Prop Shaft Systems.

[Dhutch]

I have taken on a PartC project as part of my time at university to work on the design of propellor shafts.

The Breif is:
Boats traditionally use a metal shaft to transmit torque from engine and gearbox to the propellor via a sealing arrangement. (Prevents sinking) Modern yachts are becoming much lighter in weight and cost sensitive leading to slender section long drive shafts. The project is to develop an interactive design numerical code including static dynamics and instability of the rotating shaft as a tool for boat designers / builders.

The project comes about becuase the lecture in question owned a typical modern 32ft sailing yacht which had issues with shaft vibration at high/medium revs. He thought due to to insuffience support for such a long and slender shaft. Causing shaft whirl caused by the excition of the propellor.

Basical, the shaft was flexing whirling round like a bananna inside the stern tube.



What im asking is what (if any) considerations for shaft support/length/diameter are usally considered to be normal.


I've been in and around boats all my life which is one of the reasons why i choise the project. However not overly familur with yachts (for instace ive only just found out what a v-drive is, very clever) so initally im just trying to get a feel of the industry, whats common place, what people do and dont do, etc.

I sail dingys a lot, but my other interest in in narrowboats (uk canals) and in the narrowboat industry the bulk of problems are got around by over engineering.
- Weight simply isnt an issue, becuase usally they have 8-10 tonnes of conrete flags in the bottom of them anyway.
-So typically the base plate is 10-15mm steel, hull sides 6-8mm, cabin top 3-4mm. You get the picture basicaly.

A typicl drive on a narrowboat is really very short. There counter stern hulls,and the engine sits in the swim, inline with the propellor. Theres 35-50hp four pot deisal on soft mounts, a simple engine mounted 2-1 reduction box, then usally a form of flexable coupling, a short 1.5inch tail shaft though a conventional stuffing boc sterntube gland. With a 18*18 bronze prop.

Or if you take our narrowboat, with an engine in an engine room. Theres a 15ft propshaft, 2inch diameter solid stainless, running though a stuffing box, thrust bearing, second bearing, to engine. So its supported every 4/5ft or so, and its really no going anywhere!!

We are slowly starting to see vetus water-lubricated cutlass bearings appearing in the canal world, but even these (which i believe are simular to the type used on the oceanis are rair)


Daniel

[marshmat]

A quick check against a few reference books suggests that shaft diameter is typically specified as approx. 1/14 of propeller diameter for a tobin bronze shaft, and 1/17 of propeller diameter for Monel (subtract 1 from the denominator for 4-bladed props, and always round up to the next standard size). As a rule of thumb, maximum bearing spacing without causing excessive vibration is usually around 40 times shaft diameter, and bearings should never be closer than 20 times shaft diameter.

Shaft vibration could be related to insufficient support (too narrow a shaft, or too much space between bearings); it could also be the result of too little tip clearance between the propeller and the hull (should be at least 10-15% of prop diameter, anything under 10% is virtually guaranteed to shake like crazy).

[Rick Willoughby]

................
The project comes about becuase the lecture in question owned a typical modern 32ft sailing yacht which had issues with shaft vibration at high/medium revs.
........................
Daniel

Daniel
Ask him if the shaft was inclined. The out of balance forces are greater with a 2-bladed prop having high aspect blades on an easily driven hull but they exist on any inclined shaft.

If you cannot work out the problem then I can explain more. There are a number of video clips and images on the forum that help explain the problem.

The best solution for a sail boat is a sail drive.

Rick W

[daiquiri]

Shaft vibration could be related to insufficient support (too narrow a shaft, or too much space between bearings); it could also be the result of too little tip clearance between the propeller and the hull (should be at least 10-15% of prop diameter, anything under 10% is virtually guaranteed to shake like crazy).
Ask him if the shaft was inclined. The out of balance forces are greater with a 2-bladed prop having high aspect blades on an easily driven hull but they exist on any inclined shaft.
Why don't you take into consideration simply a misalignment of the shaft bearings and the engine? :confused:
It is a fiberglass boat, flexible hull... An incorrectly aligned shaft is the first thing that I would think about.

[Rick Willoughby]

Vibration is always related to unbalanced forces. The most common source of imbalance is the inclined shaft. People spend a lot of time getting all the mechanical components perfectly aligned in a system with the shaft misaligned to flow by a huge angle. I have seen as much as 20 degrees and 15 degrees is common. They give no thought to what is actually going on with the misaligned propeller. They must assume it does not matter because it is only water or have little understanding of how a propeller actually generates force. Out of balance forces are significant with any inclined shaft but lightly loaded high aspect blades are particularly severe.

You do not need a shaft strut if the shaft is aligned with the flow. This link is a video clip of a small unsupported prop carried on a 1/4" spring steel shaft:
http://www.boatdesign.net/forums/attachments/inboards/inclined-prop-shaft-question-15524d1187343575-v7_strutless_prop.wmv
You can see the blades flashing about in the water. If you work out why this works then you will be able to reason why a shaft forced to be inclined to the flow causes out of balance forces.

So the first step in overcoming the vibration is to eliminate the exciting force. That is achieved by having the prop aligned to the flow. Once the out of balance forces are gone there is no longer a vibration issue.

daiquiri - if you do this exercise you can start to appreciate why I can get the prop efficiencies I nominate. Others will happily incline a shaft without any consideration for what the poor old prop is being asked to do and then come up with fudge factors to allow for the loss in efficiency. Most prop calculators have these fudge factors in-built.

Rick W.

[Rick Willoughby]

Daniel
If you want to add real understanding then set up a model that will demonstrate the size of the out of balance forces for various size props at various angles of inclination.

The first step in designing any mechanical system is to know what the forces are. BUT as I have said the easy solution is simply to run the shaft parallel to flow. Under normal operation the only force is a thrust forces and the shaft has to accommodate the buckling load. You also have to contend with the cases of tight turning and reversing but these are usually achieved at slow speeds so out of balance forces are much smaller.

