Why Are Big Ships Faster Than Small Ships? 

As a Naval Architect, I really wish I had access to these videos when I started my degree. Such a practical demonstration of the theoretical concepts.

Thanks for the explanation. I've often wondered the same. Years ago, I was on a 42' private vessel crossing the Panama Canal. The Canal obviously has too much traffic to let a puny motoryacht take up an entire lock, so the Harbormaster put us in with a cargo vessel -- large, but sufficiently short to accomodate us both -- for each leg of the trip. On the way up, the big ship entered first, and we behind her. When we reached the lake elevation and that ship fired up her engines, her prop wash was so intense that the only thing keeping us from getting bashed into the sides of the lock were the six lines the Canal staff gave us (delivered by the famous "monkey fist") that held us tightly in the center. Still, all open beverages were immediately spilled. When she got underway and we followed, I was amazed at how quickly she put distance between the two vessels. In no time at all, we lost sight of her. Of course she made it across Gatun Lake in a fraction of the time it took us. The Canal staff knew this would happen, so they had previously scheduled us for a departure with a different vessel. This time, her length wasn't an issue, as they put us in ahead of her, and her bow literally covered the space we occupied. You don't truly get a sense of the scale of these beasts until you have looked straight up from the deck of your boat and seen nothing but metal above your head.

I live 800 miles from an ocean, and yet I find your videos fascinating, including this one. Cheers!

I spent 23 years on aircraft carriers. Being retired, I now work around a lot of Airforce dudes. None of them believe me when I tell them carriers are the fastest ships in the fleet.

wow, I think this is the first time in my life, where someone used the term "wave length" and is actually talking about waves. (I am an audio engineer with a fable for physics, and I know, that those other waves are waves, too... but you know what I mean) And also, that was an utterly interesting video, thanks, I learned a lot^^

Those jet engines on the ship are 100% what I would do If Kerbal had sea fairing vessels.

4:40 "But of course that isn't the hull story." Proceeds to give talk about the Hull design. Nice.

I was thinking it was due to a ship 2* as long, wide, and tall having 4* the surface area (for drag), yet having an engine 8* as powerful. Didn't think about the wave interference. Good video

Really appreciate that this video is pure information. no sponsorship, no Patreon pitch, no asking to subscribe

I heard a story about a US navy carrier group deciding to have a drag race for amusement. As expected the smaller ships got the jump at the start, but it was not long before the carrier caught up and passed every other boat…much longer hull length.

I used to sail small boats as a teenager, and despite not knowing how any of this worked I remember intuitively feeling the phenomenon of hump speed and planing. Especially the boat 'falling' off the top of a wave, going from planing to hump speed, as a gust of wind dies off. Super facinating to learn how this actually works!

I've seen a boat running at "hump speed" before, a motor yacht. even to my untrained eye, it looked horribly inefficient, like the boat was trying to climb a hill.

You could easily expand this video with the hull shape aspect alone, ldl hulls are faster and lower fuel costs. a good example, an 85' loa motor yacht with a draft of 4 feet designed for displacement cruising. Top speed is 18 knots, cruising speed is 15 knots. 15 knots speed gives her 1,500 nm range on 1000 US gallons of diesel, 18 knots gets you 500 nm from the same quantity of fuel. These same factors are in play with the big cargo ships, yet most people can relate to being told if they slow down by 5 miles an hour they will travel 1000 miles further in their pleasure boat over it's cheaper to ship a 6 week trip than a 1 month trip for your overseas delivery.. *ldl = low displacement length.

I sailed on VLCCs in the '70s and their top speed was 15 knots. During the oil crisis we never sailed faster than 8 knots.

Wow. Great video. I’m completely land locked but totally in love with the ocean. Just amazing information. Mind is blown right now

Informative, no nonsense, and very well structured for my layman mind to understand. You run a phenomenal channel and are a great teacher. Thank you!

it maybe of interest: the bulbous bow works only at a limited range of speeds, because its wave has to interfere as designed with the normal bow-wave. Cargo ship hulls (including the bow-shape) are designed for a resonable range of cruising speed, to be fuel efficient. Intrestingly you can guess a ships speed by looking at its generated wave lenght at the hull. you only need to know how long the ship is, judge the length of the wave and use the formula depicted in the video to calculate the speed of the ship (through the water) ...

