Older Finnish icebreakers have 4 propellers, 2 at the bow and 2 at the aft. New icebreakers have have 3 pods and one of those is located at the bow of the ship.

Just a tip: having the two sides of the propeller blade be different shades makes the 3D effects a little easier to read. As it is now the blade just disappears when the tip is facing you. Edit: sooo many typos

One additional factor. With the propeller at the bow, the rudder is in the way of the driveshaft, making beveled gear necessary to wrap the shaft around the rudder, reducing efficiency for all types of ships.

I must say, The Little Captain is ridiculous adorable and reasonably priced. I want him! 🤔 Maybe a birthday gift to myself

I love that your videos always start relatively simply with things that I might already know or are very intuitive. By the end you've always taught me something new or taken the discussion in a direction I hadn't anticipated! You're a great teacher.

Wow. This video makes great sense in every little detail! Sometime ago I expressed my enthusiasm about the azipods to a friend of mine who was a mariner. He cooled me down abruptly, saying that they prefer the electric motors installed well deep into the ship, with magnetic couplers and big axles that they can actually see and service en route. If an azipod fails in the middle of the Pacific, they must limp at a quarter steam to their port of call; if a traditional power plant fails, they can always fix it - with the exception of a broken axle. I like the compactness and manoeuvrability of the azipods nonetheless. When I was a young kid, I had a RC boat with two axles and two propellers, and a propelling pod wasn't even a concept at the time. Thank you again for the great video. Greetings, Anthony

I would have liked a little more contrast with the example of aircraft given in the beginning, which usually prefer tractor over pusher configurations. As it is, I think that case provides counterexamples to most of the reasons given here: -Aircraft don't mount any control surfaces directly behind propellers, as they prefer "clean" (less turbulent) airflow over the control surfaces for predictable, consistent maneuvering. (Even rudders at the end of small aircraft are negatively affected by the rotating prop wash from a nose-mounted propeller) -An aircraft's body does not block the thrust of the propeller mounted in front of it, as it is designed to be smooth and aerodynamic. In fact, the cleaner airflow at the front improves the propeller's overall efficiency (As far as I know, this is also the reason to commonly mount jet engines on the wings instead of at the back of the fuselage) -Aircraft prefer tractor over pusher configurations, i.e. tension over compression in the axle/shaft and bearings. I find this intuitive, given the high tensile strength of steel (and most metals?) compared to its lower compressive strength combined with possible buckling limits. This leaves two reasons: protection from damage, where the analogy to aircraft clearly does not apply (except for gear-up landings maybe), and easier sealing (where I'm surprised the additional pressure is significant compared to the head pressure, but my intuition may be wrong here), both of which do not apply to azipods mentioned at the end? I don't intend to come across too negatively, I like this channel, and this is an interesting topic I had not come across, I just feel this video has not entirely answered the question in the title to me. Then again, I know much more about aircraft than about ships, so that might color my understanding here

It's also worth noting that there ARE multiple airplanes with the propeller(s) at the rear, and the same with jets.

The Joint Marine Systems Ghost uses forward mounted propellers to cause supercavitation around the hulls to reduce drag

The only instance i can think of where the prop is at the front is trolling motors for small bass boats. Considering how they are used and how they can be retracted to avoid damage, they are probably more analogous to maneuvering bow thrusters than bow mounted main screws.

And on the other side of things aircraft propellers are meant to pull and on multi engine aircraft the benefit of having front propellers that are wing mounted is that it provides artificial airflow over the wings making it more resistant to stalling

The little captain is love, the little captain is life.

Airplanes with front propeller reasons include better cooling . Prop planes with regular or diesel engines get intense airflow over the engine to cool it more than if the prop was behind. Its not always vital and turboprops can be different. Another poster also mentioned the lift benefit of the intese airflow over the wing behind the engine.

Will say, got my brother (who is fascinated by historial sea voyages and legends of old ships) the Little Captain, and despite him being a grown man in his late 30s, his face lit up like a child when it arrived. We briefly played a game of hiding it around the house waiting to see who would find it, which was cut short when one of the cats won a round. Now the captain is setup on a cat proofed shelf in my brother's room where he can safely watch over the chaos.

The little captain looks like E.J. Smith. He ain’t getting on my boat… 😂🤣😛

I now know more about boats and ships listening to your videos than I knew I could even know. The complexity of captaining a ship is far more complex than I ever thought it was. Watching the captain s on deadliest catch made me believe it was mostly point tge ship that way and go. Never understanding the complexities of cargo, ship design, control. . .

Video idea - talk about how sails were phased out. SS Archimedes had steam propulsion but was also fully rigged for sails. As time went on ships (specifically navy ships) had provisions for sails but they look less and less effective over time until they eventually disappear altogether. What were the last navy ships to have the option to go under sail? How did the maritime industry in general get comfortable moving away from sail and trusting in powered propulsion?

