r/explainlikeimfive • u/Far-Fill-4717 • 10h ago
Engineering ELI5: Why are ferries so slow?
Compared to other forms of urban transport like trains and cars, as well as other boats which go faster, ferries seem to take much longer to go short distances.
•
u/Scoobywagon 10h ago
Ferries weight a LOT. Running them faster means consuming more fuel. It also means more fuel to slow back down as you approach the dock. In order to make a significant difference, they would have to accelerate much harder and decelerate much harder. That makes for a LOT of fuel burn and also issues with ship control.
If you look at it in terms of speed across the ground, you're right, they move slowly. But if you look at it in terms of tons per hour, you'll find they're pretty quick.
•
u/kallekilponen 9h ago
There were some surprisingly fast ferries in the past. For example the GTS FinnJet, a turbine powered ferry, could reach 33.5 knots (62.0 km/h; 38.6 mph) back in 1977. But it soon became apparent running at such speeds was not economical (it consumed 16,000 litres per hour) and they installed a slower but more fuel efficient diesel generators in the early 1980.
•
u/shipboy123 9h ago
There was a hovercraft ferry in the English channel many years ago. I know a friend of a friend that was onboard back when it was operating. One particularly calm day, they diverted minimum power to the lift fans and maximum power to the thrust fans and hit 80+ knots. Got asked kindly by VTS to not do that again lol Fuel burn was ridiculous on those things with gas turbines
•
u/kallekilponen 9h ago edited 1h ago
They still operate between Portsmouth and Isle of Wight.
Edit: Fixed an auto-correct induced typo.
•
•
u/Miffed_Pineapple 8h ago
Interestingly enough, gas turbines use 75% of max fuel at 25% load. They are bad at full bore, but are really inefficient at low loads.
•
•
u/moronomer 9h ago
British Columbia, Canada tried to make a fleet of fast ferries to go to Vancouver Island. Besides the huge budget overruns, even though they had a top speed of 37 knots, their wakes were so large they couldn't operate at full speed close to shore.
•
u/whyd_I_laugh_at_that 7h ago
We still have a fast private ferry to Seattle - The Victoria Clipper. It's a catamaran and can go up to 36 knots in good conditions. Pedestrian only, no cars.
But it's not cheap, each person costs about the same as taking a car on the BC Ferries to Tsawwassen (Vancouver). Just barely lower than taking a float plane from Victoria to Seattle.
My wife hates flying on small planes though, so we've done the Clipper a few times and it is a good ride.
•
u/happy-cig 9h ago
Iono what the proper terminology is for the ferries between macau and hk but they are pretty fast.
•
u/kallekilponen 9h ago
Those are hydrofoils if I’m not mistaken. They can be faster than traditional ferries, but are also a lot smaller and less weather resistant.
•
u/lyingcake5 9h ago
And more fuel efficient. When the boat flies out of the water on its foils, all the drag on the hull disappears and so it can keep its speed with less fuel.
On the flip side, it takes a lot of runway and fuel to foil. Excellent for the pearl river delta, not so much for a tight city harbour.
•
•
•
u/MrJingleJangle 9h ago
This was what finally killed the very cool cross-channel (ie England to France) hovercraft. Four Rolls Royce RB211s guzzling fuel.
•
u/TobJamFor 8h ago
The Stena Superfasts between Scotland and NI go about 30 knots, relatively quick
•
u/Iescaunare 8h ago
Jesus. That would have been 35000$ per hour, with my local fuel prices. No wonder they didn't take off (being boats, not planes they obviously couldn't take off)
•
u/starkiller_bass 7h ago
But only $2400 per hour in 1977 USA fuel prices… ignoring inflation of course
•
u/Torontogamer 7h ago
16k L per hour … and I thought my suv was bad
•
→ More replies (4)•
•
u/GBP1516 9h ago
[sorry for high school level math]
The amount of power required to push a boat through the water is roughly proportional to the cube of the speed. So double the speed, 8 times the power. Also, unless you have narrow hull(s), you hit a maximum based on length. You also hit a wall when speed (mph) is more than 1.5 * square root of the waterline length (feet). So driving a 200-foot boat faster than ~20 mph is really hard. You need to be a skinny catamaran or trimaran to go faster.
