As vessels age, the weight of the vessel naturally increases. Some of this is planned in a new-build design with the addition of a growth margin.

Often times the weight growth is very significant when an owner wishes to add significant structure to a vessel in the form of additional cabins, new decks, additional equipment, etc. 

Even when an owner is not electing to add structure for commercial reasons, the weight addition is sometimes forced through compliance with new regulations. This is becoming increasing relevant in recent years as international regulations mandate the addition of substantial equipment to existing vessels in the form of exhaust scrubbers, ballast water treatment, fuel piping and systems for dual fuel consumption in ECA zones, upgrades of power distribution systems, etc.

To facilitate the increase in weight, sponsons can be added to an existing vessel to provide additional buoyancy to the hull. Sponsons can be added to the side of a vessel, but for aesthetic reasons and maintaining a faired hull form, the sponson is more commonly added to the stern where it is then called a "ducktail." In addition to increasing buoyancy, a well-designed ducktail has additional benefits including:

  1. Increases waterline length which reduces resistance underway.
  2. Shifts the longitudinal center of buoyancy aft reducing dynamic squat.
  3. Improves wake field and hydrodynamic flow.
  4. Reduce pitch motion due to increased longitudinal moment of inertia.

A further structural addition often utilized in either as a stand-alone or in cooperation with a ducktail is an interceptor trim plane. This is a vertical plate fitted to the transom of the vessel extending the width of the vessel. The plate deflects water flow downwards creating a high pressure area aft of the propellers which in turn provides hydrodynamic lift.