Why Do Fat Ships Wobble?

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------------------ABOUT THIS VIDEO------------------
In this video, we investigate why fatter ships appear to "wobble" while thinner ships tend to go in a straight line. It's a concept known as Directional Stability and is really interesting once you start looking into it.
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All content on this channel is provided for entertainment purposes only. Although every effort has been made to ensure the content is accurate and up to date, it remains the responsibility of the viewer to determine its accuracy and validity. The content should never be used to substitute professional advice or education.

[@CasualNavigation]

Thanks to Ground News for sponsoring this video. Visit ground.news/navigation and subscribe for unlimited access.

[@Wockes]

2 weeks ago, first by a mile

[@ground_news]

Another informative video Casual Navigation! Thank you for partnering in making our mission more accessible. For those who want to be better informed and compare media coverage, check out the link in the description.

[@jmannUSMC]

Didn't expect to see an ad from Ground News. I really like what they're doing

[@misterhat5823]

Seriously? An ad in the MIDDLE of a six minute video. Ground News is going on my list o' feces.

[@mytech6779]

Don't forget prismatic coefficient. It is the 3d equivalent of the block coefficient.

[@johnstobie1163]

i didnt come to Casual Navigation to feel personally attacked lmfao

[@shietnewfeature.delethethis]

I hate it when i get called a fat ship with no directional stability :(

[@miro007ist]

Being reminded that you’re obese is not a personal attack.

[@suleymanbabak1973]

@@miro007ist he's joking.

[@OfficialSamuelC]

@@miro007ist Wow. You must be fun at parties.

[@HSamee]

Getting insulted within the first minute. Brutal. 😅

[@arthanor9631]

I'm a huge fan of your videos and how informative they normally are. This one, however, is the first one that leaves me with an unanswered question. Why do fat ships wobble? Because their LB ratio is low...? That's kinda just saying a fat ship is a ship with a low LB ratio, which is pretty obvious. But how does a low LB ratio make a ship less stable? Something to do with water pressure on sides of the hull? An explanation as complete as in your ship speed and efficiency video would be quite welcome!

[@boklasarmarkus]

Yeah, I was surprised he didn't go into more detail

[@daftbence]

I essentially think of this as wheelbase with cars. A shorter wheelbase means the car is more nimble, but equally more twitchy, e.g. doesn't go straight at high speeds as well as a longer wheelbase car. Now since ships don't have wheels you can't really say wheelbase, but you can have the ratio of the length:width and voilá (you could just have the same thing with cars and it would work the same). This still might not/doesn't answer your question, but it might just be more intuitive, at least it is for me. Edit: wider cars can be more stable as well, so it's not the same as LB but is similar enough for me to consider it is an analogy.

[@jirivorobel942]

He would have to get into how the center of lateral pressure moves around and how far ahead of the center of mass it is in each ship, and what the C of (L) P is, which would probably double the length of the video.

[@arthanor9631]

@@jirivorobel942 but overly detailed explanations of maritime concepts I don't really use is exactly what I come here for. This was more: are shorter ships easier to turn? Rather than: why fat ships wobble

[@OfficialSamuelC]

Yeah this video was considerably lacking unlike his usual videos. Not great when about 20% of the video is an ad.

[@whoeveriam0iam14222]

okay but why does this happen

[@Daniel95221]

Its the same thing with old tanks, the length to width ratio determines how hard it is to turn and also how sensitive it is to do smal corrections while driving in a straight line.

[@ejaurand]

I always appreciate that you include the occasional nod to the great lakes ships. I would love to see something on our thousand footers. And the old whalebacks are my favorite.

[@terryboyer1342]

Edward Aurand 👍😁 Love the Roger Blough and Presque Isle!

[@jasonreed7522]

I grew up near the Seaway and its truly a marvel of engineering that doesn't get enough appreciation. As a kid i knew it was used for trade but i never realized that the Lakes and Seaway enable inland cities like Toronto, Chicago, and Deluth to be ocean teir port cities with none of the downsides of being on the ocean. (Military/strategic, climate change, normal storms, or even just dealing with the tides) Its also crazy how you can literally go to the locks and spend all day watching the ships for free and nobody says anything.

