Prop Shaft Systems.

[Rick Willoughby]

Originally Posted by MikeJohns

I was reading Brians 300HP+ Motor vessel post.

OK from a pragmatic point of view, presuming a sound and well built hull with strength and stability adequate to the conditions meeting SOLAS requirements

Robust and reliable propelling machinery is an equally important safety feature on power boats and the next equal on sailing vessels.

Reliable auxilliary propulsion on sailing vessels has saved more lives and vessels than most people would have considered.

Vessels founder principally on coasts which those same circumnavigators tend to avoid.

As Ad Hoc said the stored strain energy is high relative to the shaft size.

Its a great application for your boats but I'm concerned when you suggest it for a powered offshore vessel.

See post #1. It is about sailing vessels. Like I pointed out, motoring on such vessels is well down the list from safety perspective. Most critical time for a motor on such vessels is in a harbour from what I have experienced. Pointing ability and speed are the key safety factor for a lee shore.

The drives that Brian originally pointed to were all related to yacht auxiliaries I believe. He introduced the high power application in a subsequent post.

That said, I do not discard the curved shaft for ocean going vessels. You can see by many previous posts on this thread and on other threads that not many people understand how a prop actually works and why it is inherently stable when pushing without the need for any outboard support. This understanding has never been really exploited. It simply does not need rigid support. The thrust produced from Mark Drela's prop absorbing 1kW would be something like 170N. Now he has a 3mm shaft in a 10ft column not buckling under that force. To me that is quite amazing. Think about it 3mm and 1kW. Since the torque rating goes up with the 3rd power of diameter, going up to 6mm at the same rpm will give 8kW. As you can see you do not need very big shaft to get significant power transfer. Look at the size of an 8kW diesel and think of transmitting its power through a 6mm shaft. Go to 10mm if you must to increase the safety margin.

Anything can be engineered if you understand what is going on. Many do not. They just look at how it has been done in the past and think this is the only way to do it. I look for opportunities not road blocks.

When you have a big heavy shaft you have to be very careful of bumping it for fear of a slight bend. Its weight will cause significant vibration if not perfectly straight.

Very few people understand that inclining a shaft induces vibration. From an engineering perspective this is much more difficult to contend with than anything associated with designing a curved shaft.

The 1/4" shaft I used on my electric drive got bent at some point when I did not take care and placed a loaded box on it. I straightened it by eye. It is still bent but functions without problem:
http://www.boatdesign.net/gallery/da...11JE_10kph.wmv
http://www.boatdesign.net/gallery/da...11JE_Drive.wmv
It is far more forgiving than a rigid shaft.

I had disregarded the curved shaft as practical for offshore application but the more I use them and think about their robustness compared with rigid shafts the better I like them. Beats having a gearbox under the water, which is the only current method of getting a shaft horizontal at the prop - unless you want the prop in disturbed flow behind the hull. Or the belts and chain that Brian suggested.

Could you imagine going to an aircraft manufacturer and telling them to whack the engines on say 15 degrees up angle to the fuselage. They would say you were bonkers. And yet this is done without a second thought on boats with very few actually understanding the consequences. They chase shaft alignment and ballancing to the nth degree and then mount the shaft inclined - simply do not understand what is actually going on. The shaft needs to be heavy and solidly supported to handle the induced vibration. Allow the shaft to go with the flow and the large unballanced forces disappear.

Rick W

[Ad Hoc]

Rick

A flexible shaft is totally dependent upon the radius of the shaft, and are normally only used of low power transmission, for reasons already cited above.

A typical flexible shaft, used for power tools etc, for example of diameter 4.75mm has a torque rating of 1243Nm on a radius of 10". But when that radius is reduced to 4" is drops to 452Nm, a reduction of over 63%.

Flexible shafts are limited in their radius and the max torque transmission, owing to the stiffness of the material, hence low power applications only, as already noted above in whirl calc's ref.

As for 8kW, that is nothing!....i can almost swim that fast...which is what Mike is referring to. This is all low power transmission, not real power transmision.

[daiquiri]

Originally Posted by Ad Hoc

Flexible shafts are limited in their radius and the max torque transmission, owing to the stiffness of the material, hence low power applications only, as already noted above in whirl calc's ref.

Hi Ad hoc

The critical whirling speed (Nc) is indeed a variable here, but in effect there is also a (significant?) stabilizing effect of unsymmetrical hydrodynamic forces acting on inclined prop's blades. They most probably tend to shift the Nc much beyond that of a cantilevered rotor case which is usually considered.
I had made a simple qualitative analysis in my post #17 and, from a mechanical point of view, things do appear to fit well with Rick's observations. The non-symmetrical hydrodynamic forces on an inclined pushing prop appear to act as dynamic stabilization for the shaft. I don't have time right now to make a numerical calculation of the forces and moments involved in a whole prop disc, but as soon as I'll find some spare time for that, I will.