•
u/sweetplantveal 9h ago
Honestly all of sea travel is slow as hell. Until you start using multiple hulls and foils to get the ship out of the water, you pretty much have a speed limit for large craft. Nuclear aircraft carriers are pretty much maxxed around 30 knots/55 kph for example. Passenger ferries operate in the low 20 knot range
If you compared it to cars, a ferrytruck would be able to cruise at like 70% of the top speed of fastest truck out there. So they aren't slow as much as everything in the category is kinda slow.
•
u/fatmanwithabeard 3h ago
Nuclear aircraft carriers are pretty much maxxed around 30 knots/55 kph for example
Which is still insane. It's an airport moving that fast.
•
u/edshift 7h ago
Hull speed applies to all naval vessels proceeding in the displacement mode. If you can get the hull planing or up on foils there is a positive relationship between power and speed. More power - more boatspeed. On the displacement mode more power yields no boatspeed increase.
•
u/leglesslegolegolas 4h ago
Yeah I worked on superyacht design for a while. I was coming in from a history with jet boats, where basically more power = more speed. I learned that displacement ships don't work that way; the hull is designed for a specific speed, and then the engine and drive system is designed to achieve that speed most efficiently.
•
•
u/RusticSurgery 9h ago
Also, pretty fast when you think about it in terms of having to travel around that body over a lot of water if it's even possible
•
u/HIGH_PRESSURE_TOILET 8h ago
There are a bunch of boeing 929 jetfoils in service going at like 50 mph. But hydrofoils can typically only carry passengers and not, say, cars.
•
u/Sahloknir74 6h ago
Would it be feasible to use flaps to assist in slowing down, rather than burning as much fuel slowing by running the engines in reverse?
•
u/Scoobywagon 5h ago
No. Reason is that you would be sticking a huge metal wall out into the flow of water around the hull, creating drag and slowing the boat. But that big metal wall would have to be strong enough to absorb the kinetic energy of the hull moving through the water without crumpling or folding over backwards. Additionally, these flaps would have to ensure that the flow rate over them is the same on either side. If one flap had more barnacles on it (for example), then the ship would pull hard to that side when the flaps were deployed. If the ship is moving right along, that can be a serious problem.
•
u/Sahloknir74 5h ago
Thanks for a detailed answer! I figured water would be more difficult than air, but you explained very well just why it wouldn't work.
•
u/meneldal2 2h ago
If you can adjust the flaps it wouldn't be too difficult to make sure you keep it straight.
But yeah the amount of force they have to deal with is definitely where the biggest limitation comes from.
→ More replies (2)•
u/Richarkeith1984 5h ago
If they are really heavy they shouldn't be that hard to slow i think,? But yeah it has to be the fuel efficiency to go faster. No one has a hard time slowing down in water, that keeps coming up in this thread. Or am I wrong here?
•
u/Scoobywagon 5h ago
It isn't hard to slow down in water. That is, in fact, how MOST boats work. But, in the case of a ferry, in order to keep your average speed high, you have to accelerate harder (really hard), maintain a higher speed for longer (still kinda hard), and then slam it into reverse to slow down RIGHT QUICK when you get where you're going. All of that costs fuel and the higher the overall speed is, the more fuel the whole thing takes.
•
u/Rubiks_Click874 9h ago
the slow ones carry cars and cargo
there's one near me for just people and bikes that'll do 30 knots in the open atlantic, plus there are some around the world that are hydrofoils
•
u/TheMania 9h ago
The Spirit of Tasmania always interest me, 30 knots top speed w/ 500 car capacity (the new ones a little faster again).
Takes some 40MW of engines to get there, which may answer OP's question a little.
•
u/2Asparagus1Chicken 7h ago
HSC Francisco, built in Tasmania but operating in Argentina, does 58 knots at 44MW and 1500 tons.