[@terryboyer1342]

@@jasonreed7522 The big downside was all the invasive species that have been introduced to the Great Lakes. Very detrimental.

[@jasonreed7522]

@@terryboyer1342 invasives are definitely a problem with global shipping, and as great as a 2GW hydropower dam is for emissions, its also really bad for the fish. Another thing thats bad for the ecology is how we connected the Mississippi to lake Michigan via Chicago. While it lets us sail all the way around the east coast without going around the west coast (kinda meme thing to do), it also means any invasives in the Mississippi can get into the Lakes. (I believe asian carp and possibly a lamprey species are the big concerns right now that we want to contain to the the smaller Mississippi) PS: the Mississippi carries less water by 6m^/sec difference in average annual discharge. (Bigger drainage area, less water, not sure if human consumption is accounted for in the number)

[@jmdibonaventuro]

I recreated one of those whalebacks in the game stromworks once. Very interesting designs, those ones!

[@PauxloE]

For a cruise ship, having a larger length-beam ratio also helps to have a greater ratio of passenger cabins with windows.

[@cosmoelpresidente4019]

As someone who lives on Lake Superior, it’s always nice seeing some Laker appreciation

[@pennycarvalho1223]

That’s why battlecruisers (a lighter but faster battleship) we’re typically longer than their battleship counterparts yet still lighter, bc they were a lot longer but thinner, therefore making them able to achieve greater speeds but it did cost in agility as trying to turn a long plank of metal half submerged can be kinda hard

[@PianoKwanMan]

I cannot unsee the surprised face, with a pink hairdo, on the bow of the Great Laker at 4:20

[@HSamee]

I'm not chunky, I'm just big beamed.

[@UstedTubo187]

I feel like I missed the reason why "fat" ships wobble. I know he mentioned how the different shapes of ships act differently, but I don't recall hearing why a fat ship wobbles and why a skinny ship doesn't.

[@TitanicTours27]

directional stability, the bigger ship needs more rudder input to keep it straight which could look like the ships wobbling.

[@patricknorton5788]

@@TitanicTours27 yes but, why? We just heard that the fatter hull has less directional stability, so we know it'll need more rudder input, but why?

[@Eric14492]

​@@patricknorton5788 An analogy is how a figure skater will spin much faster when they bring their arms close to their body, and slower when their arms are straight. It is not the same mechanism, but the effect is the same. A longer boat has more water to push sideways to turn, further from the center. This takes more effort.

[@patricknorton5788]

@Eric14492 that makes some sense (and I have experienced the difference and trade-off between maneuverability and efficiency/tracking ability in using short and nimble/or long and fast kayaks and cross-country skis). The point was, though, that the video didn't explain it, even if you can.

[@jek__]

fluid dynamics 🤷 something to do with how higher length-beam ratio ships act as a longer lever, so the water on the sides has more impact on the motion of the boat than the water at the front and back? I dunno

[@wreilly09]

Maybe I missed something, but this didn’t seem to answer the question of why they wobble. Why do they continue to “steer” even after the rudder has been recentered?

[@RealCadde]

Maybe they are essentially aquaplaning? A knife will cut through water and find a direction to go in, whereas a baseball bat will go whichever way its momentum takes it.

[@tarekihaddaden9512]

You can experience this effect really well in a kayak. Those long skinny kayaks are really easy to cruise with and keep straight. The more stout and wide kayaks, while better in rough waters because of their lateral stability, they are constantly moving left and right as you paddle along. And if you try to go really fast in a stout and wide kayak it has a tendency to turn so much that you lose control and spin out, which can cause you to capsize.

[@Av8rwaka]

Great video! This is similar to aviation in that dihedral wings (curving upward) like you find on airlines require more effort to turn, whereas andihedral wings (curving downward) like found on large military aircraft want to turn, thus being more maneuverable.

[@samuelmerrill3392]

is that why I can't walk in a straight line?!? love the content! always love the detail, keep it up!

[@nadavm.6843]

There are even larger ships on the Great Lakes that are designed to the limits of the Soo Locks. But those can't operate out of the lakes because they can't make it out.

[@USAltefore]

Now I'm imagining a tuba player on a tug boat playing background music for a wobbly fat ship.

