Nichimo Sendai Type-L Trawl Doors, or SLs, are bottom trawl doors made of Japanese black pine.

The vertical-type SLs have a low gravity center that won't be possible in a metal door, thanks to rich resin of black pine grown in Japan that generates good buoyancy.

Their wide shoes effectively keep SLs from tilting outward.

SLs are traditional, but remarkably easy-to-use wooden doors because of their low gravity center and wide shoes.