How do surface piercing propellers work?

[ardnog]

We have a new boat, and it has surface piercing propellers. Half of the prop is above (out of) the water, and half is below. I can't quite understand how that actually works or why it would be better.

 

[jedishrfu]

I found this description online at:

http://forum.woodenboat.com/showthread.php?44600-Surface-piercing-props-question

The surface-piercing theory is based on the idea that the central portion of the prop--the hub, strut, shaft, etc., create mainly drag. In a high-speed boat, lifting this portion out of the water means that what is left---the tips of the prop, is 100% forward thrust. It's a nice match with a planing hull that likewise is mainly out of the water. In a low-speed application, such as a tugboat, the total blade area is more important for maximum thrust.

 

[ardnog]

If only the blade tips are in the water, you're also only generating a fraction of regular thrust.

I can't find a wikipedia entry for the technology. I find that unusual.

EDIT: So to get increased speed in this position, the drag from the prop shaft must be far, far greater than the drag from the hull whatever else is in the water like your interceptor plates (underwater flaps)?

 

[A.T.]

I can't find a wikipedia entry for the technology. I find that unusual.

https://en.wikipedia.org/wiki/Supercavitating_propeller

An alternative to the supercavitating propeller is the surface piercing, or ventilated propeller. These propellers are designed to intentionally leave the water and entrain atmospheric air to fill the void, which means that the resulting gas layer on the forward face of the propeller blade consists of air instead of water vapour. Less energy is thus used, and the surface-piercing propeller generally enjoys lower drag than the supercavitating principle. The surface-piercing propeller also has wedge-shaped blades, and propellers may be designed that can operate in both supercavitating and surface-piercing mode.

 

[256bits]

If only the blade tips are in the water, you're also only generating a fraction of regular thrust.

The propeller and engine are probably matched one would think so you get 'regular' thrust.
The surface propellers can be of large diameter

 

[anorlunda]

Thanks for the question. I never heard of surface piercing propellers before. I found the following. The reference paper might provide you more details, and the list of references shows 31 other papers on the subject.

Hydroelastic analysis of surface-piercing propeller through one-way and two-way coupling approaches
https://journals.sagepub.com/doi/abs/10.1177/1475090218791617?journalCode=pima

 

[Office_Shredder]

This website at least claims the primary benefits are not hydrodynamic in nature. No idea if they are right though.

https://www.surfacedrivesystem.fr/

Usually the propeller shaft must be extended to be less sensitive to the wake factor. There is no blade tip clearance from the hull, the propeller diameter can then be the maximum that fits. The designer is able to use a much deeper reduction ratio, and a larger, lightly-loaded, and more efficient propeller.

 

[anorlunda]

Usually the propeller shaft must be extended to be less sensitive to the wake factor.

I'm sure that this is one of those cases were changing one thing forces you to change a dozen other factors. The result can be very good, but analysis and explanation become extremely difficult.

Is anybody following the Americas Cup race with those AC75 sailboats? Wow! How radical can you get.

 

[A.T.]

Is anybody following the Americas Cup race with those AC75 sailboats?

I don't think they have to deal with caviation at the hydrofoils, do they? But I know the Sailrocket 2 had a caviating hydrofoil:
https://www.aerotrope.com/what-we-do/low-carbon-vehicles/sailrocket-speed-sailing.html

 

[anorlunda]

I don't think they have to deal with caviation at the hydrofoils, do they? But I know the Sailrocket 2 had a caviating hydrofoil:
https://www.aerotrope.com/what-we-do/low-carbon-vehicles/sailrocket-speed-sailing.html

In normal circumstances that's correct. But in the accident shown in the picture, American Magic used too much rudder force in a sharp turn. That caused cavitation. The foils under the rudder lost lift, the boat angled up, became briefly airborne, and capsized. You can see the rudder in the 2nd picture.

I mentioned AC75 in this thread not because of a direct analogy, but as an example of a design change so radical that it's hard to imagine at first. That surface piercing prop is really radical.

 

[Arjan82]

Surface piercing propellers are only beneficial for very high velocities (Froude number well above 1, so planing boats). There is a limit to the amount of thrust loading (or power loading/power density, what's in the name...) you can put on a normal propeller before you get thrust breakdown. This simply means the propeller is only pushing water downstream, instead of also 'pulling' the water on the suction side of the propeller. For a normal propeller roughly 2/3rds is pulling, 1/3rd is pushing.

If you really need to go to higher power densities super cavitating propellers are usually the next step. The propeller, shaft and struts (holding the shaft in place) are still entirely submerged in this case. Here you simply accept that the suction side (forward facing side) of the propeller doesn't do anything useful and you are only pushing the water. Now you can design propeller blades which are better for super cavitation.

