4 ms·
The target speed is around 10 knots. Commercial cargo vessels are designed at 20-23 knots, some lower at 15-18 knots (slow steaming), rounding it at 20. So they
by azepoi 6y ago
The target speed is around 10 knots. Commercial cargo vessels are designed at 20-23 knots, some lower at 15-18 knots (slow steaming), rounding it at 20. So they need about a quarter of the power to maintain half the speed.
- msandford 6y agoFrom the wikipedia article: "Such ships will be used on transatlantic routes and are expected to perform the journey in 12 days, longer than the current average of eight." I 100% get that half speed = 1/4 drag and thus 1/4 power. That makes sense. But I'm not sure how to reconcile half speed with not at least doubling the trip duration. New York to London is 3000 nautical miles. At 10 knots that's 300 hours or 12.5 days. If there are boats doing 20 knots then they should arrive in ~6 days rather than the 8 quoted.
- malandrew 6y agoFor a ship this size, I'm guessing that you need to factor in at least 1/2 day at each end coming and going from port, possibly a full day. It's likely that cruising speed only matters for total number of days minus 2 regardless of trip distance.
- azepoi 6y agoIt's even worse than that, for a drag force proportional to the square of the speed, the power is proportional to the cube of the speed. As for the transatlantic voyage in 12 days vs 8 or 6 I don't know on what they based it. Maybe ships don't always go at the nominal speed as it may not be economical to do so. 8 days is 15-16 knots, so this corresponds to a more economical slow steamer. As they pitch an environmentally friendlier solution, a comparison to the next best thing (slow steaming) may be more apt. Using the cube-law, ~2/3 the speed ~= 0.3 the power. And only 1/8 if you compare 10 to 20!