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A big thing in boat design is the energy usage per mile at different speeds. A typical rowing boat (human powered) will have a displacement hull. It is a stre

A big thing in boat design is the energy usage per mile at different speeds. A typical rowing boat (human powered) will have a displacement hull. It is a streamlined curved shape which can move very efficiently through the water at slow speeds. 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. A 'planing' hull is normally used on speedboats and the like. It has two efficient modes - either displacement, or planing (where the boat rises most of the way out of the water and only has a small contact area). Typically there is a region in between - a speed the boat cannot travel at a fixed throttle - it can go faster or slower though. There are lots of other possible hull designs - but in general you need to pick your ideal speed or range of speeds (and loads, and wave conditions), and optimize for minimum energy consumption at those speeds. In the modern world, you normally combine this with a cost optimization - trading off the salary and depreciation and opportunity cost of travelling slower, with the saved fuel of travelling slower. In an electric boat, where energy storage is really expensive, it is even more important to do a really good job of this optimization - and I forsee we'll see more people attempt esoteric yet theoretically very efficient hulls, like hydrofoils, to try to have a big edge in a market where energy efficiency is suddenly a big driving factor.

coryrc 3y ago
IIRC planing versus displacement is about 10x increase in energy for the same speed. That's why all ocean-going freighters are displacement hulls.
- mojomark 3y ago
  That's not really accurate. Displavement hulls are more efficient at lower speeds for the same displacement (i.e. payload mass), but at higher speeds, the speed-power relationship of planing hulls flattens allowing for MUCH higher speed transits, whereas the power requirements for displacement hulls just continues to skyrocket with increased speeds (1). Freighters are generally displacement hulls because they carry thousands of tonnes of cargo. This sizable displacement would make it impractical for these vessels to "get up on plane" (assuming their hulls were shaped like planing hulls vice displacement hulls). You can simply carry more payload in a displacement hull than a planing hull. Planing hulls like to be lightweight. It's not impossible to make a planing freighter, just impractical - and all of that inertia and mechanical stress on the hull structure while traveling at high speeds would be very difficult to design against, not to mention very difficult/unsafe to amneuver in a sea lane or inclement weather. There are certainly high speed ferries, which are generally catamarans (for added stability) with similar speed-power characteristics to planing hulls, but those are still carrying relatively lighter cargo (humans, some cars) over shorter distances compared to fairly dense bulk liquid/material/container cargo transiting accross oceans. 1. https://images.app.goo.gl/Yn3znGUBgiM9jkB4A https://images.app.goo.gl/Yn3znGUBgiM9jkB4A
  - coryrc 3y ago
    > Displacement hulls are more efficient at lower speeds for the same displacement Yes. I thought it was obvious I was comparing at displacement speeds only; I'm sorry for not being more clear.
    - coryrc 3y ago
      And, actually, the speed doesn't matter, because the fuel usage of planing hull makes it uneconomic at any speed, even if such speed is greater than displacement hull speed, even though it's infinitely more effective than a displacement hull at those speeds (since a displacement hull cannot reach those speeds basically by definition).
  - bombcar 3y ago
    I think you could argue an airliner is just an extreme form of planing hull - certainly a seaplane is. And those are used for high-value freight.
    - supportunions 3y ago
      [dead]
- coryrc 3y ago
  And, a planing hull is not a great shape for displacement. I think they have significantly more drag, just 2x or 3x though, when operating at displacement speeds.
  - coryrc 3y ago
    If anyone's confused about 10x in the GP post and 2-3x in this one, a given size boat with a displacement hull might have a hull speed of 6 kts and with a planing hull a hull speed of 5 kts. At 2 kts, they'd both probably be in displacement, but the planing hull would be using 2-3x power. At 6 kts, the planing hull would be using about 10x power, because it would be operating in planing mode while the displacement hull is still operating in the very efficient displacement mode. Only the planing hull could exceed 6 kts.
Xylakant 3y ago
> 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
Sparkyte 3y ago
sodium ion batteries are where we need to be at for the most efficient use
audunw 3y ago
> and I forsee we'll see more people attempt esoteric yet theoretically very efficient hulls, like hydrofoils Spot on. The Candela C-8 is an excellent example: https://candela.com/candela-c-8/ https://candela.com/candela-c-8/ Ripple Boats has a concept of injecting air to a space underneath the boat with a fan. Seems like a concept that would be easier and cheaper to scale up (in both size and number of ships produced) https://www.rippleboats.com/ https://www.rippleboats.com/

