professorpaul wrote:
THAT is a nasty gap. Wonder why it happened? Something sunk, so I
might be curious about the underlying support of the tub. If you are
going to try to caulk it, first clean out the area very well. You need
the caulk to stick. Make sure there is no grease, moisture, etc.
Rubbing alcohol works well here. Use a good grade of silicon caulk.
White, in your case. Fill the cavity well, and smooth it out with a
tool, or your finger. If things move,then there is some chance of this
caulk holding. Check in a few weeks and see if it holds. I have a
shower base that settled about 3/16", and had a bit of a caulking
issue with it. It seems to have held up, but I check it now and then.
New construction.

There is stuff called "backer rod", I believe, for filling space too
large for caulk alone. Comes in different sizes, and it is right next
to the caulk at my fav. store. It is a foamy, plastic round length of
stuff that you put into the space so you don't have to load the entire
space with caulk - caulk won't work when it is piled in too thickly. Do
use silicone caulk, and have the tub and wall immaculately clean - after
you get every speck of soap scum and lime off, wipe with full-strength
bleach. I am wondering, too, why such a large space - is there movement
of the tub? For the less talented of us, painter's tape is nice for
getting a nice straight line - smoothe it out with wet finger and remove
the tape as soon as you get the caulk on right.