I've got doubled-up 2x12s for my "bunks" on my lift - with a slight angle sawed out of them. These work fine.

24 inches is the amount of draft you need - but this is by no means close to how much room you need from where the bunks rest to the end of the prop/fins. Where the bunks would rest is under water from the draft line. I agree with sled that 9 inches should be enough, 12 inches is plenty.

Just be careful when the lake level drops - you are still losing however many inches thick your crossbeams are - meaning if your crossbeams are 3 inches thick, then you need 27 inches of water to clear them.

BTW (fine print) - I don't have to worry about my prop, only the fins...