Why are ships so slow?

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 Be sure to subscribe for more videos like this every other Friday Until next time thank you for watching and goodbye.

Channel: Casual Navigation

Views: 4,099,039

Keywords: casual navigator, marine, uscg, shipping, casual navigation, maritime explaination, merchant navy, sailing, marine animation, ship speed, ship engine, boat speed

Id: YPQY70z5uKE

Length: 7min 12sec (432 seconds)

Published: Fri May 03 2019