By the way, ship propellers do come part way out of the water during severe pitching. My father had the misfortune to get a cabin in the stern of the ship he was on to come to the US in 1948 and had to experience this first-hand when the ship got into a storm. Also should mention bidirectional ferries -- with these, you _can't_ get away from having some propellers up front, unless you're talking about paddle wheel ferries (which also existed well into the 20th Century).

I love that all of the mechanics and engineers are having minor fits about the thought of extra gear boxes, prop shafts and everything else this would complicate.

Having the propeller in the front would cause a challenge connecting the propeller to the engine, since the rudder is in the way. Thus needing a more complicated solution.

I have read that having the prop at the rear also helps energize the boundary layer on the hull, making it more efficient. This is less important for an aircraft where the air has less density. For propeller planes, the issue is for cooling of the engine.

0:28 such a good portrait

Brilliant question that I never considered and I am surprised by how much behind this question there actually is.

Inspired by another comment here, I just got a really fun idea: The host of this channel should go to Port Revel Shiphandling Training Center. It's a lake in France with manned 1:25 scale model ships designed to handle similar to their full-scale counterparts, used for training captains and pilots. Let him try his hand at operating a miniature cargo ship, put his knowledge to the test. Maybe send Mike Brady from Oceanliner Designs too, he would certainly have a good time there even if it isn't as closely related to his old oceanliner content.

Beechcraft Starship, AASI Jetcruzer, Piaggo P-180 all pusher propeller aircraft for anyone who is interested. I see alot of comments about the complications with push props so didn't want people to think it can't work. The 180 is one of the fastest private aircraft. Push props are more efficient but pull props are more convenient for aircraft ease of maintenance, design, visibility.

Double upvote at 1:30 because that first part is such an awesome explanation. And a triple upvote by the end. Please use this as a benchmark for future shortish videos.

I was already in the process of skipping the ad when i saw what it was ad for. Went back and watched it, cheers captain

As always, excellent video and I really like the fact that you seem to cover all bases, including details like water being pushed towards or pulled away from the seals.

S.S. Archimedes; what a great name for the first propeller-driven ship!

I think the most important reason, is to protect the propeller from possible damage if mounted at the front @4:00 . 😊

love these videos. i live a pretty landlocked life, but giant container ships and gritty sailors have always fascinated me. Also your voice is pleasant. 👍

Your animations always are a joy to watch!

Another point is stability. With the rudder in the front any drifting of the vessel will cause it to turn further without corrections. It's like pushing a trailer instead of pulling it

Kudos to your topics, graphics, history and overall approach.

Props to you for an excellent video!

At 3:37 the propeller and rudder under the hull reminds me of the inboard (as opposed to outboard and inboard-outboard) motor boats / yachts from when I was young And I remember how careful you had to be to not run them aground Even as a youth (about a million years ago) I thought the traditional inboard motor scheme was less than optimal for shallow waters (like the Gulf Coast)

Another great explainer!

It's weird, I don't have a boat of any kind and I'm not even keen on being on the water but I look forward to your videos and always find them fascinating. Thank you.

An obvious exception are tugs. Wether using pods or Voith Schneider, it's generally in the center under the hull. But that's for a good reason as tugs need to be able to provide incredible amounts of thrust in any direction, and have no need for efficiency at speed.

With the Propeller forward and rudder behind it the rudder would need to be split in two to allow for the propeller shaft to pass through.

AS usual, that was a wonderful and clear explanation of a concept which I've never really thought about.

I love that you modeled a king air, that was my favorite part.

This is a super cool video!! It actually ended up answering a question i had from a completely unrelated video i saw, where a gentleman was interviewing an engineer of a brand new cruise ship and they talked about the pods!! I was wondering why choose those, but the other video never gave an explanation other than "efficiency". This is a fantastic video!!

Really fascinating! It's obvious but only in retrospect.

A very stern lesson, indeed!

Bought a little captain can’t wait to get it!

The question answered I never thought to ask 😊

Also, when moving ahead the pivot point is aprox 1/3 of the ship's length from the bow, so having the rudder and the prop at the bow would make the ship less maneubravle bc the moment arm would be much smaller.

The primary reason for having the airscrew at the front of aircraft is space availability, and the secondary reason is engine cooling, which is markedly worse in pusher designs

My old main ship had tow azimuths under the mid part of the vessel. Was working quite good when pacing on the ship, due to the sucking from the props. Also been on a ship with a retractable in the front, which when laying standby could work like a anchor and keeping the heading quite good

Read Austin Vernon's article on "Why battery powered cargo ships are compelling". Could make an interesting vid.

