in a very general sense when you take

two similar ships just one being larger

than the other you'll usually find that

the bigger one is faster

for example these 400 meter container

ships typically have a speed of 25 knots

yet these feeder style container ships

might only have a typical speed of 15

knots

it's odd because it goes against common

sense when you think that something

bigger weighing significantly more will

be faster so why are big ships faster

than small ships well firstly we need to

remember that a ship is just a hull

displacing water when you move a hull in

the water it generates waves which we

see as the wash

at the bow the waves are generated by

the hull pushing water out of the way at

the stern similar waves are generated as

the water rushes to fill the void left

as the hull moves forwards

from the side it looks as if there's one

wave system generated with its first

peak at the bow and a second wave system

generated with its first trough at the

stern

now it gets complicated fairly quickly

if we have two wave systems so let's

just focus on the bow to begin with

at slow speed you might get a wave

profile like this the wavelength is

shortened the wave speed is well it's

the same as the ship's speed as the ship

speeds up the whole thing stretches out

increasing the wavelength this

immediately tells us that the wavelength

is proportional to the wave speed which

is directly linked to the speed of the

ship the higher the ship speed the

higher the wave speed and the longer the

wavelength

as the ship's speed increases you'll

eventually get to a point where the

wavelength is two-thirds of the ship's

length your peaks are here and here and

your troughs are here and here remember

though we said that there are two wave

systems generated the second being at

the stern starting with a trough

the troughs from the stern system and

the bowel system are in the same place

so we get constructive interference

behind the ship a massive wash is

generated as the bow waves and stern

waves add together while that doesn't

help you on a ship if you're on a small

boat pulling a wake boarder for example

that might be what you need

just find the speed that corresponds

with a wavelength two-thirds of your

boat's length and you'll have the best

water sports business in town anyway

back to ships let's see what happens as

you further increase the speed you'll

get to a point where the wavelength is

the same as your ship's length now your

stern wave and bow wave will

destructively interfere leaving minimal

wash but take a look at your hull

you have two peaks and only one trough

at this speed the wetted surface of your

hull and corresponding resistance are as

high as they can be this is known as

your hull speed which is the least

efficient speed for a displacement

vessel you can approximate it using the

formula speed in knots is equal to 1.34

times the square root of your waterline

length in feet

for example queen mary 2 with a length

of

1132 feet has a hull speed of 45 knots a

300 foot coaster will have a hull speed

of 23 knots an 80 foot small commercial

boat will have a hull speed of 12 knots

and a 20 foot small pleasure boat will

have a hull speed of only six knots

of course you can push past your hull

speed as you get faster though your

wavelength will continue to increase

until you hit the point where it's one

and a half times your boat's length

your boat's stern will sink increasing

your trim and creating the feeling of

continuously running uphill earning it

the name hump speed

with the trough at the stern again you

get constructive interference with the

sternwave generating a massive wash this

really is the worst speed to run at but

once you get over the hump there is the

potential to reach really high speeds

this is known as planing where the

wavelength can be many times your own

boat's length the thing is a

displacement hull like you find on most

cargo ships will never be able to

generate enough energy to get past its

hump speed so they can never plane

you can only reach a plane with a hull

designed for it small speed boats ribs

and things like that

anyway combining all of today's examples

together we can produce a nice little

graph with the hull's resistance caused

by the wave pattern at different speeds

notice how it's not a nice smooth curve

there are humps and bumps causing little

sweet spots at different speeds as the

length of the ship increases the speed

at which all these humps and bumps occur

changes

longer ships experience sweet spots at

higher speeds meaning bigger ships can

be naturally faster than smaller ships

but of course that isn't a whole story

you can reduce resistance from the bowel

wave using a bulbous bowel it generates

a second bowel wave designed to

destructively interfere with the first

reducing all the effects that we've just

covered

alternatively you can adapt the shape of

your hull to reduce the waves generated

finer lines like a long thin hull with a

sharper bow will need to push the water

far more gently reducing resistance and

helping the ship to get over its hump

speed this is how things like this 300

foot ferry can travel at 40 knots

despite having a hull speed of only 23

knots of course if you're not limited by

power you can always just strap on a

massive engine and not worry about the

effects of resistance and go at whatever

speed you like

unfortunately though particularly in the

commercial world that just isn't an

option which conveniently brings us back

to our original question why are big

ships faster than small ships

well they're not necessarily but it is

in a way easier for bigger ships to

travel faster as they experience

prohibitively high drag at higher speeds

due to their greater water line length

giving them a naturally higher hull

speed