Prop Shaft Systems.

No it won't. You have not thought about the forces involved. When it pulls at an angle it wants to dive rather than lift. It is inherently unstable if pulling.

You need to consider what each blade is doing. You cannot treat the prop as a single force. The forces involved can be much greater than the thrust force particularly if it is operating at low slip.

Rick W.

 

This discussion is becoming a bit more academic than I've thought but I'm glad I can leave the usual, commercial, gross engineering for the moment.
So...You're right. I've made a brief analysis (only 4 points: top, bottom, and the two opposite lateral positions) of the hydrodynamic forces acting on blades of an inclined prop.
Assuming that the rotation angle 0° is at 12 o'clock (top of the prop) I've seen that the difference in blade forces arise when blades leave the positions at 0° and 180°. The maximum difference should be at 90° and 270° so I'll continue with the analysis of only these two positions.
In effect for a pulling configuration the vectorial sum of the forces points downwards, while for the pushing prop it points upwards.
And not only. I have also noticed that the forces at 90° and 270° are not the same, neither in direction nor in magnitude, so the application point of the net force vector is not passing through the hub but is located at some lateral distance from the center.
It means that there is a moment acting at the prop's hub and it's vector is pointing upwards in case of pushing prop and downwards in case of pulling prop.
And the angular momentum equation tells me in this case that the pushing prop will tend to increase the pitch-up angle until it reaches the equilibrium with the spring action of the shaft. The pulling prop will conversely tend to pitch down until so much inclined that it becomes useful.
Is this analysis correct? I feel like I got back to school again. The problem is that it didn't make me become any younger...

 

This is not ACADEMIC. This is the CORE issue with shaft vibration that Daniel should be addressing. The analysis you describe is exactly what he needs to be doing to understand the source of vibration and the magnitude of the exciting force.

Obviously if you do not incline the shaft then you take away the largest source of vibration.

If you start to apply some realistic numbers to the example you describe then you will get an idea of how significant these forces are. Lets say for the yacht doing 4m/s (say 8kts) requires 4kW at the hull then the average force is 1000N. With a two bladed prop running aligned the force on each blade is a steady 500N.

For simplicity you can assume an efficient blade will run at 3 degrees AoA if aligned with flow. The blade force can be taken as being applied at 75% of radius as a good approximation.

Another good approximation is that the lift force on the blade is linear with the angle of attack. So at 6 degrees the force will be twice that of 3 degrees.

Now set the shaft at 5 degrees angle and work out what angle of attack will occur at the 90 and 270 degree points to achieve the same average thrust. It requires some iteration and need to consider more than just the 90 and 270 to get an idea of the average. (I am aware of people thinking they have the perfect prop because the prop actually exhibits negative slip - advances more than the pitch for each rev when mounted on an inclined shaft). The forces will be significantly different and they are producing a moment by virtue of the separation of their point of application being either side of the shaft. These set the shaft into a bow that will cause nasty deflections unless everything it rigidly attached - almost impossible in any boat.

Once you have done this analysis you will be much more concerned about inclining shafts. It is particularly so if you want efficient operation where the props are required to operate at low slip. Going up in the number of blades reduces the amplitude of the force but increases the frequency. So in considering vibration with an inclined shaft you also have to deal with the blade passing frequency. This will be generating higher forces than any imbalance on the prop or shaft.

Rick W.

 

re prop shaft problems...as Marshmat pointed out there are practical considerations to think about ,theory is one thing,practice is quite another as we have all found out at one time or another .
The simple fact is that inboard powered boats under about 35 or 40 foot have some degree of shaft angle ,and you just have to put up with it ,in practice up to 15 degrees is quite acceptable ,in a good engineered installation there will be no discernable vibration ( none that the occupants of the boat will be aware of )
As regards sterndrives being a solution to keeping the prop in the correct "flow" i suggest buying and operating one for a few years ,putting 150 200 hp through those little shafts and gears ,and you would settle for vibration any day .
As regards aircraft designers laughing at engine installations not in correct "flow",the Short Sunderland flying boat had its 4 engines angled outwards ,it certainly did not suffer for it ,also i believe some Grumman aircraft in world war 2 had angled engine installations,givinng them a nose up attitude inflight .................cheers Hartley

 

I don't dispute that you can realize significant improvements in efficiency by aligning the prop parallel to the flow- indeed, that would be the ideal situation for just about all applications.