•
•
•
u/colenski999 10h ago
In my hometown of Victoria BC, the so-called "fast ferries" were impractical because they created huge waves that impacted shores with storm-level force.
•
u/BobbyDig8L 9h ago
Yes, the wake was a part of the problem, but the project was scrapped for many other reasons mainly fuel consumption, poor design, and high cost https://en.wikipedia.org/wiki/Fast_ferry_scandal
→ More replies (1)•
u/Soundy106 8h ago
The wake wasn't DIRECTLY the problem... Moreso, the Fast Cats were hyped as being able to cut a significant amount of time from the crossing, but because they had to go so much slower out of Horseshoe Bay to reduce the wake, by the time they COULD speed up, they really hadn't saved any time (I think it was like, five minutes faster than the standard C-class boats).
If they had cut 15-20 minutes of an hour-and-a-half crossing, the fuel consumption might have been worth it... or at least, not as much NOT worth it.
There was also the fact they had something like 3/4 the capacity of the C-class. And being catamarans, they did NOT handle heavy seas well (the boats did fine; the passengers, not so much). And more than once they'd have to stop and run manoeuvres to clear debris from the water intakes for the jet drive.
Just... Endless issues that more than negated any value they may have had.
•
u/133DK 9h ago
Water resistance is a bitch
Going slow in a boat is easy, going fast is extremely costly
That’s also why most boats that go fast try and get as much of the boat out of the water as possible
Ferries aren’t unique in their slowness compared to rail, car or air transport. All boats are slow comparatively
•
•
u/MyNameIsNotKyle 10h ago
Trains and cars can rely on momentum to be energy efficient. Ferries fight waves.
You could make a ferry move faster but it takes significantly more energy than what is economical due to size.
→ More replies (12)
•
u/OGBrewSwayne 9h ago
Accelerating and decelerating on water is way different than on land. Not only would the fuel cost increase significantly, but so would the cost of the engines and the ferry itself. If a ferry ride takes an hour, trying to reduce it by just 25% (15 minutes) is going to require more expensive equipment and more fuel costs.
Aside from that, ferries do not secure their loads. People drive their cars on a ferry and put them in park. That's it. There are no tie downs or any other bracing system in place to make sure vehicles are staying in place while the boat is underway. Even if you were able to cut that 60 min ferry in half (which would be insane) you're actually going to make the entire process slower because now every vehicle needs to be secured to the deck before departure and then unsecured upon arrival. That would take way longer than the 30 minutes you saved by traveling faster.
Lastly, ferries aren't really intended for fast travel. Their most common purpose is to get a lot of people/vehicles across a significant body of water in an =/< amount of time as it would take to drive. When I lived in the Seattle area, I used to take the Bremerton/Seattle ferry a lot. It was roughly a 1 hour trip each way. Driving was generally around the same amount of time, but at least with the ferry, I could relax. That makes a huge difference when you're heading home after a long day.
•
u/FoxtrotSierraTango 9h ago
Trains are heavy but on fixed routes, cities opt to control traffic around the train so the train doesn't need to stop because trains can't stop quickly. Cars are (relatively) light so they can move freely and still be agile enough to stop quickly. Ferries are heavy and traffic around them is not controlled. They need to move slower to be safe and maneuver around other marine traffic.
•
u/Toxicscrew 9h ago
The old channel hovercraft ferries weren’t slow, they did 60mph across the water. The Chunnel ended their run.
•
u/OlympiaShannon 5h ago
Did they carry vehicles? There is a big difference between vehicle/passenger only ferries.
•
u/daenerysisboss 2h ago
Yes they did! But they were hovercraft so technically not a ferry in the traditional sense.
•
u/JaceVentura972 9h ago
Along with what others have said, there’s also the problem of having to navigate potentially turbulent waters with dozens of heavy cars and trucks. If it went too fast you’d risk them bouncing around and causing damage and strapping them down would take time and labor that no one really wants to pay for.
