Annex 3

A guide to optimum speed

The method described below for estimating an optimum vessel speed is quantitative in nature and requires a reasonable ability to collect basic performance data from the fishing vessel in question and to make calculations based on that information.

As mentioned in the section Engine operation, an important part of calculating the optimum speed of a vessel is the estimation of the value of the skipper's time, which is often indefinite and frequently difficult to specify. The method outlined below, however, can show particular vessel and engine speeds at which it would be unwise to operate under any circumstances, irrespective of the valuation of the skipper's time. The basic factor in selecting of an optimum speed is the compensation gained - through savings in fuel by travelling at a slower rate - for the "cost" paid by a skipper for arriving later than normal.

What do I stand to gain by slowing down?

The amount gained per hour of later arrival is particular to an individual vessel and its load condition - no two vessels have the same characteristics. The value gained per hour from travelling more slowly can be expressed as:

To apply this calculation, some basic information must be collected and a table drawn up to measure fuel consumption (litres per mile) against vessel speed. The vessel must be equipped with a speed log and either a fuel flow meter or an engine tachometer. The validity of the calculation is increased through the use of a fuel flow meter rather than a tachometer. A table similar to Table 10 should be drawn up and completed.

The information in columns A, B and C should be recorded at sea, under typical conditions with a typical hold load. Care should be taken to avoid the effect of wind and, if necessary, recordings should be made on both outward and return legs of a trip - both against and with the wind.

Table 10

Fuel price: US$0.30 per litre.

If a fuel flow meter is available, it is not necessary to record engine RPM, and column A may be left blank. If a fuel flow meter is not available then the information in column C must be calculated based on the current RPM (column A), the manufacturer's stated fuel consumption at MCR, and the propeller law. At any particular level of RPM, the fuel consumption can be estimated as:

In the example presented in Table 10, the vessel was equipped with an engine rated to be 154 HP at 1 900 RPM. At this speed, the manufacturer stated that it should consume 0.21 litres/HP/hour, giving a fuel consumption of 32.4 litres per hour at MCR. The fuel consumption at 1 500 RPM, for example, was then estimated:

Column D is the result of dividing the data in column C by column B, for each particular row.

Column E is calculated using Equation 4, based on the current row and the information in the row above. Taking the 1 500 RPM row as an example:

The results should then be plotted on a graph of value per hour against vessel speed (column B against column E), such as that illustrated in Annex Figure 1.

The form of the graph is very significant, as it contains not only the complex interaction of the propeller and hull, but also the implicit value of fuel. It will be unique not only to the vessel but also to the current economic conditions - other sample curves are shown in Figure 12. At speeds where the curve is relatively flat, operating speed can be increased with very little penalty, such as between 7 and 8.8 kt in Figure 11. It would be unwise to operate this particular vessel in this speed range. At speeds where the curve is steep, there are great benefits to be gained from slowing down. Preferred operating speeds are, therefore, at those points on the curve where it starts to become appreciably steeper. However, in order to compensate for the "cost" per hour of travelling at a slower speed with the savings per hour in fuel costs, the skipper's value of time needs to be estimated.

Figure 15: Sample curve of time value/vessel speed

How much is my time worth?

The estimation of how much the skipper's time is worth can be taken as the valuation of the cost of his/her arriving later. An approach would be to ask, "would I be willing to arrive an hour later if someone paid me $1 000?". In this case, the answer would probably be yes. But if the compensation was only 50 cents, it would probably be no. So the value of the extra hour lies somewhere between 50 cents and $1 000. The questioning process should be repeated, reducing the value from $1 000 downwards until the decision becomes uncertain and an upper limit of time value can be estimated (for example $25). Likewise, the questioning should be repeated, increasing the lower value from 50 cents until again the decision becomes uncertain, so that a lower limit is reached (again for example $15). The valuation of the skipper's time lies between these two and can be estimated as the average (in this case $20). This is the valuation per hour of the cost to the skipper of arriving late. It is worth noting that it is not so important to get a precise estimate of the skipper's time value, as the form of the vessel's savings curve may be such that some operational limits can be established by common sense.

Figure 16: Other sample value/speed curves

Combining the sample vessel data in the graph and the sample valuation above, an optimum operating speed is estimated to be a little more than 9.5 kt, at about 1 680 RPM.