While steaming at 12.3 knots, your vessel burns 168 bbls of fuel per day. What will be the rate of fuel consumption if you increase speed to 13.5 knots?
• Fuel consumption vs. speed – how fuel use typically changes with speed for displacement vessels • The cube law approximation – fuel consumption is often assumed proportional to the cube of speed for exam problems like this • Using ratios to scale from old speed/fuel to new speed/fuel
• First, think about how to form a ratio between the old speed and the new speed. What is the fraction new speed / old speed, and what do you do with it if fuel is proportional to speed cubed? • After you adjust for the speed change using the appropriate power (exponent), what should you multiply by to get the new fuel consumption? • Once you get your calculated fuel rate, which of the choices is closest to that result?
• Be sure you are using consistent units (both speeds are in knots, both fuel rates in barrels per day). • Double‑check that you applied the exponent correctly (did you square it, cube it, or leave it linear?), based on the typical exam assumption. • After computing, make sure your new fuel consumption is higher than 168 bbls/day, since the speed increased.
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