Assuming the boat you are sailing by has no number on its hull or sail to indicate its length.
Is there a simple way (rule of thumb or a trick) to estimate/judge the length of a sailboat on the water?
Off course...there is....
The lenght of the boat in question is equal to the distance from the bow to the stern...
Then what you do is you go by and ask really loud:
HOW LONG's YOU'R BOAT????
If you're inside that boat...and it's not yours, just ask the owner