Re: Dumb Question...i Know.>>> But???
If you think of the transom bracket as an upside-down 'L' and 'J' put together, measure form where the interior edge of the L and J meet LJ. You can also measure your actual transom- Using a measuring diagram I found on Google by searching "long shaft of short shaft" I noticed that a boat's transom shown as 16 in. was using a short shaft "15 in". Don't split hairs. There is, almost always, either 15" (short shaft) 20" (long shaft) or 25" (extra long shaft). If you find yourself within a few inches of one of these measurements, you've done it right.