I have heard once that for aircraft, propellors at the back improve efficiency, however, at the front they provide extra lift as they move more air over the wings.

Good presentation. 👍💯😀

Honestly the propeller in the back is very intuitive

👍 Great stuff (as always) !!

Thanks!

Had no idea there were so many reasons for this. Great engineering content

If I might offer a suggestion… I should be very interested to see you do a video on the evolution of the propeller itself. I seem to remember you doing one on the number of blades (or maybe that was someone else; I do consume a lot of nautical and maritime related content on KZfaq), but nothing covering, or even touching upon the quite radical difference in blade characteristics and design, from their beginnings to now, and their several permutations over the years in between.

nice one!

Another point on running aground…. If the propeller, rudder, etc is at the rear of the ship it’s more likely to be available to help you back off of whatever you just ran into. Whereas up front, there’s a good chance it’s either damaged, or literally stuck from being driven into the ground.

The Frisia 3 ferry uses the Voith Schneider drive and therefore has 4 propellers. 2 in front and two in the back. The ferry is built to be symmetrical at the front and back and rides equally well in either direction. Its a German ferry.

Now I can put a little sea man on my desk!

The very first intuitive idea on teh question for me has been: Pushing the water against the frontal hull would create a lot of pushback, deminishing the pulling. Everything else has been cream on top in my head. ut for a split-second intuition, i could imagen the people of the past are well aware of a number of problems with a frontal mounted propeller.

Azipods are not used in cargo ships because of the cost. They are more expensive. Also cargo ships sail long distances at the same low speed. Simple salutation works well. Nowadays Finnish icebreakers have Azibods too.

Another reason is that a ship's hull is relatively large and the propeller relatively small compared to a plane's propeller and fuselage. The airplane's propeller must be large because the density of air is much lower than water. And aircraft have to be light. The bulk of the wash from an airplane propeller is near the tips so most of it can get around the fuselage. Jet and turbojet powered aircraft do put engines in different locations, because the front wouldn't work well.

Can you do a video on who owns cargo containers and how their logistics works?

Love these videos. I was traveling back from the Channel Islands on a tri-maran fast ferry would like to see a video of the characteristics on one of these.

The main point is missing here. The efficency of the propeller in the wakefiled of the vessel is much higher becaus the water getting into the propeller streams slower and thus the propeller can better excellerate the water.

Hey, being a sailor, I'm quite sure you'll have your own boat. Don't have to be nothing fancy, could just be a small launch with a pair of motors, but maybe you could do a video on her?

Video idea: how did the big sailing ships manouvered without an engine back in the days for docking

Hi NavCas!

I say let's give paddlewheels another chance! I want to see paddles on cruise ships, container ships, and aircraft carriers, lol. On a serious note, the Little Captain reminds me of Captain Smith. Maybe he should stay off boats?

Have you done a video about cylindrical sails yet?

Worth saying that a number of aircraft have used rear propellers (most famously the first plane from the Wrights, but there have been others). Arguably these days it's most popular to mount the engine in the middle of the plane, on the wings.

My initial guesses were; 1. Protection, the stern is the least likely place to get smacked. 2a. Access. 2b. Hydrodynamics. Why is this paired with access? Well, a bow screw would require a choice, exposed and easy to work on in dry dock or with added Hydrodynamic shielding, a complex prospect as such a structure would have to try to minimise obstruction to the screw while reducing drag. 3. Convention. Like bicycle drivetrains being on the right or the best chocolate coming in purple wrappers, it's a well established convention to whack a screw aft since so many early ships did so therefore giving early experience for such designs. 4. Mass management, no doubt keeping extra mass aft helped earlier ships open up hull space be it for goods or passengers, this goes double with motor vessels which can wedge the engine anywhere along the hull if need be.

Can we just take a moment to appreciate what an awesome name "Isambard Kingdom Brunel" is?

If you've ever tried backing a motor boat out of a slip you'll understand how much easier it is to control it the other way. Its due to the way the hull of the ship moves through the water and how the water moves over the hull. If you lay a piece of paper on a flat tabletop then blow across the paper you may understand that last part. The forces acting on the hull become very unpredictable when water is being pushed between it and the main body of water rather than when it is being pushed through the water only. Turbulence is created that can make the boat go left when you think it should go right, or shimmy back and forth with increasing intensity.

