6 ms·

I know it's a bit impolite to doubt their math on displacement and speed and all that, but it seems like a bit of a stretch to me. The boat is 200m long and th

I know it's a bit impolite to doubt their math on displacement and speed and all that, but it seems like a bit of a stretch to me. The boat is 200m long and the sails only occupy about half of that. The all-down configuration is 45m and the all-up is 105m so I'd guess the sails are about 70m tall. So 70 x 100 = 7000 sqm of sail area. That's approx. equivalent in area to a wind turbine with an 83m diameter rotor. Doing one of those online wind turbine calculators with an efficiency of 40% (limit of 59% ala Betz) comes up with about 1.5MW or a little under 2000 horsepower. Most of the big boats like this start around 10,000 hp and only go up from there so I can't help but wonder what's going on here. I guess it could have a very efficient shape and go slower but that seems unlikely to make that big of a difference to me.

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 ago
  From 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 ago
    For 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 ago
    It'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!
jabl 6y ago
A typical large windjammer from the final years of sail carried about half that sail area on a vastly smaller ship; lengthwise about half, but the displacement/tonnage difference would be huge. So yes, this seems quite underpowered, even taking into account that the modern airfoil shaped sails are certainly more efficient and have less windage than a traditional square rig. My guess is that the idea is that in low winds they use the engine to boost speed. E.g. https://en.wikipedia.org/wiki/Herzogin_Cecilie https://en.wikipedia.org/wiki/Herzogin_Cecilie
- jasonwatkinspdx 6y ago
  Keep in mind a solid/3d airfoil is about 3x more efficient than a bermuda rig, so it's going to be dramatically more efficient than a square rigged tall ship, particularly upwind.
mrDmrTmrJ 6y ago
While I'm not a naval architect, but your comment is nice reminder that naval hull drag is cubic with speed: For reference, see this document [1] from the US Naval Acadamy. "It will be observed from the figure that the doubling of speed of the Navy YP from 7to 14 knots increases the power by a factor of 10!" [1] p.7 "Ship power is roughly proportional to the cube of the speed" [1] p.8 Hence the installed power you estimate may just work out. I do have many questions about the about sail handling and rigging. Stowed and raising these things are non-trivial! Telescopic mechanisms can have all sorts of issues, especially when loaded - like in a building storm! So while the entire concept really excites me, I'd love to see them iterate at smaller scale before trying to go full scale. [1] Chapter 7, RESISTANCE AND POWERING OF SHIPS U.S. Naval Academy: https://www.usna.edu/NAOE/_files/documents/Courses/EN400/02.07%20Chapter%207.pdf https://www.usna.edu/NAOE/_files/documents/Courses/EN400/02....
- jasonwatkinspdx 6y ago
  > "I do have many questions about the about sail handling and rigging. Stowed and raising these things are non-trivial! Telescopic mechanisms can have all sorts of issues, especially when loaded - like in a building storm!" I had some downtime while living on a marina for a bit, so I read a bunch of the research literature. Freestanding wingsails that can fully rotate like this have the neat property that they can "weathervane" with very little drag. In fact a bare mast on a traditional sailboat with the sail down generates a lot more drag than the wing, due to turbulence vs laminar flow. It's not clear from these renders, and I'm not motivated enough to dig into it, but most designs I read about in this category are self trimming. Saildrone is a good example, where it uses a 2nd smaller wing on a boom to control the overall angle of attack of the wing. Change the setting on the "tail" to change the angle of attack. The wing will follow the direction of the wind no matter how it shifts. So controlling these boils down to a small electric actuator, which is easy to make controllable via a screen, software, or what have you. In fact one of the motivations for people experimenting with this stuff at more modest scale is it could be huge for adaptive sailors, because it makes sail handling so trivial.

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 ago
From 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 ago
  For 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 ago
  It'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!

jabl 6y ago

A typical large windjammer from the final years of sail carried about half that sail area on a vastly smaller ship; lengthwise about half, but the displacement/tonnage difference would be huge. So yes, this seems quite underpowered, even taking into account that the modern airfoil shaped sails are certainly more efficient and have less windage than a traditional square rig. My guess is that the idea is that in low winds they use the engine to boost speed. E.g. https://en.wikipedia.org/wiki/Herzogin_Cecilie https://en.wikipedia.org/wiki/Herzogin_Cecilie

jasonwatkinspdx 6y ago
Keep in mind a solid/3d airfoil is about 3x more efficient than a bermuda rig, so it's going to be dramatically more efficient than a square rigged tall ship, particularly upwind.

mrDmrTmrJ 6y ago

While I'm not a naval architect, but your comment is nice reminder that naval hull drag is cubic with speed: For reference, see this document [1] from the US Naval Acadamy. "It will be observed from the figure that the doubling of speed of the Navy YP from 7to 14 knots increases the power by a factor of 10!" [1] p.7 "Ship power is roughly proportional to the cube of the speed" [1] p.8 Hence the installed power you estimate may just work out. I do have many questions about the about sail handling and rigging. Stowed and raising these things are non-trivial! Telescopic mechanisms can have all sorts of issues, especially when loaded - like in a building storm! So while the entire concept really excites me, I'd love to see them iterate at smaller scale before trying to go full scale. [1] Chapter 7, RESISTANCE AND POWERING OF SHIPS U.S. Naval Academy: https://www.usna.edu/NAOE/_files/documents/Courses/EN400/02.07%20Chapter%207.pdf https://www.usna.edu/NAOE/_files/documents/Courses/EN400/02....

jasonwatkinspdx 6y ago
> "I do have many questions about the about sail handling and rigging. Stowed and raising these things are non-trivial! Telescopic mechanisms can have all sorts of issues, especially when loaded - like in a building storm!" I had some downtime while living on a marina for a bit, so I read a bunch of the research literature. Freestanding wingsails that can fully rotate like this have the neat property that they can "weathervane" with very little drag. In fact a bare mast on a traditional sailboat with the sail down generates a lot more drag than the wing, due to turbulence vs laminar flow. It's not clear from these renders, and I'm not motivated enough to dig into it, but most designs I read about in this category are self trimming. Saildrone is a good example, where it uses a 2nd smaller wing on a boom to control the overall angle of attack of the wing. Change the setting on the "tail" to change the angle of attack. The wing will follow the direction of the wind no matter how it shifts. So controlling these boils down to a small electric actuator, which is easy to make controllable via a screen, software, or what have you. In fact one of the motivations for people experimenting with this stuff at more modest scale is it could be huge for adaptive sailors, because it makes sail handling so trivial.