So, for a while I've been using a Rodenstock Vario ND. It says it stops up to 5 stops etc. What I've found is it's good at dawn and decent at sunset but in the daylight it's next to useless for anything over 2 seconds. I've been reading up on the Lee Big Stopper, but I was just curious if that would be decent in daylight or suffer the same fate?

I stumbled across an old wrecked WW1 ship and wanted to get a 30 second exposure with clouds etc but the max I could get was 2 or so seconds, so I know I need something better.

Does anyone have any ideas if the Lee would work well for me?