Rick W.

[daiquiri]

This link is a video clip of a small unsupported prop carried on a 1/4" spring steel shaft:
http://www.boatdesign.net/forums/attachments/inboards/inclined-prop-shaft-question-15524d1187343575-v7_strutless_prop.wmv

Hi Rick,
the video is no more there. Do you have some other link?

[Rick Willoughby]

Hi Rick,
the video is no more there. Do you have some other link?

I tested it yesterday and it worked but not now so I have attached here.

Rick W

[Dhutch]

Thankyou for your replys.
- To answer the question i beleave the shaft is inclined. I e, sloping down from the engine thro the hull to under the boat?

Is that really a major problem?

Im also not sure what the video is actaully showing, sorrt to be a bit slow prehaps but what are we actually looking at. I can see the one hull member, and i think i can see what is a propellor freely wondering around the right of it.
It the propellor coming from the hull? Camera on another?

Daniel

[marshmat]

Well, firstly, I agree with Rick on the shaft angle thing- if you do the math, you find that the prop works best when its shaft is parallel to the flow.

On most boats, the shaft is inclined. When you go to place a prop, shaft and engine in a hull, what you'll often find is that to get a suitably large diameter prop, the shaft has to be angled in order for the engine and gearbox to end up inside the hull instead of below it. Shaft angle is a necessary evil in many, if not most, inboard-powered hull shapes. Balancing the added efficiency of a larger diameter / slower turning prop against the loss of efficiency from an angled shaft is not necessarily a straightforward procedure, and there may be other considerations- eg. drag from the running gear when under sail instead of engine.

In theory, and to some extent in practice, it's possible to support a prop freely on a shaft without a cutless bearing. I can't think of any case where I would consider this to be a good idea, though. If put even slightly off balance- eg. by picking up reeds or bumping a piece of driftwood- such a setup could vibrate itself to pieces in short order, where a supported shaft may be substantially more resistant to balance irregularities. And the underside of a boat is a long way from being an ideal environment for any kind of mechanical system.

[Rick Willoughby]

.........
Is that really a major problem?

Im also not sure what the video is actaully showing, sorrt to be a bit slow prehaps but what are we actually looking at. I can see the one hull member, and i think i can see what is a propellor freely wondering around the right of it.
It the propellor coming from the hull? Camera on another?

Daniel

Daniel
YES it is a major problem and few people realise it. Out of balance forces are significant for any inclined shaft. You need robust strut and shaft to contend with these forces. Most builders just make the shaft much heavier than necessary to handle the induced forces. These induced forces are much greater influence on vibration than whirling and thrust buckling. I prefer to eliminate the source of the vibration - namely out-of-balance forces.

The video shows a propeller supported on an 8mm aluminium shaft that just dangles in the water completely unsupported at the outboard end. You can see the propeller blades flashing in the water and prop moving from side-to-side as the boat turns.

The point of the video is to demonstrate that a propeller is self stabilising - very few people understand this. There are large forces on the blades that align the propeller to the flow. If you force the propeller to be misaligned by inclining the shaft then the forces are no longer balanced and they produce vibration.

I am suggesting that you understand the source of the vibration and eliminate that in the first instance. If you must incline the shaft then determine how much out-of-balance you produce by doing that.

What you need to think about is how propeller blades generate thrust. The sort of high aspect blades fitted to easily driven hulls like yachts work at very low angle of attack. If the shaft is aligned to flow then under normal running the AofA is around 3 degrees. Now think about what happens if the shaft is aligned at 10 degrees. Clearly the angle of attack varies with angle of rotation. The forces on the blades vary continuously.

In the above example you end up with the upward going blade working at around -1 degree AofA and the downward going blade working at +7 degrees. It means very high forward thrust one down-side and small reverse thrust on the up-side. This creates a large bending moment in the shaft that is in phase with the shaft rotation and the frequency is rpm times number of blades. It is worse with 2-bladed prop but is present with any number of blades. I have actually bent small shafts as a result of these forces.

The attached photo shows the strutless shaft out of the water. You can see the way it just hangs in the air. It operates quite happily with a thrust up to at least 150N and it is 8mm thick aluminium. (I sometimes use 1/4" spring steel in similar application)

The other thing to remember is that the water dampens oscillation in the shaft. All the equations you see for critical speed are in air. This only applies to the length of shaft between the gland and the gearbox - usually a short distance so critical speed is rarely an issue.

Eliminate the source of the vibration and problem solved. Much smarter than just making everything heavy to contend with induced forces.

Rick W.

[Rick Willoughby]

..........
In theory, and to some extent in practice, it's possible to support a prop freely on a shaft without a cutless bearing. I can't think of any case where I would consider this to be a good idea, though. If put even slightly off balance- eg. by picking up reeds or bumping a piece of driftwood- such a setup could vibrate itself to pieces in short order, where a supported shaft may be substantially more resistant to balance irregularities. And the underside of a boat is a long way from being an ideal environment for any kind of mechanical system.

Matt
The forces on the prop are highly stabilising. I have not tested it but I believe a single bladed prop would happily work around a central position on a flexible shaft. The water provides tremendous damping and any blade will work toward a constant angle of attack throughout its angle of rotation.

You are right about builders using inclined shaft but it is just poor practice. Imagine if you suggested to an aircraft builder that he had to run the prop at an angle to flow. They would laugh at you. For yachts you have sail drives and for powered craft you have stern drives or surface drives. Any displacement craft can be designed to accommodate a decent prop with a horizontal shaft.

Rick W

[daiquiri]

Daniel
...
The video shows a propeller supported on an 8mm aluminium shaft that just dangles in the water completely unsupported at the outboard end. You can see the propeller blades flashing in the water and prop moving from side-to-side as the boat turns.
The point of the video is to demonstrate that a propeller is self stabilising - very few people understand this. There are large forces on the blades that align the propeller to the flow. If you force the propeller to be misaligned by inclining the shaft then the forces are no longer balanced and they produce vibration.
...


