Driveshaft Friction losses

[Banjo Dick]

We all want to put lots of power to the prop, yet it seems obvious that driveshaft friction or drag may reduce what's actually delivered. Please advise me if this topic has already been researched; I read a lot on this forum but maybe haven't found the conclusions yet.
So, I'm thinking, using a constant voltage source to initially approximate 30,000 RPM using my test motor, how much friction loss is there (consider all in a round, greased, stuffing tube) (and maybe later an "oiled" stuffing tube)
1. in a straight solid shaft, 3/16"
2. in a piano wire shaft
3. in a flex shaft 0.187"

and then

4. piano wire S curved shaft
5. flex S curved shaft
6.7.8.9. with a teflon liner
10. for my own amusement, a straight solid shaft in a square stuffing tube.

I'm thinking about conducting an experiment (unless it's been done before) to very accurately measure the current to the motor from the constant voltage circuit.
Well, if all this turns out negligible, so be it. But if it's not, maybe I'll have a market for my own design micro constant velocity joints for articulated shafts...
Your comments and advice and suggestions will be greatly appreciated and I'll gladly share my experimental results.

[Fluid]

There has been a lot of testing done by many folks. Most testing was rubbish as they neglected to apply a realistic load to the drive line. Wire and cable will whip and flex under load differently than they will unloaded, giving much different frictional losses. Most testers just confirmed what they wanted to show.

What has been proven on the water:

Wire has less friction (if set up right) and is used in many of the fastest FE boats in the world.
Cable is easiest to set up and powers some of the fastest FE boats in the world.
Compared to wire or cable, solid shafts are quite inefficient at pushing the boat due to their terrible thrust line.

BTW wire does not like an "S" bend in the stuffing tube.

BTW part of my day job is studying and reducing friction in long steel shafts whipping around in bent steel tubes.



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[NativePaul]

I haven't seen any testing good or otherwise and would like to see your results.

I have switched from flex to wire but friction was only part of the reason why, drag and weight were big parts of it too, and while I think my wires may have less friction than my flexies, I have felt some flexies in liners that amazed me.

Jay makes a good point about load and whipping, I will add that both flex and wire will whip like crazy if used with no bend at all so you would need some bend.

A plane prop gives a steady consistent load and no mess, but if you want to test flexies with liners you will need to get water up it to keep the friction down, so maybe a prop submerged in a sealed container would work best.

As Jay says solid shafts are inefficient at pushing a boat due to the thrust line, but the lift given can make a boat run more efficiently, which in turn can be enough to negate it's own inefficiencies, ECO type boats have a submerged prop on or near the CoG and are still split between straight shafts and bent ones, I am mainly a surface drive guy and only have one submerged drive race boat (bought) it has a straight shaft, if I was to build one it would have a bent shaft, but there are still plenty of folk racing with straight shafts, so I would be interested in the comparison.

One comparison that I would be really interested in you have not mentioned, it is between the 2 schools of motor position in Monos. Is it better to have the motor at the back of a mono with a relatively short flex but a tighter bend, or nearer the middle with a longer flex but a gentle curve.

[tlandauer]

Sorry: don't mean to hyjack, but this is the kind of posts that I print out and put in a binder. This is why I don't mind going back to school.