe results in the sail being inverted (turned inside out). An inside gybe requires you to pull the clew through from between the sail and the forestay.
Asymmetric spinnakers work better on the reach (either beam reach or broad reach) with symmetrical spinnakers better designed for broad reaches and downwind sailing. If you wish to sail higher on the wind with a symmetrical sail then setting the pole end down and forward creates a ‘virtual bowsprit’. Alternatively, to sail an asymmetric sail deeper downwind you should ease the tack line which opens the luff and enable the sail to be flown deeper down wind.
For shorthanded and family cruisers a great alternative to the asymmetric or symmetrical spinnaker is the rather more conservative cruising chute.