I wrote:
> I've had great luck cutting gains with just a backsaw and a
> chisel.

John replied:
<<>>>>

Because the gains taper away at a constant rate, I begin the
cut at the end of the strake, taking the kerf down about
halfway through the strake, then lower the heel of my
backsaw with each stroke until it just touches the plank at
the start of the gain.  (I cut with the toe of the saw at the end
of the strake with heel toward the middle.)   Making the kerf
too deep at the start of the gain won't hurt much --epoxy will
fill the kerf--but go too deep at the end of the plank and you're
in trouble--it could split.

Paring away the veneers with a sharp chisel is a joy, and you
can use the glue lines as a reference to keep the gain
tapering away at a constant rate.

I cut gains only on the upper-edge, outer side of the strake,
leaving almost a feather edge at the end of the gain.  The
other method is to go only half the thickness of the strake,
and then cut a matching gain on the lower-edge, inside of the
adjoining strake.  This later method, though stronger, is
unnecessary on plywood/lapstrake/epoxy boats, IMHO.

-Gil