question about rudder axis of rotation

[icecreamviking]

I've noticed on many, perhaps most ships, that the axis on which the rudder rotates is apparently canted or raked at a noticeable angle. and having found some pictures that show the hinge pins conform to this angle, it seems like the rear of the rudder would sweep upwards perceptibly out of alignment with the bottom of the keel when it rotated. I've attached some mocked up pictures to demonstrate what I mean.

my question is, is this in fact accurate or have I misunderstood something? and if it is accurate, why is this desirable?

 

[Corsair]

It is accurate. However, the rudder doesn't rotate that much in use. on larger ships probably only 20 - 25 degrees to the side. More on smaller ships. if you have a 20 degree angle at the leading edge of the rudder with the rudder set to one side at 25 degrees, it would only be about a 6 degree force in the vertical plane. And actually, this would drive the bow of the ship down probably helping to make it a turn rather than crabbing or skating to the side but remaining pointed in the same direction.
Just some educated guesses on my part. I've not found any source for this information.

 

[icecreamviking]

It is accurate. However, the rudder doesn't rotate that much in use. on larger ships probably only 20 - 25 degrees to the side. More on smaller ships. if you have a 20 degree angle at the leading edge of the rudder would only be about a 6 degree force in the vertical plane. And actually, this would drive the bow of the ship down probably helping to make it a turn rather than crabbing or skating to the side but remaining pointed in the same direction.
Just some educated guesses on my part. I've not found any source for this information.

thank you once again! awesome info

 

[AllanKP69]

I've noticed on many, perhaps most ships, that the axis on which the rudder rotates is apparently canted or raked at a noticeable angle. and having found some pictures that show the hinge pins conform to this angle, it seems like the rear of the rudder would sweep upwards perceptibly out of alignment with the bottom of the keel when it rotated. I've attached some mocked up pictures to demonstrate what I mean.

Your question is a great one, but how to explain this I do not know. Regarding the angle that it can turn, the following is from Goodwin's The Construction and Fitting of the English Man of War, page 129.

To the fore edge of the main piece the bearding was fayed. This was a relatively thin strip of elm fayed, dowelled, and bolted along the entire length from the heel to the point where the sternpost terminated. It was triangular in cross-section, with the angle set at 30 degrees between 1650 and 1720, and 45 degrees from 1720 onwards.
The drawing below is for Enterprise 1774 and shows the tiller has room to swing nearly 45 degrees with no problem.

Allan

 

[Roger Pellett]

Bear in mind that the flow of water around a ships hull and the way that this flow created forces that could allow the vessel to steer a straight course and to maneuver are complex and were poorly understood until relatively recent times. Rational analysis has only really been possible since the advent of finite element techniques linked to CAD displays in the 1980’s.

Also, most of the time the purpose of the rudder is to cause the vessel to steer a straight course, not to maneuver. This requires very slight rudder movement. Corsair is right, even when maneuvering, maximum rudder rotation is about 20 degrees or less. Beyond this point flow over the back edge separates and the rudder becomes ineffective. Like stalling of an airplane wing.

Roger

 

[Smithy]

And actually, this would drive the bow of the ship down

I think you’ll find that the small force in pitch applied by a rearward leaning rudder would tend to drive the stern downwards.

Imagine the rudder with a 90 degree deflection and it becomes clearer.

Would a rearward leaning rudder increase the force in roll which tends to make the ship lean to the inside of a turn? I’m not sure about that one. Do we have any real naval architects here?

 

[Senorbob]

the main reason (especially in the past) was just do to ship construction. reducing keel length and wetted surface. with modern materials the keel can be built as long as you want. modern rules require a rudder to move from 35 degrees on one side to 30 degrees on the other side in 28 seconds or less. 28 seconds seems like an eternity

 

[Corsair]

I think you’ll find that the small force in pitch applied by a rearward leaning rudder would tend to drive the stern downwards.

Imagine the rudder with a 90 degree deflection and it becomes clearer.

Would a rearward leaning rudder increase the force in roll which tends to make the ship lean to the inside of a turn? I’m not sure about that one. Do we have any real naval architects here?

Yes, you are correct.

 

[AllanKP69]

Beyond this point flow over the back edge separates and the rudder becomes ineffective. Like stalling of an airplane wing.

Live and learn. Thanks Roger. I do wonder why they would have changed things after 1720 if the extra movement would not be used anyway. Seems like a lot of change if it would not be used. How nice it would be to have a WABAC machine with Sherman and Mr. Peabody to give us a ride to a shipyard operating in the 18th century.