[@karlwolf9805]

Great video as always. In the future could you include explanations for why things like "fat ship wobble" and "chunky ship wobble" occur? Those are always what I found most fascinating in your videos.

[@Enzo_Gaming00]

Great Lakers, and I loved how you used the Edmund Fitzgerald by the looks of it

[@cr10001]

I've felt the same thing on a kayak. It just didn't want to go straight, just paddling one stroke each side alternately wouldn't keep it straight, you constantly had to give more strokes on whichever side it was trying to turn towards - and be ready to swap to the other side the moment it began to swing the other way. I think it's because the streamline flow at the rear prefers to flow diagonally one way or the other. I've also heard that certain airliners do the same, they tend to crab slightly, albeit they have a whacking great tail fin to restrain the tendency.

[@kevin-parratt-artist]

I found this when rowing. A long boat is easier to control than one that is shorter and wider. There was no rudder on either of them. Granted, the longer boat had a deeper keel, and cut through the water beautifully.

[@40arpent]

So I've been to an island wedding on the St Lawrence. The rumor was that the lock operators would time it to where a cargo vessel would pass by during the wedding.

[@daftbence]

Having the QM2 in a vid of yours always makes my day! I loved working on that ship :)

[@narri214]

Sea kayak vs recreational kayak makes this very noticeable. The tracking stability of a sea kayak is very noticeable compared to a recreational boat, which often turns direction every time the paddle is applied to the water. Even more extreme a white water kayak is short and stubby to make turning as easy as possible.

[@cr10001]

Yes I noticed this the one time I used a kayak (in a calm lagoon). Very frustrating, the bloody thing just didn't want to go in a straight line at all.

[@Bhoenix]

Ok but why do Length to Beam Ratio and Block Coefficient impact Directional Stability? What’s the mechanism here? I feel like you didn’t answer the question in the title tbh

[@TarenGarond]

Video title : "Why Do Fat Ships Wobble?" Answer in video : "Fat ships wobble." XD

[@beeble2003]

@@TarenGarond Actually, the "answer" in the video is that fat ships wobble because they're fat, since "low length-to-beam ratio" just means "relatively wide for its length", i.e., "fat".

[@Eulemunin]

Having grown up on Superior, seeing Lakers used was fun. But the block coefficient and L-to-B make an interesting set of parameters to design around.

[@ryanlegrand141]

I think this video could use a follow up diving into why exactly the fat ship wobble happens. I wonder the physics are similar to the overbanking tendencies or adverse yaw found in Aviation?

[@TheRalliowiec]

Most "Lakers" don't traverse the St. Lawrence locks as that would make them "Salties". But they do traverse the Wellad Canal and the "Soo" locks so the constraint still applies.

[@derHutschi]

great video during my time in subscripted military service I spent a lot of time on the wheel (vessel was 50 metres long) it was mostly thinking 5 seconds ahead to keep it on course :D

[@punkypink83]

fantastic vid. where i work we have 2 boats, one is 55x12 ft the other is 57x6'10" most people have definitely found our narrowboat to be the easier to drive of the 2. they've put it down to the narrowboat being on a tiller while the widebeam being on a wheel, but i reckon the fat ship wobble plays a part too

[@Roytulin]

Huh this is interesting. Aeroplanes have similar performance characteristics so it’s interesting to know how attitude stabilities affect ships as well. In aviation, we say that planes that keep increasing in the relevant attitude axis once control input returns to neutral have negative stability on that axis, 0 if it maintains its attitude, and positive stability if it returns itself towards the flight trajectory. Negative stability is primarily used in fighters.

[@thomasnelson5010]

I never would of imagined there was so engineering involved in building and operating ships!!!! Thank You!!!

[@ryanbekkers3423]

I love your channel as an Engineering Cadet!

