there is a common latin expression that my mother (well, it is
mother's day, you know!) saw on a sailboat that means something like
"if not this, then nothing", or "without this, nothing".  she liked
the name very much and couldn't remember the latin.  can you find out
what latin expression this is?

The Latin phrase you're looking for is "Sine qua non."

"Conditio sine qua non 
 Condition without which not - i.e. "indispensable." 

Wikipedia
http://en.wikipedia.org/wiki/List_of_Latin_phrases

"Sine Qua Non 'Without which nothing,' an absolute necessity" 

Classics Technology Center
http://ablemedia.com/ctcweb/consortium/latinmottoes2.html

"sine qua non (SIN-ay kwah NAHN) 'That without which nothing.' 
 Indicates an essential element or condition."

Math Academy
http://www.mathacademy.com/pr/prime/articles/latin/

Google search strategy:

Google Web Search: "latin" + "sine qua non"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=latin+%22sine+qua+non

I hope this helps! If this is not the correct Latin phrase, or if
anything is unclear, please request clarification; I'll be glad to
offer further assistance before you rate my answer.

Best regards,
pinkfreud

very fast, with references to boot.  very pleased indeed!   see that
expression everywhere but never totally understood its meaning.

thanks, pinkfreud

the motto could also be: "aut Caesar aut nihil", that means "either a
Caesar or nothing"