Sharrow counterrotating propeller seems to have negative stagger and positive stagger

[Robert Biegler]

I saw this picture in a news piece promising a lot greater efficiency:

Each blade of the Sharrow propeller is like a box wing. The forward propeller, down in this picture, looks to have negative stagger. The roots of both parts of each blade are at about the same horizontal level. The aft propeller, up in this picture, has positive stagger. The roots of both parts of each blade are well separated vertically.

Can anybody tell me why that is?

 

[seasquirt]

Hi Robert, I went to their site and didn't see anything there like in the picture you show. I'm no expert, but I suspect it is a multi fit item for LH and RH prop applications, saving on tooling, and being equally effective both directions, in whatever it does, (not really sure what it does but I'll have a crack). I doubt it is for counter rotating setups, but I could be wrong.
If fixed to the rear (top) prop, on the same splines, in that position shown, it looks like the lower leading edge forces flow into the low pressure zone behind the first drive blade, and the lower concave is jetting water into the gap, and on to the front face of the following drive blade. The back internal face of the lower concave making a low pressure to direct / aid the following negative angled lower front face to push water up into the low pressure zone behind the next drive blade. Assuming the lower section has the same potato peeler type gap as the propeller; hard to tell for sure in that pic.
It could be just two items unrelated, but stacked nicely to make an impressive (and confusing) chrome picture for the article.
I studied it for a while, as a non expert, and only replied because no one else did. Maybe an expert here will correct me.

 

[Robert Biegler]

Hi Robert, I went to their site and didn't see anything there like in the picture you show.

I first found it at Ultra-efficient looped propeller gets contra-rotating double upgrade https://newatlas.com/marine/sharrow-contra-rotating-toroidal-propellers/ On Sharrow Marine's site, the counterrotating drive is one of the items on their product page: Our Products - Explore Sharrow Marine’s Award-Winning Propellers https://sharrowmarine.com/pages/our-products

If fixed to the rear (top) prop, on the same splines, in that position shown, it looks like the lower leading edge forces flow into the low pressure zone behind the first drive blade, and the lower concave is jetting water into the gap, and on to the front face of the following drive blade. The back internal face of the lower concave making a low pressure to direct / aid the following negative angled lower front face to push water up into the low pressure zone behind the next drive blade.

Do you mean that the trailing part of the blade portion acts like a slotted flap to the leading part of the blade? Could be, but that would not explain why the first propeller is different. I noticed that all the single propellers on the Sharrow Marine product page are like the aft propeller of the counterrotating drive in that they show positive stagger.

The forward propeller has negative stagger, like this box wing plane:

On the other hand this drone (see Looped propellers: A noise-killing game changer in air and water https://newatlas.com/aircraft/toroidal-quiet-propellers/) has toroidal propellers that have negative stagger:

 

[seasquirt]

On closer look at the bases of the lower blades, they do look like both having a rake angle suitable for counter rotating operation, both bases angled opposite to the above propeller. Holding a straight edge to the photo it looks to me like the lower blades do have a very few millimetres of height difference between the leading and trailing bases of each whole blade, but nothing like the above prop shape; it could be an optical illusion too; scaling off a photo is not good practice. I can understand the top prop, but the lower, leading one is baffling, especially being counter rotating. Maybe it sets up the flow to maximise the following prop's efficiency, (like in some turbines), without doing as much actual driving. The inventors probably won't go out of their way to explain it, to slow down copiers from flooding the market.
So really it is still a mystery to us both, and I cant help with an answer. Pretty picture though.

 

[jehardiman]

The term stagger is an aerodynamic one and not used in most hydrodynamic propeller design. Unlike the pressure effects of stagger in aerodynamics, those small pressure effects are overshadowed by the kinetic mass effects of the water which is ~800 times heavier and much more viscous. The issues with these types of props are that they are fixing self imposed 'restrictions' in design (mostly diameter and rpm issues). Much like kort nozzles you are trying to improve an overall defect in the powering design. Bigger, slower turning wheels will get you more.

 

[DogCavalry]

Bigger slower turning wheels is best. These are probably meant for outboards where nothing is changeable except the props. In which case these are probably an expensive improvement. Which seems a bit excessive, until you consider Mercury's 600hp V12, and then suddenly they seem like entry level stuff. A minor cost.