- The starting point is to know the "sea speed" and the SHP required to make that speed.
- Add 10% to that power to allow for "sea state"
- Then, add to that SHP the "SHP" needed to generate the electric power (I will discuss that separately)
- Add 10% for deterioration of the plant
- Multiply the SHP by 1.045 to allow for split plant operation, where applicable
- Compute the burnable fuel rate: (SHP x 0.9 lbs/SHP/hour)/2240 lbs per ton
- Compute normal range: sea speed x normal fuel load x 0.97895/(burnable fuel rate) (the factor accounts for unusable fuel, among other things)
- Compute max. range: sea speed x max. fuel load x 0.978595/(burnable fuel rate)
There are issues caused by the variable displacement and the power required to reach the sea speed under different loading. I'm pretty sure that the calculations described here are just designed to give the average performance. I suppose that you could integrate over time to get a more accurate figure, as the required power decreased, as fuel is burned.
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