What Separates a Propeller from a Waterjet?

No doubt, the math of propulsion devices is complicated. But it is all the same physics. By dividing each of these devices into their essential tasks, we find new opportunities. New methods of propulsion from different combinations of old tricks. Insight into this continuum allows us to better classify marine propulsion and dissect past successes to search for new combinations.
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References
[1] J. Carlton, Marine Propellers and Propulsion, Burlington, MA: Elsevier Ltd., 2nd Edition, 2007.
[2] T. Schmidt, "Propeller Simulation with PropSim," Human Power, Technical Journal of the IHPVA, vol. 48, pp. 2-7, 1999.
[3] Wikipedia Authors, "Trailing edge NACA 0012," Wikimedia Commons, 29 July 2008. [Online]. Available: commons.wikimedia.org/wiki/Fi.... [Accessed 23 Jan 2022].
[4] Wikipedia Authors, "Twin Linear Aerospike XRS-2200 Engine edit," Wikimedia Commons, 3 Jan 2007. [Online]. Available: commons.wikimedia.org/wiki/Fi.... [Accessed 23 Jan 2022].
[5] Wikipedia Authors, "Trinity explosion film strip," Wikimedia Commons, 10 Sep 2005. [Online]. Available: commons.wikimedia.org/wiki/Fi.... [Accessed 23 Jan 2022].
[6] Machinery Spaces, "Fixed Pitch Propeller," Machinery Spaces, 2010-2016. [Online]. Available: www.machineryspaces.com/Propel.... [Accessed 23 Jan 2022].
[7] Becker Mewis, "Becker Mewis Duct," Becker Marine Systems, 2022. [Online]. Available: www.becker-marine-systems.com.... [Accessed 23 Jan 2022].

[@SpecialEDy]

I've always thought of the physics of propulsion in even simpler terms: reaction mass. For every action, there is an equal and opposite reaction. Every method of propulsion, land sea, air, space, biological, chemical, or mechanical, all accelerate mass in the opposite direction of thrust.

[@shaider1982]

The pressure surface is where the reaction force pushes the vessel.

[@DrZygote214]

This is a great video thank you. Anything getting to the fundamentals is great. I rly hope you can get to Icebreakers eventually. I wanna know all their secrets of design lol.

[@DatawaveMarineSolutions]

I do keep intending to do icebreakers. It's a bit daunting, because there is so much to cover with them.

[@roybeltran4424]

This video is Great! I love the way you lay things out! Going to website now!

[@DerekWoolverton]

Always welcome a visit from our friendly neighborhood naval architect. Been watching some DIY'ers build river jet boats, and an interesting discussion was going on about the benefits of multi-stage impellers for quicker self-priming. Again a trade off of having more pressure differential that doesn't help specifically with thrust, but deals with the inconvenient fact of air going where you might not appreciate it.

[@martinrousseaux8319]

Thank you! Very nice all your videos.

[@shaider1982]

Thanks for the table, I now know why my favorite submarine channels refer to a submarine's propulser as pumpjet and is not interchanged as waterjet.

[@caseydamiano269]

Hi Nick! 'Love your channel! But this video really dope-slapped me into remembering my Engineers who have to fix these things when they break! When I was an Academy Cadet, I thought Voith-Schneider props were the cat's a**. But when they break, it costs your Operator a fortune!

[@DatawaveMarineSolutions]

Very true. I admit that Consultants like myself don't get the best information on costs for the equipment. We are aware of it. But it's very hard to get accurate details. Most people don't respond well when you ask them to share intimate financial details about their operations. Most of our details come from the equipment vendors, who always paint a rosy picture.

[@Eugene2ndW]

I am curious about designing a propeller. Once speed, power, wake is determined, how does one determine blade area, pitch determined. I know this is a complicated subject, just asking about basics. Also, the relation between prop dia and pitch.

[@brianmckerrow817]

Excellent. Hydrostatics man.

[@ianstanley-maddocks9262]

Thanks for this great explanation of propulsion. I made a realization while listening that sails are really no different from propellers: they also generate a pressure difference, and then apply that difference to a surface. It's the source that is different: sails just rely on capturing wind energy rather than using rotational mechanical energy. The same physics apply to both. Now that I'm writing this, I feel stupid. Of course they are the same! They're both foils.

[@DatawaveMarineSolutions]

Great point! That thought normally takes a while even for the professionals to realize it. Yes, sails and propellers follow many of the same physics.

[@kitten_processing_inc4415]

Where should propulsion by an oar fit in to the classification? Would old time transverse axis paddle wheels even be in a class of their own?

[@lincolnlincoln27]

What is you opinion of rim drive technology scaled up to yacht size?

[@DatawaveMarineSolutions]

Yes. It works. Probably better, because the larger electric motors frequently require water cooling. From a pure physics perspective, I don't see any problem with rim drives at larger scale. I think there may be some practical drawbacks. Specifically, a rim drive thruster has no access to the motor for maintenance. I haven't worked with enough electric motors to say if that would matter. Smaller electric motors are virtually maintenance free. But large electric motors, the size of a car . . . those get more complicated.