Hi Rick. Too bad the video doesn't really show much. We can barely see something moving underwater. :)
But my doubt is another. It is all clear what you said: Incline the prop and you'll get the periodical flexural exitating force acting on the shaft.
But in the middle photo I can see your prop shaft IS inclined. I'm not talking about the curvature due to the gravity, but about the inclination. So how come it is still self-stabilizing? :confused:

[Rick Willoughby]

Hi Rick. Too bad the video doesn't really show much. We can barely see something moving underwater. :)
But my doubt is another. It is all clear what you said: Incline the prop and you'll get the periodical flexural exitating force acting on the shaft.
But in the middle photo I can see your prop shaft IS inclined. I'm not talking about the curvature due to the gravity, but about the inclination. So how come it is still self-stabilizing? :confused:

The shaft is flexible. The aluminium round bar is only 8mm in diameter and 4ft long. The attitude you see is it just hanging in space with the weight of the prop causing the curve. When it operates in a pushing direction the propeller rises up and thrust forward to force the shaft into a natural curve. The stabilising forces are such that the shaft acts as if it was rigidly supported in the stream.

The attached pictures the boat that recently set the human powered distance record of 245km in 24 hours. It shows the shaft as it was used for the record. When the boat stops the shaft just hangs in the water. Once the prop rotates the prop lifts up and curves the shaft so the prop sits perpendicular to the flow and the shaft at the prop perfectly alignied with flow. The angle of inclination at the drive box sets the operating depth for the prop.

The shaft on this boat is 1/4" spring steel. The reason for this configuration is to reduce drag by eliminating the strut. A normal rigid struts costs about 5W for this size boat.

So rather than fight the prop forces I let the prop find its own position.

I am not saying that this configuration is practical for all applications but I use it to make the point that the forces on the blades are highly stabilising for a pushing prop. Likewise if the shaft is forced to be inclined it produces very large out of balance forces. This leads to the shaft and supports being much heavier than if the shaft was aligned.

I am certain very few people understand the forces created by inclined shafts. If they did they would work much harder to avoid them. The issue of shaft vibration comes up often and you get all the numbers on critical speeds but no one ever asks what actually excites the vibration. They work on the symptom rather than curing the problem. Simple solution is to avoid inclined shafts. They will spend an ordinate amount of money getting everything perfectly aligned and balanced then set the shaft at an angle. Once it is angled at even a few degrees all the money and effort spent on aligning and balancing is an absolute waste. Sit down and work out what the bending moment is on a high efficiency prop on a shaft inclined at 5 degrees. They are serious forces. Now do 10 or 15 degrees and you see why vibration is a common problem.

Rick W.

[daiquiri]

This is a bold solution and I really love how you did it. :)
I have a suggestion for you. Why don't try the pulling configuration next time, leaving the shaft behind and exposing the prop to a truly free stream. You will avoid the shaft buckling instability and therefore will be able to further reduce the shaft (or spring) diameter. It will make you gain another bit of efficiency. ;)

[Rick Willoughby]

This is a bold solution and I really love how you did it. :)
I have a suggestion for you. Why don't try the pulling configuration next time, leaving the shaft behind and exposing the prop to a truly free stream. You will avoid the shaft buckling instability and therefore will be able to further reduce the shaft (or spring) diameter. It will make you gain another bit of efficiency. ;)

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.

[daiquiri]

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...

[Rick Willoughby]

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.

[hartley]

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

[marshmat]

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.

[Rick Willoughby]

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


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

[Rick Willoughby]

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

[Rick Willoughby]

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

[Rick Willoughby]

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

[daiquiri]

If nothing else daiquiri has seen the light and is on the right track.

Thanks for making me see the light.
:D :D :D :D
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. :D

[Rick Willoughby]

Thanks for making me see the light.
:D :D :D :D
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. :D

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

[marshmat]

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.)

[Rick Willoughby]

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

[Dhutch]

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

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.
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?

[Dhutch]

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 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

[marshmat]

Hi Daniel,

Belt drive is actually fairly common in electric motor launches. I've heard of it being tried with engines, usually by backyard DIYers, although I'm not aware of any commercially produced boat with such a setup.

Coaxial counter-rotation brings some other issues with it, mechanical complexity being perhaps the big killer. (Take a look at the parts drawing of a Bravo 3 or Volvo DP and you'll get an idea of just how bad this problem is.) Coaxial props also have the potential to introduce a nasty resonant vibration issue (you get a pulse every time two blades pass each other), hence why they generally have a different number of blades on each of the two props. And you'd never do this on a long or inclined shaft- although theoretically possible, it would require more engineering time than you'd need to either straighten the shaft or coax a bit more efficiency out of the off-axis prop.

[Rick Willoughby]

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?

Daniel
Before you move on from this comment you should sit down and draw the velocity vectors for a blade at various angles of rotation on a horizontal shaft with the blade set at AoA at 3 degrees at say 75% radial position. Just doing this will help you understand propellers. Now incline the shaft at say 5 degrees and redraw the vectors in each location.

Until you do this exercise you do not understand the problem as you have not even tried to determine the forces you need to handle. You will probably find your lecturer will be intrigued with this and may help frame the solution. Particularly if you suggest to him that he could use a much thinner shaft to solve the vibration problem providing it is nicely curved such that the prop shaft is horizontal at the prop.

Ideally the prop is larger diameter than 12" - maybe 16". A sailing boat tends to have a larger diameter prop because the draft does not constrain the diameter. For any given prop the thrust from a given power level is a function of diameter:
Thrust = (Power/(4/pi/9/rho)^0.5 x D)^2/3
This is the ideal case excluding prop losses.

Rick W

[marshmat]

Hi Rick,

I know this discussion sort of went dead there.... but I was giving some more thought to the flexible shaft idea on the train home yesterday. (For some unknown reason, I had put the work that actually needed to be done in the suitcase....).