What remains as a big question mark is if and how can it be employed for boats whose safety at sea is tightly related to their propulsion system, which means nearly all motor boats / ships (as MikeJohns has pointed out).
If you read my post #41, I've expressed the similar doubts over the release of the stored spring energy, but yet this concept might be worth some more investigation - even if just for better understanding where are the limits for it's practical use. One big limit was stated by Rick W., i.e. the impossibility of going reverse, because the system is stable only when in pushing mode. But it can be resolved, like anything else can. Not having to deal with shaft alignment and mis-alignment problems is a nice motivation for investigating into this concept, imho.

Now that we know it works well for pedal boat, the question imho becomes: what other types of boats and up to what power, rpm, sea-state or else could benefit the simplicity of flexible shaft systems?

[Ad Hoc]

Hi D

Just read your post in #17.

It seems your analysis is based upon an open water free flow?...is this correct? The presence of a hull and its wake iwo of a prop, makes the analysis totally different ball game. There have been endless studies and papers trying to estimate/predict the effects on the dynamic aspects of props in close proximity to a hull and even worse, when inclined. Hence the comment that your calc's can only be for open water free flow, which 99% of all boats do not have!

But the prop, whether balanced or not, is spinning owing to the torque in the shaft. A shaft can only transmit a certain amount of torque and this relationship is directly proportional to the modulus of the shaft, ie the radius and material properties.

So, a small amount of power is fine, the shaft diameter is small hence modulus is low and hence the max toque transmitted is low. Change the material property, changes again. Try twisting steel compared to soft rubber.

But try transmitted a large amount of torque in the same small diameter, can't do. Unless something changes, material properties and/or radius.

Similarly, whether the flexible shaft is 1.0m and works, if the same conditions remain, except extend the shaft to 10.0m in length it wont work. There requires a change to the original parameters to maintain equilibrium of dynamics and statics.. Why?, because to maintain the same angle of twist over a fixed length the radius must change.

The Nc always plays a role...no matter how one looks at it.

Small flexible shafts are fine for low power applications, but not in higher power/torque transmissions.

The strain energy when twisting is again directly proportional to the modulus (T^2/GJ)....conditions for strain energy in the shaft must be maintained, whatever it is doing.

Hence fine when used in low torque/power applications...and on a prop in free flow only. Unforseen loads and eccentric loads, also do not help, as would be the case in a 'normal' boat.

[MikeJohns]

Originally Posted by Rick Willoughby

....................not many people understand how a prop actually works ........

Could you imagine going to an aircraft manufacturer and telling them to whack the engines on say 15 degrees up angle to the fuselage. They would say you were bonkers...........

Rick W

Rick

What you say is not that applicable for a displacment monohull. You might find it sensible to have a moderate shaft angle on some hulls.

The propeller operates in the wake of the hull and when under power the whole wake field can be highly non-uniform at the best of times (In a seaway in a smaller vessel its anyones guess).
However in calm water (recirc-test tank ) with tuft tests and local velocity measurements it can be illuminating to see just what the wake does. You find a large vortex development where the water flows up from the bilge but down from the waterline and it can even flow foreward at the stern centre-line, this is all modified locally by the 'actuator disk' but the infeed is all highly non-uniform.

Surely if you start trying to use lifting line theories in this environment to predict vibration it's lacks validity?
As soon as you have skewed blades it gets even harder and more complex.

Advanced and expensive prediction for performance vessels requires that wake field be defined before you start your prop-position- angle blade shape design. The whole analysis will then be Ranse or similar. Then you may well find a moderate shaft angle beneficial and it's a design spiral since the prop flow modifies the wake.

I think that given the turbulent wake a trailing prop on a flexy shaft would be operating in and even without the vessel ahead constantly pitching and heaving the prop would be constantly out of balance and would be 'searching around' constantly for that non-existant steady inflow to align to.

What do you think?

[Rick Willoughby]

Originally Posted by MikeJohns

Rick

What you say is not that applicable for a displacment monohull. You might find it sensible to have a moderate shaft angle on some hulls.

The propeller operates in the wake of the hull and when under power the whole wake field can be highly non-uniform at the best of times (In a seaway in a smaller vessel its anyones guess).
However in calm water (recirc-test tank ) with tuft tests and local velocity measurements it can be illuminating to see just what the wake does. You find a large vortex development where the water flows up from the bilge but down from the waterline and it can even flow foreward at the stern centre-line, this is all modified locally by the 'actuator disk' but the infeed is all highly non-uniform.