The reason airplanes usually don't use pusher props is because of how the propeller interacts with the airflow coming off the fuselage or wing ahead of it. It creates vibrations and other issues that negate the efficiency benefits. Cooling can also be an issue for air-cooled engines, and the prop diameter is limited by the landing gear (more so than traditional airplane propellers)

Push/pull done

Thinking of the comparison between airplanes and ships, I realized another major factor the video missed. Having the propeller at the rear - be it of an airplane or ship - is more efficient overall as it re-energizes the boundary layer, though the propeller itself suffers in efficiency due to turbulence entering it. This improvement is more pronounced on ships than on aircraft for a couple reasons. Ships have skin friction - water dragged along the side of the ship as the ship moves through it - as a greater portion, I've heard even the vast majority, of their drag. The propeller re-energizing this helps a lot, the ideal thing would be to draw in all this water that's been slowed down and speed it back up, resulting in nearly zero water movement in either direction behind the ship. Not to mention that the size of the boundary layer is probably larger relative to the propeller than it is for an aircraft, which has a much larger propeller due to less dense air. Airplanes have skin drag as well, but it's a smaller portion of their overall drag, as they're in contact with a lot less mass of fluid, and much of the drag is induced and wingtip-vortex drag from the wings, a lot of drag being a by-product of the need to generate lift that doesn't apply to ships. Meanwhile the water flow behind a ship's hull is probably a lot less turbulent than behind an airplane, as ship hulls are designed only to be as streamlined as possible, disturb the water as little as they can, while aircraft have to redirect air downward in order to generate lift - this is very visible in videos of airplanes flying low over dusty ground, punching in and out of clouds, or the patterns of smoke and flames behind military aircraft dispensing flares. This "wake" of downward-moving air behind an airplane is such a big factor that aircraft must be carefully directed to avoid being too close behind one another, especially a smaller aircraft behind a larger one, and there have been several crashes caused by this. Having the propeller operating in this much more disturbed air would reduce the propeller's efficiency, while likely also causing vibrations and noise as the propeller moves through the wake - certainly a lot of noise on some pusher-propeller aircraft (most famously the Piaggio P.180) from engine exhaust passing through the blades.

I have seen that some ferry has a propeller at the bow and the stern, and they are not double ended like those Staten Island ferry. Is it so that they could run in both directions without having to turn? Or are they helping in maneuvering of some sort? BTW, of all the propellers, I think the Voith Schneider is quite a unique one.

Nice King Air hehehe

Those old time sailers knew their stuff! 🎉

I agree with some of the other comments - You never came back to aircraft to complete the comparison. For example: WHY are props in front on airplanes? Then, there are the anomalies that are XB-35, B-36 Peacemaker, Beechcraft Starship, Reaper UCAV, etc, etc…that DO have the props on the back.

Next time on Casual Navigation: Why is the deck on top and the hull below 😅

I would assume at the stern you are also less likely to run into plants, ropes or other similar objects that will wrap themselves around the propeller?

Please more about Azipods and jet driven boats.

On an outboard motor, then, does the turning moment increase with speed, as the propeller mount (or whatever that sticky-outy bit is called) acts as a rudder?

3:45 That is exactly what many very small boat with outboard motor does, they can retract it back when in shallow water.

Propellers on unloaded ships are often exposed. As are propellers on ships pitching or rolling severely. So much less so than paddlewheels, but they are not always fully submerged

Can you talk about the forward propellers (bow thrusters) on ships with a bulbous bow?

I would have thought the most important factor is something you didn't mention at all: Steering stability. With the centre of pressure behind the centre of mass of the ship, the thrust acts opposite to the perturbation, returning the vessel to its initial course. If the centre of pressure is ahead of the centre of mass, this is not the case and the vessel tends to turn off course. The same is true of rockets.

the propeller will also be in the water most of the time at the back

Was following the compression theory except for the glaring fact that ships have reverse, which then makes the stern the bow so to say, but the shaft doesn't pull out even at back full while under way.

Hi Casual Navigation, question: Tomorrow I am going to Belfast on the Ferry, Any recommendations for medicines to avoid getting seasick?

Also, when a vessel is making way ahead the pivot point is approximately 1/3 of de LOA from the bow, meaning that if you have the rudder in the bow, the lever would be 2 times smaller making the maneuver much difficult

I'd like to point out that a propeller at the stern doesn't always stay submerged. In heavy seas it is not unusual for the propeller to become visible or even completely being out of the water for short periods. This isn't a very good thing for the engine of course, as it suddenly loses its load. This leads to a quick increase in the RPM until the propeller submerges again.

me at any given time: "I have no questions about boats, ships, or nautical vessels of any kind" *casual navigation posts a video "...hey wait yeah why IS the propeller at the back?"

Propellers on planes generate extra lift because the propwash is so much faster than the general airspeed, and lift varies by the square of the speed. Pusher prop planes and jet planes don't have this benefit, which greatly helps shorten takeoff runs.

Lotta drill ships have pods for and aft. Don't forget the voith Schneider drive that is also mounted fore and aft.