Ship Trim and Draft Changes

TRIM - Outcomes

• Trim refers to the difference between the forward and aft drafts of a ship.
• Trim is influenced by the movement of masses, which can be either solid or liquid.
• Loading masses lead to bodily sinkage and an increase in draft.
• Discharging masses results in bodily rise and a reduction in draft.
• TPC (Tonnes Per Centimeter Immersion) is related to the draft and displacement of the ship.
• MCTC stands for Moment to Change Trim by One Centimeter.
• Change of Trim is proportioned to forward and aft sections in proportion to the location of LCF (Longitudinal Center of Flotation).
• Trim can affect ship structure and operations,

Longitudinal Change in Draft

• Trim can be seen as the longitudinal equivalent of the transverse list angle, resulting in changes to forward and aft drafts.
• The mean draft remains constant as long as no mass is added or removed.
• Longitudinal GM (Metacentric Height) changes occur in the forward and aft directions when masses shift.
• COG (Center of Gravity) and COB (Center of Buoyancy) have dimensions in transverse, longitudinal, and vertical directions, and cannot be observed in isolation.
• Longitudinal measurements are taken from the AP (Aft Perpendicular).
• Transverse measurements are taken from the C/L (Center Line), while vertical measurements are taken from the keel at the MBL (Moulded Base Line).

Describing Ship Trim

• A ship can be described as being on an "even keel" when the drafts are equal forward and aft.
• "Down by the head" means the draft forward is greater than the draft aft.
• "Down by the stern" means the draft aft is greater than the draft forward.
• Movements occur around the waterplane and the LCF (Longitudinal Center of Flotation), not the LCOG (Longitudinal Center of Gravity).
• Mean draft is determined as the average of the forward and aft drafts.
• Movements of mass already onboard do not change the mean draft.
• The total change of trim (COT) must be apportioned based on the distances of the masses relative to their distance from the LCF, not the LCOG.
• The LCF and LCOG do not necessarily coincide.
• Apportion the final trim as a ratio of LBP (Length Between Perpendiculars).

Calculating BML and MCTC

• The formula to find BML for any shape is given by BML = IL / V, Longitudinal 2nd moment of area / Displacement Volume.
• MCTC is the Moment required to Change the Trim by one Centimeter.
• Shifting a weight from aft to forward affects trim.
• If a weight is shifted from aft to forward through distance d, the CoG of the ship moves from G to G1 causing a trimming moment. The ship will trim in an effort to restore the original state of equilibrium
• MCT 1 cm = (W x GM_L) / 100L where W = displacement, GML = longitudinal metacentric height and L = vessel length in metres

Even Keel Shift

• When weight shifts aft from initial position the ship will trim aft until G and B are again aligned.

Answering Trim Questions

• When answering trim questions, observe the initial trim conditions (fwd and aft drafts) and note the position of the LCF.
• Notice if weight is added or discharged and use correct formula.
• The change in draft can be calculated using the formula: Δdraft = Wadded / TPC or Δdraft = Wdischarged / TPC.
• Determine the Change of Trim (COT); this is the total change due to the added, discharged, or shifted weight.
• The NETT Trim Moment / MCTC should be used in the case of multiple movements of weights.
• Apportion the COT proportionally between forward and aft.
• Summarise findings, tabulating information.

Calculations

• When shifting a mass there will be a difference in answers even though two vessels are of equal length because the LCF is at a different position relative to the LBP.