Boat lengths , waterline or deck ?

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When an Ad. gives the length of a boat, is it measured at the waterline or the deck ?. Only on a speed boat it can make a big difference at the bow. Also am I correct in assuming that the length (which ever way) wont include the drive (inboard engine) poking out the back of the transom ?. This is all with regard to storing on land & buying a trailer .
Thanks
Martin

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Thanks to all for replying, 23/24 meters I don't think will ever be a problem for me even if I won the lottery ! 7/8 meters is all I am looking at.