The plastic company I have looking for now skylight glazing suggested this
material:

http://k-mac-plastics.com/polycarbonate-lexan-mr10.htm

MR-10 Lexan with a silicone hardened surface.

Looks like the cat's pajamas. Strength of Lexan but abrasion resistance
increased from 35 to 1-4 (lower is better), almost a good as glass (.5). It
is recommended for use in building windows so I don't imagine yellowing and
weathering would be a problem with the boat covered several months a year.

Anybody heard of this stuff or have any experience?

--
Roger Long