Unless the sand is extremely firm the keels and rudders will sink in. This is usually not a problem. If you wish to keep the boat a bit higher on the hard
it's possible to place boards under the keels and let her settle on these.
We ideally mark a sand bank with a weighted float at low tide. This way you get to test the sand to make sure it's not "quick sand". Position boat on sand bank at high tide after first checking the next high tide is high enough to float her off.
A stern anchor
may help keep the boat positioned on the sand bank.
Get ready for a swim, and then tie a rope
between each of the chain plates. Dive down and make sure they go the shortest route
under the boat, that will be towards the aft end of the keels. Grab your "jetty boards".
(You've got those? They are boards, usually 1.2 to 1.5 metres long that we place outboard
of our fenders when tying up next to jetty pylons. They usually have a lanyard attached to each end.)
Tie the jetty boards to the rope
going under the keels with prussik knots, or a few clove hitches if your knottically challenged, like me. Wait for the boat to settle on the boards which are protecting the keels and keeping the rudders nice and high.
Finally, hope that the incoming water
isn't too choppy or swelly as the boat re floats and that your job goes pearshaped before the tide returns. There is nothing so stressful as having a dismantled saiłdrive and an incoming tide!
Here's to dry bottoms!