Boat zero drag folding prop to learn from

I cannot comment on the efficiency once open but, dragwise, there is obviously something to learn from when folded. Too bad, diameters start at 11.5in or 290mm.


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Zero drag, my ass! That doesn’t happen. Ever. Pull something in water, it will take some force…


To start reducing drag, let’s find a powerful smaller motor like Flite does to reduce propellers pods and hubs size.

Anyone with some fluid dyn skillset that can tell if these type of subsystem optimisations can realley make a difference for an assist or wave efoil? I doubt it based on my limited knowledge since the area exposed by a existing folding prop is so much less than what lars and adam mentions as bigger issues to fix first.

This looks very juicy, only 41mm in diameter, with a custom folding prop, it can actually work

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I tried one of the DD motors, it was crap. Above 100mohm phase resistance and not enough torque to get up on foil. It burnt even though i tried optimising it with a few different smaller and lower pitch props.



Their clam-shaped design has no equivalent in the efoil world. The closest is probably the Plume folding prop.


Interestingly, their smallest hub diameter (HD) with 73mm is close to our motor diameters but the price isn’t. The price range starts at 2400usd or 3000€, too much for an efoil adaptation…

On the above page, GORI publish their folding and fixed blade propeller drag test figures:

GORI Race – 0 Newton - could not be measured.
2-blade Maxprop – 2.0 N (Tested with blades “hidden” behind the Saildrive leg which cannot be done in ‘Real’ life – a saildrive stops wherever & the feathering propeller blades cannot be aligned)

2-blade Std GORI – 2.2 N
2-blade Flexifold – 3.1 N
2-blade Varifold – 3.1 N
2-blade Variprop – 7.0 N
2-blade Volvo – 9.7 N
2-blade fixed – 99.9 N held / 198.7 N free rotating

I’m assuming the prop has been optimised for low drag being in its closed position, where it spends 99.9% of its life. In the open position it will work, but not be very efficient. Not an issue running on diesel, but more of an issue running on Batteries.


Especially because we’re running on batteries, this clam or pebble design should be taken into consideration when searching for the minimum drag once closed. Once open, this curved blade design is called “cup blades” and is said to provide a stronger lift.

The Plume efoil prop, released with optional alu blades in 2024:
220801 Folding prop

That diserves to be tested or improved. That’s where 3D printers work wonders…

Let’s summarise the advantages of the pebble (clam) shape:

  • closed: more streamline, see GORI figures,
  • open: well designed, cup-shaped blade props provide a better lift
    Looks like we could have a winner…
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Curious to know what Copilot (Microsoft AI) can extract from specialised websites about cupped blades (to be verified in practice with prototypes): easy to adapt to existing designs so a serious way towards the smallest drag propellers…

What are the advantages of cupped-blade propellers over straight blade ones ?

Cupped propellers offer several advantages over straight blade propellers. Let’s dive into the benefits of cupping:

1-Reduced Cavitation
Cupping involves bending the trailing edge of the propeller blades toward the pressure face (the blade surface facing aft). This creates a noticeable curve or “cup.” The camber introduced by cupping helps reduce cavitation. Cavitation occurs when water vaporizes due to low pressure near the blade surface, leading to performance loss and potential damage. Cupping minimizes this effect, allowing the propeller to operate more efficiently.

2-Increased Performance
By adding a cup, the effective or nominal pitch of the propeller changes. Essentially, cupping adds camber to the blade shape. This change improves performance by enhancing thrust and reducing energy losses. The small alteration in the blade’s trailing edge shape significantly impacts overall propeller efficiency.

3-Stability and Grip
Cupped propellers provide better grip in the water. The curved trailing edge allows the propeller to maintain better contact with the water, especially during high-speed maneuvers. This stability contributes to improved handling and responsiveness.
Quicker Planing: Cupping helps boats plane more quickly. The increased thrust and reduced cavitation allow the boat to reach planing speed faster, making it ideal for water sports, fishing, and recreational boating.

4-Optimized Existing Designs:
Cupping can be applied to existing standard propeller designs without major changes to patterns or tooling. Manufacturers can enhance performance by adding cup to their propellers without redesigning the entire blade.

!!-Rule of Thumb Sometimes an advantage doesn’t come alone
When using a cupped propeller, reduce the design pitch value by 1-2 inches compared to a non-cupped propeller. This adjustment keeps the effective pitch the same while benefiting from the cup’s advantages.

As a conclusion, cupping is a subtle but effective modification that enhances propeller performance without compromising other design aspects.

What is a “cupped” propeller and what benefits does it offer? Michigan Wheel

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Last but not least, the Plume propeller has interesting bent (round) tips. A step towards more safety without duct ?

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How about this one :
half the weight, 50mm diameter, same power, double the torque at 8.5NM, half the price…

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It won’t meet those numbers, too small size basically.

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