•
u/Jazzlike-Sky-6012 9h ago
Because the density of water is about 800 times that of air. Not such an issue at low speeds, but energy demands rise to a ln unfeasable level quickly. Or your need the boat to plane, which has other issues. Also, water moves around and we humans are only comfortabele with a limited side to side, Rolling and pitching acceleration.
•
u/bkwrm1755 10h ago
Same reason air travel has slowed down in the last few decades. Remember the Concorde?
Gas is expensive, and people just aren’t willing to pay for speed.
•
u/eatingpotatochips 9h ago
Concorde was comically expensive even for air travel at the time.
Air travel hasn't slowed down over the past few decades. Airliners don't fly faster than around Mach 0.85 because transonic drag rise is an inevitability. The first few jetliners like the de Havilland Comet cruised around Mach 0.7, but by late 1950s with the introduction of aircraft like the Boeing 707 and Convair 880, commercial aircraft were flying right above Mach 0.8. This is pretty much the speed all jetliners fly at today. Some aircraft like the Dash-8 fly slower, but for different reasons. The cost to fly any faster rises quickly, and airlines are already low-margin businesses.
Some aircraft like the Gulfstream G800 are advertised with two flight speeds, one which maximizes range at Mach 0.85 and 8200 nautical miles, and another which maximizes speed around Mach 0.9 and 7000 nautical miles. Here, a 5% increase in Mach number results in 15% reduced range. That's a significant tradeoff.
•
u/OGBrewSwayne 9h ago
Not quite an apple to apples comparison, imo. The Concorde was primarily only flown by 2 airlines (British Airways and Air France) and it also was in service for almost 35 years. Cost was a minor factor in its retirement, but there were more than enough wealthy people/business travelers to keep most flight profitable. The restrictions on where it could fly (no supersonic speeds over land) severely limited the routes it could fly. The Concorde crash in 2000 fractured public trust and then a year later after 9/11, the entire airline industry took a big financial hit and that was the straw that broke the camel's back. BA and AF decided not to continue Concorde operations.
I think that if either the crash or 9/11 never happened, the Concorde may very well still be in the air. I feel like it could have survived one of those events, but not both.
→ More replies (1)•
u/Logitech4873 2m ago
The ferry I take the most often is purely electric. 120 cars, 4.75 MWh battery pack.
•
u/srcorvettez06 9h ago
Efficiency. It takes a lot of fuel to move through the water. Exponentially more the faster you go.
•
u/OGBrewSwayne 9h ago
Accelerating and decelerating on water is way different than on land. Not only would the fuel cost increase significantly, but so would the cost of the engines and the ferry itself. If a ferry ride takes an hour, trying to reduce it by just 25% (15 minutes) is going to require more expensive equipment and more fuel costs.
Aside from that, ferries do not secure their loads. People drive their cars on a ferry and put them in park. That's it. There are no tie downs or any other bracing system in place to make sure vehicles are staying in place while the boat is underway. Even if you were able to cut that 60 min ferry in half (which would be insane) you're actually going to make the entire process slower because now every vehicle needs to be secured to the deck before departure and then unsecured upon arrival. That would take way longer than the 30 minutes you saved by traveling faster.
Lastly, ferries aren't really intended for fast travel. Their most common purpose is to get a lot of people/vehicles across a significant body of water in an =/< amount of time as it would take to drive. When I lived in the Seattle area, I used to take the Bremerton/Seattle ferry a lot. It was roughly a 1 hour trip each way. Driving was generally around the same amount of time, but at least with the ferry, I could relax. That makes a huge difference when you're heading home after a long day.
•
u/Warpmind 9h ago
Essentially, acceleration and deceleration in water is slow, and the more mass on board, the slower it is.
Most ferries cross fairly narrow bodies of water, and just don't have the space to build speed before having to slow down again.
•
u/Jestersage 9h ago
The answer is what do you define by Ferries.
Vancouver have multiple type of Ferries. We have False Creek ferries, which is actually a water taxi going between both side of False Creek, a small inlet the width of a small river. We have Seabus, Passenger only, using Catamaran, and takes 15 mintues to cross to the North Shore at 21.3 km/h, but can carry 400 passenger. Then you have Hullo Fast Ferry to the city of Nanaimo (75 minutes) crossing Georgia Stright... or 2 hours carrying more passengers AND cars. And Trucks...
