Power losses occur in the propeller’s slipstream. It is easy to notice the white swirl behind an efoil propeller. @Toto44has published an interesting link where a stator positioned up stream to the propeller guide one side of the flow in the opposite direction to the propeller rotation. The combination of opposite deflections is a straight flow behind the propeller and not a rotating one.
Some jet engines use a stator in their nozzle to deflect the flow aft the impeller in its opposite rotation direction.
As we need a protecting nozzle around our propellers that adds a non negligible drag, why don’t we use this interesting example to deflect water in the opposite direction to the propeller ? If the example is right, we might cancel the drag induced by the nozzle.
What do you think ?
This idea comes from
1- 1’24: Energo Flow up to 10% saving
2- 1’37: EnergoProFin: up to 5% saving
3- 1’48: Nozzle: 5 to 15 % saving
4- 1’57: EnergoPac: up to 6% saving
We tried solution n°3 and didn’t come to the same conclusion: we move faster without nozzle so it adds drag.
However, solution n°4 “EnergoPac”, a mast behind the propeller is @Riwi’s attempt in “The other way round” thread, a passive solution that calms the propeller flow with zero added drag. Maybe is it time to revive it ?
Solution n°2 is interesting. Would it be better than @Flying_Rodeo recent boss cap ensuring zero noise and vibrations ?
The fundamental flaw with this is that the prop will be hit by everything in your path. Sticks, weeds, any debris will hit the prop first and potentially damage it. A damaged prop would then automatically have less efficiency and also damage the motor as it would be badly unbalanced. The foil mast provides a great “plough” to shift debris out the way of the prop thus preventing damage. It is also very obviously safer if it were to ever hit another living creature.
Now for another strange thing…why does everyone want to use whats efficient for tugs and ships for efoils? The use case is completely different in every sense other than they are both in the water. If you want to figure out what works well for efoil’s you need to look at what works well for high performance, low drag sports watercraft etc…
Yeah, I’d agree that there is not much difference in efficiency. And forward facing way worse for safety.
The dual props are great but make the drive way to complex for our needs!
I still need to get my single motor dual jet belt drive design finished. I think it would be pretty interesting to see how it handles.
Interestingly most solarboats (who aim for efficiency) have pullers instead of pushers. I know the Delft team had their’s designed by Marin - marine design firm and they came up with a super efficient puller.
With regards to the main topic - the prop (or rotor for a jet) imparts a tangential velocity to the stream. You can win this back and increase pressure difference with a stator before or after the blades that cancels this tangential velocity. I’ve been working on a parametric impeller and dug up my turbo- machinery literature which explains exactly this. I found the below link useful as a summary that actually talks you through the calculations (MIT for the win ) UNIFIED PROPULSION LECTURE #1
I wouldn’t agree with going duo prop for efficiency since what makes the most impact is disk area. This is why helicopters are substantially more efficient than multirotors.
So one big, slow moving (no seperation/ turbulence) disk area is what will yield the best efficiency.
The key for efficiency in FR prop, in my opinion, is reduced blade thickness. That’s what most does the trick. Any add on like the above tug boat solutions will add drag above a minimum speed. As proof of concept just try with and without duct (any style of duct).
@MaB I think the point made is: if you’re running a duct might as well add features to improve efficiency.
@Riwi that’s interesting. Do you mean two separate props or counter rotating? Can’t imagine two separate props could be more efficient but I’m no expert.
Printed and tested on testing pool at work, definitely an exercise of style. Crappy performance though. 3d printed with a Statasys, lasted only a few run.
At hi rpm the less on water the best
3d ASA or ABS did not withstand the abuse, we tried the prop just out of curiosity and enthusiasm for such fancy design, on a bowthruster pod, measuring rpm, thrust, power absorption and noise. The thrust was ridiculously low, lot of drag because power request was unusually high respect to the classic 4 blades propellers, even the 3d printed ones (we use such, at first, before to CNC the couple of chosen models.
Anyway, it’s printable, a little tricky to CAD properly and to be printed without solvable supports…but surely doable. If it won’t work…still an attractive object to keep on your desk!
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