a22andrew:

That is uncaulkable. Even if you do caulk it, it will look terrible, even if I did it.

If you must have this gap closed, hire a solid surface fabricator to make a custom piece to do the job. That caulk joint will look beautiful and function perfectly for years.

Where are you trying to caulk? It's not supposed to be against the wall, it's called freestanding for a reason. It's not intended to have a shower above it. In the days of showering in a claw foot tub, there'd be a curtain all around it.

Are you trying to caulk the floor behind the tub? You would do that when the tile is done, before the rib goes in.

Totally agree with amberm145. You're trying to use something in a manner that it was not designed to be used. That is a soaking tub ONLY. It's not designed to be used with a shower. You need a separate shower, or a different tub.

You're going to get massive water damage in this situation. You need to correct your design.

Thanks all

Never said anything about a shower above. For the sake of ensuring water doesn't go through the hole below (after just splashing out on the wall side), the bottom rim needed caulking / sealing.

Also - don't care if it's not meant to be against the wall. That's the way it is, and it looks great..

For future readers, an older plumber came up with the solution immediately. There are rubber seals designed to go around doors, about 6mm wide. In Australia, Raven is one company that sells them. They are adhesive on one side (not that it matters with the weight of a stone bath) and rounded on the other (http://www.raven.com.au/domino/raven/RPCEN.nsf/$defaultview/4882AC561F008837E9256B6E00085B1D).

The plumber stuck 5 meteres of that to the base, while the tub was suspended on two paint tins in the middle. Three of us then managed to lower the tub down so that it sat evenly on the rubber. It was tricky to lower without dislodging the rubber, but we did manage.

I'm now confident no water will get below the tub.

a22andrew-
That sounds like a good solution. I would have suggested a seal that could be affixed to the bottom edge, but I mistakenly thought you had the tub already installed and were looking for a way to caulk it in place. Anyway, it's resolved...good for you.

I never said this tub couldn't be next to a wall. I (and likely a few others) thought you were trying to caulk the rim of the tub against the wall. I asked where you were trying to caulk in order to clear up the confusion.