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Maximum rudder angle before stall?


blackrose

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Is there a general rule or does it vary from boat to boat and the type of boat? I've read some yacht forums and people are suggesting 25 - 35 degrees each way from centre, but would this be different on a bigger, heavier and slower boat? I imagine fluid separation on either side of the rudder (and drag) which leads to rudder stall is velocity-dependent? I'm just talking about a balanced rudder here, not Schilling, etc.

 

My current flat plate rudder moves about 40 degrees either side of centre and seems to steer fine at the stops, but I'm changing it for a this one below as it has a better profile. I need to weld a vertical piece on top to act as a rudder stop. Should I just put it in the same place as the existing rudder stop?

 

utf-8BU3RyYXRmb3JkLW9uLUF2b24tMjAxNDA0MD

 

 

On the subject of rudder stall, this is an aircraft wing but the principal is the same.

 

585px-StallFormation.svg.png

 

 

This is a good link:

 

http://www.stirling.saradioyachting.org.au/Sailing_Hints/rudder.htm.

 

 

And then I found this which blew my mind:

 

For a turning ship, except at the start of the turn rudder angles of attack α are much smaller than the rudder angle δ relative to the midship plane. Comparing these small attack angles with the large stall angles of Table 3 for full-scale Rn, it is obvious that larger maximum rudder angles than the usual 35° are a simple, cost-efficient means of improving the turning ability of ships.

 

However, in yaw checking large rudder angles opposite to the turning motion would lead to stall and the accompanied decrease of rudder transverse force. This is easily avoided by limiting the shaft moment which the rudder gear can generate to about the stall angle moment; then in yaw checking the rudder will just operate at the stall angle, producing maximum lift. This better manoeuvring for smaller maximum rudder gear moment was also observed practically by FSG on trial trips. For large ships this may result in difficulties to satisfy the IMO requirement of turning the rudder, at full ship speed, within 28s from 35° on one side to 30° on the other side. This rule, which is criticized also because the 28s limit should better depend on ship size and speed, should be substituted.

 

http://www.nap.edu/openbook.php?record_id=9771&page=633

Edited by blackrose
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I would say on my boat that about 60 -70 degrees will give me the maximum sideways thrust, but you have to look at the flow coming off the back of the rudder. Usually to start with the wash is split, but with a turn or two of the rudder you can see the flow attach to one side of the rudder and virtually all the flow will go one way. It is a basic function of fluidics which I learnt about over 40 years ago.

 

Yet how often do you see boats trying to turn with the tiller hard over and the wash splitting about 45%/55% and struggling to turn? Get it right and the boat will turn almost in its own length. It's not just my boat; when I was working in a marina i moved many boats from different builders and usually they all followed the same pattern.

 

ETA: Of course, the big difference between a narrowboat rudder and the aerofoil section diagram above is that the nb rudder has a significant part of its area in front of the rudder stock. I doubt that making the rudder in an aerofoil section will really make that much difference compared to watching how the wash attaches to one side of the rudder blade.

Edited by dor
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Somebody will probably shoot me down but I agree with David Mack on this one.....At the moment I have 2 boats one has an unbalanced rudder and turns best with fairly small helm unless you are trying to use the prop walk at very low speeds. The other has a balanced rudder and if you commence the turn with about normal helm and as the head starts to swing go to 90 degrees she will then spin on the spot with virtually no forward motion. The balanced rudder is on a narrow beam cruiser and has a large amount of "balance area" I would estimate about 60 to 70% of the area of the main part of the blade

 

edit for duplication of word

Edited by John V
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I watch any number of boats wind above the top lock in Stoke Bruerne (not a very good place to wind IMHO) or get way off line trying to enter the lock. The ones who push the tiller as far over as possible always struggle but the ones who don't let the end of the tiller go outside the line of the cabin are the ones who can turn on a sixpence without trying to destroy the opposite towpath edge or hit the gates with a fair old bang! 35 degrees is about the max efficiency - no need, in my view, for complicated physics - just look what happens when an aeroplane stalls - it fails to fly!

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.......... 35 degrees is about the max efficiency - no need, in my view, for complicated physics - just look what happens when an aeroplane stalls - it fails to fly!

 

if it is a canard it doesn't stall........if all rudders were the same and all swims were the same you could make a "one rule rule fits all" your rule may well hold true for a large proportion of narrowboats as many seem to have a very basic plate rudder with a fairly large balance area, very different from the one in the OP's photo.

 

Going back to OP IMHOP the airfoil diagrams aren't really a fair comparison as they seem to show the flow coming from the leading edge whereas in a rudder the centre of the flow is at the pivot pont

Edited by John V
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I can't believe there is a "one size fits all" rule on this.