As a retired Captain now involved in ship construction I find this presentation fantastic. Thanks for sharing, keep up the good work.

Very happy about your new pace of publication, especially seeing that the quality of the videos didn't go down a notch! Good work!

also worth mentioning: the drag force is proportional to the cross sectional area of the ship. by the square-cube law, doubling the scale of a vessel will quadruple the drag, but allow 8 times the internal volume for engines to be put in. thus, if your engines take up the same amount of space, you'll have twice the power, and sqrt(2) times the speed

Beautifully explained. I’ve heard the waterline length rule touted for years and only recently learned it was due to a longer wavelength of bow and stern waves. You closed the loop for me, easily explaining that the wavelength was itself proportional to speed. Thank you!

Still awaiting that tugboat video. It'll be epic. Because tugboats are epic.

Brilliant as always. I just really enjoy how he doesn’t talk down to his audience. I learnt something watching this…thank you.

Excellent video, I heard the term "Hull speed" but nobody had a good explanation as you put it here as to what happens. Keep up the good videos and knowledge spreading you do!

I did not know the hull speed formula. It's nice to see it confirming my guesstimated speeds for my D&D ship's speed tho (130 ft 3 masted schooner at waterline, assumed her top speed to be around 12 knots. Which is less than hull speed, but considering wind and stuff it's within ballpark!)

Overcoming the hump speed is mechanically the same as breaking the sound barrier. You wffectivly turn a wave into a straight line, saving tons of energy. But it needs tons of energy to overcome the hump/barrier first. And even then it needs more energy then any speed that doesnt approach thr hump/barrier.

As an instructor of maritime topics, thank you for another video that makes me really think!

I really enjoy your work and efforts uploading these, not too technical and not too simplistic. 👍❤️🇬🇧

This was extremely informative! One of my favourites of this channel so far. Very well explained and so interesting.

"If you want to build a ship, don't drum up people to collect wood and don't assign them tasks and work, but rather teach them to long for the endless immensity of the sea" --Antoine de Saint-Exupery

I like how you answered the question again but this time with the context of everything we're talking about so the more literal explanation makes more sense. Great way to sum it up at the end of the video.

Going into this, I thought it would be a matter of larger ships having larger, more powerful engines. That's a really interesting cause, though, and it shows just how much thought goes into every aspect of sailing.

Perfect timing. I was bored so this is great

As a random guy on the internet I have to say this is absolutely facinating so congrats to y'all naval engineers who choose an interesting major

Nice. I had some inkling of what is working here, but having it neatly encapsulated like this clarifies my thoughts and fills in the holes of my practical observations.

I had assumed this was just going to be a fairly straightforward square-cube thing, where the drag on the ship scales with the surface area of the ship but the size of the engine it can hold scales more with the volume

Finally i understand why my uncle always told me it's better to go fast with the boat so it starts planing!

Informative video. Could you please cover the large, high speed, passenger catamarans invented by Incat in 1990 and developments of that design such as the Austal trimaran you showed at the end of this video? I've always been amazed that craft over 100 metres long can travel at up to 50 knots and still be commercially succesful.

Always a treat. Keep making these!

I went fishing a lot growing up with my Dad in our 19ft jet boat, I’d always wondered why the boat pitched and accelerated differently before the boat got on step, and how it would fall off step. this makes perfect sense

I was under the impression that the hull speed was the last efficient speed (assuming a slippery hull) before the "wall" of resistance you get as you approach hump speed, due the destructive wave interference outlined in the video. So in the graph it would be found at the bottom of the little dip rather than at the hill before it.

Great, now you've got me visualizing a cargo ship planing the waves like a speed boat by force of several strapped on jet engines.

