Why Are 4 Blades Better Than 3?

✩ABOUT THIS VIDEO✩
In this video, we investigate why the number of blades on a propeller matters and why 4 blades can be better than 3. We also touch upon why Titanic might have had a different number of blades on her propellers and what experiments they may have been running.

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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.

[Ensign_Cthulhu]

2:40 Actually, the very first screw propellers WERE Archimedes screws, but it didn't take at all long for designers to realize that blades could do the same job more efficiently.

[grondhero]

*_Props_* to Casual Navigation for bringing this to our attention. 😉

[pizzaivlife]

an intersting situation is that the Iowa class battleships and liner SS United States all have 4 propellers, with the inner ones having five blades and the outer's having four. This combo was found to be effective when trying to reduce the shaking the North Carolina class battleships made at speed. The Iowas and SS United States were designed for speed so took that lesson and added another rudder for fast turns at speed as well. This design was classified at the time and SS United States only got to use it as she was designed to be usable as a high-speed troop carrier in case of war

[mikefochtman]

Years ago, we used to listen carefully to surface ships to determine their RPM and blade number. It helps submariners determine just what sort of ships are in an area. The details of making super-quiet propellers was always a highly classified aspect of subs. Many times in photographs of subs in drydock, you'll see the propellor area blurred or covered in canvas to try to keep secret the details.

[Michael_Brock]

I heard that the Victorian engineer who "invented" the first prop had actually started with an Archimedes screw, but it broke, rather than waiting for a new casting he just continued with his steam boat the tiburnia, and the prop worked better than he thought.

[kylej]

Kudos to your animation of the Titanic not having smoke come from the fourth funnel.

[nekomakhea]

There's also really weird propellers, like toroidal propellers, and the 7 (or more) blade swept propellers on modern subs that somehow sustain 30+ knots while also being extremely quiet and low-cavitation

[flamingchuucygnet]

I've heard a story from my father who manages a boat making company and his friend who makes propellers. It appears that some boat owners (especially in context to law enforcement and defence) prefer 4 bladed/even numbered props so if one of the props are damaged/cut off, they can cut the opposite side and make it a 2 bladed prop and maintain relative stability/minimal vibrations.

[santoast]

Its fun to note that things like Nuclear Submarines, which desperately need to keep cavitation to a minimum, tend to have tons and tons of blades, and I believe some even have variable pitch propellers like you see in many aircraft, allowing you to even more finely optimize your blades to your immediate circumstance

[elcheapo]

Once again the parallels between aviation and seafaring are uncanny. Very well explained and animated, as always, Mr. C.N.

[RobKaiser_SQuest]

I used to have this toy set called Capsela for building your own motorized vehicle toys, it included a propellor with 4 blades that could be adjusted so you could see the effects of high or low rake on your little boat, and which best fit your design as it paddled around the bathtub

[colinpovey]

Remind people, this also applies to aircraft propellers. The calculation numbers are VERY different, because air is a lot less thick that water, but the same factors apply.
With aircraft, 2 blades are generally favored, but in general, as horsepower increases, so do the number of blades.

[doggonemess]

3:30 I wanted to point out that screws are very rarely made of steel for ships. Most of the time they are some alloy of bronze. Great video!

[robertzeitz]

I wouldn't know how interested I am in this stuff without this channel existing. Thanks.

[jimsvideos]

Cutters for milling machines and similar have the same challenge; more edges on cutters has costs and benefits.

[Bryzerse]

I'm pretty sure the mystery of Titanic's central propeller has been solved by the discovery of the order for a central 3-bladed propeller a couple years back

[garfieldslasagna]

I have officially watched all of Casual Navigation’s videos. The quality of information has always been top notch and the visual aids keep getting better and better!

[charliemaybe]

i would love a deeper dive into propeller theory

[tonenuff]

I really look forward to your videos & smile when I see a new one has posted 🤗 They are always very well done & properly explained. The animation you use is easy to watch & always extremely helpful in explaining your point. Thank you for doing these 🙏🏻

[jaconbob]

these videos have a rediculous level of quality. Soo good. Technical depth and animations everything is on point. Big fan!