Re: How do you rig a trysail?
The foot is not attached to the boom. You lower the boom to the deck, and strap it down tight.
Two sheets are attached at the trisail clew, each being led out somewhere close to the quarter, and hauled tight.
Fortunately I've only ever done it as an exercise, never for real. After rigging it the owner decided to keep the sheets permanently attached to the sail, so it would be one less thing to worry about when times get hard.
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Next time, it'll all be different.