Anyway, I was looking at a sailing catamaran that, in order to have a suitably sized prop at a zero-degree angle, would have put the ~40hp engines almost amidships with six-metre rigid shafts. So I thought of your model earlier in this thread and wondered if it might be possible to use a flexible shaft about two metres long, bending maybe eight or ten degrees over its length. That would allow a V-gear and second shaft to get the engine up in an out-of-the-way place where it would be much more accessible.

The question thus became, what materials are suitable for this kind of shaft, and how does one go about supporting it where it passes through the deadwood without wearing out the cutless bearing? You'd need something springy, fatigue-proof, corrosion resistant, relatively easy to work, and galvanically compatible with the rest of the boat... and does it even have to be metal?

[Rick Willoughby]

Matt
Engineering the system for a high power application could prove challenging. You would probably end up with a shaft much smaller in diameter than you would normally be comfortable with. For example I know Mark Drela has tested a 3mm shaft that is about 3m long for transmitting the sort of power generated in a pedal power boat. The water dampens any vibration as it does with my 1/4" shaft.

My view on the best system would be to determine the diameter required to transmit the rated torque with a safety factor of say 3. It will likely be a lot smaller than you expect. The flexibility will mean it is quite tolerant of shock loads. Work on a high quality stainless steel with a yield of say 1000MPa. (I have not been able to readily locate these materials in the sizes I am using so normally use painted spring steel. Spring steel would have limited life in salt water. If you have a shaft material you like already then use that material for calculations and see what it looks like.)

Once you have the diameter you can determine the minimum bending radius that will give infinite life. I can show you the calculation for this.

I currently use stainless steel or glass ball bearings in my shaft strut. These bearing actually take the bulk of the thrust load so I do not force the shaft into a tighter radius under thrust. So the strut is transferring the thrust to the hull.

The through hull bearing does not need to be flexible but needs to set up to maintain the right curve. Again I would probably consider a ball bearing with shaft seal. An ordinary gland would be OK but you need to be careful not to score the shaft such that it creates a stress raiser.

The output of the "V" drive also needs to be set up to match the curve of the shaft and will have a bending moment applied to set the curve.

The fact that you are not producing vibrations means you can use a much lighter shaft.

The first step is to determine the rated shaft torque and a shaft material so the diameter can be determined. If you do this I can see if the minimum radius will allow a viable set up.

One of the things I have thought about is using the flexible shaft as a means of lifting the prop out of the water to reduce drag. The strut needs to lift vertically till the prop clears the water. The allowable stress under this condition could be much higher because the frequency is much lower than what it sees in normal running.

Rick W

[marshmat]

Hi Rick,

Interesting idea about integrating a thrust bearing where the cutless bearing would ordinarily be. If I'm not mistaken, the possibility of a shaft buckling under the axial thrust load is the main reason why they usually have to be thick and rigid. Eliminate the axial load and you're left with pure torsion, much easier to handle in a shaft small enough to be slightly flexible....

I will get back to you on this in about a week. (The two most brutal exams of my academic career to date are coming up... fast.) The project in question is quite a few years off still- I have a Rideau canal cruiser in the 8-9 metre class on the drawing board first (and hopefully on the shop floor, as soon as I have a shop).

Cheers,

[Rick Willoughby]

Matt
I intend to play around with this idea for something like 5kW at 1000rpm but like you my time is tight right now. I will post details when I have some results.

Some of these things are significant departure from current approach so they are bound to have plenty of unforeseen challenges. My experience at the low power level has been extremely encouraging.

Rick

[MikeJohns]

Prop shaft angle is an issue with speedboats , I'd be careful extrapolating this to displacement boats (not ships), the flow field around the hull is not very predicatable at the best of times, and once in a seaway with the boat heaving yawing and pitching and its anybodys guess. You can ususally feel the prop shake as the hull is pushed around by waves, then it settles down again as the flow becomes more orderd.

In powered tuft tests the tufts are generally aligned locally with the prop shaft in the region of the prop even at extreme angles, then the real question is what happens to the pressure field from the action of the inclined prop and how does it react with that of the hull.

CFD is not all that usefull for these sorts of predictions yet ..unfortuantely.

[Frosty]

A flexible shaft transmitting torque will "mangle" this can be common on car prop shafts that will throw the gearbox all over the place when high torque is transmitted.

Take a straight piece of rope and twist it, it will eventually buckle into a knot, That is what the shaft is trying to do.

[Rick Willoughby]

A flexible shaft transmitting torque will "mangle" this can be common on car prop shafts that will throw the gearbox all over the place when high torque is transmitted.

Take a straight piece of rope and twist it, it will eventually buckle into a knot, That is what the shaft is trying to do.

I use 1/4" shaft from 4 to 6ft long transmitting up to 1kW and Mark Drela thinks I am using overkill. He has used 3mm shafts for the same power level that are up around 10ft long. The thrust is carried by the shaft as well.

The water provides a dampening column and the shafts are hard to damage because they simple flex out of the way. I run mine over river logs and sand bars without causing damage.

This is just a different approach that works quite well. I am not sure a piece of rope will work but maybe it will once in the water. I would not dismiss it until I had a closer look. Pushing props are self stabilising and have very large stabilising force. When they are forced to operate at an angle to flow they vibrate badly particularly if only two bladed.

Most people are skeptics until they see what is possible.

Rick W

[Frosty]

1KW ????

I think we are talking different stuff here I was talking mangle and I have spent many hours trying to cure it, however I was talking considerably more than 1 KW.

I did'nt suggest you use a piece of rope Rick.

