Except for the fact the colors are reversed, it looks like a giant version of the signal flag "A" (which is sometimes used for divers). But, again, the color scheme is wrong.
I would venture that it is some kind of riding sail (used at anchor, or sometimes with a sea anchor, to keep the bow into the wind). It is a funny shape, but I've seen some odd looking ones that were bent over a backstay to create an almost kite-like sail.
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT