Why Do Fat Ships Wobble? 

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i didnt come to Casual Navigation to feel personally attacked lmfao

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!

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.

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.

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

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

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?

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.

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.

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.

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

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.

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

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.

okay but why does this happen

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.

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

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.

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.

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

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

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.

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

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.

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?

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.

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

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

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

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.

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

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.

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.

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

I've never steered a large container ship. Thanks for the explanations. I always see them under fine control when attended to by tugs. I'll have to watch them in the Bay and see if I can detect the wobble when they are not in tow.

I love your channel as an Engineering Cadet!

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

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

Very interesting. Thanks for sharing this with us.

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

I always learn something new with your videos man!

Dude you’re more reliable than google when it comes maritime info

This is super evident on leisure crafts such as Day Cruisers that wobble like there’s no tomorrow having an LB of 3 or so.

Fascinating. Thanks for sharing.

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.

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

Very informative content⚓

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.

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

Love the videos

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

That's a great sponsor!

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

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

I did not quite understand your explanation of the block coefficient, could you perhaps review that again in a future video?

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?

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

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.

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

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

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

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.

This is precisely why ikea carts are so difficult to handle

Never thought a video on ships can be useful for scale modelling

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

Hey, love your videos as do my kids. This made me think of the Norwegian navel and seismic survey vessels the Ramform design. Flat bottomed and 150m long and ~70wide for the latest Titan class. The previous classes were narrower and were supposed to be stable but rolled massively in the slightest long period swell, the tank tests they did for designing clearly missed something... fun but not that fun to work on for weeks at a time. If you ever wanted to do an episode on them I worked on them for years so happy to answer questions. The Banff was a ship oil storage facility built before the seismic vessels that cost millions to fix due to not being stable, and same mistake went on with the first seismic vessels, could be an interesting story to cover all of them.

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

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

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)

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.

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

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

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.

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.

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

Ok but why does the length to beam ratio and block coefficient affect directional stability?

Sounds like an extra factor that was at play for the Evergreen, outside of the positioning on the canal?

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

Water an air work in very similar ways, i think a good way to illustrate this is to imagine a ship like a dart or ball Ships that have a larger LB ratio are more like darts, the center of mass of the vessel has a large surface behind it to create drag and self stabilize the ship. Just like when you throw a dart, the fins keep the dart straight along the arc it's travelling on even when the direction it's pointed is disturbed On the other hand Ship's with lower LB ratios are more like balls, there's less of a surface behind the center of mass to act as flukes, and when you throw a base ball or basketball it very easily spins in the air, and In the case of a baseball this spin can very easily change the direction of the ball (though that's a different effect entirely)

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.

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

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.

on small leisure sport cruisers (8-12m) at non planing speeds you also need to steer a lot. is that the same reason? those are normally having len/beam ration about 3:1... those are normally at water line rather pointy (low block ration). do those principles translate to those small boats?

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.

More ship design videos!

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

How does this translate to yachts? Many modern sailing yachts have a shape that looks like a clothing iron from above, and quite flat bottom-Hull. Or does the keel counteract this a lot?

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.

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

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.

It is observed easily in the ship simulator as well especially during mooring.

So it IS about size and shape over "the motion of the ocean"! I've been lied to!

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?

Alot of this seems to also be assosiated with trailers. A smaller trailer might be 12ft long and 6 ft wide. They turn insanely fast but hate going straight, your often seasawing the wheel to get it to go straight back. However a 30ft trailer that's 8 ft wide will be tough to get started turning but keeping it straight is much easier. I don't know if they're related but it seems they are. I have an additional question as well though. Does a single screw ship of the exact same build with this fat wobble turn easier then a ship with two screws assuming everything else remained the same, including thrust, rudder size, etc?

How do you make your videos ? Which software do you use ? I have a college project so asking for the same reason.

How does this work for catamarans? A 20m long catamaran with a beam of 10m and 2, 2m wide hulls - is the length to beam ratio 2, 5, or 10? Does it wobble?

Where can I get that Ship turning simulator in the start of the video?

How does a twin Hull catamaran experience the ratio and coefficient?

That as progress bar definitely doesn’t move uniformly!