You are correct about the weight differences. White and sugar pine
are about 25-27 lbs/Cu.Ft, Cedar 22 Lbs/Cu.Ft, Redwood 28 Lbs/Cu.Ft
and Mahogany and Shortleaf (yellow?) Pine run about 36 Lbs/Cu.Ft.
Depending on the amount of mahogany the weight gain over just Cedar
would be small if the mahogany were decorative pieces/strips. More
mahogany = more weight. A large percentage of mahogany would need to
be used to make a major difference. White pine has been used by some
builders to strip the entire bottom below the waterline for
design/contrast and as an aide in spotting a capsized craft without
adding any weight.
Cedar, Redwood, White Pine and Mahogany are all commonly used in
building strippers, Cedar as the base wood and the others mostly for
adding contrast and design to the individual boat. Other woods are
also used for decoration. Rosewood, Ebony, Holly and dyed woods are
usually used as an accent, sometimes inlayed, sometimes part of the
structure, but almost always in small amounts. Take a look at other
web sites for ideas on how the different spiecies are used for
design and decoration.
The other consideration on weight is the application of the epoxy,
how many layers and the weight of the fiberglass cloth. Adding the
extra layer on the bottom for abrasion will make a difference as
will 4 oz cloth vs 6 oz. cloth. You have to make a decision based on
the use of your boat (touring, racing, family knock-about) and see
where a comprimise can be made. A racing canoe on calm waters can
use a lighter mix of cloth and thinner wood than the same canoe
frequenty used in white water or by the kids on a rocky shore. I
hope this helps.