I am helping a friend replace his blown lower unit on a 1993 115HP Mercury in-line 4 cyl. My friend asked if he should switch to a long shaft or stick with what he has now. Frankly, I don't know if he has a long or short shaft now. First question, How do you measure the shaft length? I need to know which type he has now to order the replacement. Second question, How do you determine which lower unit would be right for a given boat? Is there some rule of thumb for shaft length to match a boat?<br />Thanks,<br />Mcgurk