Quite correct, fuel and power are NOT directly proportional to speed. They are proportional to speed to the third power. In other words, double the speed from 8 to 16 knots, power requirement goes up no less than EIGHT times- and hence fuel consumption does. This assumes that propeller efficiency remains the same, which in reality is not the case; the lower a propeller's rpm is, the higher its efficiency. However, for our purposes, it's good enough.
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