 

It must be highly influenced by (at least!).....

 

1) How much "balance the rudder has, forward of the shaft it mounts on

2) How big the prop is, (because it is a combination of the size of the balance versus the size of the prop that determines how much of it is "covered" on the side you are steering away from).

3) Separation between rudder/balance and prop.

4) Whether you "work" the tiller when trying for the fastest possible turn, or just hold it over at a fixed angle.

 

I think you are trying to apply too much science to it, (and probably the wrong science as well!).

 

EDIT: To clarify next bit was a new addition that has rather confusingly merged on to the end of the above.....


I have steered this boat enough to know that if I only held the tiller at 35% or 40% from "straight", that it would not have a cats chance in hell of turning anything like as tightly as it does in this video.

 

On other narrow boats holding the tiller like this would be totally counter-productive, but I present this as very firm evidence that different boats have very different needs.

Edited by alan_fincher
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As an home made aerodynamic guy, and propeller guy, I have some insight in aerodynamics, I don't know all, but some, humble,

 

The max lift - stall depends on angle, not so much speed, but some, Of course it depends on the airfoil, a sharper leading edge often means a sharper earlier stall. the stall angle also depends on aspect ratio. a lower aspect ratio (height vs. length) and stall angle increase. so a longer rudder turn better, but a high aspect ratio rudder, make less drag at small steering angles.

 

Most of the times it stall at 15-20 degree, and as is said the actual angle is reduced when the boat turn. so it can go further out and still have attached flow.

but also a wing profile make lift after the stall angle! just think that you blow air or water at 90 degree angle on a plate, still pressure on one side. but a lot of drag.

 

A schilling rudder have its max lift at about 40 degree from neutral in the boat, but still produce lift at 65-70 degree at that angle it turn the boat on the spot, it even make the boat reverse with propeller going forward.

 

before I converted to a schilling rudder, I had some bad vibrations at 30-40 degree angle, it is now gone, my schilling rudder look like my avatar picture.

 

Lately they look at the prop wash that come off the propeller in a spiral, meaning the angle the water hits the rudder is different above and below the prop shaft line, and closer to the shaft line the angle is greater then close to the propeller tips. so turning the rudder in the prop wash means top or bottom half of rudder stall earlier then the other half. depending on prop rotation left or right, and steering direction.

 

so called twisted leading edge rudders have more and less camber, (camber is the mean line in the airfoil) see the example of the airplane wing in this tread. with more camber close to shaft centre and less camber top and bottom of rudder, and turned each way top half and bottom half to meet the swirling prop wash.

 

The balance area help reduce the steering forces, it also have the effect that it "cover" more of the propeller disk, or prop wash. with hand force steering, the balance should not be more then 24% of the total chord. some schilling rudder have 40% but then it is not manual input. some sailboats have 17% balance area. to much balance area and it will be difficult to get the rudder back to neutral.

 

End plates at top and bottom of rudder help the water flow to stay on the rudder and not spill over the edges.

 

Dalslandia's rudder have the stock at 24%, the airfoil is 25% thick at 20% of the chord length.

Edited by Dalslandia
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Blimey! Why make something easy so complicated? Why do you *need* a rudder stop? Why don't you just try it out to find by experience what position of the tiller maximises the amount of turning effect for your boat/rudder combination?

 

We have a rudder stop, it's what caused the rudder to get bent when she was a hire boat

 

Richard

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Blimey! Why make something easy so complicated? Why do you *need* a rudder stop? Why don't you just try it out to find by experience what position of the tiller maximises the amount of turning effect for your boat/rudder combination?

 

Why does my existing rudder have a stop? Think about it... It's very difficult to tell where the stall point is without rudder stops so it's very easy to over-steer.

 

And another thing, without a rudder stop the rudder could go around 360 degrees and hit the prop. Not nice if you accidentally reversed into something (or some idiot steering wants to see what will happen...)

 

Does your rudder have stops or does it just swing freely? Do you think the boat builders just guessed or did they have some angle in mind? Where did that angle come from?

 

It may be easy for you, because you didn't design your boat, but you can be sure that someone had to think about the complexity of fluid dynamics, and angles of attack when they designed boats with rudders.

 

Your post reminds me of people who fly in an aeroplane without giving a thought about how it's staying up there. It's simple isn't it?

Edited by blackrose
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- no need, in my view, for complicated physics - just look what happens when an aeroplane stalls - it fails to fly!

 

Yes, and that's exactly why there is a need for complicated physics! Without knowing the maximum angle of attack planes would be falling from the skies!