Surely if you start trying to use lifting line theories in this environment to predict vibration it's lacks validity?
As soon as you have skewed blades it gets even harder and more complex.

Advanced and expensive prediction for performance vessels requires that wake field be defined before you start your prop-position- angle blade shape design. The whole analysis will then be Ranse or similar. Then you may well find a moderate shaft angle beneficial and it's a design spiral since the prop flow modifies the wake.

I think that given the turbulent wake a trailing prop on a flexy shaft would be operating in and even without the vessel ahead constantly pitching and heaving the prop would be constantly out of balance and would be 'searching around' constantly for that non-existant steady inflow to align to.

What do you think?

I am principally talking about sailing vessels where the prop is essentially in undisturbed flow.

However if you consider any outboard or high speed planing hull the prop operates in undisturbed flow apart from what the drive leg or shaft creates. Much effort is applied to make these streamline shapes.

When you consider a 10 to 20mm shaft transmitting the power to say a 600mm prop then it is not creating much wake. Any restraining strut does not have to be very stiff laterally because a prop running in line with the flow is powerfully self-aligning particularly if it is an efficient large diameter prop have high aspect blades.

If you consider modern sailing vessels there is no concern about high aspect keels 2 to 3m deep and even higher aspect rudders maybe 2m deep. With a large diameter 2-bladed prop on a curved shaft it can be raised up behind the transom to clear the water to reduce drag under sail. Or with a cat the shaft comes down from the bridge deck above the waterline and is supported off the aft crossbeam. Can be easily raised to completely clear the water when under sail. As simpler as tilting an outboard.

As far as shaft critical speeds are concerned, try to play a stringed instrument underwater. I have not calculated what thickness of shaft would start to run into to problems with critical speed, if at all, when submerged but 10mm steel does not have a critical speed because the damping of the water is too great. Also the prop acts as a rigid support once turning even if there is no strut.

Basically you need to unlearn what you have been taught about props and shafts and think about this idea from scratch.

Under ideal conditions you would run a large diameter prop off a horizontal shaft in undisturbed flow to get the best efficiency. In practice there is a draft constraint so shafts either extend behind the hull, angle down from the hull or are off outboards hung off the stern with the smallest possible gearbox underwater to reduce turbulence. All have fundamental performance issues. The curved shaft offers a means to overcome the limitations of these other options. From what I have seen very few understand what is going on and no one has really explored curved shafts.

When it comes to shielding the prop from foreign objects or going aground it has compliant support so just bounces out of the way or bumps along. The hulls on my pedal boats draw about 100mm. The prop is around 400mm in diameter and normal draft is about 500mm, set by the prop. However I can easily operate in water just deep enough to float the hull by raising the prop and operating it as surface piercing. Not very efficient but it enables me to negotiate very shallow water.

What diameter shaft of spring steel do you actually need to turn a prop absorbing 100kW at 1000rpm with a safety factor of 2.5 if critical speed, corrosion and mechanical damage are negligible issues?

What diameter shaft do you need to handle the bending moment from the p-factor produced by a 2-bladed prop absorbing 100kW at 1000rpm doing 10kts with a shaft inclination of 15 degrees.

Rick W

[yipster]

Originally Posted by Rick Willoughby

The curved shaft offers a means to overcome the limitations of these other options. From what I have seen very few understand what is going on and no one has really explored curved shafts.

not true, '63 pontiac tempest comes to mind, oil drill's also use them and offcourse there are flexible couplings
what i havent seen yet is a recreational CR propshaft

[Rick Willoughby]

Originally Posted by yipster

not true, '63 pontiac tempest comes to mind, oil drill's also use them and offcourse there are flexible couplings
what i havent seen yet is a recreational CR propshaft

No dispute about the oil drill string being curved. I think the Pontiac had the "Rope" shaft - not sure how it compares with the curved shaft described here. However the discussion is ON propeller shafts.

The understanding I am referring to is around the critical speed of a shaft in water (what diameter does it occur) and the apparent fixity of a pushing prop. Associated with the latter is the p-factor in angled flow to a prop. Something that aeroplanes worry about but apparently no concern for boats. The common wisdom among boaties is that the shafts vibrate due to being near critical speed or some misalignment in the drive train - few actually give thought to how the blades produce thrust.

Have a go at the comparisons I posed in my previous post. It will give you some idea of what I am on about. It did not bother doing the calculation until I bent a 12mm 2011 T8 aluminium shaft in a pedal boat. Once I understood what was going on I found I could operate with a 6mm shaft without a problem. Compare the bending strength of a 12mm shaft to a 6mm shaft. Amazing what can be done when you understand the forces involved.

