The increased popularity of motor homes has led to a desire to travel with both a boat for water travel and a smaller car or other wheeled vehicle for more convenient and economical travel around towns or on side trips.  Many travelers have been forced to drive both a motor home and auxiliary vehicle under their own power and to pull the boat on a trailer behind the motor home or auxiliary vehicle.  Although this approach has the increased reliability of two vehicles it creates a caravan out of a travel trip and requires either increased communications equipment or greater patience to compensate for the missed turns and other coordination problems which result from two vehicles traveling together. 
Previous attempts have been made at creating trailer assemblies which are capable of carrying both a boat and auxiliary vehicle in order to overcome the problems indicated above.  Many of these attempts have focused on trailer designs which place a boat on a frame or secondary trailer which is mounted above the auxiliary vehicle.  Some of these devices use an elevator structure which is lowered to allow installation of the boat.  This approach has required that the auxiliary vehicle be removed from the trailer prior to either installation or launching or other removal of the boat.  The requirement that the vehicle be removed prior to installation or removal of the boat necessarily entails extra work, time and inconvenience. 
Prior art trailer structures which utilize secondary boat trailers necessarily are more costly because of the extra tires, wheels, bearings, springs and related equipment which is duplicated.  Such devices are also typically cumbersome and heavy to haul These devices also require the secondary boat trailer to be removed from the primary trailer before the boat can be launched rather than allowing the boat to be directly launched from the primary trailer. 
In order to solve at least some of these limitations of the prior art, the current inventor created the boat and motor vehicle trailer described in U.S. Pat.  No.  4,560,316.  This trailer represented a substantial step in the art of dual load boat and vehicle trailers.  Unfortunately, it had certain disadvantages associated with requirements that the frame structure be adjusted relative to the trailer wheel carriage in order to adjust the center of gravity and balance of the trailer when changing from single load conditions with just a boat, compared to dual load conditions with both a boat and vehicle.  It also presented substantial amounts of surface area under the boat to create wind resistance, particularly during single load conditions with just a boat. 
The current invention provides a novel dual load boat and vehicle trailer which is capable of launching a boat whether the vehicle has been removed or is still riding on the trailer.  It also is convertible between single load and dual load modes of operation so as to allow a boat carried alone to be positioned low on the trailer.  It is also advantageously constructed to automatically redistribute the weight of the boat and its supporting boat carriage when changed between single and dual load conditions.  These and other benefits and advantages of the invention will be described or apparent from the detailed description given herein.