I also don't dispute that a self-stabilizing, strutless pusher prop can be done successfully- witness Rick's tests above. It can and does work.

The reason why I don't think- at present- that flexible shafts with no cutless bearing are a good idea has more to do with practical concerns. Most of us have had to deal with a fouled prop, a rock, or a piece of driftwood at some point. And most boats don't have a convenient place in undisturbed flow to put a self-supporting shaft and prop, without exposing it to damage.

Angled shafts are not ideal, but they have been made to work for as long as there have been propellers, and if properly installed exhibit little or no tendency for excessive vibration. Yes, there are substantial off-axis forces created at the blades of an inclined prop, that would induce vibration in a free shaft. But that is why, when we incline the shaft- often the only way to get it to fit- we support it well enough that those off-axis forces do not translate into nasty harmonic oscillations in the shaft. The Benetau (IIRC) from the initial post likely has an inclined shaft, that is not properly supported for the forces it is carrying. On a sailing auxiliary, modifying the hull to accommodate a better engine setup would likely not be worth the compromises to the sailing characteristics, so it isn't done.

Rick's setup is one, apparently quite effective, way of supporting a propeller. As an experimental platform it is very intriguing. And getting the prop as close as possible to parallel with the flow is certainly a good goal to strive for. But I don't think it's right to label inclined- or supported- shafts as inherently wrong, when there is lots of empirical evidence to suggest otherwise.

 

An inclined shaft is not "good" engineering. It is a poor compromise at best. It is done because very few people have bothered to understand what it does and are happy to use low efficiency props that are not too bothered by shaft inclination. It will become less tolerated as designers seek more fuel efficient applications with low slip props.

Aircraft engines may not have the prop shaft aligned with the wing or fuselage but they are aligned with the flow. They can be offset slightly to suit the stream flow as it approaches the wing under normal cruise conditions.

What I am proposing is for Daniel to work out the size of the forces related to an inclined shaft. Understanding the forces is the basis of any mechanical design. If you realise you can dramatically reduce the forces simply by aligning the shaft then you become less tolerant of inclined shafts.

If you buy an under engineered stern drive then tough. Like all things buyer beware. It is not a good reason to replace it with an inclined shaft. Inclined shaft systems are not "engineered" they are a mechanical concoction usually developed through trial and error with little to no analysis of the forces. That is why so many suffer vibration problems and you get the seat of the pants engineers proposing better alignment and better balancing. Both a waste of time and money for an inclined shaft.

Rick W

 

Matt
I am not proposing a shaft unsupported at the outboard end as being practical. I can tell you it is a nuisance if you want to go astern because it wants to dive or even approach a beach because it hangs down once it stops rotating.

The point in showing the unsupported shaft it is to get people thinking about the forces involved as very few people actually understand what is happening. Even fewer have actually done calculations on the significance of the out-of-balance forces related to shaft inclination.

If nothing else daiquiri has seen the light and is on the right track. Hopefully Daniel will take a proper engineering approach and work out the forces so he has a basis for shaft design. (That is of course tolerating shaft inclination.)

I believe there is a possibility for a nicely engineered curved shaft but it becomes a material issue. If you look at the required torque and the damping provided by the water I expect you could get away with maybe a 1/2" shaft in a sailing boat.

I thought my 1/4" shafts were small but Mark Drella has used a 3mm shaft successfully transmitting up to 1kW. So when you look at pure torque requirement you do not need much meat. My 1/4" shaft has a very large safety factor on torque and this will comfortably get me to 10kts.

Rick W

 

By the way I have actually seen people worried about a bent shaft in an inclined application. The bend induces nothing like the forces created by the inclination. Complaints about vibration are common. The first question to ask- Is the shaft inclined? If it is you are 90% of the way to solving the problem because you now know the cause. You have to do things to reduce the force or accommodate them. Straightening shafts, balancing and aligning is just a waste of time.

Rick

 

By the way, the reason why I developed a compliant shaft was to make it tolerant to striking solid objects. I compete with paddled craft in a log infested river and anything rigid would just get broken or bent when it gets dragged over a log. The flexible shaft simply bounces over objects so it is much less prone to damage.