 

 

I think you are trying to apply too much science to it, (and probably the wrong science as well!).

 

 

But the science IS there to explain exactly why things behave as they do whether people like it or not. Application of science is another point, but I don't think naval architects would have been using that science over the centuries if it was wrong. However, if you know better then I'm all ears.

As an home made aerodynamic guy, and propeller guy, I have some insight in aerodynamics, I don't know all, but some, humble,

 

The max lift - stall depends on angle, not so much speed, but some, Of course it depends on the airfoil, a sharper leading edge often means a sharper earlier stall. the stall angle also depends on aspect ratio. a lower aspect ratio (height vs. length) and stall angle increase. so a longer rudder turn better, but a high aspect ratio rudder, make less drag at small steering angles.

 

Most of the times it stall at 15-20 degree, and as is said the actual angle is reduced when the boat turn. so it can go further out and still have attached flow.

but also a wing profile make lift after the stall angle! just think that you blow air or water at 90 degree angle on a plate, still pressure on one side. but a lot of drag.

 

A schilling rudder have its max lift at about 40 degree from neutral in the boat, but still produce lift at 65-70 degree at that angle it turn the boat on the spot, it even make the boat reverse with propeller going forward.

 

before I converted to a schilling rudder, I had some bad vibrations at 30-40 degree angle, it is now gone, my schilling rudder look like my avatar picture.

 

Lately they look at the prop wash that come off the propeller in a spiral, meaning the angle the water hits the rudder is different above and below the prop shaft line, and closer to the shaft line the angle is greater then close to the propeller tips. so turning the rudder in the prop wash means top or bottom half of rudder stall earlier then the other half. depending on prop rotation left or right, and steering direction.

 

so called twisted leading edge rudders have more and less camber, (camber is the mean line in the airfoil) see the example of the airplane wing in this tread. with more camber close to shaft centre and less camber top and bottom of rudder, and turned each way top half and bottom half to meet the swirling prop wash.

 

The balance area help reduce the steering forces, it also have the effect that it "cover" more of the propeller disk, or prop wash. with hand force steering, the balance should not be more then 24% of the total chord. some schilling rudder have 40% but then it is not manual input. some sailboats have 17% balance area. to much balance area and it will be difficult to get the rudder back to neutral.

 

End plates at top and bottom of rudder help the water flow to stay on the rudder and not spill over the edges.

 

Dalslandia's rudder have the stock at 24%, the airfoil is 25% thick at 20% of the chord length.

 

Thank you. Finally someone who understands the science.

Make sure your rudder stop doesn't leave your rudder vulnerable in locks Mike

 

Richard

 

That's a fair point, but then again I generally work locks alone so even if I could tuck my rudder away at 90 degees under the counter, without restraining the tiller it would just swing out again.

I would say on my boat that about 60 -70 degrees will give me the maximum sideways thrust, but you have to look at the flow coming off the back of the rudder. Usually to start with the wash is split, but with a turn or two of the rudder you can see the flow attach to one side of the rudder and virtually all the flow will go one way. It is a basic function of fluidics which I learnt about over 40 years ago.

 

Yet how often do you see boats trying to turn with the tiller hard over and the wash splitting about 45%/55% and struggling to turn? Get it right and the boat will turn almost in its own length. It's not just my boat; when I was working in a marina i moved many boats from different builders and usually they all followed the same pattern.

 

ETA: Of course, the big difference between a narrowboat rudder and the aerofoil section diagram above is that the nb rudder has a significant part of its area in front of the rudder stock. I doubt that making the rudder in an aerofoil section will really make that much difference compared to watching how the wash attaches to one side of the rudder blade.

 

You have 60 - 70 degrees on total or on each side of centre?

 

We have a rudder stop, it's what caused the rudder to get bent when she was a hire boat

 

Richard

 

Someone must have reversed into something. Imagine the damage if there were no rudder stops...

Edited by blackrose
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Boating used to be a simple, enjoyable activity, learned by experience with a generous application of common sense, often aided by the more experienced. You got a feel for what worked with a particular boat and got on with it. I've never heard of rudder stops on canal boats, unless the " feet" left on the uxter plate by some builders count in that way. I'm saddened by the new wave of technological overkill that threatens to swamp the enjoyment of boating, at least to my naive yet experienced eyes. And no, I don't understand the science at all, it certainly wouldn't enhance the pleasure of being afloat. Sorry!

 

Dave

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I was intrigued by the bit about twisted leading edge rudders in Dalslandia's post and was doing a bit of looking up.(incidentally thank you for an extremely interesting post)

I am unsure if it is relevant to the low thrust and slow speeds of canal boats...I was also refreshing my memory about Schilling rudders, Kort nozzles and Kitchen rudders, all very interesting and excellent engineering but I wonder how easy they are to free up from the remains of an old mattress round a prop, while your head is down a weed hatch.