coryrc 3y ago

IIRC planing versus displacement is about 10x increase in energy for the same speed. That's why all ocean-going freighters are displacement hulls.

mojomark 3y ago
That's not really accurate. Displavement hulls are more efficient at lower speeds for the same displacement (i.e. payload mass), but at higher speeds, the speed-power relationship of planing hulls flattens allowing for MUCH higher speed transits, whereas the power requirements for displacement hulls just continues to skyrocket with increased speeds (1). Freighters are generally displacement hulls because they carry thousands of tonnes of cargo. This sizable displacement would make it impractical for these vessels to "get up on plane" (assuming their hulls were shaped like planing hulls vice displacement hulls). You can simply carry more payload in a displacement hull than a planing hull. Planing hulls like to be lightweight. It's not impossible to make a planing freighter, just impractical - and all of that inertia and mechanical stress on the hull structure while traveling at high speeds would be very difficult to design against, not to mention very difficult/unsafe to amneuver in a sea lane or inclement weather. There are certainly high speed ferries, which are generally catamarans (for added stability) with similar speed-power characteristics to planing hulls, but those are still carrying relatively lighter cargo (humans, some cars) over shorter distances compared to fairly dense bulk liquid/material/container cargo transiting accross oceans. 1. https://images.app.goo.gl/Yn3znGUBgiM9jkB4A https://images.app.goo.gl/Yn3znGUBgiM9jkB4A
- coryrc 3y ago
  > Displacement hulls are more efficient at lower speeds for the same displacement Yes. I thought it was obvious I was comparing at displacement speeds only; I'm sorry for not being more clear.
  - coryrc 3y ago
    And, actually, the speed doesn't matter, because the fuel usage of planing hull makes it uneconomic at any speed, even if such speed is greater than displacement hull speed, even though it's infinitely more effective than a displacement hull at those speeds (since a displacement hull cannot reach those speeds basically by definition).
- bombcar 3y ago
  I think you could argue an airliner is just an extreme form of planing hull - certainly a seaplane is. And those are used for high-value freight.
  - supportunions 3y ago
    [dead]
coryrc 3y ago
And, a planing hull is not a great shape for displacement. I think they have significantly more drag, just 2x or 3x though, when operating at displacement speeds.
- coryrc 3y ago
  If anyone's confused about 10x in the GP post and 2-3x in this one, a given size boat with a displacement hull might have a hull speed of 6 kts and with a planing hull a hull speed of 5 kts. At 2 kts, they'd both probably be in displacement, but the planing hull would be using 2-3x power. At 6 kts, the planing hull would be using about 10x power, because it would be operating in planing mode while the displacement hull is still operating in the very efficient displacement mode. Only the planing hull could exceed 6 kts.

Xylakant 3y ago

> 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

Sparkyte 3y ago

sodium ion batteries are where we need to be at for the most efficient use

audunw 3y ago

> and I forsee we'll see more people attempt esoteric yet theoretically very efficient hulls, like hydrofoils Spot on. The Candela C-8 is an excellent example: https://candela.com/candela-c-8/ https://candela.com/candela-c-8/ Ripple Boats has a concept of injecting air to a space underneath the boat with a fan. Seems like a concept that would be easier and cheaper to scale up (in both size and number of ships produced) https://www.rippleboats.com/ https://www.rippleboats.com/