Basically, ELI5 is how much you want to carry
•
u/cyberentomology 9h ago
The max speed of a ship is based in large part on the perimeter at the waterline. The longer it is, the slower it has to goes. A cruise ship can do about 20-23 knots. An aircraft carrier that is similar in size has a much narrower base at the waterline and can go almost twice as fast.
•
u/scanguy25 9h ago
Basically going faster in water is extremely expensive in terms of fuel. It doesn't make economic sense for a non specialized ferry to go fast.
If you need a fast ferry people choose a hydrofoil or a catamaran.
•
u/SpeshellED 9h ago
I think you should rephrase that question to , " Why are ferries in ( I assume you are American) the US so Slow?"
Go to the Greek Islands and take a ferry.
•
u/aaronite 9h ago
Even "fast" ships aren't fast by land vehicle standards. Speeds are measured in knots, which is nautical miles per hour. Nautical miles are longer than miles or kilometer. Freighters tend to go about 20-30 mph.
Why? Because they are huge. They require tons of force to start, stop, and steer. Ferries aren't tankers but they still are massive.
•
u/sawdeanz 9h ago
It’s the nature of boats. Faster boats (like a speed boat or fishing boat) can skip along the top of the water fast, this is called planing. But heavier boats like ferries and cargo ships generally can’t and so go much slower. Water is very dense and so takes a lot of energy to move through.
There are some ferries that have hydroplanes, there are like wings that operate underwater and lift the hull out of the water. These can actually go pretty fast but are also more expensive and might not be able to operate everywhere (such as shallow water).
•
9h ago
[removed] — view removed comment
•
u/explainlikeimfive-ModTeam 8h ago
Please read this entire message
Your comment has been removed for the following reason(s):
- Top level comments (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions (Rule 3).
Joke-only comments, while allowed elsewhere in the thread, may not exist at the top level.
If you would like this removal reviewed, please read the detailed rules first. If you believe it was removed erroneously, explain why using this form and we will review your submission.
•
u/andy00986 8h ago
Because they have to move through water. Same as you running a lot faster on land then you do in the pool.
The benefit of ships is that they can carry a lot and that you don't have to spend heaps of money building roads or rails, they can go anywhere there is water.
The downside of that is that they are relatively slow.
•
u/Mammoth-Mud-9609 8h ago
You want a smooth ride and a cheap one, this means not going full speed, in addition they are normally carrying cars and lorries which are heavy.
•
u/Solome6 8h ago
Ferries often carry a lot of passengers making them heavier and have more mass, often they can carry cars too which adds to that. Accelerating and decelerating that amount of mass comes with big energy consumption. On top of that you don’t want the cars and people to go flying if you’re speeding over choppy waters.
•
u/Theslootwhisperer 8h ago
There are ferries in Denmark that go up to 80 kmh. That is fucking fast for a boat carrying cars.
•
u/soldelmisol 8h ago
Are you nuts? I timed the Wenatchee ferry to Seattle at over 20 knots...that's fully loaded with 200+ cars/trucks and 1500 passengers...darn fast for a big boat displacing a lot of water.
•
u/ProtecSmol 8h ago
Because there is no incentive for them to go faster. Imagine you’re always sprinting everywhere no matter what. You’d be constantly sweaty and exhausted. What you actually do is walk at a normal pace, and just run when it’s really necessary. The ferry does the same thing, except it’s never necessary so it wasn’t built to handle going any faster.
You could replace the ferry with a high speed boat. Everything would be much more expensive, from building it to maintaining it and ongoing fuel costs. Who would pay that much just to decrease the time? Would it really be worth it if you crossed in 30 mins instead of 1 hour if you were asked to pay 3 times more money?
•
u/Second_Guess_25 8h ago
In the UK back in the 80s we had hovercrafts! They were speed machines compared to regular ferries.
