Not so. The geometry of the folding beams is designed so that the side shrouds hold the mast stable whether the boat is open, folded or in between. Or one side folded and one out. Many times, people motor with the mast up and just one side ama unfolded, to get around inside a marina.Just looking at DrJudyB's trimaran pictures, I guess I can answer my own question. It looks like the mast is stayed to the ama's so as long as the mast is up, I believe the ama's would have to be out and locked in place.
The Hobie TI has an unstayed mast which makes collapsing the ama's easy but probably not the case for the larger trailerable trimaran.
The F24 is quite stable when motoring with one ama out. Not all the Corsairs are as stable when folded; it varies by specific model.
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