Hull coatings and roughness play an important part in the minimisation of the skin friction component of resistance. Hull surface roughness comprises the sum of two elements; permanent roughness and temporary roughness. The former refers to the amount of unevenness and condition of the hull plating in terms of the bowing of the ship’s plates, weld seams and the condition of the steel surface, while the latter principally accounts for fouling and deposits that build up on the hull surface. Fouling commences with slime, comprising bacteria and diatoms, which then progresses to algae and in turn on to animal foulers such as barnacles, culminating in the climax community.
The tin-based marine coatings, particularly tributyltin (TBT), were excellent in keeping underwater hull surfaces free of fouling and, in so doing, reducing fuel consumption. However, they were exceedingly detrimental to the environment and were eventually banned in 2008 following discussion at IMO MEPC.
Currently a number of alternative marine paints have come on to the market such as copper-based and synthetic biocide paints; nevertheless, further work is proceeding to find alternatives. Silicon based paints have also been marketed and, while relatively expensive, can be effective in preventing fouling when used in the correct circumstances.
Research work is progressing to find ecologically friendly alternatives. One such method is based around electrochemically active coating systems. This concept produces regularly changing pH values on the surface of the hull and thereby effectively prevents fouling colonization without having to use biocides. Initial tests have been promising in proving product stability and efficiency inpreventing bio-fouling.
Propeller improvement devices
Wake Equalizing and Flow Separation Alleviating Devices
In general, wake equalization and flow separation improvement devices are features to improve the flow around the hull that were developed to prevent propeller problems and/or added ship resistance caused by suboptimal aft hull forms. As such, they are less effective when the ship geometry has been designed correctly, with an eye at optimizing the flow to the propeller and avoiding the generation of hydrodynamic effects such as bilge vortices. The most common wake equalization and flow separation devices are Grothues spoilers, Schneekluth ducts and stern tunnels
Pre-swirl devices are hydrodynamic appendages to the hull aiming to condition the wake flow so that a rotation opposite to that of the propeller is imposed on it, thus improving the angle of attack of the flow on the propeller blades over the entire disk.
Also, the pre-swirl rotating flow counteracts the rotation flow induced by the propeller. As a result, the flow leaving the propeller disc can be made to contain minimum momentum in the circumferential direction, thus requiring less kinetic energy to produce thrust. Pre-swirl devices have been designed and installed both as retrofits to existing ships and as an integral feature of newbuildings. Normally, they can be made to work in nonoptimal flows (the ducted type in particular) but they work best in already optimal nominal wakes. In this sense, they can be considered as fully complementary to other optimization approaches with the exception of nonsymmetrical stern lines.
The role of post-swirl devices is that of conditioning the flow at the aft end of the propeller. In a number of cases, this means trying to convert the rotational components of the flow created by the propeller to useful axial flow. In others, it is just a matter of either suppressing detrimental flow characteristics (such as the propeller hub vortex) or diverting it to improve rudder efficiency.
In turn, this might allow the use of a smaller rudder, hence reducing overall ship resistance. Because these devices attempt to condition the flow behind the propeller, they are almost invariably associated with the rudder design. In fact, some considerable overlaps should be expected between possible improvements in propulsion thrust and rudder efficiency benefits, so the design of the assembly should take both aspects into consideration. Since the performance of post-swirl devices and rudders are so closely linked, it is important to verify the effectiveness of both parts and the absence of detrimental side effects for all rudder and propeller operating conditions, particularly in terms of strength and fatigue.
Post-swirl devices can be fitted in tandem with a pre-swirl setup. However, because the pre-swirl device would already decrease the rotational flow past the propeller, a reduced effectiveness of the post-swirl device should be expected.
In general, larger diameter propellers with fewer blades operating at lower RPM are more efficient than smaller, faster counterparts, for a given required PE. However, this general principle is balanced by the need for reasonable propeller clearances, the nominal wake distribution behind a given hull form, and the need to match propeller and engine best performance. This type of optimization is done routinely at the design stage, when the principal propeller characteristics, and its detailed geometry is optimized to achieve best performance for the design speed and draft.
However, there may be interest in revisiting propeller options where slow steaming is considered for a given ship on a longer term basis. In this case, the additional cost of operating the ship in off-design conditions for a long period might well justify re-examining the vessel’s propeller design. Similarly, when examining the design of a newbuilding, it might pay off to optimize both the propeller and hull hydrodynamic performance not just for the design speed and draft, but also for those off-design conditions that the ship is most likely to encounter during its life. It has been demonstrated that optimization around the design speed and draft does not guarantee acceptable performance in off-design conditions.
-ABS Ship energy efficiency measures advisory, status and guidance