## 62 Power prognosis using the admiralty formula

The 'admiralty formula' is still used today, but only for a very rough estimate:

The admiralty constant C is assumed to be constant for similar ships with similar Froude numbers, i.e. ships that have almost the same CB, CP, V/L, Fn, V, etc. Typical values for C in [t2/3 • kn3/kW] are:

general cargo ships 400-600

bulker and tanker 600-750

reefer 550-700

feedership 350-500

warship 150

These values give an order of magnitude only. The constant C should be determined individually for basis ships used in design. Volker (1974) gives a modified admiralty formula for cargo ships with smaller scatter for C:

rD in this formula may be estimated by one of the above-mentioned empirical formulae. Strictly speaking, the exponent of V should be a function of speed range and ship hull form. The admiralty formula is thus only useful if a ship of the same type, size and speed range is selected to determine C. It is possible to increase the accuracy of the Volker formula by adjusting it to specific ship types.

More accurate methods to estimate the power requirements estimate the resistance as described below:

MacPherson (1993) provides some background and guidance to designers for simple computer-based prediction methods, and these are recommended for further studies.

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