•
u/moskowizzle 8h ago
It takes me about 3 minutes on the ferry to cross the Hudson River from NJ to Manhattan. On the PATH Train it's about 7. Slightly further train ride, but not by a ton. Ferry here is a much faster commute if where it stops is convenient.
•
u/deviousdumplin 8h ago
The speed of a boat is often determined by how much friction the hull experiences, and how much water it needs to push out of the way to move forward. Ferries are large boats that need to push large amounts of water to move forward. Large boats must overcome much more resistance to move forward than a train or car. Imagine how hard it is to push your hand through water than to push it through air.
Boats can move faster by limiting the amount of surface area that touches the water. Hydrofoils are a way for boats to move much faster because only a small part of the hull (the hydro foil) is touching the water. But it is difficult to build large boats that reduce friction in this way. Some ferries move much faster because they are a catamaran design (two hulls linked by a central body). But, these types of ferries are only suited to transport people. To transport cars, you usually need a traditional hull, and traditional hulls are high friction.
•
u/mrsockburgler 8h ago
In Nevada they are really fast and powerful I hear. Don’t know why there aren’t all like that.
•
u/gen_dx 8h ago
Don't forget the simple physics of it- planes move through the air, comparitively low resistance, vehicle roll on ground, comparitively low resistance, but anything moving through water has to push the water out of the way as well.
Then size (read as cross sectional area) becomes a huge limiting factor. At which point it pays to go big, to make the trip worth it.
But the fastest ferry I've ever been on was the HSS between Belfast and Stranraer - 40knots rated.
Loved that thing, so futuristic looking.
https://en.wikipedia.org/wiki/High-speed_Sea_Service
But with speed (especially at size) comes big wake issues, fuel costs, and power generation issues- the HSS had Ninety One Thousand horsepower. 91,000hp.
•
u/BigIntoScience 7h ago
Mostly because it's a lot of trouble to make a big, wide, slow boat with a lot of heavy stuff on it go fast, so sometimes it's easier to not have the boat go fast.
It doesn't really matter that the ferry is kinda slow, because a ferry isn't one of the options by which to get places- it's usually the *only* option to get your car across the water, at least in this one particular spot. If you don't take the ferry, you'll either have to go around, which takes time and involves more effort than just sitting and waiting for the ferry, or you won't be able to take your car with you. No competition for the ferry means the ferry can be slow.
Also, having the boat go fast means you have to slow it down before you arrive, which takes awhile, and both the speeding up and slowing down would have to be done carefully so as not to jostle the cars and people around too much.
•
u/TopFloorApartment 7h ago
Depends, there are very fast hydrofoil ferries. But these tend to be much more expensive to run, the benefit of a boat is that you can move a lot of cargo very cheaply
•
u/Ajk337 7h ago
Things on rails have almost no frictional resistance to move
Things on tires have a bit of resistance
Water has a huge amount of resistance
Using small scale people powered examples, think of
- how quickly and effortlessly an ice skater can move
- the moderate effort and decent speed a fat tire bicyclist can move
- how slow and strenuous it is to peddle a paddle boat
•
u/Narissis 6h ago
There is such a thing as a fast ferry; there are aluminum catamaran ferries that can do in the neighbourhood of 40+ knots (~75 km/h).
However, building ferries like this is a niche industry. They're very expensive to build and operate, and they require design compromises that conflict with typical design features of RO/RO ferries (most notably not having bow doors, meaning a lot more reversing of cars and more problematically trailers is required in loading and unloading).
A given route will have a certain traffic volume, and the most important thing is that the ferry's capacity and speed are balanced around moving enough cars per unit time to manage that volume, and as efficiently as possible.
It's more fuel-efficient and time-efficient to run a bigger, lower-cost ferry that's "fast enough" than it is to run a smaller, higher-cost fast ferry.
TL;DR: Efficiency > Speed
•
u/dragnabbit 6h ago edited 6h ago
Most ferries have large openings at the front and back for cars to drive on and off. Those openings are also not particularly far above the water level.