Rick W

[Rick Willoughby]

For those who have come into the thread recently I have posted the picture of an unsupported curved shaft when out of the water. You can see it just dangles down. After all it is only 1/4" diameter and 6ft long supporting a 380mm diameter prop.

As soon as the prop starts spinning in water and producing thrust it just aligns perfectly with flow.

Some have questioned how is it possible to get 85% efficiency from a prop at relatively low Reynolds number. Well when you have:
high aspect twisted foils for the blades,
operating on a large area,
with very low velocity ratio,
prop perfectly aligned to flow,
only minute drag from a tiny 1/4" shaft and
no appendage drag
it is quite easy.

Like I say - do not think anyone has really explored the possibilities with curved shafts and props as most look at the attached photos with disbelief.

By the way that boat in that configuration has done 9kts.

Rick W

[yipster]

thanks for showing and no disbelief, questionable but amazing yes, applicability..
ok must read back a bit

edit see car was allready named and had teh shaft in a curved tube

[MikeJohns]

Originally Posted by Rick Willoughby

I am principally talking about sailing vessels where the prop is essentially in undisturbed flow.

Which is why a lot of interesteing ideas for smaller vessels that look so good in the testing are compromised in waves.

Originally Posted by Rick Willoughby

Basically you need to unlearn what you have been taught about props and shafts and think about this idea from scratch....

No problem with the physics at all and I am quite happy that it works, but how to apply a thinner curved shaft robustly in a marine environment is the tricky bit. I look foreward to your sulutions/suggestions there.

Keep up the good work.

[Rick Willoughby]

Originally Posted by MikeJohns

.... I am quite happy that it works, but how to apply a thinner curved shaft robustly in a marine environment is the tricky bit. I look foreward to your sulutions/suggestions there.

Keep up the good work.

Mike
Thanks. I have had far greater challenges in paid work. Admittedly significantly greater resources than my personal resources that I can bring to bear on this challenge though.

Do you happen to know Dave Sugden - spent 10 years trying to get one of his inventions a commercial reality - with limited success.

Rick W

[MikeJohns]

I think he's concentrating on violins now.

[Rick Willoughby]

Mike
Leave you to figure out what invention of Dave's I spent a good deal of my working life trying to exploit. Not many people have patents covering such a wide range of engineering design.

Rick W

[brian eiland]

Originally Posted by Rick Willoughby

Mike
Leave you to figure out what invention of Dave's I spent a good deal of my working life trying to exploit. Not many people have patents covering such a wide range of engineering design.

Rick W

I noticed the "compact high torque hydraulic motors". Did you happen to ever look at this hydraulic motor I included on my website:
http://www.runningtideyachts.com/power/
(I'll repeat some of it here, as I may take it out of my site soon)


While working on a new bow thruster design, a Hungarian gentleman has developed and built several entirely new and unique gearless, twin-rotor, angular-piston, hydraulic motors. This patented mechanism employs two rotors which are interconnected by angular pistons that are all contained and bearing mounted in a common housing. The linear displacement of the pistons in one of the rotors is directly converted into the rotational motion of the other rotor, interactively. The geometrical and mechanical relationship between the two interconnected rotors provides optimum conditions for direct conversion of linear displacement of the pistons into rotational displacement of the rotors. Barring frictional losses, the conversion is 100% efficient. None of the commercially available axial piston motors have such direct and efficient means for converting linear displacement into rotational motion.

And maybe more importantly, the design is greatly simplified by the elimination of the traditional angular swash plates, sliding shoes, and related components. There are many fewer moving parts in this motor, which should significantly increase their reliability while decreasing the maintenance and manufacturing cost of these units. Simultaneous linear and rotational motion of the pistons within the rotor bores results in even wear, increasing the life of the motors.

Employing suitable valving, the motor can be operated at two levels of speed and torque output at the same operating pressure and flow rate of fluid. Simultaneous pressurization of the pistons in both rotors produces the "wedge effect", driving both rotors with equal power and higher torque. The interactive function of the pistons assures optimum transmission of the driving torque between the rotors by distributing the load to all pistons. This exceptionally high torque efficiency allows direct drive of the propeller without gears, and the using only medium rather than high pressure pumps.

The motors are bi-directional, and the direction of the rotation can be selected externally by valves, or internally by interchangeable valve plates. The motors have automatic hydraulic pressure and propeller thrust compensation. The replacement of the mechanical transmission by hydraulic components provides a stepless speed control from stand still to full speed, as well as instant reversal, reportedly assuring the availability of full power at any moment.

Having no mechanical connections between the engine and the propeller drive units, the marine designer has complete freedom in the positioning of single or multiple propeller drives and the engine units themselves.