[daiquiri]

Rick, I was thinking a lots about your flexible shafts and their possible use on commercial boats (yeah, it is growing on me :) )...
Have you ever tried to investigate on how they behave in heavy seas? Talking about inertial forces acting in any possible direction on the prop.
The water will dampen the oscillations, we agree on that. And it is also true that the shaft, being pre-bended upwards by the prop rotation, will enhance the rigidity of the system when the prop hub is moving upwards - though the back of the medal is that it will restitute elastic energy during the opposite motion.
But I believe (can't prove it) that there will be at least one amplitude peak (for a single excitation force) before it dampens. And since the excitation force is random, both in direction and in amplitude, there could happen a bigger-than-expected peak. Don't you think a freely travelling prop could then hit the hull or an eventual rudder support structure (assuming that rudder is placed behind the prop)?

[Chris Bretter]

Hi all
I am starting to build a 38 ft Wharram i would like to use a centre mounted Nissan SD22 with a 2-1 reduction box and a dropdown longshaft. All fairly simple Has anyone any ideas.Also perhaps a ducted prop with a rudder to assist low speed manouvering.You guys think out of the box.Rick you got some crazy stuff going on so cool.
Regards Chris South Africa:cool: :idea:

[Rick Willoughby]

1KW ????

I think we are talking different stuff here I was talking mangle and I have spent many hours trying to cure it, however I was talking considerably more than 1 KW.

I did'nt suggest you use a piece of rope Rick.

I realise you did not intend using rope but I was making the point that rope might actually make a workable shaft. Have you tried it? Can you prove it will not work?

I am talking 1kW at around 500rpm so around the same sort of torque you would have in a moderate displacement sailing boat using 3 to 5kW at higher rpm. Can you imagine a 1/4" shaft handling that. If you do the stress calculation you do not need a particularly large shaft to transfer a couple hundred kW when there is no concern for critical speeds.

Rick W

[Rick Willoughby]

Hi all
I am starting to build a 38 ft Wharram i would like to use a centre mounted Nissan SD22 with a 2-1 reduction box and a dropdown longshaft. All fairly simple Has anyone any ideas.Also perhaps a ducted prop with a rudder to assist low speed manouvering.You guys think out of the box.Rick you got some crazy stuff going on so cool.
Regards Chris South Africa:cool: :idea:

Chris
My interest has been chasing down losses. For the last 5 years I have been playing with pedal powered craft. I have lots of time to think when I am exercising on the water wondering how I can go faster for the same effort. I have better feel than most about what is efficient and what is not because I feel it directly in my legs.

The curved steel shaft is very good providing it is properly designed. I operate in fresh water most of the time and these days use spring steel for most shafts. The low diameter bar I use is not the best you can get but it is close to it. The yield strength is 1600MPa. It is surprising how far it can be bend before it yields. More like rubber than steel and yet surprisingly tough. I used to get worried about stress raisers with connections but now I just grind flats and sink grub screws into the flats. It is very tough but not highly resistant to corrosion. You can cut it with a hacksaw but it will dull the blade in one cut.

You can get high strength stainless but it is not easy to come by. I am going to play around with low diameter spring steel up to 9kW to see what is possible but I think you could make a practical system with a lift-up prop where the shaft just curves up out of the water.

In a few days I will post some numbers on the sort of shaft you would need to handle around the 10kW mark and what sort of bending radius is possible.

I used to bend my shafts with my big props until I worked out they needed to be aligned with flow. The attached photo is not a shaft problem but shows what can happen due to imbalanced loads on an angled shaft. The prop shown was not particularly efficient but is just one of many ideas I played with.

Life gets much better once the prop is aligned to flow.

Rick W

[Rick Willoughby]

Rick, I was thinking a lots about your flexible shafts and their possible use on commercial boats (yeah, it is growing on me :) )...
Have you ever tried to investigate on how they behave in heavy seas? Talking about inertial forces acting in any possible direction on the prop.
The water will dampen the oscillations, we agree on that. And it is also true that the shaft, being pre-bended upwards by the prop rotation, will enhance the rigidity of the system when the prop hub is moving upwards - though the back of the medal is that it will restitute elastic energy during the opposite motion.
But I believe (can't prove it) that there will be at least one amplitude peak (for a single excitation force) before it dampens. And since the excitation force is random, both in direction and in amplitude, there could happen a bigger-than-expected peak. Don't you think a freely travelling prop could then hit the hull or an eventual rudder support structure (assuming that rudder is placed behind the prop)?

I would never consider a completely unsupported shaft in any way practical. Greg K had his unsupported during his record breaking run but he only ever went forward.

I use a very light tension strut that is pivoted. It allows me to pull the prop up out of the water to remove weed. It also stops it from diving when I go in reverse.

The forces on the prop are powerfully aligning so it tends to track a straight course unless it is constrained. The boat can move all over the place on waves and the prop will just continue constantly aligned with the flow. So if you turn it hangs out to the side or curves in under the hull. Even with my strut I can get a situation when travelling with a beam sea and the prop down wind that it begins striking the hull. So if it is not constrained in some way then it will move all over the place.

The feature of the curved shaft is that you can use a very flexible shaft to get the prop aligned and overcome the nasty forces associated with an inclined shaft. It is far supperior to things like universal joints working under water. There is also the possibility of simply lifting the prop clear of the water to reduce drag in a sailing vessel. Even now I prefer to use the shaft strut to take the thrust load rather than adding the buckling stress to the shaft. They can handle this but it is another design consideration.

The flexible shaft is compliant so will take bumps and just bends out of the way rather than being damaged. With pedal power I have to compromise between the bending stress for the curve and low torsional rigidity for shaft wind-up that makes pedalling inefficient. With a motor the latter is not a constraint so you can build in significant torsional compliance that will give the system resilience if the prop strikes something solid. I get no damage when the prop rides over a log - at least so far.

Rick W

[marshmat]

I came across an interesting setup recently that seems to predate anything you've come up with, Rick....
A '61 (I think) Pontiac Tempest. Transmission in the back, with a seven-foot spring steel driveshaft bent into a curve. "Ropeshaft", they called it, c/o DeLorean. And, apparently, it worked.