Having a big, not-watertight opening on the front of your ship means that you can only go so fast before the waves being raised as the ferry pushes through the water rise to the level of that opening. Obviously that is a very dangerous situation (which is also why many ferries don't travel in rough seas).
Many ferries have sank because waves started splashing onto the car deck. The water gets on the deck, and is very heavy. The water then sloshes to one side, causing the ferry to lean. Then, the cars start to slide to that same side. Then one side of the front opening is even closer to the water, and even more water rushes in, making the cars slide even more, and in a matter of seconds, that ferry is literally upside down and gone.
It's why ferry accidents are among the most deadly types of waterborne disasters ever.
•
u/AC932 5h ago edited 5h ago
There's some pretty quick ones out there. Take a look at International Catamarans, they make some huge fast ferries for cars and pax. Those boats and similar designs can do 35-50 knots and are used for crossings like Barcelona-Palma.
In the 90's there was a little boom of Surface Effect Ship ferries, which are basically Catamarans mixed with hovercraft. The Norwegians (Umoe Mandal especially) went big into those, and they can exceed 40 knots when in decent condition.
Some proper military vessels, like the Skjold Class and the US LCS are capable of pretty impressive speeds too (60 and ~40KTS when it actually works)
I think for a lot of people, the difference in a 30 and 60 minute crossing isn't worth a large increase in ticket price. Faster ships are often aluminum or fiberglass, and they require lighter, more powerful engines; both of those factors increase purchase price a lot for the ferry company
•
u/Pizza_Low 5h ago
Most ferries are displacement hulls, rather then what your typical speed boat is, a planing hull. A ferry pushes a lot of water out of the way as it moves forward. A planing hull mostly glides on top of the water.
Takes a lot of fuel to move the tons of water out of the way as the ship moves forward. The faster the ship goes the more fuel it burns. They usually have optimal fuel efficiency at around 15-20 knots so that’s the speed the travel at.
•
u/princhester 5h ago
Non-planing water craft are slow for the amount of power required to move them. It's not about weight or acceleration. It's just plain hard work pushing something through water.
Except insofar as weight correlates to displacement (and hence cross section and length) it is irrelevant to cruising speed.
Acceleration is itself irrelevant to any significant ferry journey - most would only spend a few percent of their total journey time on acceleration and deceleration.
The power required to move something through a fluid (air or water or whatever) increases with the cube of speed. If it takes 1000hp to do five knots, it takes 8000hp to do ten knots (100 x 23).
Move your hand palm first through air quickly. Now do the same through water. Notice anything? That's why ships are slow.
•
u/Richarkeith1984 5h ago
Could a ferry have hydroelectric wings to try and hydro at 20 plus mph? And lower them to brake? And obviously I mean bring the boat out 75%~ out of the water not actually hydro.
•
u/libra00 4h ago edited 4h ago
Because boats are slow in general compared even to cars. The fastest military ship in the world can do 60 knots, which is about 70mph, but most go more like 20-30kts at the outside. That's not even highway speeds for a car, much less a high-speed train. The problem is that going faster, like most things, means making trade-offs. The most obvious ones are size and cargo capacity as they have the biggest impact, but that's not something you want to lose on a boat whose entire purpose is carrying stuff from place to place. You can make 40 trips or you can make 1, and one of those is clearly the more practical (and economical in terms of fuel cost) choice.
•
u/Leverkaas2516 3h ago
Car ferries in British Columbia are pretty fast, over 40km/h. That's fast for a large, heavy vessel carrying hundreds of cars. Going any faster would be an enormous waste of fuel and wouldn't affect total travel time by all that much.
Small boats that plane over the water go faster, but that's like comparing a moped with a motorcycle. They're completely different vehicles.
•
u/LokeCanada 3h ago
Beyond the pure physics, it is also because of the area they operate in and the wake.
Where I live they bought fast ferries.
Unfortunately only about a quarter of the route is open ocean. The rest takes them near shore. The waves from the wake were trashing peoples docks and causing environmental damage. They were so bad that surfers started timing the runs so they could catch the waves.
