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.
Yes. I thought it was obvious I was comparing at displacement speeds only; I'm sorry for not being more clear.
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.
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,
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 (It’s grossly simplistic and outdated concept, but a workable estimate).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.
oof I never thought about that
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.
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
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.
Spot on. The Candela C-8 is an excellent example: 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/
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.