Tandem boat trailers have two axles and four wheels as opposed to standard boat trailers with one axle and two wheels. Whether you need a tandem axle trailer or not depends on several factors, including the size of the boat being hauled, fuel efficiency desires, road handling needs and your maintenance capabilities.
Tandem axle boat trailers typically have a much higher gross weight rating than single axle styles. This allows tandem trailers the capability of carrying larger and heavier boats. All trailers are required to display the gross weight capacity of the trailer on the unit. Checking this capacity against the boat's weight that it is intended to be used with can indicate which style is required.
Some users choose tandem axle trailers even if weight is not an issue. Tandem trailers usually tow more smoothly, without the tendency to fishtail that some single axle models have. Additionally, in case of a flat tire it is much easier to keep the tandem trailer under control. Some tandem trailers are capable of tire changes without the need of lifting the trailer with a jack. Also, the tires are larger which makes replacements easier to find.
However, since it is larger and heavier, a tandem trailer increases the fuel consumption of the vehicle towing it more than single axles do. A tandem trailer also requires more maintenance because it has more parts subjected to road wear and tear.