about to purchase a boat from an individual and it comes with an 07 ameratrail tandem axle trailer with disc brakes. all aluminum and stainless. he swears it is a 21' trailer and says that trailers have to be 2' longer than the boat (which is 19'). is this true or is the foot length of the trailer measured by what it will haul as in the length of the boat? i think the trailer is 19'. it isn't a huge difference in the value but i need to know for the bank.