[Paddlewise] Beam reduction/speed

Assuming I take the basic dimensions (17'8" x 21'8") and
shape of a CD Caribou (maybe take the rocker out of the
tail) and reduce the beam to 19", what increase could be
expected at 5 mph cruise speed?  I know I've seen formulae
on the list, but I neglected to save them.

Thanks.

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