With an additional two drains of 1.5", you'd get
approximately 8 times the drainage speed of a single .75"
drain. (forget reynolds numbers for this simple exercise. A
= pi x radius x radius. If you leave the original one
connected, your cockpit will drain 9 times as fast.
Try an expericment -- fill up the cockpit with water to the
sill of the door. Time how long it takes to drain. Now
divide by 9. Is that fast enough for you?
Best,
Judy b