This is the outside strainer I use on both my charter boat and trawler. The screen opening is 9" x 4".
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I get zero grass coming through it, which can be a big problem on Chesapeake Bay. The flat surface is so large that there isn't enough vacuum for debris to get pulled against it. The screen slides out after removing the 2 screws (replaced mine with 1/4" SS bolts). My diver can easily keep the screen clean with his bottom brush. Some of the watermen in the marina will remove the screen, bead blast it, and coat it with either Propspeed or the zinc spray paint. I still have strainer baskets on the inside of the boats as they came with them and I saw no disadvantage to keeping them.
I wouldn't own an in water boat without one. Now if you don't keep the boat bottom clean, I can see this getting covered with growth after a few months in Florida.
Ted