Trim, hull and propeller design and optimisation

Mariners are used to know the best possible trim for their vessel using sea worthiness as main and only criteria. Usually best trim is 30.48 cm astern. Even keel loading in the port is quite common especially with the small ships operating from small harbours. Today more important criteria are energy efficiency and emission reduction and if these two criteria fills up with the trim, result is excellent. Trim is normally defined as the difference between the aft draft and the forward draft. Optimum trim is trim where required propulsive power is minimal. Optimum trim is achieved via the proper planning and ship ballasting plan. A ship’s resistances and trim are closely related to each other. Trim affects to hulls wetted surface area and therefore increases or decreases forces that is slowing the ship (MariEMS 2017).

Using optimum trim, it can have 2% to 4% reduction on fuel consumption. However, this reduction can be higher or lower depending on the vessels type and operation draft. And when there are ways to save environment and money all possible measures should be taken care of. Therefore even 5 to 10 centimetres difference on trim can cause higher fuel consumption and this causes emission increases.

Nowadays the accuracy has improved so much that, trim tables based on use of Computational Fluid Dynamics (CFD) software tool calculations can be comparable to the results from resistance model test. However, both resistance test and CFD methods tend to ignore the impact of the propeller: this may have significant impact on evaluations of vessel with light drafts. Normally when Trim is being calculated the test are made at both forward and aft trims. Often forward trim is not even possible for the lighter drafts due to restriction in the propeller’s submergence. Because of the recent energy efficiency regulation, more shipping companies have chosen to do trim optimization calculations using the CFD software (MariEMS 2017).

When talking about energy efficiency and emission reduction on ships, the trim isn’t the only factor that affects to ship’s propulsion fuel consumption. One almost as big thing to keep on mind is hull and propeller cleaning and maintaining.

Hull surface roughness can be divided to two categories, physical and biological. These two categories are normally divided to two subcategories micro roughness (less than 1mm) and macro roughness (more than 1mm). The physical micro-roughness is normally human made mistakes or mechanical failures like, mechanical damage, failure of the applied coating and even improper preparation of the surface and/or improper application of a new coating. Biological roughness (fouling) comes when some organic growth sticks on the hull of the vessel (slime, algae, barnacle etc.). Roughness has significant impact on resistance. Light slim covering the entire wetted surface can increase total resistance by 7-9 %. Heavy slime increases resistance by 15-18%, and small barnacles and weed can push up to 20 to 30% increase in total resistance. Therefore, hull resistance has significant factor to fuel consumption and emission reduction and energy efficiency. Cleaning the hull and changing the hull coating to better working coating can save 10-12% on fuel consumption (MariEMS 2017).

Like the hull roughness, Propellers suffer also decreased performance due to surface roughness. The absolute reduction in ship energy efficiency due to propeller roughness is less than those seen on hull roughness, but propeller roughness can increase propulsion fuel consumption up to 6%. Propellers suffer physical surface roughness created by corrosion, cavitation erosion and impingement attack. These damages can be cleaned and polished which will reduce the propellers frictional and rotational losses. Fouling is not problem on propellers that are constantly moving, therefore vessels that are anchored or in harbour for extended periods should rotate their propellers for short period once a day to prevent fouling on the propellers. In a long run the fuel saving that have gained by preventing the fouling by rotating propeller outweighs the fuel consumption that is used to do so. Polishing roughened propeller can decrease fuel consumption by 3%. Cleaning and polishing the propeller can give 6% improvement on fuel consumption. Divers can clean a 5 blade 10 m diameter propeller in about 3-4 hours and that costs about US $3,000 in Far East and about double of that in Europe (MariEMS 2017).