Thanks for the replies guys. The photo of the ring beneath the vent cowl on Sirocco is a dead ringer for the vent ring under my current makeshift cowl. The difference is that my cowl was replaced by a previous owner with an ill fitting snap-in Nicro cowl. The bottom (verticle section) of a new Nicro plastic snap-in style deck ring cut off becaue it didn't fit inside of the aluminium ring, but was too small for the deck cut-out. The remaining horizontal portion of the plastic deck ring was attached to the original aluminium ring, which looks just like the aluminium ring on Sirocco.

The pastic ring was attached by sheet metal screws and lots and lots of silicon. The plastic ring was badly cracked and the installation (which was entirely bedded with silicon) showed signs of leaking.

Frankly I find the idea of a snap-in plastic ring a bit frightening in this location. I like screw-in deck plates for off shore sailing.

Naturally, curiousity got the best of me, and I decided to mess with it before I discovered that the deck cut-out on my boat is exactly four an one eight inches in diameter. The Nicro four inch vent requires a four and three eighth inch cut out and the three inch vent requires something less than four inches.

It would be really nice to find a screw-in cowl vent that fits the threads on my original ring. Otherwise I will have to go with a larger set-up and cut a larger diameter hole.

Thanks for the tip on the Rostand catalog. Also my copy of the Ariel maintenance Manual shows a deck plate on the hand written packing list ffor Ariel #24 as: "9114 1 ea deck plate 3" Rostand"

Although it is conceivable that this plate was installed for the purpose of filling the water tank by passign a hose below through it, the deck plate would in that case seem superfluous since the forward hatch is more proximate to the water tank inlet below than is the deck plate on my boat #330 or Sirocco (see photo above), which is over the anchor locker in the forepeak.