Reversing

[blackrose]

Prop walk !

 

It has been called the paddle-wheel effect for the past 50 years, what is different now..

 

It's been called lots of things for the past 50 years, not just the one term that you happen to have latched onto.

[carlt]

Prop walk !

 

It has been called the paddle-wheel effect for the past 50 years, what is different now.. A propeller is close to having a concentric symmetrical performance when comparing one side to another.. The big difference is seen when you compare the top to the bottom, the propeller is very much more efficient in deeper water than in shallow water.

 

There is always a considerable sideways force which is proportional to the axial force, when going forward the effect is not noticeable because the steerer automatically compensates with the tiller, when reversing the steerer does not have that level of control.. As the bottom of the prop. is producing more thrust than the top the stern of every boat will always be pushed to one side depending on the direction of the prop rotation..

 

With a smaller propeller the the depth difference between the top of the prop and the bottom will be much less, therefore the paddle-wheel effect will be less, the only single prop. vessels with near zero paddle-wheel effects are submarines.

Prop walk, propellor walk, asymmetric thrust, asymmetric blade effect, P-effect it's been called all sorts of things.

 

Submarines, btw, if their blades are not protected by tunnels certainly do experience prop walk.

 

It is mainly influenced by the prop shaft angle. No angle (with the vessel's direction of travel), no prop walk.

 

If it was dependent on depth in the water, how come planes and helicopters experience it?

[alan_fincher]

Interesting,

 

Web searches yield two quite different answers....

 

The one Carl gives above, where it is claimed to solely be a result of shaft angle, and where "propeller walk" seems more commonly used.

 

But other sources give the explanation I'm more familiar with...

 

e.g.

 

Paddle-wheel effect is a result of the fact that pressure increases significantly with depth (technically 'head' - the amount of 'head' of fluid above the point in question). In this case the point, or points, in question are the propeller blades. Blades at the bottom of their rotation are subjected to greater pressure from the water than blades at the top, therefore enabling them to produce more force than those at the top.

 

"Paddle wheel effect" seems to be more commonly used for this, but this may just be by chance on articles I looked at.

 

So are both explanations actually valid, and can canal boats suffer from a combination of the two ?

 

As most modern ay narrowboats have their propshaft virtually parallel with baseplate and gunwales, then it will only be at an angle to the waterline by the amount the boat is trimmed higher at the front than the back. In the case Carl has posted, if the narrowboat is ballasted level, there should be no prop walk at all.

 

Ours experiences it a lot, and does sit very bow high. However I can't say I've noticed any improvement when a full water tank brings the bow down by at least 5 inches.

Edited July 19, 2008 by alan_fincher

[tidal]

Hmmmmm!

back when I first started with boats (only some 37yrs ago admittedly so I am a relative newbie compared to some on here*) both terms were used but for different effects which would be those approximating to Alan Fs definitions above.

 

Also in those days there were a few more props that rotated in the reverse manner to most modern ones.

 

Prop walk was then (and is now from my experience) at its most emphatic and noticeable when either accelerating from a standstill, when there is a significant change in rotational speed of the propeller or when the prop changes rotational direction and as far as my knowledge stretches is primarily caused as Carl said by slight misalignment by shaft and hence prop angle BUT is also is affected by the actual direction of rotation of the prop.

 

The ONLY time I have not experienced it was on some twin prop vessels and on one craft which had the single prop in a tunnel

 

On those now fortunately rare boats which have/had their props BEHIND the rudder it was a very useful tool for maneuvering.

 

 

* but I do have the beard already

[howardang]

.

 

It is mainly influenced by the prop shaft angle. No angle (with the vessel's direction of travel), no prop walk.

 

If it was dependent on depth in the water, how come planes and helicopters experience it?

I'm sorry, Carl, but the angle theory just doesn't stand up. The effect of transverse thrust, propwalk or whatever you want to call it certainly can be felt with a prop shaft at zero angle. I would think 99% of boats on the canal have their prop shafts at or near zero angle and yet they experience it. In fact the vast mojority of ships fall into this category too.

 

As far as aircraft are concerned, isn't the similar effect they experience caused by the gyroscopic effect of the airscrew spinning at high speed which causes precession at right angles to the direction of thrust.

 

Howard

[howardang]

Interesting,

 

Web searches yield two quite different answers....

 

The one Carl gives above, where it is claimed to solely be a result of shaft angle, and where "propeller walk" seems more commonly used.

 

But other sources give the explanation I'm more familiar with...

 

e.g.

 

 

 

"Paddle wheel effect" seems to be more commonly used for this, but this may just be by chance on articles I looked at.

 

So are both explanations actually valid, and can canal boats suffer from a combination of the two ?

