Optimal gearing for human powered watercraft

[philSweet]

Here's a useful document to support the design of the pedal drive. Realize that unlike on a bike, the prop isn't attached rigidly to the ground - it slips. So any torque variation produces a pulsing thrust and rpm change. It will feel like pedaling up a steep hill in too high a gear. This is bad for people engine efficiency. Using a prop with low slip and a higher torque coefficient than ideal based on standard selection methods can help here. You are trading off a bit of friction for better legs. Also consider mild egg gears.

https://www.uni-konstanz.de/mmsp/pubsys/publishedFiles/QuDaSa15.pdf

Also be very conscious of stiffness of all drive components. Any windup of shafting or lash in the chain drive is felt as slip at the pedals.

Realize also that people engines make huge peak torque, even if they are only at around one horsepower. I used to compete on the bike, and at 135 pounds, I could leg press 1000 pounds. If I'm making 1 hp at 90 rpm, that's like a 40 hp tractor at full chat at 3600 rpm. I've looked at using small bush-hog transmissions for angle drives. Better is to use a hub gears in the B bracket to get the rpms up immediately, perhaps even gearing back down a little bit at the prop. Six to one is what I came up with for endurance racing, and I don't see why a sprint would be different, just that everything has to be ten times as strong.

Note that Rick W's craft were mostly endurance racers. His 1/4" shaft was marginal even then. You need at least ten times the stiffness modulus in your shaft after accounting for any gearing differences.

One other point - on racing bikes, aerodynamics is crucial, and the bottom brackets are actually too narrow for best effort. Widening the pedal spacing about 2 inches over what is the norm for a racing bike makes for better ergonomics on a pedal boat. Unfortunately, it may create yaw issues unless you train on a swing or turntable.

And there is nothing wrong with simplication, either. At least run the numbers on a two-bladed straight 1:1 direct drive surface piercing prop. Try small helicopter tail rotor extrusions for blades, and clock the blades to the pedal crank for torque matching. Give the shaft some yaw angle for best efficiency (A little bit of paddle wheel effect). Varying the yaw angle slightly is one way to vary the gearing a little bit, say to account for wind.

 

[philSweet]

As far a props are concerned, only recently has there been a full analytic method for handling propeller and turbine optimization. It has taken well over 100 years to produce a closed form series of formulas that let you optimize induction flow and the blade geometry at the same time.

Here is that paper - https://openscholarship.wustl.edu/c...le.com/&httpsredir=1&article=2174&context=etd

and some prop design and analyics software - http://web.mit.edu/drela/Public/web/xrotor/

 

[jehardiman]

We have no set physical constrictions when it comes to the watercraft, only that it has to be human powered and reach 'a maximum top speed over 100m', I would interpret this as trying to break the 1991 record set by Drela. In regards to the propeller, we were thinking skinnier longer blades as this would allow us to have a higher top speed (high speed, low torque) so I think 20" may be appropriate?

Check to see if the requirement is 100m from a standing start or just through a 100m speed trap. You would use very different propellers/propulsors.

 

[DogCavalry]

As far a props are concerned, only recently has there been a full analytic method for handling propeller and turbine optimization. It has taken well over 100 years to produce a closed form series of formulas that let you optimize induction flow and the blade geometry at the same time.

Here is that paper - https://www.uni-konstanz.de/mmsp/pubsys/publishedFiles/QuDaSa15.pdf

and some prop design and analyics software - http://web.mit.edu/drela/Public/web/xrotor/

Interesting as that paper is, it is about peddling, not props. But thanks anyway.

 

[philSweet]

Fixed the link.

 

[DogCavalry]

Thanks!

 

[DogCavalry]

I feel, without running the numbers, that a 2 bladed surface piercing prop would have a brutally unsteady load. Thered be the moment when the rising blade burst out of the surface, but the falling blade wasn't wet yet. But that's just my visualization.

 

[philSweet]

Yes, that's the whole idea, or half the idea. When pedaling, you only make torque half the time, so load the prop in sync with the pedal power. The other half is that I can eliminate a whole bunch of lossy junk, and get enough of a power boost at the prop to make up for its, erm, odd proportions. I originally looked at this for the R2AK race where they have to pedal or row their craft a few miles into inlets at checkpoints. Typically 30 foot catamarans and such. Since they already have a big boat, agricultural-looking equipment isn't as big a problem as when trying to package it in a flyweight hull.

 

[DogCavalry]

Ah! Asymmetric loading to match asymmetric leg power... Clever.

 

[portacruise]

Another possible consideration for a record breaker is to keep the weight as low as possible, since some of the pedaling energy is used to keep vehicle and Rider above the water, with a hydrofoil. CF or inflatable pontoons might help the weight situation, especially if they can be jettisoned, if it is allowed.

DC, I think I know what you mean, but aren't all propellers that have a Twist to their blades variable pitch, ie. the pitch at the root is different than at the tip? How does one go about building a controllable pitch propeller for a HPB? I have seen some model airplane propellers that I have a changeable pitch when at rest, but nothing in the range for HPB which can be adjusted on the Fly. I wonder if any of the faster HPB foil boats have used either adjustable gearing or a controllable pitch propeller? A Controllable pitch propeller might be helpful if there is a standing start involved, seems to me.

 

[DogCavalry]

Ah yes, terminology. I did mean controllable pitch, not varied pitch.

A relatively simple addition, given the complexity of the rest of the build. It seems to me. All prop driven aircraft, outside the very lowest performers, use one because the cost is unequivocally worth it.

The hub with the control mechanism is a commonly available component in sail boats. The pilot should have little difficulty adjusting pitch on the fly. Twist control, and some practice.

 

[portacruise]

I've always wondered how optimally designed surface piercing vs. 1 blade propellers compare in efficiency...

 

[BlueBell]

There are awesome adjustable pitch R/C plane prop hubs and blades available from
many European countries that allow pitch adjustment shore side, not on the fly.
Pricing is more than reasonable, blade selection varied.

These 100m speed traps are just that, typically.
So, a flying start through a timed 100m distance.
Either average speed or peak speed, or both.
It's a little ridiculous Goom, the OP, doesn't know.

 

[Tiny Turnip]

There are awesome adjustable pitch R/C plane prop hubs and blades available from
many European countries that allow pitch adjustment shore side, not on the fly.
Pricing is more than reasonable, blade selection varied.

Yes, indeed, and a fair amount of precedent for using model R/C props for hpbs. It wouldn't be beyond the wit of man to come up with a variable pitch prop which could be used on the fly from the drivers seat, if I'm thinking straight. use a tube as the drive shaft to the prop, bevel gears to take the drive from the pedals to the tube-shaft, and the control rod for the pitch up the middle of the tube, out and beyond the bevel gears to a control lever system.
Whether its feasible to do so in a robust and mechanically efficient way is another challenge. Only worth it if a standing start and a short distance?