[@gregeconomeier1476]

Thanks, I appreciate your clear explanations. Now, could you please apply all that knowledge and make a more efficient outboard motor propeller for my boat? These new gas (petrol) prices are getting difficult. 🙂

[@DatawaveMarineSolutions]

That's ironic, because the idea for this video started when I considered how to improve the propeller further. My basic conclusion: you can't really improve the propeller much more. There are a few add-ons that help. But we are at the limits of propeller physics. I think the next big breakthrough will require us to completely rethink propulsion. I have a few ideas on that front, but nothing actionable. The next move will be just as big as when commercial shipping moved from wind to steam propulsion.

[@DatawaveMarineSolutions]

@@satunnainenkatselija4478 Hard to say. I have seen this style of propeller applied before in wind turbines. The physics are largely the same between wind turbines and propellers. But I never saw systematic tests in that case either. I did find a video from UofM testing on the sharrow propeller. kzbin.info/www/bejne/o6i5ooFvdqdpars So it appears the prop was tested formally. And the video does support the claim that the Sharrow Propeller reduces tip vortices. But there are some big caveats. First, that video had to be recorded with the prop and water flow operating below normal operating speed. I know know this because the video shows die in the water to trace the water flow. If you operate at full speeds, those dies immediately diffuse into the water and you never see the streamline. On the whole, there is a theoretical basis for the claims of improved efficiency in the Sharrow propeller. But in 5 minutes of searching, I did not find a 1:1 comparison that proved the Sharrow propeller was tested under the same conditions as a conventional propeller. There is a very good chance that data exists. Sharrow Marine may just want to keep confidential to protect their investment.

[@Blue_Maxxx]

Ok I need help. I got involved in a conversation that has turned into a argument about Iowa class and Montana class Battle ships. I believe that Montana would use more fuel to maintain it's cruising speed because it's a larger ship with les power than Iowa and I think Iowa has better hydrodynamics than Montana.Yes I know we never built a Montana.

[@anuardelcastillo2627]

are rotating propellers more mechanical efficient than fish or whale fin? whales move fast many tonnes with very little movement and friction. why boats no add some fins?

[@DatawaveMarineSolutions]

This has been a long debate in the marine community. I'm not sure if rotating propellers are more efficient than fin propulsion. I can tell you that we studied fish fin moments. And the main reason we don't mimic the fish is that their movements are insanely complicated. You can see a video of this here: kzbin.info/www/bejne/f2azmZpqppp4eLc Any type of fish propulsion would require numerous underwater joints and be extremely difficult to create. This is similar to the argument of why didn't animals evolve rotating wheels for land propulsion? Because a rotating wheel is incredibly difficult for biology to create. Some things are easier with biology than with mechanical equipment. And fins are much easier with biology, but extremely difficult to reproduce with mechanical systems.

[@anuardelcastillo2627]

@@DatawaveMarineSolutions thanks so much! for your reply. I will check the video

[@anuardelcastillo2627]

my naval architect friend, I am ver pleased to have found an answer! look for hobbie mirage kayak. there are videos showing how powerful it is and faster than kayak prop. systems!!!

[@franciscomartinezzea8531]

There is a ting called positive displacement... I wonder if things like roots pumps have ever been used to generate thrust. always bear in mind that I am not an engineer...

[@TrabberShir]

I was really expecting some examples of systems that use air as the working fluid.

[@franciscomartinezzea8531]

thing*

[@lincolnlincoln27]

When are you going to speakup about carbon fiber hulled submersibles????

[@DatawaveMarineSolutions]

I'm intentionally not speaking about the Titan submarine incident. We have very few facts, and far too many opinions right now. I believe that it is unprofessional and irresponsible to provide any public engineering opinion without a sound factual basis to form that opinion.

[@lincolnlincoln27]

@@DatawaveMarineSolutions you are overthinking it... the question is what is the best method material for a deep diving submersibles if you are going to use the provrn bathysphere materials and shape.... you no gut INMHO. carbon fiber mated to titanium rings is not practical.... of course it appears the weaving of the cfiber section was not cross woven, like cfiber scuba tanks are????...

[@lincolnlincoln27]

You neeed to upgrade your video "jets" below the water line work or US navy work not be able to use in propulsor style

[@DatawaveMarineSolutions]

True, waterjets do work below the waterline. This gets more into classification and terminology. I classify a waterjet specifically as the device that mounts on the transom of a boat, using a duct to suck in water from the bottom and exhaust it out the stern, above water. There are other devices that work very similar to a waterjet but the outlet is underwater. We call them something different than a waterjet, just to show that they are a different device. This isn't a large debate over physics, it's semantics. And it may seem annoying, but I have found I need to be consistent and specific with my semantics.

[@lincolnlincoln27]

@@DatawaveMarineSolutions be useful if you made that distinction in you bvideos, people maybe considering below the water line locations for pump-jet type propulsors,,,,

[@amunderdog]

Three thoughts. Water does not compress. They termed it a screw for a reason. VELOX - AGILE AMPHIBIOUS SWIMMER