This idea is intriguing. Why not use a large diameter thru hull with no clam shell nor screen which has a pipe attached which extends above the waterline? Then you have an elbow which leads to an inline strainer and then to the engine etc/whatever. To clean, unscreww elbow, insert a cylindrical stiff bristled long handled brush to clean the tube. Reattach the elbo.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.