Trim is the fore-and-aft tilt of a vessel, calculated as the difference between forward draft and aft draft readings.
Types of Trim
- Even keel — forward and aft drafts are equal, no trim
- Trim by the head — forward draft is greater than aft draft (bow sits deeper)
- Trim by the stern — aft draft is greater than forward draft (stern sits deeper) — the more common condition on most vessels, particularly for propeller immersion reasons
Why Trim Matters
- Fuel efficiency — a ship trimmed outside its optimal range burns more fuel for the same speed, and modern trim optimization software helps crews find the most efficient trim for a given loading condition
- Propeller immersion — trim by the stern helps ensure the propeller remains fully submerged for efficient propulsion, particularly important when not fully loaded
- Under-keel clearance — trim affects which end of the vessel has the least clearance in shallow water, critical information for safe passage planning
- Visibility from the bridge — excessive trim by the head can reduce forward visibility
Calculating Trim
Trim = Forward Draft − Aft Draft (with the sign convention indicating direction — trim by the head or by the stern)
Adjusting Trim
Trim is adjusted primarily through ballast water distribution between forward and aft tanks, or through cargo distribution during loading planning.
Exam Relevance
Trim calculations, optimal trim for different conditions, and its relationship to draft and stability are core Ship Stability and Cargo Work topics in deck officer competency exams.