The dealer just hit you with the long or short shaft question and you have no clue as to what you need or even what he's talking about. You reply, "Im not sure, I think its a long". As a dealer I don't really like hearing this because there is a chance it could go either way and you have to get it right. So from there, I will try to help out by asking questions about your boat such as what type it is and the length. With some boats I have a pretty good idea as to what length motor should be hung on them. If you said you have a pontoon or a larger style boat such as a bass boat or speed boat, I would be pretty confident in saying it needs a long shaft. If you said something smaller than that, thats where it can get tricky. Smaller 12-14 foot boats can require either a short or long shaft outboard depending on the style of the boat and the brand.
If we run into that scenario, I would still show you the different options of motors and go over pricing, but I would also ask you to hold off and do one thing before you pull the trigger. I would ask you to go back and measure the transom of your boat. I mean it is better to be safe than sorry, am I right? The transom is the back of the boat where the motor is hung. To do so, you will want to measure at the exact center of the transom from the very top to the very bottom. If it measures 15" or with in a inch of that you will need a short shaft outboard. If it measures anywhere from 17" to around 22", then your going to want to go with a long shaft motor. Anything bigger than that your obviously going to need an extra long shaft.
When the right length motor is hung on the transom of a boat, the cavitation plate (the large plate that extends out over the propeller) should be even with the very bottom of the boat. This placement is not only recommended but it will give you the best performance compared to something that is too long or too short. If you went too long it would still work but it may lack in performance. You will also risk hitting bottom or submerge objects more frequently such as rocks or logs. If you go too short your more than likely going to cavitate and loose power. Cavitation is when the propeller is out of the water due to the boat forming a wake of "air bubbles" or "vapor cavities" around the prop. This is probably the worst case scenario out of the two. At least if its too long you can raise the motor or "build up" your transom.
Thats it. Pretty easy to figure out but like I said its crucial to pairing a motor with a boat. I would also recommend finding your maximum horsepower rating which is located on the yellow coast guard tag towards the back of your boat. Lastly, knowing the year of your motor and what style of controls you have (side mount, concealed side mount, or binnacle) is also a plus because it will tell the dealer if your current controls are compatible with the motor you are purchasing. Using your existing control box and gauges will save you quite a bit of money in the end. I hope this helps someone out in their search for a new or newer outboard this season.