为什么船舶航行速度如此慢

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Ships have a reputation for being large bulky lumps that slowly plot around the world
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Even a passage from Southampton to New York on a modern ocean liner now takes the best part of a week
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Traveling a let's call it twenty knots ish in an airliner. You can do the same in a matter of hours rather than days
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They have a typical cruising speed of 500 knots ignoring the likes of Concorde
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Of course
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even your own Transport has typical speeds of 70 miles an hour about
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60 knots when traveling on the motorway in the UK at least
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So what is it about ships that makes them so slow in terms of physical size
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Ships engines are enormous. They're so big
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The larger ones are typically known as Cathedral engines the Emma Maersk is powered by one of these
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She is an eClass container vessel almost 400 metres long and was the largest container ship ever when she was launched back in
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2006 of course since then musk have bought out the triple e class which is slightly longer and wider and
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Other companies have bought out ships even bigger still her main engine weighs twenty three hundred
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Tonnes and pumps out a whopping 109 thousand horsepower now
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it's easy to compare that to a car as
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They all publish the horsepower of their engines the typical cars you're looking at about a hundred horsepower depending on the configuration
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You've chosen at the extreme end. You've got f1 cars which can be pushing a thousand horsepower
Power
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It is a little trickier to compare that to an aircraft as they use jet engines. Which produce?
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Thrust instead of the mechanical horses that we use everywhere else. The maths is complex, but the best estimates are found, but typical
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747 engines around the hundred and fifty thousand horsepower mark
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So for power it makes sense that the plane is the fastest but the ship comes second and she is still slower than the car
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So there must be more to it than just the horses
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With movement and things another factor always makes an appearance mass
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it is in the energy formula with kinetic energy being half MV squared and
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It forms acceleration force being mass times acceleration
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So, how can we add mass into this discussion?
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We can look at horsepower per ton instead for the plane
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we can take the 747 they come in anywhere between three and five hundred tons and are powered by four of those jet engines a
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Crude effort gives the power per ton
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let's say
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1,200 horsepower per ton the car
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we said was around 100 horsepower typically and you can assume in normal car weighs between 1 and 2 tons and
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Averaging that out gives you about 75 horsepower per ton of course with f1 cars
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They're somewhat lighter and more powerful. So their figures are closer to the figures
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We got for the airliner now the ship fairly obviously she will weigh the most a fully loaded Emma
Terminal Velocity
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Maersk will tip the scales at
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rounding off slightly
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200,000 tons
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With her engine delivering 109 thousand horsepower. We our horsepower per ton around
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0.5 horses per ton the vehicles are now starting to settle in the correct order
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The plane is still out ahead the car in second and the ship is trailing a long way behind
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But for the ship from here on it only gets worse
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Do you remember working out terminal velocity at school is when the force produced by an engine?
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matches the resistance force from the medium
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The object is moving through for example
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You drop a ball and it will fall faster than if you drop a feather, the wind resistance of the feather is much greater
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so it's velocity ends up being slower the same applies with vehicles the plane experiences air resistance the car a
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Combination of air resistance and friction with the ground but the ship is moving through water a comparatively dense medium
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This means she's experiencing the greatest force against her the drag equation
Drag Equation
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explains that drag is proportional to a cross-sectional area and
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The square of the speed the cross section will get from the breadth of the ship times its draft
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Which we can assume to be a constant for most ships
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Of course if you reduce the draft
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Like when there's no cargo on board the ship will be able to go faster as there's less drag
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Otherwise drag is determined by the square of the speed. You double the speed you quadruple the drag
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You can sort of assume the force produced by the engine is constant
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There are variations due to the water flow
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but we can ignore those for now all the while the engine produces more force than the resistance the ship will accelerate as
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She speeds up the Dragon greases according to the square of the speed
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Once the drag force matches the engine force no more acceleration occurs
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The ship has reached terminal velocity for the Emma mask
Fast Cats
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This is around 25 knots and that's typical for most large ships
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For smaller ships. This is usually slower
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And that's because their engines produce more power
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But crucially the cross-section doesn't reduce in proportion to that change in engine power
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There are fairies that do go significantly faster than normal ships. You'll often heard them called fast cats
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Fast cat is short for fast catamaran and a catamaran is just a boat that has two hulls
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Instead of a single box shaped hull the cat has two thin hulls
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The separation between them produces the transverse stability that they need and the buoyancy is just produced by the combined
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underwater volume of both hulls
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Clearly there's less buoyancy
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Which means less carrying capacity which is why they are usually only passenger ships
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The key here is that you've drastically reduced the cross section that produces the underwater drag
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reduce the drag and you increase the
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Theoretical maximum speed add on a few jet turbines and you've got yourself a ferry that's capable of speeds far higher than a typical ship
Small Speed Boats
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And what about small speed boats? Well again, they reduce their cross sectional area allowing higher speeds
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But rather than change the shape of the hull they're designed to rise above the water instead of pushing through it
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We call it planing above a certain speed the water flow lifts the hull
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Reducing the cross section reducing the drag and increasing the speed
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Hydrofoils do a similar thing except they have an underwater wing to produce the left as slow speeds the whole hull creates
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Resistance as the speed increases lift is generated
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lifting most of the hull clear
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reducing the cross section
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increasing the speed is the same theme for every
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Watercraft that's capable of high speeds is all about reducing that cross section
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Reducing the drag and allowing a higher speed if you've enjoyed today's discussion
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Be sure to subscribe for more videos like this every other Friday
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Until next time thank you for watching and goodbye.