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