5 ms·

> It has a maximum speed, and if you try to push it beyond that, efficiency drops a lot and you put in a lot more energy for small speed increases. The most in

> It has a maximum speed, and if you try to push it beyond that, efficiency drops a lot and you put in a lot more energy for small speed increases. The most interesting thing about the maximum speed of a displacement hull is that it depends on the length of the waterline. Larger (longer) hulls have a higher top speed. There are some hull shapes that allow higher speeds while in displacement mode,

iancmceachern 3y ago
Interesting! I wonder if this has to do do with the width of the boat vs length. For the same displacement, a narrower front, and therefore longer boat will clearly win. It's like a missile vs a peterbuilt
- two_in_one 3y ago
  May be it's because usually the longer the waterline the bigger the engine(?) And the size of the engine is proportional to L^3. While the cross section is only L^2. I doubt with the same engine the bigger waterline has any positive effect...
  - jabl 3y ago
    You may doubt the effect of the waterline length, but it's really basic naval engineering: https://en.m.wikipedia.org/wiki/Wave-making_resistance https://en.m.wikipedia.org/wiki/Wave-making_resistance
    - two_in_one 3y ago
      I'm familiar with fluids dynamics, thanks. Yes, bigger ending or smaller cross section is "better". You can downvote again if you have nothing to say. I don't care really.
  - Xylakant 3y ago
    My previous post is cut short, something something computers :( The same engine in a longer waterline may have some effect. The underlying physics is that the length of the waterline influences the wake system that a boat creates between its bow and stern wave. Those waves interfere constructively and at a certain speed, they start adding up to significant waves - the bow wave starts to pile up more and more. The boat needs to ride up the wave. Drag rises sharply at that point - so a stronger engine helps little at this point. This used to be called the “hull speed”, the maximum speed that a hull can achieve. (It’s grossly simplistic and outdated concept, but a workable estimate). Going beyond requires either a longer waterline or switching from displacement to planing. Some hull shapes can exceed their nominal hull speeds somewhat, but not indefinitely. Some hulls (keelboat for example) can never transition to planing. Hull speed is a concern if you tow a small vessel with a larger vessel that can run faster than the small vessels hull speed. You can sink a boat by creating a large enough bow wave. https://en.m.wikipedia.org/wiki/Hull_speed https://en.m.wikipedia.org/wiki/Hull_speed
    - aledalgrande 3y ago
      > You can sink a boat by creating a large enough bow wave. oof I never thought about that
    - pjc50 3y ago
      This sounds similar to the "sound barrier", except at a different speed due to the different value of rho?
      - jabl 3y ago
        Sort of, but not really. The speed of sound in water is 1480 m/s (at 20C), and is independent of the length of the vehicle. Ships are sort of special because they operate at the surface of the water. Submarines operating submerged have no such "hull speed", and their wave-making resistance is insignificant.
    - ben_w 3y ago
      > Hull speed is a concern if you tow a small vessel with a larger vessel that can run faster than the small vessels hull speed. You can sink a boat by creating a large enough bow wave. This explains something that didn't make sense to me when I first encountered it 25 years ago. There's a board game, Star Fleet Battles, which is superficially set in a fork of the (original) Star Trek animated series[0], but the mechanics are all much closer to ~WW2-to-cold-war naval combat[1]. The specific mechanic you've just explained is that if you grab a small vehicle like a shuttle craft in a tractor beam and drag it along the map too fast, the shuttle will be destroyed by going too fast. [0] and therefore has Kzinti [1] Q-ships named as that; "drones" that act like torpedoes, photon torpedoes that act like missiles
- jabl 3y ago
  Yes, it has to do with the beam, and some other things. A widely used method for estimating the propulsion power requirement for ships is the Holtrop-Mennen method. It has a dozen or so parameters, which you can play around with at an online calculator https://www.mermaid-consultants.com/resistance-prediction-holtrop-menon-method.html https://www.mermaid-consultants.com/resistance-prediction-ho...
- chipsa 3y ago
  The width of the boat doesn't matter too much, surprisingly. It's mostly the length. From the waterline length you can guess how fast a ship is, regardless of the displacement. It's part of why the rumors that the Nimitz class can go 45 knots aren't believed by most people who know a bit about naval engineering: it doesn't matter much how much power the ship has, it's not going to go much faster.
  - Xylakant 3y ago
    Though it would make for an impressive spectacle to see a Nimitz class carrier transition to planing.
    - dotancohen 3y ago
      It would also make the F-18 landings a tad more spectacular as well!
  - jabl 3y ago
    > The width of the boat doesn't matter too much, surprisingly. It's mostly the length. Well.. The length and the speed determine the Froude number the ship is operating at, and yes, the length to a large extent determines the achievable top speed. However the beam, prismatic coefficient ("fineness" of the hull shape) and a host of other factors do definitely affect the total resistance quite a lot. Ships designed for high speeds, like warships, tend to have large length/beam ratios and low prismatic coefficients. > From the waterline length you can guess how fast a ship is, regardless of the displacement. So are you saying the Seawise Giant was considerably faster than the Nimitz, considering it had a waterline length of ~450m (!!) vs. a "mere" 317m for the carrier? ;) But yes, I'm quite sure Nimitz doing 45 knots is unrealistic. I'm also quite sure that given the hull shape and size, and engine power, are known, that potential adversaries who have a naval engineer or two on the payroll have a decent idea of what the actual top speed is.