The other feature of the set up is having the prop running beside me and being able to lift it clear of the water to remove fouling. It is near impossible to yield a 1/4" spring steel shaft to permanently bend it because it flexes so readily. It takes on an extreme curve before it yields so you have to be trying to yield it to get a bend.


Rick W

 

Thanks for making me see the light.

Now seriously, I've learned something new yesterday and I'm grateful for that.
Hope that I'll be able to give back the favour. I can teach you some good recipes with pasta, if you want.

 

I have been looking forward to the calculation of the out-of-balance forces. You have the basic physics right, now you just need the numbers. Not many have bothered to work it out. If you look at typical pleasure craft shafts they are much heavier than they need to be simply to contend with the induced forces from inclined shafts.

I am glad I was able to get someone to understand to the point you have. Most just dismiss it because inclined shafts work. They also happily tolerate prop efficiencies of under 60%. That was acceptable when the globe was huge and human kind did not have the impact it does now on our resources. Hopefully by understanding and being clever we can achieve more with less.

Rick W

 

Agreed, Rick.

My own analysis (admittedly rather brief as I'm a bit occupied with the physics of much, much smaller things these days) has me agreeing with you that most vibration issues- and that annoying side thrust problem- can be mitigated simply by keeping the prop shaft parallel to the flow.

Being at a significant angle to the flow is not a natural state for a propeller. Large ship designers have known this for a long time- when was the last time anyone saw a 600' freighter with inclined prop shafts?

In a primarily sail powered vessel, where the engine is really just for puttering around the marina, anchoring, and occasionally helping claw away from a lee shore, I'd happily accept a (well supported) inclined shaft if the alternative meant a hull shape that would detract from the boat's sailing ability. Which, in a shallow canoe body hull with fin keel, it might.

In a vessel expected to spend significant time under power, I would expect the drive system to be engineered accordingly. The use of 15-20 deg. shaft angles on modern power cruisers is a product of marketing beating out engineering; the marketing department says they'd rather sell a 600 hp, 25-knot twin inboard 32-footer with an overloaded deep-V hull than what the engineers would want- which would likely be longer for its weight, much less powerful, and with a hull shape and drivetrain optimized for maximum efficiency. So to cram 2x300hp out those props with such a short waterline length, you end up with a big prop in a tunnel with a steep shaft- not ideal, shakes like hell, etc.

And before you ask.... all of the "Matt's next boat" candidates currently on the drawing board, except for one, have large diameter, relatively slow turning props that are either at 0 degrees to the flow, or are on trimmmable sterndrive legs that can be brought to 0 degrees. (The odd one out is a jet drive, chosen mainly for its ability to handle rocky conditions and driftwood- which claim several props a year in my home territory.)

 

Matt
A viable solution for serious sailing craft not wanting to instal a sail drive (why I do not know) would be to consider the much smaller flexible shaft. The outboard end nicely supported of course. This has the advantage of a tiny shaft and accordingly reduced drag. It would need to be a high strength stainless steel with good endurance capability. This is something I have not been able to get hold of.

Another idea I have thought about is a prop on a flexible shaft and an elevating strut so the prop can be lifted out of the water when not required. This would dramatically reduce drag. The problem with this set up is getting around the rudder but if it had twin rudders then the shaft would just run between them. So a very simple method of stowing the prop and inspecting it if required.

Rick W

 

I've finally got time to properly read and research the comment on here.

Moving on from that comment, would twin counter rotating props help?
- Im trying to understand the forces that would act blade on blade, and the affects of having it at an angle to the flow, as it would be on an inclinded shaft.
- If you had two props (typical each with two blades i guess) counter rotating?

 

I like the idea of that.
- As you say, the material would have to be right. And you may have a fatiuge issue at the sort of hours that would be expected of the setup?

Failing that, especially as the power levals are so low (10horse power or so) why are there no belt or chain drive systems on boat.
- Take V-drive system of reversing the engine and gearbox one step further and drop the output to bildge leval (or close to) , at the belly of the boat. Then run the shaft out stright and horizontal. Such that by the time it gone far enough aft, it clears the hull enough for the prop?

What sort of size props are we talking about typical? 12*12 twoblade case bronze prop?


Daniel