[@stilltoomanyhats]

"I like low length-to-beam ratios and I cannot lie, you other skippers can't deny"

[@benjocaz45]

This explains so much about my skiff, my new one is wider than my old one and it just won’t hold straight

[@jamesday7339]

A simple explanation as to why that happens. You have centre of mass of the vessel and centre of propulsion as well as a centre of resistance which are all in a straight line when the ship is moving forwards. Ships are rear steer objects so the centre of thrust moves out of line which then presents a bit of the beam to the forward flow of water across the hull effectively moving the centre of resistance across in the opposite direction so the vessel will continue turning about a centre of rotation usually near the centre of mass. This happens every time there is a rudder input. This happens more for wider vessels as a certain angle of turn causes a greater movement in the centre of resistance. Long thin ships tend to get stuck with nice flows along both sides of the vessel which are hard to break so difficult to imitate turns. Both of these problems are solved by proper rudder design and a ship which is excessively prone to either of these conditions could be rejected by the customer at the sea trials stage. The ship with a high block coefficient has the wobble problem as it is effectively a brick in water and the forces acting are higher for a comparable vessel of same length&beam. As a naval architect you can spend months conducting operability and control assessments on a design in CFD and other simulation softwares before designing your rudder.

[@alex_zetsu]

I kind of wish before showing a rudder corrected course you showed us a simulation of a ship with a length to beam ratio of 3 making a turn and then keeping the rudder in the center to let us see the constantly changing direction.

[@saladiniv7968]

this just made me remember that the popovs existed. late 19th century russian warships with a length to beam ratio of quite literally 1.

[@Meddlmoe]

As an aerospace engineer, I suggest to increase the yaw stability volume. Maybe itroduce a deployable vertical stabilizer on the long distance legs in order to reduce fuel consumption and crew workload.

[@andrewnagl]

This is precisely why ikea carts are so difficult to handle

[@alphaadhito]

Lesson of the day: Titanic would've miss the iceberg if it is chonky

[@KitsuneBleu]

I live so far inland and have no personal or professional need to know about shipping. WHY AM I OBESSESSED WITH THIS CHANNEL?!?!??!

[@amandahugankiss4110]

This may well be true. We all want that long fat ship though.

[@tonifakerman9639]

This is amazing, these same concepts are fully in play in the world of motorsports as well

[@Cyberguy42]

6:05 "ships with a lower length-to-beam ratio... are generally faster and more efficient" I'm confused, I thought we just learned that higher length-to-beam ratio was more efficient due to not needing to use the rudder as much when traveling straight

[@beeble2003]

Yeah, I think he mis-spoke there. Should be _higher_ LB and lower block-coefficient.

[@mellissadalby1402]

While you are right about the Great Lakes Lakeboat bulk carriers that can go through the Saint Lawerence seaway, there are a number of "footers", which ar 1000+ feet long and hence "lake bound". Just adding detail to your story, not trying to argue.

[@WalterBurton]

Your narration and (importantly) the choice and volume of the background soundtrack is pretty damned good. 👍👍👍

[@SigzTT1]

Ok but why? You just gave it a name. It is quite analogous to transverse stability. The hidrodinamic force created in a turn lies far fwd for a fat beam and have a bigger arm increasing this turn. It is the same as if the ship was trimmed by the head

[@KillingDeadThings]

I've been on a ship once in my life. I'm 44 now. In all likelihood I'll not be on one again in my lifetime yet, here I am watching your videos.

[@bcfairlie1]

I found this episode very interesting. I had no clue that this is a thing. Now I know. Even though I still don't get most of the math

[@MrRodgerMoore]

My boating experience is very limited yet my enjoyment of these videos is very high!

[@strakhovandrri]

"This is 9/10, but..." Nice one.

[@robbabcock_]

Fascinating! I'd never heard of such a thing.

[@jamesneuman7083]

It's kind of like a bicycle vs a tricycle at high speed. You'd think the tricycle would be more stable but at speed the bicycle has it beat

[@thechumpsbeendumped.7797]

There’s a similar phenomenon with smaller vessels such as dragon boats. The 20-man version has a ratio of roughly 10.5:1 and is relatively simple to keep straight and inversely difficult to turn, however, the 10-man boat has a ratio of probably less than 7:1 and is a pig to keep in a straight line and likes nothing better than to zigzag down the course, given the slightest provocation. Ask me how I know.

[@lianvitos]

So, how do you know that?

