The hole needed is 68mm, it needs to be as close to the deck as you can get it or in the bottom of the splash well if you have one. you also need to make sure the trunk wont fowl the propeller.
Has anyone used a non-return valve on the elephant trunk, so the water doesn't get back onto the deck with the boat at rest? I'm thinking of the plastic fittings that trap a ball (size of a squash ball), so it floats and plugs the hole when its under water. I'm sure they're meant to be used without an elephant trunk, but just wonder whether you could put a trunk on the outside of one.
You could do then at least the NRV stops working you can just pull the trunk up.
You could also glue a cork to the tube so it floats at rest but the water drains when underway.
