A distant sail offers a tempting shortcut for a notebook caption: copy the number and move on. Give it another look first. A crease, another sail or a reversed view through the cloth can make a character surprisingly difficult to read.
Preserve the whole identifier
Copy visible letters as well as digits. Keep spacing in a separate transcription if you are unsure whether it belongs to the identifier or merely to its arrangement on the sail. The RYA sail-number guidance explains the identification purpose of these numbers; an observation in your notebook is still only what you could actually see.
Leave doubtful characters doubtful
Write “third digit unclear” rather than silently choosing an eight over a three. Save the original photograph beside the note so you can revisit the evidence. Do not turn an uncertain number into a confident boat name, ownership claim or race entry match.
For a shore-based exercise, compare two photographs of the same sail. Record which characters become clearer and which remain hidden. That produces a better observation than enlarging one blurry image until it appears to confirm your first guess.