[Rick Willoughby]

Matt
I did some googling and found many references to the ropeshaft but no specification. It seems it ran in a tube in the car and I am thinking it might be the same when used in a boat. It is claimed it provided a performance advantage by improving weight distribution.

Curved shafts have been around for a long time and was one of many ideas suggested to me by one of the many people who I chat to about such things. I do not know if I actually have original thoughts these days.

Even the unsupported prop was being done by another fellow before I copied it. He was using it on a small electric powered prop on a creek boat. He sent me videos of this thing just bouncing over logs and rocks while pushing the boat forward. That discussion evolved after I did the Murray River race and ran into problems with fixed prop getting damaged from hitting logs as well as getting stuck on sand bars that the paddlers could easily cross.

The current 1/4" shaft goes through severe vibration at critical speed in air but there is no critical speed in water because the damping is so high. So I expect 99% of all prop shaft vibration that people experience is due to inclination. THere may be a rare example where the unsupported length in the hull is sufficient to have a critical speed.

As I have said quite a few posts back you do not need very large diameter shafts to handle the torque with good safety factors. I expect most prop shafts are sized on the misconception and calculation for critical speed in air. This has nothing to do with the situation when in water. Sure if you incline the shaft it needs to be mighty large otherwise it will bend due to the high force imbalance.

I could have run the numbers quicker than producing this post.

Rick W

[LyndonJ]

Rick
This app of yours is interesting but its not really in the same ballpark. When you put significant power relative to overall drag into spinning the fan its going to create its own flow alignment and angle is not a problem. Theres a lower pressure area ahead and behind the prop and the flow through the disk will be normal. The more power the more aligned.

If its behind a non planing hull its operating in the hulls mess and the effects of a powerfull prop considerably alters the flow over the aft sections.

I think your observations are valid for your application but there's no way thin shafting is going to work in a seaway as someone has already said.

Ducted props are a good solution for any reasons.

[Rick Willoughby]

Rick
This app of yours is interesting but its not really in the same ballpark. When you put significant power relative to overall drag into spinning the fan its going to create its own flow alignment and angle is not a problem. Theres a lower pressure area ahead and behind the prop and the flow through the disk will be normal. The more power the more aligned.

If its behind a non planing hull its operating in the hulls mess and the effects of a powerfull prop considerably alters the flow over the aft sections.

I think your observations are valid for your application but there's no way thin shafting is going to work in a seaway as someone has already said.

Ducted props are a good solution for any reasons.

As I said earlier I am designing for efficiency. That includes the whole system. I have velocity ratios as low as 1.01. The hull/s is/are long and slender so very little disturbance. My props operate in undisturbed flow.

The idea has merit for sailing craft and modern low power displacement pleasure craft. I expect the proportion of high power pleasure craft to decline into the future as overall operating efficiency takes priority.

The original question here was vibration in a sailing boat drive and how to design a shaft to accommodate it. My answer was intended to encourage thinking about the cause of the vibration rather than just beefing up the shaft.

The more blades you have and the higher the power the less noticeable the problem. I cannot say for sure how the flow will alter as the velocity ratio goes up but you would expect the far field flow will have diminished consequence.

I have explained what I believe is a practical means of using a thin shaft. It involves a thrust bearing shaft strut that is closely coupled to the prop. There is no way you can leave the shaft unsupported if you want something practical for operation in most boats. You may not have noticed it but in the photo of the catamaran there is a rope looped around the shaft running to the back of the seat. This is a crude tension strut that allowed me to support the shaft when reversing.

Rick W

[brian eiland]

...Failing that, especially as the power levels are so low (10horse power or so) why are there no belt or chain drive systems on boats. Daniel
I've long be a proponent of horizontal prop shafts over inclined ones. My original thoughts back in the 70's was to utilize what in those days were referred to as 'gilmer belts'. Fast forward to modern days and make use of those industrial power belts by Gates, kevlar and now carbon belts. I'd love to be involved with some of this development if there were a client willing to support the work.

http://www.runningtideyachts.com/power/ (http://www.runningtideyachts.com/power/)

There have been some fits and starts on such projects, but not much carry thru. PYI products developed a chain drive that was quite interesting, but then have stopped production of their two different power units. In recent talks with them I have discovered they are willing to build the larger size unit on a custom basis. Here are a couple of reference postings I've made on this subject:
http://www.yachtforums.com/forums/7567-post9.html
http://www.yachtforums.com/forums/15453-post14.html

...and a chain-drive duo-prop arrangement...not near as complicated as the current duo-prop drives
http://www.runningtideyachts.com/dynarig/Tennant_Hull_V_ChainDrive.php

[Rick Willoughby]

Brian
The curved shaft is hard to beat.

When you consider typical prop shafts you see that they are inclined so they generate a vibrating force. The shaft is beefed up to avoid bending under the out of ballance force, also to avoid its critical speed and strong enough to avoid accidental bending - solid and heavy contributing plenty of drag.

With a curved compliant shaft you can run the prop shaft in line with flow at the prop. This avoids the vibrating force in most circumstances - have to take care when making tight turns. You can use the water damping to avoid any issue with critical speed. You have a compliant shaft that flexes over things to avoids damage when bumped. It can also be easily raised behind the transom so the prop is clear of the water if you want to get the best sailing performance.

If you are only designing for the torque on a high strength shaft you find it ends up being surprisingly small diameter. It also provides torsional compliance.

Rick

[brian eiland]

Rick,
But will it work on 300 plus diesel hp with a vessel that is driving out thru a hard chop and slamming quite a bit??

I was just on a 35 Bertram the other morning, going out into the Atlantic for some tuna fishing. We were driving north into a chop created by an unusual (for this time of year) northerly breeze that was giving us a wave form that was just not well suited for that hull length. Fortunately that was a good solid vessel, but on a more flexible vessel I could just imagine what those shafts were going thru, particularly when those props grabbed a little air. Really makes you think about what forces are involved at both ends of the shaft, both in torque and thrust loading, and in implusive, non-steady state conditions.

