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[Guest625101138]

Originally Posted by SolomonGrundy

...
So my question is: Could a flexible shaft be appropriate in this application, if not, why?
I'm wondering if a flexible shaft would be simpler and more reliable by removing the shaft and one bevel gear.
Thanks, SG

The torsional compliance in a pedal system is an important criteria. If the shaft is rubbery in torsion it unwinds when the pedals are in the flat spot and winds up when the pedals have most leverage. A shaft that is not stiff torsionally is like pedalling in mush.

The RMS power ends up considerably higher than the average power so there is more effort to deliver the same boat speed. I have values for the shaft torsional spring constant that feels bad and similarly what feels good. So it is important that you check this information for the particular shaft.

I have used an 8mm aluminium shaft that is 1.2m long with 1:5 step up and the best speed possible was 12kph. Changing to 1/4" steel 1.2m long got to 14kph. With 9mm steel 2.2m long I got to 17.3kph. The only change was the shaft.

Rick

[OldNick]

So do I have entirely the wrong picture?

[OldNick]

I just watched a couple of YouTube vids of the European HPB championships.

These covered a 100M speed run and a 10KM race.

In both, the guy who was out front was actually riding a "water cycle" hydro foil, in that he rode a HF in conventional cycling position.

In the 10KM event he seemed to have gefallen off . But this was tight at a sharp turn and was caused by (a) hydrofoil instability or (b) interference from the 3-man craft that was his serious contender or (3) simple fatigue.

Whatever, the normal cycling position seemed to be winning until failure, and certainly won the 100 M dash over other hydrofoils that were recumbent.

Don't want to start a fight, but is the main gain for recumbent simply aerodynamics? I know that RC bikes seem to be slower uphill in competition.

This guy was obviously professionally set up, and it may well that it was all about his pedalling ability. Also the RCs were pedalling like mad, and I winder if they were wasting their power by too low a gearing (whether prop size etc).

So many questions.

[SolomonGrundy]

The prop is on the nacelle about half way down the fin keel, the ballast bulb (foil really) is on the bottom of the keel. If I extend the chain further aft of mid-ship, I could have a nearly straight shot with about a 40" shaft extending half way down through the keel from aft to forward. The material would need to be stiff, flexible, and durable...any suggestions?
As always, thanks for your input Rick & Nick.

[OldNick]

Ok. Glad you thanked me for input, not help I am just learning all this and that theoretically so far. I had the wrong idea about your setup.

Rick suggests 8mm spring steel. IIRC also a couple of times he mentioned that the shallower the curve, the more efficient and durable the shaft was. I am assuming that was why Rick got better performance with a longer shaft. 40" sounds very short, particularly if your prop is some way down the nacelle. I suppose the question is, "what angle is there in the bend, to go from optimal gearbox angle at the chain, with the the prop being vertical?". Then a minimum length could be sorted out, I would think.

Also, something comes to mind. The flexi shaft idea works stably for a push prop (which I still have to grasp). If you use it as a pull prop, will this cause friction and wear at the prop bearing? Or is that negligible? Rick? I ask because I can see the sense of a pull prop..I think...less wash around the prop from the nacelle.

Nick

[Guest625101138]

Nick
I believe the question was directed at a flexible shaft as you see in a weedeater with wrapped springs rather than a solid curved shaft.

These shafts are available with adequate torque rating but I have not seen values for the torsional spring constant. If you ever make a rubbery system you will know what I am talking about.

Rick W

[OldNick]

Sorry Rick you have lost me.

I thought I was remembering a post you did about your curved shaft trying to dive if you reversed.

I had wondered about using the coiled spring idea, because you would seemingly get less loss from curvature. But figured it would need too much support. All the applications I have seen for it require, or at least use, a housing: weedeaters, drills etc.

What is a rubber system?

Nick

[Guest625101138]

The word should have been rubbery. Meaning the torsional compliance is high.

The coiled spring shafts operate in a tube. The tube provides the support. So it does not matter if it faces forward as the shaft/tube mounting will stop it from moving about.

Here are some examples:
http://www.sswt.com/standard.htm

With 1:5 speed step up the tolerable stiffness is about 15Nm/rad. Wind-up and unwind of the shaft while being cranked will become annoying and sap energy if the stiffness is less than this value. I have not seen stifness data for the coiled spring type shafts.

Rick W

[Submarine Tom]

OldNick,

Sorry to interject here again, but go with Ricks recommendation.

Stiff, crisp, rigid, unforgiving vs soft, spongy, mushy. His "drive shaft"

or "propeller shaft" is brilliant, tested and a lucky stroke of good fortune

for him, you, and any one else looking to build what you guys are making.

In my humble opinion.

-Tom

[OldNick]

Thanks for that Rick. Now I understand.

So I am assuming that Solomon Grundy is asking about a curved shaft, but you reckon they are talking of a "proper" coiled shaft? Sorry. I just don't want to cause confusion. SG?

I can certainly see why too much rubberiness would cause trouble. I can almost feel it!

Nick

[OldNick]

not a problem Tom. As I said I had thought about the coiled idea, but having used weedeaters and drain unpluggers, I know there is a lot of spring in there as well as a tendency to coil themselves even in a housing.

Nick

[I57]

There is a boat/bike that uses a coiled shaft.
http://www.shuttlebike.com/
It uses a coiled shaft to transfer power from the rear wheel to a prop, won't set any speed records but it shows what a coiled shaft can do.

Ian

[OldNick]

Cool idea. It did about 6.5km/hr for an hour, which is a reasonable walk.

Bet it's not cheap though!

I was just wondering whether SolomonGrundy was after coil or shaft.

Nick

[SolomonGrundy]

Since I tend to trust Rick's judgment in these cases, I think what I am after is more of a shaft than a coiled spring type...but even that is up in the air as the "torsional" losses are hard to estimate considering the unknown values of shaft size and other properties. I'm imagining that a "scaled up" weed-eater type of shaft might be achievable, but I'm unsure if those things are available and the pro's and con's of using such. It seems simple enough, but would it be robust or even possible

[rambat]

I am sure this has moved onto more technical aspects, but for a few years our family (some creative types) held a fun pedal boat race. Rules were to use bike parts.. We had flipper, multi oars, Dog pedal (best) momentum assit, fully enclosed wheel and good ol plate wheelers. A great hoot... enjoy some summer images.