As Kracker Jack stated above, a scupper is nothing more than a way to let water out of the boat automatically....it can be a hole or a channel. On most small boats it's a hole, and on large boats it's a channel. A check valve, which can be simply a flapper at the end of the hole or a more complicated ball assembly inside the hole keeps the water from coming back in. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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