The Schilling looks more plausible for our shallow and often rubbish laden waterways but I wonder if it is worth the extra cost / complexity for a canal boat.

Edited by John V
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On other narrow boats holding the tiller like this would be totally counter-productive, but I present this as very firm evidence that different boats have very different needs.

 

 

That's interesting. So in answer to my original post, there must be other factors coming into play on different boats.

 

This is my boat turning at full rudder (about 40 deg) at about 1000rpm with no bow thruster used.

 

http://www.youtube.com/watch?v=YbfzCz0cnqc

 

Should I just put the stops in the same place on the rudder? I can't see it getting much better than that.

Boating used to be a simple, enjoyable activity, learned by experience with a generous application of common sense, often aided by the more experienced. You got a feel for what worked with a particular boat and got on with it. I've never heard of rudder stops on canal boats, unless the " feet" left on the uxter plate by some builders count in that way. I'm saddened by the new wave of technological overkill that threatens to swamp the enjoyment of boating, at least to my naive yet experienced eyes. And no, I don't understand the science at all, it certainly wouldn't enhance the pleasure of being afloat. Sorry!

 

Dave

 

The "new wave of technological overkill" that you speak of has been used on ships and boats for centuries - you were just unaware of it.

 

Most canal boats have rudder stops of one kind or another - even if that's just the rudder hitting the back of uxter plate or the stern of the boat.

Edited by blackrose
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Someone must have reversed into something. Imagine the damage if there were no rudder stops...

 

It happens when the boat drifts backwards going down in locks. The stern fender overhangs the cill, so the boats next point of contact with the lock is the rudder. That gets pushed under the counter until the stop engages, then the rudder bends

 

Richard

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Boating used to be a simple, enjoyable activity, learned by experience with a generous application of common sense, often aided by the more experienced. You got a feel for what worked with a particular boat and got on with it. I've never heard of rudder stops on canal boats, unless the " feet" left on the uxter plate by some builders count in that way. I'm saddened by the new wave of technological overkill that threatens to swamp the enjoyment of boating, at least to my naive yet experienced eyes. And no, I don't understand the science at all, it certainly wouldn't enhance the pleasure of being afloat. Sorry!

 

Dave

Hi ya,

Yes, Advancement or the Butchering of boats

gives us all something to talk about,

I actually heard of a guy who put an 'Engine' in a boat of all things !, Yep an Engine I tell you.

Whatever next !. Bring back the days when we could cut trees down an make our own Dug out, but I guess some bright spark will come along and make a paddle or worse a sail or something something !. Then we need a Keel, A Mast, A Rudder. OMG Boatings ruined !.

 

Obviously I say the above Tounge in cheek, I'm afraid it just Human Nature to tinker and not always for the best !.

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Hi ya,

Yes, Advancement or the Butchering of boats

gives us all something to talk about,

I actually heard of a guy who put an 'Engine' in a boat of all things !, Yep an Engine I tell you.

Whatever next !. Bring back the days when we could cut trees down an make our own Dug out, but I guess some bright spark will come along and make a paddle or worse a sail or something something !. Then we need a Keel, A Mast, A Rudder. OMG Boatings ruined !.

 

Obviously I say the above Tounge in cheek, I'm afraid it just Human Nature to tinker and not always for the best !.

 

 

When I was in the merchant navy there was a saying...."when engineers were kept in their place.....chained to the oars!"

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Does the angle of stall really matter in a canal boat?

 

When I raced Fireball sailing dinghys, yes the angle of stall mattered, going too far over with the tiller caused drag, hence a reduction of speed and lack of efficiency, not what you want in a racing dinghy.

Also an aerofoiled rudder and centerboard were essential in lowering drag.

 

To my knowledge the speed of a canal boat is not critical.

Edited by Ray T
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I hadn't realised that there was a similar thread running already, (questionaire ....Kitchen rudders) I see it started in February while I was busy so I missed it. The consensus there seems to be " it might work but it's too complex/fragile/expensive for its advantages".

The only sophisticated design of rudder that seems to get a cautious thumbs up is the Schilling.

 

I know that I certainly wouldn't change the simple balanced plate rudder on my narrowbeam cruiser as it does everything that I have asked it to. I would love to fit something more efficient on the Humber barge but it only has an unbalanced rudder and I would be unwilling to do too many alterations to an eighty five year old boat.

 

 

edit for spelling

Edited by John V
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