Lewmar vs. Pompanette (Bomar)
The cutout for the Lewmar 40 is 16 9/16 x 16 9/16. The cutout is not the only consideration. You also need to consider do you want a flat base or a flange base. Lewmar makes this model in a flange base and a flat base. The original was a flat base. I could not find one with a flat base, with correct deminsions, built by Pompanette. I did not want to reduce the actual opening by introducing the flange base through the opening. I did not want to expand the opening. My concern on expanding the opening was the effect of disturbing the joint of the v-birth ceiling and the deck. So I went with the Lewmar because it was the only flat base hatch I could find with the correct cutout size.