On another vessel I was taking from Fla down to Venezuela. I actually twisted off two 2" shafts just aft the strut due to a slight misalignment of the shaft line. I was lucky the props didn't fly up thru the bottom of the vessel, but rather deep-six.

[Rick Willoughby]

Rick,
But will it work on 300 plus diesel hp with a vessel that is driving out thru a hard chop and slamming quite a bit??

.....

I am proposing the idea for easily driven hulls that are set up for efficiency. Like auxiliary in a sailing boat. Fewer people are able to afford to run a 300+HP these days.

That said there is no reason it cannot be scaled. The main reason for poor prop efficiency on boats is draft. The curved shaft allows the prop to be easily elevated to reduce draft and work as surface piercing when in shallow water.

Once you have a compliant shaft the alignment is less critical. The whole thing is more forgiving.

The idea was given me by a fellow Vic Garza who made fibreglass shafts for electric propulsion in shallow creeks. He sent me the attached video. It shows how it just climbs over things. Going "with the flow" rather than fighting it so to speak.

Rick W

[Rick Willoughby]

Another of Vic's videos.

I go quite a bit faster in the Murray Marathon and my prop is much higher aspect with more fragile blades. I have damaged the prop but it takes lots of hits and mostly bounces out of the way.

You can design props to be heavy and solid so they destroy most things they hit or you can make them lighter and compliant so they move around things that are not part of their normal function. For example outboards will usually survive a hit with a log but not a high speed inboard.

Rick W

[MikeJohns]

Consider that for a vessel (rather than a sheltered waters paddleable boat) the propulsion system is the most important safety device aboard. A thin shaft will whip itself into destruction the first time the prop is fouled under power.

On an ocean going vessel a prop shaft should be robust enough to cause the clutches in the gearbox to slip or to stall the engine if the prop is jammed. This is the torque that should be applied to the shaft design.

Corrosion allowance is sensible, thin shafts are going to be very prone to shear failure following a little pit corrosion.

Note for interest that you should always paint a thin shaft, a highly stressed component will fail relatively quickly from fatigue in immersed situations unless well painted. Ditto for stress-corrosion. The S-N curves are for dry shafts once they are wet fatigue failure is greatly accelerated.

[Ad Hoc]

MikeJohns

Also the design case of all the strain energy in the shaft suddenly being realised radially..hence straight shaft...does wonders for the whirling calc's too!

[Rick Willoughby]

I put staying afloat the #1 safety feature followed by staying upright then pointing ability. There have been many circumnavigations without a motor so difficult to see how the prop shaft relates to the most important safety device aboard.

Like all things new they take development. I certainly did not know that a pushing shaft operates quite happily without being supported until I tried it. From what I have seen very few people realise this. This realisation opens new avenues.

Setting torque limits are not difficult. With electric drives you just set it as you please. Similar with hydraulics and the pressure relief. A mechanical slip clutch may not be as precise in setting but it can be set to suit. The torsional compliance of a thin shaft gives it time to gently absorb the motor inertia before the limit operates.

Corrosion resistance requires the right sort of design. I have not been able to get any material better than common spring steel. I paint it for my shafts but they are predominantly used in fresh water. I am considering a fibreglass wrapped shaft to get a more robust protection system. Just a matter of development time and effort.

Try a big solid shaft in the creek that Vic was negotiating.

Rick W

[Ad Hoc]

Rick

"...I certainly did not know that a pushing shaft operates quite happily without being supported until I tried it..."

Your "bent/curved" shaft will have a minimum 2 bearings, on on the prop and one on the transmission side. In this case, the geometric axis of the shaft is not coincident with the axis of rotation. As such centrifugal forces will tend to make the shaft deflect until they are balanced by the restoring force owing to the stiffness of the shaft. Ergo a low stiffness shaft is balanced by a low force, your peddling.

The speed at which balance is achieved between these 2 forces is called the whirling speed of the shaft. All you have is a low stiffness shaft in a low force environment that is matched to its whirling speed.

As soon as you increase the loading, the shafts stiffness must increase to balance the load. An increase in stiffness of the shaft renders a "bent/curved" shaft from easy to hard to difficult to impossible as the forces increase.

Otherwise the out of balance force will create significant inertia loads that are impossible to contain and major damage is the result.

[Rick Willoughby]

Here are a few more examples of what can be done with curved shafts.

The first shows a sketch of a craft that Mark Drela built and tested using a 3mm shaft that was supported at both ends but unsupported for over 8ft. This section transmitted the thrust force to the gearbox. These guys were capable of producing about 1kW or more at that stage of their life. Mark got decavitator to over 18kts for 100m so he was certainly fit and strong.

One photo shows my V11J boat "beached" sitting flat on the ground and the s-curve of the shaft as it curves outward. This is the same shape it takes when I pull it up for inspection for weed.

The second set of photos show a strutless prop on an 8mm aluminium shaft.
The video shows this shaft operating at low speed so you can see how the shaft aligns with the flow. Watch carefully for the flash of the prop as the boat is turned. I have used that shaft to push that boat to 9kts, requiring 570 to 600W.

Most people simply do not realise these things are possible but when you chase efficiency as Mark and I have done you look for novel approaches. You are forced to understand how things actually work rather than take the accepted practice. I design from basic physics as I have a solid understanding of the physics involved rather than relying on rule books or past practice.

The boat that Greg K used to set the world record used a strutless shaft . It was only supported at the gearbox above the water. It was 1/4" 6ft long.

Rick W

[MikeJohns]

...............I put staying afloat the #1 safety feature followed by staying upright then pointing ability............ There have been many circumnavigations without a motor so difficult to see how the prop shaft relates to the most important safety device aboard.

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.