This is all well and good, but if you want to go to even higher power densities, the drag of anything in the water becomes enormous (and starts to cavitate and possibly erode and all that). So the beauty of super cavitating propellers is that there is only the flat surface of the hull and one blade of a propeller that is in contact with the water at any point in time at these very high speeds. The propeller shaft and anything else is all entirely above water to minimize drag, which is a real benefit at these high speeds. Secondly another advantage is (I think) that you are accelerating most of the water through the air, instead of generating a jet in the water. Just as for waterjets, this is beneficial for efficiency since the goal of both a waterjet and a surface piercing propeller is to generate as high as possible water velocities (look at the trail of water above the water surface aft of a surface piercing propeller at high speeds).

But please note the following simple physics: momentum (or thrust) is mass(flux) times velocity. If you double the mass, you double the thrust. And if you double the velocity you also double the thrust. However kinetic energy is mass(flux) times velocity squared. Thus doubling the velocity takes four times as much energy! This is why it is always better for efficiency to accelerate a lot of water a little-bit instead of a little-bit of water a lot. So increasing the water velocity is never a very efficient option. At these speeds you have no choice however.

 

[Arjan82]

This is the video out of which the picture that @anorlunda shared is take I think:


This is so crazy fast...

 

[ardnog]

Thanks for information.

Very surprised how terse the Wikipedia article is. One paragraph, no photos, and not even under its own entry.

Foiling cats are super hard to sail. Surprised they don't pitchpole when a fish looks at a foil the wrong way.

Long distance, sail is still the fastest thing on the water. 40 day circumnavigation vs 60 under engine power.

 

[davenn]

Thanks for the question. I never heard of surface piercing propellers before.

Likewise
interesting subject and some good learning

 

[cjl]

Surface piercing props are really interesting - as mentioned above, they're mostly a thing when you're trying to push a very high amount of power through a relatively small propeller. Normally, that's limited by cavitation on the suction side of the prop, which can be damaging. However, a surface piercing prop entrains enough air to maintain a constant air cavity on the suction side, entirely preventing cavitation. This also means that you effectively have a reduction in prop area as the boat speeds up, which is beneficial for the top speed/acceleration tradeoff (a large prop is good for acceleration, a smaller one can give a higher top speed, so long as you don't run into cavitation or breakdown issues).

They also look really cool, thanks to the huge rooster tail.

Long distance, sail is still the fastest thing on the water. 40 day circumnavigation vs 60 under engine power.

I'd still probably bet on a nuclear ship for that particular title. Quick envelope math says it should be <30 days for a modern carrier. Still though, impressive that sailing ships are as good as they are.

 

[Arjan82]

However, a surface piercing prop entrains enough air to maintain a constant air cavity on the suction side, entirely preventing cavitation.

Preventing cavitation is not the main benefit for surface piercing propellers. Supercavitating propellers cavitate a lot but that is fine as long as there is no erosion. There is no erosion because the cavity implodes downstream of the propeller blade (because 'super' in supercavitation means the cavity is larger than the blade). There is however a velocity at which the cavity does end on the blade, however you don't buy these kind of boats to sail at such a low velocity ;).

Also I read somewhere that 'generating the cavitation' would cost energy and therefore reduce efficiency. This is not even remotely significant if at all true.

(a large prop is good for acceleration, a smaller one can give a higher top speed, so long as you don't run into cavitation or breakdown issues).

Why would this be true? Larger diameter propellers in general have larger efficiencies for equal thrust (if the propeller RPM is properly adjusted such that your propeller pitch can be equal). This would imply higher top speeds (same power, higher efficiency -> more thrust -> higher velocity). Larger propellers also have more area to distribute the thrust over and therefore generally have less cavitation (i.e. thrust breakdown occurs at higher thrusts).

Also higher acceleration implies higher thrust. Higher thrust implies higher top speed?

 

[ardnog]

I'd still probably bet on a nuclear ship for that particular title. Quick envelope math says it should be <30 days for a modern carrier. Still though, impressive that sailing ships are as good as they are.

Nuc power record is 60 days 21 hours for a US submarine.

I'm sure modern ones are better than that, but I'm also not sure that they're designed to be turned up to full for extended periods. How hot would it get?

Mechanical power would certainly be more consistent. You could go in either direction and not worry so much about the weather.

 

[cjl]

Nuc power record is 60 days 21 hours for a US submarine.

I'm sure modern ones are better than that, but I'm also not sure that they're designed to be turned up to full for extended periods. How hot would it get?

Mechanical power would certainly be more consistent. You could go in either direction and not worry so much about the weather.

I don't see any reason they couldn't run flat out for a month+ at a time. I'm sure they don't have cooling issues there, I bet they just haven't bothered to set the record.

 

[ardnog]

I don't see any reason they couldn't run flat out for a month+ at a time. I'm sure they don't have cooling issues there, I bet they just haven't bothered to set the record.

If it was just a fuel problem, I'd be asking why the diesel trimarans with pre-positioned tankers aren't any faster either.

EDIT: I mean racing trimarans, of course. I also understand that a warship has better things to do than go racing but I don't think anything to do with the ocean is as simple and risk-free as it might appear.