It's been short aeons.....till the term "wavelength" made this much sense to me! Thanks a bunch👍

All the info I needed about something I have no practical use for. Subscribed

I love how these videos are extremely educational and are practically like having a real “ship” course.

Back around 1890 when they were building the first USS Texas battleship an argument ensued with the designer, the Bureau of Construction, and the actual builder. There were worries that the weight calculations of the ship had been in error and the builder suggested that the ship be made ten feet longer but the keel had already been laid (but could have been modified at extra cost). The builder said that the ship would have the added benefit of being faster as a result. However, the argument grew so heated about modifying the ship at this stage that the Bureau of Construction threatened to cancel the vessel due to the added cost...however the builder recalculated the weights and stated that while the lengthening could have added speed, the weights were not an issue so the project continued at the design and the Texas, although having an extremely long build time nonetheless was delivered on it's original budget to it's original design. As it turned out the USS Texas might have used an extra knot or two during the Battle of Santiago in 1898 but the Battle turned out well in any event for the United States.

Very interesting, and very clear! In case you were wondering whether a longer, more detailed video on this topic would find its audience, know that you'd have at least one enthusiastic viewer.

Every now and again the RU-vid recommendation algorithm comes up with a real gem. Subscribed.

I assume the trough shown in the resistance/speed chart found after Hull Speed must have a name? I don't think it is called "Sweet Spot." This appears to be where the ratio of fuel consumed and time expended in transit would be best for ships.

The fastest ship ship I've ever been on was the USS Carl Vincent. We were going WAY faster than the 32 knots Wikipedia says it capable of. Seriously. WAY faster than that. It was mind blowing. Aircraft Carriers can move.

Hull shape! When I was in maritime school, this crazy old ex-Navy captain used to tell us, "whatever the question is, the answer's usually hull shape." And it's pretty close to correct, although he didn't like it much when the least-mature adult in the class tried to turn that into a running gag. That ex-captain never could get me to understand hull speed, though, and I never really understood the physics behind planing and the way a small craft's bow rises out of the water at hump speed. Never heard the term "hump speed" before. I've never been able to find this stuff in the American Seaman's Manual, either. But thanks to Casual Navigation, it all makes perfect sense. This is a gem of a video. And then we have the bonus of a big container ship getting up on a plane, and then being fitted with giant jet engines. That made me smile.

Fascinating. Thanks random RU-vid recommend and the people that produced this excellent video!

"A ship is just a hull displacing water." Dictionary of Casual Navigation

Wait, so the local min is between Hull speed and Hump speed, but what is the effect of the waves on the boat that makes it face less resistance? It seems like you only talked about the speeds individually, but maybe I missed something I loved the bit about the bulbous bow, I had no idea that’s what that was for!

Your videos have real peaked my interest into boats and sailing. I've never had any interest before this! I think I'm going to have a ask a friend with a boat to take me out so I can learn more!

Great video! I have been fascinated with this since I was just a lad learning about the great America’s Cup yachts from back in the day. And about the simpler sloop I served on. :)

Now I want to see a cargo ship planing.

I guess something could be said about fuel consumption? I assume the smaller vessels choose to run at a more optimal hull speed because it makes sense financially. Unless, they have a customer willing to pay the extra price for express delivery speed?

omg, I always wandered and never understood why boats had that feeling during some acceleration/deceleration process... thank you so much!

Thanks for making these purely informative videos. So cool when mathematical concepts appear so clearly like this!

20 yrs ago, My wife and I took a cruise from Vancouver BC to Hawaii on Princess cruise lines. The ship kept a steady spead of 24 knots for 2300 nautical miles to Nawiliwili on Kauai. When we docked, I got a chance to speak with a ship crew member while they were fueling. He said that they burned 800 tons of bunker oil. I told him that the trip was really smooth. He said that they cruise at that speed because its about physics and the hull design. He stated that the ship could go a little faster than that but, it would burn to much fuel. While we were underway ftom BC, we passed a smaller container ship under the Matson flag. Everything that YOU have stated here is CORRECT. Thank you sir!

Me and my dad had almost identical boats the only difference was mine was 45’ where as his was 34’. For years how we couldn’t figure out why my boat was so much faster then his.