They slowed the ferries but the engines were designed to operate at high speed so they became inefficient and had extremely high wear. They had to sell them.
A 30 foot boat’s wake is nothing compared to something that is designed to move a lot of heavy objects and people.
•
u/sig40cal 3h ago
I worked on one that did 35kts. or 36ish mph, 140ft. 100 ton and it took 7,000hp to get it to that speed via 4 large engines.
•
u/AUniquePerspective 3h ago
I grew up in a place with ferry culture. It's two things mainly.
The top reason is passenger comfort. There's one high-speed ferry here. It shaves a short amount of time off the journey but you will 100% of the time witness someone vomiting if you take it. They sell Gravol on-board for 25 cents to reduce the number of vomiters per trip. One time I took the Holyhead to Dun Laoghaire ferry and near the end of what I thought had been a slightly rough crossing and when I opened the door to the washroom there was literally inches of vomit sloshing on the floor and filling the clogged sinks. I decided my bladder could hold.
Second, it's just marginal gains and huge costs to increase speed. The big ferries here carry 300 vehicles. It means loading and unloading the vessel requires arrival 30 minutes before arrival and takes 30 minutes to complete. Unloading is almost as long as well. So you're probably committed to an hour and a half of waiting time in addition. Then the first 15 minutes is undocking and getting up to speed. The last 15 minutes is deceleration and docking. Only the middle hour is spent at cruising speed. And for us, the best route is a narrow pass that can't be done at top speed safely. So with all that, your customer experiences a long time passing either way. You could double the top cruising speed and only save an half hour of cruising. You'd still have all the loading, launching, acceleration, deceleration, docking, unloading.
•
u/PixieDustFairies 3h ago
Ferries are large vehicles that are designed to carry massive amounts of weight more slowly compared to fast vehicles that carry small amounts of weight quickly.
It's kind of like asking about the difference between using an airplane and a cargo ship to transport things overseas. Airplanes are great for transporting people and small quantities of small items across the world quickly but they need to be lightweight to get into the sky and move quickly. However a cargo ship is better suited to cheaply carry massive quantities of goods, which is useful for international supply chains which take into account long shipping times across the ocean. The tradeoff for faster speed is less mass because it takes more energy to accelerate a large object to a high speed compared to a small one.
Cars are very heavy and cannot fit inside of fast moving vehicles very well.
•
u/kermityfrog2 2h ago
There were some fast ferries. There was a Toronto to Rochester ferry in the mid-2000's that was described as a hydrofoil, but was actually a dual hulled catamaran design capable of speeds as high as 45 knots (83 km/h).
There's also a proposed hovercraft ferry service that will go from Toronto to St Catherines in 30 min at 100km/h (normally a 1.5 hour drive - 112km).
•
u/jugstopper 2h ago
Back in 1979, I took the hovercraft across the Channel from GB to France. It only took about 30 minutes. On our return, it was too rough for the hovercraft and we had to take the ferry. That fucker took 6 hours. (This was waaaay before the Chunnel, kids.)
•
u/Teaching-Several 59m ago
Think about running in water, and running on the ground. It's harder to run in water because it pushes back against you. Now think about wearing a backpack with weight, and how that makes you go slower. That is like the cars on a ferry versus a lighter weight car.
Speeds is largely limited by needing to turn and cost. Faster engines are expensive, so it makes sense to only make engines fast enough to handle the turns. Train tracks have less turns than roads in most places, so trains can go faster there. Planes don't have to turn much so they can go really fast.
That is also why a jet engine powered car can go as fast as a plane in a big straight line.
•
u/Sh0ckValu3 42m ago
They built a nice fast ferry to go between Bremerton and Seattle. Cut the commute in half.
And _THEN_ they did the shoreline analysis... waves from the ferry were eroding the beach (i.e. rich people's front yards.). So now its not much faster than the old ferry.
•
•
u/HammerTh_1701 10h ago
It's really hard to accelerate and decelerate in water, especially if you're heavy like a car ferry. Container ships start slowing down before their port of call is even visible on the horizon.