 

As most modern ay narrowboats have their propshaft virtually parallel with baseplate and gunwales, then it will only be at an angle to the waterline by the amount the boat is trimmed higher at the front than the back. In the case Carl has posted, if the narrowboat is ballasted level, there should be no prop walk at all.

 

Ours experiences it a lot, and does sit very bow high. However I can't say I've noticed any improvement when a full water tank brings the bow down by at least 5 inches.

 

As I have commented elsewhere, in my opinion the theory about prop walk etc being caused by trim angle is a fallacy, and if you draw it out the change of angle from a full water tank to an empty one, using a boat typically 50 ft long with a 5 inch change of trim would be hardly noticeable. Most vessels on the canals and many elswhere have prop shafts which are parallell or nearly so to the water surface) and yet suffer the effect.

 

I would also say that, in my experience, every propeller acting in water will generate an element of transverse thrust unless it is either in a tunnel (many lifeboats have this arrangement which I'm sure Carl will be familiar with) and props in Kort nozzles,where the thrust generated is axial.

 

The prime cause of transverse thrust, which has been taught in ship and boat handling circles since Adam was a lad, is that the bottom blades meet more resistance when turning than the top blades. This is caused by the greater churning effect of the blades nearer the surface, and therefore the bottom blade meeting more resistance with the more solid wall of water, generates a sideways force which we call various things. I call it transverse thrust because I first learned the term relating to ships, but like others I have come across prop walk, paddlewheel effect etc. The net cause, however, remains the same - to cant the stern to starboard when going ahead with a righthanded propeller and vice versa when going astern.

 

There are other effects which make themselves felt in the rudder/propeller area but which are small in effect c ompared to transverse thrust.

 

Howard

[tidal]

I was referring to alignment with the "keel/central/dorsal line" of the boat (I thought Carl was too) On lesser craft even a small mis-alignment can have a significant affect on directional stability.

 

However, as always, I am happy to be corrected.

[alan_fincher]

No angle (with the vessel's direction of travel), no prop walk.

 

 

I was referring to alignment with the "keel/central/dorsal line" of the boat (I thought Carl was too) On lesser craft even a small mis-alignment can have a significant affect on directional stability.

 

However, as always, I am happy to be corrected.

 

Ah, I missed that !

 

The confusion comes that sources like this.....

 

http://www.castlemarine.co.uk/download/propwalk.pdf

 

refer to....

 

Prop walk is caused by the angle of the propeller shaft to the water surface; if there is no angle, then there is no prop walk.

 

which clearly is not the same definition of "at an angle" as Carl is using.

 

Clear as mud, really!

[tidal]

I also have to confess that I have never really researched the reasons why in any depth, I have always looked for it, assessed the effects for each individual boat and factored that into my maneuvers....

 

until I came back to canals it was rare that I came across this as being a problem in reverse as it only affected the first few metres of travel as most boats I have played with have had deep and effective rudder systems that would actually steer a boat in reverse above a certain minimal speed (with the exception of certain long-keeled sail boats.)

 

As a result of the many threads on this subject I am wondering if i will have to budget for a bow thruster so I can cope

[howardang]

Ah, I missed that !

 

The confusion comes that sources like this.....

 

http://www.castlemarine.co.uk/download/propwalk.pdf

 

refer to....

 

 

 

which clearly is not the same definition of "at an angle" as Carl is using.

 

Clear as mud, really!

The trouble is that this diagram from Castlemarine ignores the major factor about "propwalk" altogether - namely the more disturbed water nearer the surface as I tried to explain earlier and which is the main cause. This diagram has been quoted before and I think I even got into correspondence with the writer because his basic premise is wrong, namely that with a zero shaft angle there is no prop walk. This is patent twaddle!

 

I think an easy way to try to explain how prop walk works is to think about a propeller turning righthanded with the waterline at the propeller hub level; in other words with only half the prop immersed. The blade as it enters the water pushes the stern across to starboard because the blade at the top of the rotation is only pushing against air therefore nosideways force to speak of.

 

Now, progressively ballast the boat down and the bottom blade will still have this push to starboard, although it will be countered by the increasing effect of the upper blade at the top of its rotation as the water deepens. Eventually, when the propeller is totally submerged the two blades (top and bottom will generate opposite forces which, however, are not equal, because the top blade is in less dense water than the bottom (because of the agitation which is more pronounced at or near the surface) hence the "prop walk" effect is still more pronounced to starboard than to port.

 

Now, increase speed, and the faster the prop is going, the less the effect is in relation to the axial thrust which is progressively increased.

 

Howard

[alan_fincher]

I agree with you, I have to say.

