The width and height of the rollup door is important for those wanting to store a boat or other trailer in a Self-Storage Facility. Typically, most boats over about 16’ will have a trailer with fenders in excess of 8’. For most ski boats and tandem snow mobile trailers you will need a 10’ wide door, which corresponds to at least a 12’ wide storage unit. The tongue of the trailer and outdrive (or outboard) will add about another 4’ to the required depth of your self storage unit. Remember that the listed dimensions of a self storage unit are the exterior dimensions. The interior length could be as much as 14" shorter, and the width about 5.5" narrower (if you subtract the 1.5" corrugation of the side, and the 2" x 4" metal framing) than the listed dimensions. Measure your self storage unit and overall length of your boat and trailer to verify it will fit.
Also remember that tandem axle trailers don’t pivit all that well. At A-Plus Mini Storage we have 12’x30’ units with 8' high, 10’ wide doors. The actual opening of these doors is 10' wide and 7' 9" high. The actual inside dimensions are 11' 10.5" X 28' 8". If you are looking for winter storage it is okay to work a little to get the boat out of the elements. A large boat can be a tight squeeze into your unit, but it is worth it. We have a 30’ access aisle between the rows of our 12'x30' units. We have successfully stored boats up to 22’ in length, but I can say from personal experience that it takes patience and skill to make the cut when backing a boat of that size into a storage unit. You will need a second set of eyes to help you back in. Short wheel base vehicles aren't so good for towing on the highway, but can come in handy when backing your boat into a tight storage space. Single axle trailers are easy to disconnect from the tow vehicle to pivot into position. Two people working together are usually strong enough to push a boat on a single axle trailer in and out of the self storage unit if necessary. If you want to be sure to have a self storage unit of this size for winter storage, make arragements in early September. These size units suddenly come into demand around that time of year.
Our 10x20 and 10x18 self storage units have an actual door opening that is 8' wide and 7' 9" high. Again, these units are 14" shorter and 5.5" narrower than the listed dimensions (including the thickness of the corrugation, and 2" x 4" framing). So it could handle smaller aluminum fishing boats, and some drift boats. Our access isle between these storage units is 25', but these sized boats usually only have single axle trailers, so they can be manually placed into the unit.
As an example, the fenders on the trailer of my 15' Gregor, Baja Special were right about 6' 6". If you consider the outboard motor, and tongue of the trailer you might squeeze that into the 10x18', but the 10x 20' would be the safer choice. If you are a skilled carpenter, consider building a mezzanine in the rear of the unit that would clear the height of the rear of the boat. That would give you plenty of room for gear, and keep the floor clear for the boat. I have done this at other self-storage facilities where I rented in the past. It can be done without drilling any holes into the walls of the storage unit.