I just got back from taking out my 2015 Tandem Island with Spinnaker kit. Downwind, Broad Reach, and even some beam reach makes the boat much faster. In 10 mph winds, the TI really moves with both sales deployed downwind. I really like the speed.
There is some added complexity. I rigged it wrong and had to fix it on the water... twice, ugh... Also, some more things to remember. My girlfriend found the mast tip rotator in the sand as we were leaving. I forgot to stow it.. Whew..
The Spinnaker is easier to deploy facing downwind and easier to stow in a headwind. In the beginning, I had problems getting it back into the snorkel bag, but, I got the hang of it.
Downwind, the Spinnaker really lifts the bow ! This is great when you are riding swells coming into the marina near the breakwater. It really helps keep you from nose diving when riding swells.
On a almost beam reach with both sales tucked in tight, the TI is quite a site with both sails, it really moves.
Here are some pics and a video. Downwind, coming back, I had the spinnaker left and the main sail to the right. In this case, the Spinnaker Halyard line does seem to limit the main sail movement to the right, but, I think you could reef the main and tuck it under if you wanted to.. I would like to get some opinions on that. I guess to get a true "butterfly", you have to reef the main under that halyard line and then unfurl it behind the halyard line (back stay) ?
Anyway, in the video, the boat is picking up lots of wind and the boat is moving at a good clip. Yay !!
Video:
https://www.youtube.com/watch?v=dJsXqxXCURwPics:



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