 

I always understood it to be about the difference between the lower bit of the prop, and the upper bit of the prop, rather than anything to do with shaft angle.

 

But having said that, there are many, many references to shaft angle in many documents, including the "no angle, no prop walk" bit, often repeated. So if it is twaddle, it's much repeated twaddle.

 

Anyway, I'll dip out before this all goes in the heated direction these things often seem to!

[Nickhlx]

I was following the debate until the bit about "less dense water" ...

 

Of course water is the same density at the sort of differences in depth we are considering ( 18" ), but yes, it can be made "less dense" by agitating violently enough to introduce aeration, but I have rarely managed this except under near maximum revs which I think is not the situation here.... perhaps it is ?

 

Nick

[dave_2A_]

7 pages? Can't we just all agree that most, if not all,narrowboats are a bugger to reverse?

[Grace and Favour]

7 pages? Can't we just all agree that most, if not all,narrowboats are a bugger to reverse?

 

 

Dave_2A

 

Maybe your post isn't quite as technical as some of the previous on this thread . . .

 

but it's certainly one I can understand

[tidal]

Having spent some hours with a calculator and density depth tables I now also have trouble with the density explanation as it stands and so offer this alternative hypothesis:

 

Whereas the actual density differential across the prop itself is NOT sufficient (IMO) to cause the effects mentioned, it is (again IMO) probable that we have neglected to consider that a prop is actually moving water and that water has to have somewhere to go.

At the bottom of its rotation the propeller is moving water into a body of water that has itself to be accelerated and displaced in order for the water moved by the prop to "fit in". as water is incompressible this means that a certain level of water pressure develops

At the top of the rotational cycle, the water moved by the prop can actually be displaced easily upwards forming a "hump" on the surface on the body of water.

this would cause a fairly significant pressure and thus thrust differential across the seemingly insignificant radius of the propeller blades. It would also indicate that this would be also relative to the different pitches of the blades.

 

The case for shaft angle is a good one also, as any deviation from the horizontal would increase the above affect proportionately as the basic propeller shape means that the blades would be rotating asymmetrically in relation to the moving water column.

 

This hypothesis would also account for the increased effect of prop walk when either accelerating from a standstill OR when reversing as the differences in water displaced in the latter case would be affected by the hull itself.

 

With this theory in mind, I have perused the net again and have found that at least one person agrees with me and has provided a rather clear description of ALL the various factors which may combine to cause this effect

 

I provide a link for your perusal and consideration

 

prop walk

[Chris Pink]

Having spent some hours with a calculator and density depth tables I now also have trouble with the density explanation as it stands and so offer this alternative hypothesis:

 

Having spend seconds running the data through the intuition programme on my analogue computer i have come up with the following alternative hypothesis;

 

(it's my theory, my name is chris)

 

the result of running the propellor is a tube of spinning water extending horizontally out of the back of the propellor. that spinning water has drag (as evinced by the fact that it don't go on for ever), that drag causes the whole column to move sideways in the direction that the column rolls thus creating a need for something to fill the space which the boat obliges by moving the other way.

 

Questions? Nobel nominations?

[tidal]

[Pete of Ebor]

I'm intrigued by all these suggestions - differentioal water density [not convinced at all !] prop walk [experienced it !] but surely from the point of view of reversing, poor swim design must be high up on the list. Most modern swims are simply flat plates coming to a point. Surely the angle of the swim must make a big difference to reversing ability. I'm no mathematician or naval architect but it seems obvious [or should I say intuitive] that a long narrow swim would be better than a short blunt one - though there will be the obvoius trade-off in engine space, but what about parabolic-curved swims. Has anyone ever researched swin design ?

[Night Hawk]

What a learned bunch....Having a mooring in Swanley Marina, it's always been quite difficult to steer forwards into one's mooring with the howling broadside winds that will often send you banging into your neighbour's shiney boat. However, with the calm summer's evenings we've had of late (???) I've plucked up the courage and reversed into our mooring. I have a bow thruster (disconnected). FOr the first time in my boating history, I've managed to execute two near perfect reverse parks in full view of a number of Swanley residents. I took things very slowly with several gentle 'forwards' and 'reverses' and slipped into our space.

 

I 'spose the non technical point is that we get to know our boat's foibles...we've had ours for nearly six months now. Ours will do what it's told and reverses wonderfully provided we don't rush - aim the bow in the direction you want to reverse in and off it goes. I haven't yet managed to turn a corner in reverse....I will find a secluded spot and have a go when no one's looking!

