Wouldn't a simple solution be to have the intake tube go straight up into the boat before having a hose clamped to it above the waterline? You would want this anyway so that you could stick a brush down it to clean it out, as Maine Sail has suggested in his posts on the topic. You could have another tube slide down inside the straight intake tube ("sleeved") with a mesh or screen covering on the end. If you set the mesh end so it was even with the outside of the hull, it would have less drag than a clamshell. Flow would be the same as with a clamshell or screen. If you thought anything might be clogging the screen you could undo the hose clamp, pull up the inner sleeve and clean it off. Best of both worlds. Seacock might be a problem, but you could remove the sleeve if you wanted to close the seacock.