iancmceachern 3y ago

Interesting! I wonder if this has to do do with the width of the boat vs length. For the same displacement, a narrower front, and therefore longer boat will clearly win. It's like a missile vs a peterbuilt

two_in_one 3y ago
May be it's because usually the longer the waterline the bigger the engine(?) And the size of the engine is proportional to L^3. While the cross section is only L^2. I doubt with the same engine the bigger waterline has any positive effect...
- jabl 3y ago
  You may doubt the effect of the waterline length, but it's really basic naval engineering: https://en.m.wikipedia.org/wiki/Wave-making_resistance https://en.m.wikipedia.org/wiki/Wave-making_resistance
  - two_in_one 3y ago
    I'm familiar with fluids dynamics, thanks. Yes, bigger ending or smaller cross section is "better". You can downvote again if you have nothing to say. I don't care really.
- Xylakant 3y ago
  My previous post is cut short, something something computers :( The same engine in a longer waterline may have some effect. The underlying physics is that the length of the waterline influences the wake system that a boat creates between its bow and stern wave. Those waves interfere constructively and at a certain speed, they start adding up to significant waves - the bow wave starts to pile up more and more. The boat needs to ride up the wave. Drag rises sharply at that point - so a stronger engine helps little at this point. This used to be called the “hull speed”, the maximum speed that a hull can achieve. (It’s grossly simplistic and outdated concept, but a workable estimate). Going beyond requires either a longer waterline or switching from displacement to planing. Some hull shapes can exceed their nominal hull speeds somewhat, but not indefinitely. Some hulls (keelboat for example) can never transition to planing. Hull speed is a concern if you tow a small vessel with a larger vessel that can run faster than the small vessels hull speed. You can sink a boat by creating a large enough bow wave. https://en.m.wikipedia.org/wiki/Hull_speed https://en.m.wikipedia.org/wiki/Hull_speed
  - aledalgrande 3y ago
    > You can sink a boat by creating a large enough bow wave. oof I never thought about that
  - pjc50 3y ago
    This sounds similar to the "sound barrier", except at a different speed due to the different value of rho?
    - jabl 3y ago
      Sort of, but not really. The speed of sound in water is 1480 m/s (at 20C), and is independent of the length of the vehicle. Ships are sort of special because they operate at the surface of the water. Submarines operating submerged have no such "hull speed", and their wave-making resistance is insignificant.
  - ben_w 3y ago
    > Hull speed is a concern if you tow a small vessel with a larger vessel that can run faster than the small vessels hull speed. You can sink a boat by creating a large enough bow wave. This explains something that didn't make sense to me when I first encountered it 25 years ago. There's a board game, Star Fleet Battles, which is superficially set in a fork of the (original) Star Trek animated series[0], but the mechanics are all much closer to ~WW2-to-cold-war naval combat[1]. The specific mechanic you've just explained is that if you grab a small vehicle like a shuttle craft in a tractor beam and drag it along the map too fast, the shuttle will be destroyed by going too fast. [0] and therefore has Kzinti [1] Q-ships named as that; "drones" that act like torpedoes, photon torpedoes that act like missiles
jabl 3y ago
Yes, it has to do with the beam, and some other things. A widely used method for estimating the propulsion power requirement for ships is the Holtrop-Mennen method. It has a dozen or so parameters, which you can play around with at an online calculator https://www.mermaid-consultants.com/resistance-prediction-holtrop-menon-method.html https://www.mermaid-consultants.com/resistance-prediction-ho...
chipsa 3y ago
The width of the boat doesn't matter too much, surprisingly. It's mostly the length. From the waterline length you can guess how fast a ship is, regardless of the displacement. It's part of why the rumors that the Nimitz class can go 45 knots aren't believed by most people who know a bit about naval engineering: it doesn't matter much how much power the ship has, it's not going to go much faster.
- Xylakant 3y ago
  Though it would make for an impressive spectacle to see a Nimitz class carrier transition to planing.
  - dotancohen 3y ago
    It would also make the F-18 landings a tad more spectacular as well!
- jabl 3y ago
  > The width of the boat doesn't matter too much, surprisingly. It's mostly the length. Well.. The length and the speed determine the Froude number the ship is operating at, and yes, the length to a large extent determines the achievable top speed. However the beam, prismatic coefficient ("fineness" of the hull shape) and a host of other factors do definitely affect the total resistance quite a lot. Ships designed for high speeds, like warships, tend to have large length/beam ratios and low prismatic coefficients. > From the waterline length you can guess how fast a ship is, regardless of the displacement. So are you saying the Seawise Giant was considerably faster than the Nimitz, considering it had a waterline length of ~450m (!!) vs. a "mere" 317m for the carrier? ;) But yes, I'm quite sure Nimitz doing 45 knots is unrealistic. I'm also quite sure that given the hull shape and size, and engine power, are known, that potential adversaries who have a naval engineer or two on the payroll have a decent idea of what the actual top speed is.