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.
Little Cat entered in the singlehanded Vallejo I+II race a couple of weeks ago. We didn't have the best race. I messed up the start and got caught below the line against the tide and with just a breath of wind - crossed the line 25 minutes late. The wind was down to 5 knots or less by the time we reached Pt San Pablo and the tide had turned already. The long slow haul across San Pablo Bay had its moments when the wind came around onto a reach and we started to gain on some of the monohulls at the back of the pack. We made it to about a mile or two from the Vallejo Breakwater and called it a day as the wind died completely.
But all was not lost - the weather was fantastic and I had a great day out! The best part was in the dying stages of the wind - it came aft and I popped the new spinnaker, and managed 2-3 knots for a while against the tide with what must have been around 5 knots or less of wind. Huge credit to the F24 crew that won the race in the light-air conditions.
After pulling out of the race, we turned around and rode the tide back to Pt Pinole for the night. I also called in a DNC for the return race the next day as it was
starting the next morning against the tide and with a no-wind forecast. That race was doublehanded and was won by an F27 despite the adversities.
Pt Pinole pier ruins emerge in the morning fog
Next morning saw dense fog with visibility only a few hundred yards. We headed back to the Bay navigating by GPS and staying out of the shipping lane and Vallejo Ferry routes. Stopped for lunch at Red Rock until the fog cleared and then had a great sail home into a freshening Westerly. Great weekend!