[@alwaysbearded1]

I'll have to watch them next time I'm out sailing as I see them in the estuary training. They do also have boxy, ungainly looking hulls. I don't know if adding any lateral surfaces like a keel or just aft a skeg would help. It would add drag and wetted surface. Looks fun. I should try paddling a dragon boat someday.

[@thechumpsbeendumped.7797]

@@alwaysbearded1 I’d recommend it. It’s a great way to get fit and meet people and in the 8 years I was involved in the sport it took me all over the world for social and international race events. What part of the world are you from?

[@alwaysbearded1]

@@thechumpsbeendumped.7797 Got too many irons in the fire as they say now. Got the boat I sail on, the house, car.... busy life. Maybe when I retire in a few years.

[@William_Bryant]

Seawise Giant casually steaming with a tuba playing in the background.

[@YouTube_user3333]

There are many optimal ratios that go into engineering a vessel that runs stable and efficient. Beam to length ratio is probably the most basic of all.

[@jek__]

It strikes me that the ship with the lowest possible block coefficient would be a four pointed star, which would probably have terrible directional stability even with a high length to beam ratio lol

[@Hamsteak]

I see the Great Lakers all the time. I live right on the Welland Canal

[@noscopesallowed8128]

Tugboats are just the most adorable things from a top view lmao

[@jamesgcrawford]

Lakers don't run the St. Lawrence River, they stick to the lakes AFAIK.

[@zdb79]

I imagine that in a certain way, there is the angular momentum. When turning a ship must also add angular momentum to its own referential.

[@keatoncampbell820]

Ok so according to rudimentary math I've based on this, the most efficient hull shape is a 1 dimensional line, with a ratio of inf, and the most maneuverable hull shape is a horizontal 1 dimensional line, basically just the first one but perpendicular, as it's ratio is infinitely close to 0

[@adzib1823]

Mathematically, maybe. Physically, probably not. Think about it: in your second case, you've still got to rotate essentially a metal plank through a load of water. If we're talking 'as the limit of x approaches y' scenarios, orientation doesn't really matter anymore. I'd go so far as to say an LB ratio of 1 is the least directionally stable. I agree with your first statement though about the most efficient hull shape.

[@whatever57010]

so why do fat ships wobble? the only question that made me watch this and I got no answer...

[@applesthecat]

I just wanted to tell you that the thumbnail of this video combined with a couple of others in my recommended more or less just said “too fat? HORRIBLE FATES. Too Late!” With an ad for sonic corn dogs right above it. It certainly made me laugh.

[@applesthecat]

It also had a thumbnail right below it with a speech bubble that said something along the lines of “come on do something”

[@roceb5009]

My ship don't wiggle wiggle, it floats...

[@nehemiahmathews8683]

A 4-minute educational movie in 3 minutes of it is commercial

[@christopherg2347]

As I understand it, there is similar stuff with Airplanes. The average Airliner wants to stay right level, flying straight. Which makes them harder to turn, but more more fuel efficient in cruise. While combat aircraft are designed less stable - to actively unstable like the Eurofighter - as that increases maneuverability.

[@JeffreyHeydenKaye]

You might as well start exploring spaceship movement. You explain the concepts so well.

[@shingshongshamalama]

Is that why the Iowa-class have those enormous snouts so they can be really thick and chunky to fit those massive gun turrets on but still maintain a low block coefficient and improve directional stability at their cruising speeds?

[@nicopostigo123]

I always learn something new with your videos man!

[@robinj1052]

Excellent video, as usual. Nice to see the block coefficient explained. You don't come accross this term very often, but I used it a lot when I was designing fender systems in a previous live.

[@4ofwands]

Thanks for the explanation! I have a science background and the ship dynamics you described are clear to me as I took mechanical physics course. I’m thinking that describing the ship itself as a rudder and how the efficiency of that effect is affected by certain variables may be a good metaphor to explain. What do you think?

[@beeble2003]

But there wasn't any explanation! All he said was that fat ships wobble because they have a low length-to-beam ratio. But "fat" just means "relatively wide compared to its length" or, in other words "has a low length-to-beam ratio". So, in ordinary English, the video just says that fat ships wobble because they're fat.

