Energy losses in a small trawler

Only about one-third of the energy generated by the engine reaches the propeller and, in the case of a small trawler, only one-third of this is actually spent on useful work such as pulling the net.

In a vessel that does not pull a net or dredge, of the energy that reaches the propeller:

35 percent is used to turn the propeller;  27 percent to overcome wave resistance;  18 percent to overcome skin friction;  17 percent to overcome resistance from the wake; 3 percent to overcome air resistance.

So, where can gains be made, or at least losses minimized?

Engine. Most of the energy generated by the fuel burnt in the engine is lost as heat via the exhaust and cooling system, and unfortunately there is not a lot which the operator can do to usefully recuperate this energy. In certain cases, some of this can be regained through the use of a turbocharger but, in general, the thermal efficiency of small higher-speed diesel engines is low and little can be done to improve this. However, some engines are significantly more fuel-efficient than others (especially among different types of outboard motors).

Propeller. The energy lost in turning the propeller is controlled by two principle factors - the design of the propeller (how well suited it is to the engine, gearbox, hull and fishing application) and its condition.

These factors can be influenced by the vessel operator:

Mode of operation. The effect of wave resistance, although determined principally by the dimensions and form of the vessel, increases dramatically with speed. Significant fuel savings can be made by maintaining a reasonable speed for the hull, irrespective of vessel type. Fishing operations also influence energy consumption and efficiency through gear technology and operating patterns, particularly trip length. Neither of these are particularly easy to change in practice.

Hull maintenance. The significance of skin friction is controlled principally by the quality of the hull's finish - hull roughness as well as the amount of weed and marine growth that is allowed to accumulate on the hull. Both of these factors are under the direct influence of the operator's maintenance programme but, depending on the type of vessel and fishery, a significant expenditure on hull finish is not always worthwhile.

Reference: FAO - Food and Agriculture Organization of the United Nations FAO FISHERIES TECHNICAL PAPER 383