Prop Shaft Systems.

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

 

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).

 

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

 

Why don't you take into consideration simply a misalignment of the shaft bearings and the engine?
It is a fiberglass boat, flexible hull... An incorrectly aligned shaft is the first thing that I would think about.

 

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/at...estion-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.

 

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.

 

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

 

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

 

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.

 

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.

 

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

 

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?

 

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.

 

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.