[@KenNeumeister]

also, implications for navigation through stormy conditions, high and variable winds and waves

[@arcenaljohntimothy]

So glad to actually understand the real life scenarios of the things I just discussed in scho

[@andrewp6387]

I'd love more Great Lakers information from this channel. I noticed that the model of thr Laker looks like the Edmund Fitzgerald

[@AlRoderick]

It could very well be a reference but also the Edmund Fitzgerald is still considered a pretty modern ship by Laker standards, she was not terribly old when she sank so they haven't actually made too many more since then, if she was still afloat she'd probably still look pretty current.

[@Eric14492]

A good example of this is the difference between a white water kayak and a flat water kayak. Most people are familiar with flat water kayaks. They are long and thin, so they track very well going straight but are harder to turn. You may have seen a white water kayak in the Olympics or on some KZbin channels. They are much shorter, so they are highly maneuverable. This agility allows you can get around rocks. You can actually turn standing in place. An analogy is how a figure skater will spin much faster when they bring their arms close to their body, and slower when their arms are straight. It is not the same mechanism, but the effect is the same.

[@cr10001]

My one experience of a kayak (on a calm lagoon) must have been a white water kayak, since the bloody thing just *would not* go straight. It was just frustrating. Absolutely the wrong place for it.

[@Operngeist1]

So what I learned from the video is that fat ships and chonky ships are not the same.

[@aqueousone]

And the question of “why” remains unanswered.

[@Legendendear]

I guess thats because thin, long, pointy ships have the advantage of water "guiding" the way of the ship?

[@NovaAge]

When zoomers become naval engineers: "how the hell does your ship turn so fast!?" "L+B ratio scrub"

[@JamesOKeefe-US]

Ahhh, fat shiP....I was so confused. Definitely want merch for this :) Love this channel!

[@Janos0206]

That's a great sponsor!

[@theendofmyropemydude]

This video was only missing tuba noises every time a fat ship tried to turn

[@BimDaTitanicNerd]

I feel personally attacked

[@iceslayer777]

Very interesting to say the least but it all comes down to what the ships purpose is and whether you're trying to maximize one thing or another.

[@vincentwondermensah2710]

Ships with mid ship propulsion system has the worst case of directional stability

[@marcellalewis8766]

Nobody is gonna talk about how he managed to get the fake funnel on the titanic wrong?

[@benjif2424]

Why ignore the third main factor (arguably the first), the keel? I live in Tübingen where we have the "Stocherkahn", an unique riverboat pushed by long wooden rods. It is completely flat on the bottom and thus drifts a lot (which can be exacerbated by wind or a strong flow)

[@beeble2003]

Your "unique" boat sounds exactly like the punts that you can hire in Oxford or Cambridge.

[@iankynaston-richards883]

I was about to say... sounds like a right punt!

[@benjif2424]

@@iankynaston-richards883 and a frigate sounds like a destroyer. There both ships with a motor.

[@timothyprice9064]

So ships are like cars. A short wheelbase is great for curves but it can be twitchy, while the long wheel base is smooth and steady but turns like a pig.

[@ericcriteser4001]

Fascinating. Thanks for sharing.

[@flavvsdasilver6442]

Ohhhhh! "Ship" "fat ship wobble" With a "p", not describing my desperate attempts to get to the loo after a heavy chilli salsa night

[@Gamatech123]

Okay, this explains the maths really well- but what about the physics? What stops or maintains their momentum during a turn? I thought we'd learn about what physically causes the phenomenon...

[@neondemon5137]

I thought I was drunk, guess the boat was just a fat wobbler.

[@CrankyPantss]

Very interesting. Thanks for sharing this with us.

[@klugshicer]

That as progress bar definitely doesn’t move uniformly!

[@Alexx120493]

With a design like a WW2 battleship I wonder what effect prevails. Bismarck for example has a length to beam ratio of 6,71 but a low block coefficient with its double ogive hull shape.

[@garrysekelli6776]

A fat ship is like a portly gentleman. He tends to wobble.

[@mithral07]

I wonder how this applies to catamarans. My buddy has a 55ft catamaran that has a beam of 29ft. Each hull is stream lined and only gets to about 7ft wide. We were having trouble keeping her in a straight line and were going to see if there was slack in the steering. Wonder if it's just the boat dimensions.