Naval Architecture & Hydrostatics

Ship Trim Calculator

Analyze a vessel's initial floating equilibrium, longitudinal trim, and dynamic draft variations caused by weight shifts, ballast transfers, or cargo loading.

Quick Load Vessel Presets:
Unit System:
Metric (m / tonnes / MCTC) Imperial (ft / tons / MT1")
1 Initial Floating Drafts
Measured in Meters (m).
Measured in Meters (m).
2 Vessel Hydrostatic Characteristics
Distance FP to AP.
Distance from AP to LCF.
Measured in tonne-meters/cm.
3 Weight Adjustment Operation
Weight in Tonnes (t).
Distance moved.

Interactive Vessel Floating & Trim Diagram

Trim Profile: Active
AP (Stern) FP (Bow) LCF (Pivot) Shift Weight Ta: 3.20m Tf: 2.80m

Principles of Vessel Trim & Longitudinal Hydrostatics

In naval architecture and yacht engineering, Trim describes a vessel's longitudinal tilt or inclination in water. It is defined as the numerical variance between the draft measured at the Aft Perpendicular ($T_{\text{aft}}$) and the draft measured at the Forward Perpendicular ($T_{\text{fwd}}$):

$$\text{Trim} = T_{\text{aft}} - T_{\text{fwd}}$$

Key Floating Trim Configurations

  • Even Keel ($\text{Trim} = 0$): Draft forward equals draft aft. Baseline design state for static structural loading.
  • Trim by the Stern ($\text{Trim} > 0$): Aft draft is deeper than forward draft. Preferred configuration for motor yachts and commercial ships to maintain propeller/rudder immersion and directional stability.
  • Trim by the Bow / Head ($\text{Trim} < 0$): Forward draft exceeds aft draft. Unfavorable condition causing bow plunging, steering instability, and higher fuel consumption.

Mathematical Mechanics of a Longitudinal Weight Shift

When a mass ($w$) on board is shifted horizontally by distance ($d$), it produces a Trimming Moment ($M_{\text{trim}}$) around the vessel's Longitudinal Center of Flotation (LCF)—the geometric centroid of the waterplane area:

$$M_{\text{trim}} = w \times d$$

The resulting total change in vessel trim ($\delta t$) is calculated using the hydrostatic constant MCTC (Moment to Change Trim 1 cm) or MT1" (Moment to Trim 1 Inch):

$$\delta t = \frac{M_{\text{trim}}}{\text{MCTC}} \quad \text{(in cm)} \qquad \text{or} \qquad \delta t = \frac{M_{\text{trim}}}{\text{MT1''}} \quad \text{(in inches)}$$

Because rotation occurs specifically about the LCF, the change in draft at the stern ($\delta T_{\text{aft}}$) and bow ($\delta T_{\text{fwd}}$) is distributed proportionally to their respective distances from the LCF relative to the Length Between Perpendiculars (LBP):

$$\delta T_{\text{aft}} = \delta t \times \left( \frac{\text{LCF}}{\text{LBP}} \right) \qquad \text{and} \qquad \delta T_{\text{fwd}} = \delta t \times \left( \frac{\text{LBP} - \text{LCF}}{\text{LBP}} \right)$$