[Rick Willoughby]

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/data/500/V11JE_10kph.wmv
http://www.boatdesign.net/gallery/data/500/V11JE_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]

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 (http://www.boatdesign.net/forums/inboards/prop-shaft-systems-24636-2.html#post234082) 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 (http://www.boatdesign.net/forums/inboards/prop-shaft-systems-24636-3.html#post241287), 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]

....................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]

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]

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]

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]

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.


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]

.... 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]

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.

[Rick Willoughby]

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)

.......

Brian
I did see it. I do not like hydraulics for power applications. Losses are too much - about 2 to 3 times higher losses than electric. Hydraulic motors are best for compact, high torque applications where drive system inertia might be difficult to manage.

The stuff that Dave designed was primarily used for raise drills where the low inertia gave low impact stress on jamming - a necessity to protect the string particularly when the head is close to the drive. I think they still rank as the highest torque for size of any hydraulic motor. Something like 64kNm from a 500mm diameter motor.

The curved shaft has much lower drag than any hydraulic motor mounted underwater irrespective of how compact the motor is.

Rick W

[marshmat]

I do not like hydraulics for power applications. Losses are too much - about 2 to 3 times higher losses than electric.
I'd agree with you, Rick, if you're looking at the kind of hydraulic drive one might piece together from readily available earth-mover components and such. But a dedicated hydrostatic transmission should be able to achieve a total power loss, from engine crankshaft to propeller shaft, of about 10-12%. That's very much on a par with a good brushless DC generator, controller and motor. Hydraulics, of course, are a very different field from electrics, and have their own set of potential pitfalls to watch for. But I certainly wouldn't reject them out of hand, especially for applications such as a sailing catamaran where it might be very desirable to have only one engine but two independently controllable sets of running gear.

For pure, optimal efficiency, of course, it will always be very hard to beat a simple mechanical shaft to a properly matched propeller. But there are many situations- auxiliary power on sailboats being a big one- where overall efficiency is less of a concern than the system's physical size, ability to adapt to changing load conditions, ability to survive nasty conditions, etc. (If sailing auxiliaries cared about drivetrain efficiency, they wouldn't have 12" folding props- drive efficiency is sacrificed in favour of low sailing drag.) In these situations, right now I think it's a toss-up between electric and hydraulic: the former being preferable when house loads are a significant fraction of the total, the latter being quite attractive for primarily propulsive applications.

[Rick Willoughby]

Matt
I was comparing the curved shaft to a hydraulic drive mounted underwater. The curve shaft enables the prop to be pulled clear of the water so prop drag eliminated when sailing.

The hydraulic motor drive linked by Brian offers both drag when sailing and poor overall mechanical efficiency when motoring - neither desirable. The curved shaft has negligible losses when motoring and sailing.

There is no discussion regarding close coupled hydrostatic or hydrodynamic transmissions; different application.

I have engineered the curved shaft for my application so that it is robust and reliable. No other propeller driven craft could go where I go. The challenge is there to engineer it for other applications. If you want the best overall efficiency of a water propeller I doubt that you will find anything better.

Rick W

[Çemberci]

Very intresting strut arrangement in Bodrum

Oktay Çemberci
İstanbul/turkey

[MikeJohns]

Very intresting strut arrangement in Bodrum



:)
Its a home-brew affair , a little bit of marine growth on that lattice will seriously obstruct the flow to the prop, would have been much better to have a V support with less area and better inline forces and a lighter structure.

[Tigawave]

As a stern gear supplier that answers many of the problems associated with noise and vibration, things to look at improving are.

1. Rubber cutless shaft bearings by their nature allow a shaft to flex. Rigid composite bearings elliminate some of this movement, this improves the situation.

2. A standard cutless bearing carrier is rarely checked to see if it is in alignment to the shaft line. P bracket or stern tube. Using a clearance fit composite bearing bedded on epoxy, the alignment of the bearing carrier can be checked as the bearing should be able to spin on the shaft and in the carrier. This is a very likely cause of your lecturers vibration issue along with the fact that he probably had a rubber bearing.

3. Run the shaft in a tube as we do with the Seatorque system, this gives cleaner water flow to the prop with the added support of extra bearings in the tube. Running in oil uses less hp so more gets to the prop.

The issue of alignment in a system where the rubber engine mounts take the thrust is not that relevant, th ereason being as soon as you put the drive in geart and thrust loads are applied the engine will move and pivot so however accurate you have been with the vessel at rest in our out of the water it will all go out of alignment when in use.

[mark775]

"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so." -- Mark Twain

Tigawave
2. A standard cutless bearing carrier is rarely checked (who told you this? I have never heard of it not being checked!) to see if it is in alignment to the shaft line. P bracket or stern tube. Using a clearance fit composite bearing bedded on epoxy, the alignment of the bearing carrier can be checked as the bearing should be able to spin on the shaft and in the carrier. This is a very likely cause of your lecturers vibration issue along with the fact that he probably had a rubber bearing. (if you will erase your post, go back and read the thread, then post something of your sales pitch that is relevant to the discussion - I'll erase this post before anyone notices)

3. Run the shaft in a tube as we do with the Seatorque system, this gives cleaner water flow to the prop with the added support of extra bearings in the tube. Running in oil uses less hp so more gets to the prop. (Less horsepower than what? BTW, Is that Seatorque aluminum?)

The issue of alignment in a system where the rubber engine mounts take the thrust is not that relevant (Who is teaching you this? Every time there is a side load on a shaft, it is flexing and wearing out, applying side loads to whatever bearing is downstream, transmitting vibration to the hull through the mounts and gland, wearing out packing and wearing out the mounts) , th ereason being as soon as you put the drive in geart and thrust loads are applied the engine will move and pivot so however accurate you have been with the vessel at rest in our out of the water it will all go out of alignment when in use (The engine will move, yes, but the better it is aligned at rest, the smoother and more efficient while running)

[Tigawave]

Thanks for the comments Mark,

Maybe it's just the boats I see or work on? but you're entitled to your views.

Sorry for mentioning the product,