I've heard them referred to as "barrel nuts" but if you google "sex nuts" (gotta love that) you'll find them.

West Marine carries a few but the selection is limited and very expensive.

I'd buy them by the box online.

They have some that have a flat flange and some that have grooves to create friction against the deck to help prevent them from unscrewing. I think our boats used the grooved ones but better check yours.

-bryan

:: I am remounting some of the hardware and adding new
:: hardware.
::
:: I noticed that there are nuts on the underside that have
:: the washer built in and it is closed like an acorn nut.
:: I can not find the name of what these are and where to
:: buy. Any suggestions?
::
:: Also, I have to put some lewmar track on for the
:: adjustable genoa cars. What have others used? #10 square
:: bolts, etcs.
::
:: Thanks,

:: Don