So I'm curious: is it possible that a small sailboat could be "invisible" to
radar once it
get close to a large ship?

jeff

Absolutely.
You will need one of the "techies" to spell out the numbers, but, one factor
which may help, is the size of the "scanner" used on ships (techies?).
I read where a minimum of 25' was possible, but not practical, so that the
number in reality is somewhat higher.
However, there are other factors which play an important part, and these
include shipboard structures causing blind spots.
If you look at the positioning of scanners on many ships, you will note things
such as containers, stacks, cargo gear, which are in the way.
On most ships, there is a diagram on the bridge, which shows potential blind
spots, and on most ships, you will find one astern.
In many cases, an additional scanner has been installed, close to the bow (and
in some cases, close to the stern) where there is a known and serious blind
spot.
When passing between 2 buoys (300' spacing) on 1/4 mi range (I rarely drop that
low) from a ship, I will normally (not always) lose radar sight, after I start
passing through them ... in many cases they may still be there, but they blend
in and are not overly useful.

lotsa variables

Shen