Spinnaker

Did some research and found a great second hand spinnaker from Bacon Sails. It is a tri-radial Tempest Class spinnaker at 19.5' luff, 13' foot, and 14' mid-girth giving 236 square feet.

 

The spin has its own halyard through a block at the top of the mast (above the forestay)

I have used it quite a bit and this size certainly works on the Tiki 21. It is not large, in the sense that the max practical luff would be around 21', and the foot could be, say, as much as 16-18', but it works well and is well suited to single-handing. Because the Bay is generally windy and gusty, it also doesn't hurt to be on the modest side of size. It is 0.75oz fabric weight and I find that this will fill and pull in under 5 knots of breeze. The tri-radial cut allows the sail to carry up to a broad reach.

The two guy lines can be seen in this downwind pic

Many folks set the spin from a single tack point between the bows and use a single sheet, but after reading Richard Woods excellent article on catamaran spinnakers (http://www.sailingcatamarans.com/index.php/articles/11-technical-articles/49-sailing-under-spinnaker), I decided to go with guys and sheets from each tack/clew.

Sheet set-up: 40mm turning block on aft beam and 57mm autoratchet running to a hand-held or cleated sheet

The advantage of the single tack attachment is that the lines are simpler, but the tradeoff is that gybing is difficult. Set up with guys and sheets to each tack/clew, I can merrily gybe back and forth downwind with good control and minimal effort.

See below for a short movie of it all working in a light breeze.

Update: After many miles, my use of the spinnaker has shifted from a dead downwind sail (although I still use it that way), to a reaching sail. For beam reaching, I set it up with the two guys attached to the tack, holding it in the center of the boat (single tack attachment). With the apparent wind at 90 degrees Little Cat really flies set up like this. Gybing is tricky, but gets better with practice. Gybes can be inside (clew pulled in front of the forestay) or outside (clew pulled around the outside of the sail), but probably a bit easier on the outside once there is a bit of breeze. This spin is fairly small which suits the windy Bay. Have had some great runs set up like this in 20 knots of true wind speed.

Like this