Exactly, the genius of those props is that opening and closing are only depending on centrifugal forces therefore pure physics. They appeared on boats in th 70’s so it’s old stuff.
You can have a look at the LIFT thread where Nick Leason shows how they work.
To cut folding prop cost down, I think that STL printing (UV+ resin in eg Photon printers) (maybe FDM) has an important role to play among DiYers as it is strong with a nice finish.
Torqeedo: flexofold inspired, composite boss with stainless steel needles + bronze blades + heavy internal gear
Not tested yet but here is an idea:
1- apparently, LIFT don’t use any nozzle protection with their folding props
2- if you look p97 of the Test yachting magazine here
LIFT and Torqeedo folding prop were inspired by the Flexofold 2 blade…
A two-blade version of the Flexofold, its performance was only slightly down on the three-blade, and in fact better for astern thrust. With better test results across the board than some three-blade units, it exploded the commonly held myth that twin-blade props are inefficient.
Plus, this description kinda goes along the same lines. If you want to use an efoil to surf swells and waves then this might be a good option but otherwise, is there any other reason why someone would pick a two blade folding prop versus the regular 3 blade?
No straight answer. The LIFT Folding prop costs 1000USD . Not sure it will be popular if you’re not a surfer or doing downwinds frequently. Performance wise, only a good data logger can tell if there is equivalence. IMHO, folding props can only improve.
Found on the Flexofold site: These modern Flexofold folding propellers offer thrust comparable to fixed propellers in forward drive (...) thanks to improved weight distribution, blade area and shape
Fact is, the effizenz the falteble Props is no only proof.
No current, no speed, no rpm what the Prop need.
Only proof too expensiv.
FR Prop AL without too current, speed and rpm? I have it not found.
No there is no data on the foldable props yet as they are only available for Lift boards. The price will always be higher than a standard prop though as there’s slightly more machine time involved when making them.
Too expensive is a relative term. It might be too expensive for you, but for others that can afford it and are happy to pay for it, its the right price.
There is some data on the FR prop in the 65161/65162 motor thread. I have been running one on my setup. But bear in mind that all data is also relative to what foil you are using. I am very happy with the prop and it was definitely worth buying it from my perspective. It far outperforms the other boat props on the these motors. The simple reason being that the blades can be much thinner than boat props as the mass it needs to move is a lot less so there’s less blade flex.
FR certainly also used CFD to optimize his propeller. He does really have good performance + blades are not more than 2mm thick, which adds greatly to the overall performance like you said.
This is a nice work Frank However, you are missing 2 nuts on top i think
DIY costs totally depends on what material you are using. We are talking about brass CNC and aluminium 5083 or 7075-t6. Nylon or plastic grade is indeed another option.
Material is POM…
Handmade, no CNC
CNC only makes sense with larger quantities.
POM is expensive than aluminum, but well suited for prototyping and such applications.
Mother does not need it, thread is cut, the last courses self-locking.
There is no link to it …
Is my drawing.
Only give them free if it works well.
. Not sure it will be popular if you’re not a surfer or doing downwinds frequently. Performance wise, only a good data logger can tell if there is equivalence. IMHO, folding props can only improve.
However, you are missing 2 nuts on top i think 
