Trim and draft optimization

The trim and/or draft of the ship influence the hull resistance and therefore the fuel consumption. In general, limited regard to optimal trim and draft is taken when loading the ship and therefore optimal conditions will often not be achieved. By actively planning cargo loading and by that optimising the trim and draft, one can save fuel and reduce the emissions accordingly.

Applicability and assumptions

Trim and draft optimization is applicable for all vessel types and vessel ages. Some vessels have less flexibility on trim as for instance cruise vessels which are designed for passenger comfort and facilities for the passengers. Further, full-body ships where resistance from viscous friction is higher than wave friction (e.g. tank and bulk) will generally have a less reduction by optimising the trim and draft and similarly for ships with limited ballast flexibility. Better trim and draft will reduce the resistance and therefore less propulsive power is required which leads to a lower fuel consumption.

In order to be able to optimise the trim and draft additional equipment is required such as a better loading computer or a dedicated trim optimizer. In addition, the crew need training in the use of such equipment. Some of the available tools to be used for trim/draft optimization are:

• Trim tables where the crew themselves must calculate optimal trim and draft
• Self-learning trim and draft programs that use historical sailing data to present optimal trim and draft
• Advanced computer programs that calculate optimal trim and draft using CFD models before the voyage is carried out.

The measure can be retrofitted on existing ships. For trim tables, the technical lifetime of the measure is estimated to be the lifetime of the ship, as static trim tables and dynamic trim tables will be valid provided that the ship is operated in the same way. Self-learning systems are usually license-based, and the technical lifespan of these is therefore estimated to be between 1 month and 1 year, depending on the supplier and license agreement. One should note that if a vessel is retrofitted with a new bulb, existing trim/draft tables would need to be revised.

Below are some examples of applicability:

• Trim tables:
  • The cost of trim tables depends on how many combinations of draft, trim and speed the trim tables are limited to. Given high quality drawings of the ship and a detailed operational profile. 20 combinations will usually be sufficient for bulk carriers, tankers and passenger ships.
  • Ferries that have a limited opportunity to change trim, and that often sail with one given draft and two different speeds, often need 10-12 combinations.
  • Container vessels that have varying amounts of cargo on board will need several combinations, often up to 40 conditions.
• Self-learning systems:
  • Self-learning trim and draft systems use automatic sensors that log historical sailing information and fuel consumption to find the optimal trim and draft. The cost of self-learning systems will vary depending on which sensors and equipment the ship is already equipped with and intends to install.

Cost of implementation

The cost of trim and draft optimization will vary dependent on ship type, size and the operational profiles. Sister ships can use the same trim tables and will therefore reduce the cost. However, on the other hand if several combinations are needed will the cost increase. The costs assume that one can provide the geometry and operational profile of the ship. The price for CFD models will often reduce if one assesses several sister vessels (since one CFD model is generated per sister class).

The cost is typically in the range of $10,000 – $60,0000 (USD).

All costs will be similar for both newbuilds and retrofitting on existing ships.

Reduction potential

Optimized trim and draft are estimated to be able to reduce fuel consumption on the main engine by 0.5 – 2% for the vast majority of ship types. For ships that often use partial loads (e.g. container, ro-ro, etc.), the effect can be up to 5% in sailing mode. These estimates are derived from full-scale trials and detailed calculations carried out on a number of different ships in different routes. For PSVs, a saving of 4% has been verified, comparing two sister ships operating with and without optimal trim. This potential will be similar for new builds and existing ships.

A total fuel consumption of 0.5 – 5% is estimated.

Other References

1. Meyer et al. (2012) Slow Steaming in Container Shipping / Overview of the slow steaming history as well as the widely assumed coherence between a ship’s speed and its fuel consumption
2. Kongsberg (n.d.) Vessel performance optimizer / Paper on cost efficient vessel operation
3. Regulated Slow Steaming in Maritime Transport / An assessment of Options, Costs and Benefits
4. Cui, et al. (2018) Energy Efficient Ship Operation Through Speed Optimisation in Various Weather Conditions.