Keep up the great work! Love your voice! 😍

step one: get a big ship step two: apply for boat racing event step three: get rejected step four: say the a big ship is like a boat but bigger step five: they still reject you step six: give up

you might want to add, that to gain speed you need x times more power, that is a bigger reason why speeds are low. as for a 150 meter ship hull speed will be 29 knots but yet they sail 15. half of it. Also if you sail hull speed, with a wave top at bow and stern and a trough mid ships, you loose alot of stability. (not to mention bending moments that come into play at such situations)

As a mechanical engineering student I found this video super interesting! Love your content!

I live very close to the St. Clair river and frequently watch the Great Lakes freighters over 1000 feet in length and the salties (smaller ocean ships) down to just a few hundred feet as they travel up and down the river and into the Great Lakes. For years I have wondered about the wave pattern as some ships would create a massive wave at certain speeds while others moving at the same speed would produce hardly any waves, yet the same ship would then produce a huge wake at a slightly different speed. I just assumed that it was cargo weight that accounted for this. Usually I notice 3 wake patterns as one comes from the bow, another set from mid ship and the last from the stern. This answered a lot of questions for me. Thanks for the video.

On a related note, how much of a large ship's drag is from wave drag vs. viscous drag against the ship's hull? I always got the impression wave drag is the dominant factor. Thinking of viscous drag would be another factor in favor of larger ships, at least in terms of efficiency. More volume (mass, cargo volume, displacement) per surface area, assuming a similar shape.

Strap on a few jet plane engines lol

Excellent video demonstrating theoretical concepts. Well done, wish I could like it twice!!

I have learned much and will now calculate the optimal wake surf speed for my boat thank you.

Has anyone taken a closer look at ancient sailing ships specifically from around Greece, Rome...basically the Mediterranean area? I noticed that some of those ships had a bow with a 90° square to the natural water flow where a bulbous bow should be. Kind of like a stop sign of sorts, only under water and having 4 sides instead of 6. That big under water square bow seems counterproductive to me but maybe the ancients living in the bronze age knew something about fluid dynamics that are simply beyond me.

The big'uns generally tend to have a faster 'hump speed', in my experience.

I had to laugh out loud when I saw that massive commercial cargo vessel using the engines to start "climbing"!

My favorite example of this is the j class sailboats which have a design rule to limit waterline length but adapted by adding overhangs that enter the water and increase waterline when the boat heels over

Does a similar thing apply to airplanes? For example, the 747 max speed is faster than the smaller 737.

25 knots? Jesus, that's faster than many naval capital ships

Really fun windsurfing..when you are planning the first time you think something is wrong as all " water " resident is gone ..lots of fun now .. 😉🐿️

Your videos are diamonds! Thank you!

Brilliant

Awesome video from a fantastic channel. Thanks

Well there's an answer to a question I never had, I appreciate this new knowledge.

Thanks! That was information that I didn’t know I need till I knew it

Really informative video! I happened to literally ask myself this question recently lol

I have no idea how I got to this video or why, but it was quite fascinating!

Naval architects in my country had the adage: _Länge läuft_ (loosely translates to length runs)

This has to be your best video yet!!

This video was very informative. Great work

Really cool explanation ! Thanks !

I don’t even like ships, they scare me, but i’m such a geek that i can’t help being fascinated by all the technical elements and math involved. These video’s are amazing for people with a similarly geeky disposition as my own

Amazingly well explained!

A great explanation! Thank you!

Just from the thumbnail, I can tell it's going to be something to do with the "motion of the ocean". ;-)

Learn something every day, thanks, worthwhile and educational :)

Perfect explanation. Thank you

Good instructional flick for those students and the odd interested outsiders. Getting more tricky handling mass at slower open ocean speeds New England through Midlantic with the new speed limits.

Well that was interesting, thank you for the great video!

I'm not in any kind of naval anything yet I watch every video over time haha. I'm a pilot and I watch ship videos 😮

Another great one, cheers

Awesome explanation!!!! Thx