[SteveE]

Having to reverse off the water point at Great Haywood today to go back under the bridge for diesel, with lots of boats about and not a few gongoozlers, I found the boat making a graceful arc backwards to end up at 45 degrees so that a bit of judicial forwards with the tiller well across took it nicely through the bridge and was able to look nonch and as if I did it like that all the time. If you get lucky make the most of it and if not glare at the boat, the water, the wind, or anything else that obviously cocked your reversing manouevre!

SteveE

[mykaskin]

Right, a few points...

 

IMHO I don't think water density has anything to do with prop walk, and I think the already mentioned fact that water has somewhere to go to at the surface is the correct one. Imaging this thought experiment: The reason it's called paddlewheel effect is that there is a side profile to the blades, and if you remove the "forwards" components from the prop (i.e. the screw element) you effectively end up with a paddlewheel. Now if this paddlewheel is half in the water it would be quite effective! However it's all in the water - as the wheel turns water "spills" off sideways (in all directions), but because of the surface water having somewhere to go reduces the pressure and thus the thrust.

 

If anyone has actually noticed, you will often see that more water from the prop runs along one side of the boat (this will increase the paddlewheel effect even more by pushing the boat sidewards).

 

Steering a boat in reverse is the most important thing you can do, and if you get it wrong it's far worse! If you are lucky the sidewards effects (from water turbulence, prop walk, wind etc) is less than your ability to control then you can really get going backwards well.

 

The propeller on the boat was placed to close to the swim when built. Recently Dad put a longer prop shaft (by 3 inches) in the boat, and I'm sure it performs better in reverse than it did before... (Cirtainly runs a lot smoother going forwards).

 

I've found that once a boat bow is "lost" when reversing, the quicker you fix it the better. However you can use it to your advantage if you need to turn the boat, and I turned in nearly one go at Autherley junction.

 

Am I the only one here that has been told off for going too fast past boats in reverse!

 

Mike

 

ps. Managed 4.5 mph in reverse (not while passing boats BTW) - the boat runs much better backwards than forwards in a shallow canal, lets put a big tiller at the bow, and run all boats in reverse!

Edited September 18, 2008 by mykaskin

[down the north]

ha ha , good banter on this one,,,,

I used to own Anne, bluetop motor, 23" blade, and the longest we had to reverse, ( with a butty in tow) was up the macc from top of bosley to oakgrove winding hole, whats that a mile or so, so the procedure was , get the boat pointing in the right direction, put the tiller strings on,(these hold the rudder central and are rarely seen on modern craft) then engage reverse, walk up to the forend and just prod it now and then with a 15 ft boat pole to keep the stern in the channel,

works ( worked) for me ( why were we reversing from bosley ?? cos we had just delivered 27 tons of limestone for the new towpath in 1984 !!!!) struth its an eternity away !

martin

[SteveE]

ha ha , good banter on this one,,,,

I used to own Anne, bluetop motor, 23" blade, and the longest we had to reverse, ( with a butty in tow) was up the macc from top of bosley to oakgrove winding hole, whats that a mile or so, so the procedure was , get the boat pointing in the right direction, put the tiller strings on,(these hold the rudder central and are rarely seen on modern craft) then engage reverse, walk up to the forend and just prod it now and then with a 15 ft boat pole to keep the stern in the channel,

works ( worked) for me ( why were we reversing from bosley ?? cos we had just delivered 27 tons of limestone for the new towpath in 1984 !!!!) struth its an eternity away !

martin

 

So was the butty still strapped to the back or did you bring it round and tie the rudder to Anne's T-stud?

SteveE

[alan_fincher]

The propeller on the boat was placed to close to the swim when built. Recently Dad put a longer prop shaft (by 3 inches) in the boat, and I'm sure it performs better in reverse than it did before... (Certainly runs a lot smoother going forwards).

That's interesting Mike.

 

When our boat was surveyed at purchase the surveyor was unhappy with what he considered to be an excess length of prop-shaft between the bearing on the stern-tube, and the propeller itself.

 

He reckoned that if the prop strikes something substantial it greatly increases the risk of bending the shaft, and thus throwing things out of balance and wearing the bearing.

 

We were advised to have about 3 inches cut off of the prop-shaft, something which was done at the first docking.

 

So what your dad did was to create the situation we were advised to get rid of.

 

I wonder if others have an opinion on maximum or minimum amount of unsupported shaft between the bearing and the back of the propeller boss ?

 

Certainly ours is not the easiest boat to reverse....

 

Alan

[Timleech]

I wonder if others have an opinion on maximum or minimum amount of unsupported shaft between the bearing and the back of the propeller boss ?

 

 

 

Alan

 

 

The usual recommendation is 1.5 x the shaft diameter.

It seems to work as a reasonable compromise between strength and water flow, though how much you need for a decent flow will depend very much on other factors. Some boats don't really need any stickout for a decent flow.

 

Tim