 

[Corsair]

Live and learn. Thanks Roger. I do wonder why they would have changed things after 1720 if the extra movement would not be used anyway. Seems like a lot of change if it would not be used. How nice it would be to have a WABAC machine with Sherman and Mr. Peabody to give us a ride to a shipyard operating in the 18th century.
View attachment 517175

I would be from a different cartoon...."HELP ME MR. WIZARD ! HELP ME !"
Bonus points if you know the cartoon.

 

[AllanKP69]

Tooter Turtle (I looked it up as it was a new one for me.) Sorry to go off track, but I am still not convinced the extra movement provided was there for no reason.
Allan

 

[Rebus]

I would be from a different cartoon...."HELP ME MR. WIZARD ! HELP ME !"
Bonus points if you know the cartoon.

Thinking trying to get it. I can hear him saying it too.

 

[Smithy]

Beyond this point flow over the back edge separates and the rudder becomes ineffective. Like stalling of an airplane wing.

Especially when the trailing edge is a foot thick and squared off.

 

[Philski]

Why worry about the dynamics of the rudder other than getting it to look correct on the model? Unless you're building RC boats, the rudders may occasionally see some fluid pressures when you turn on the ceiling fan......

 

[Roger Pellett]

My post above was based on studying Naval Architecture in the 1960’s and the writing of the late Dr Harry Benford one of my professors.

I did, miss one point made by my friend Allan. Sailing ships have to perform one maneuver not required of engine powered vessels. They have to tack. For square rigged vessels, this involves the ship heading into the wind and coming to a full stop when sails are backed. The helm is then reversed as the ship begins to back down in order to fall off on the other tack. For this larger rudder angles were probably required.

Philsky, the dynamics are being discussed because Icecreamviking asked the question!

Roger

 

[Smithy]

Why worry about the dynamics of the rudder other than getting it to look correct on the model? Unless you're building RC boats, the rudders may occasionally see some fluid pressures when you turn on the ceiling fan......

I like to understand the thing I'm trying to model, how it works, the historical context, circumstances of production. For me it makes the hobby more fulfilling.

 

[AndyA]

There may be some overthinking happening here. I believe that the angle of the rudder was not usually a primary design consideration but the angle of the transom and sternpost was. In other words, the transom and sternpost have a significant effect on the shape of the hull, which is primary to how the vessel moves in the water and reacts to waves. The angle of the rudder is dictated by that design choice and has a much smaller effect. I sail a 16 foot Sea Bright skiff, based on the design by John Gardner in his book Building Classic Small Craft. Hannah has a heavily raked stern in keeping with the original use of her kind as beach boats. I have not noticed any effect that I could attribute to the swing of the rudder. That doesn't mean that there is no effect but surely it is a minor one. As others have pointed out in earlier posts, the rudder is usually swung within a narrow range while sailing.

Drawing is from Gardner's book. Photo shows Hannah sailing at Mystic Seaport Museum. Fair winds!

 

[Smithy]

There may be some overthinking happening here. I believe that the angle of the rudder was not usually a primary design consideration but the angle of the transom and sternpost was.

Certainly. I think we are all chatting about very minor effects of a raked rudder axis for the fun of it. And because it is a bit interesting. Hydrodynamics stuff. Like what you said.

~~~~~~

Why do they have that broader chord at the bottom?

 

[Philski]

My post above was based on studying Naval Architecture in the 1960’s and the writing of the late Dr Harry Benford one of my professors.

I did, miss one point made by my friend Allan. Sailing ships have to perform one maneuver not required of engine powered vessels. They have to tack. For square rigged vessels, this involves the ship heading into the wind and coming to a full stop when sails are backed. The helm is then reversed as the ship begins to back down in order to fall off on the other tack. For this larger rudder angles were probably required.

Philsky, the dynamics are being discussed because Icecreamviking asked the question!

Roger

By the way, I used to teach sailing when we were in Japan. On 16' Snipes. Hey! It had a mainsail and a jib! The rudder and centerboard were retractable. Tacking and jibing.....fun!

 

[AndyA]

Sailing ships have to perform one maneuver not required of engine powered vessels. They have to tack. For square rigged vessels, this involves the ship heading into the wind and coming to a full stop when sails are backed. The helm is then reversed as the ship begins to back down in order to fall off on the other tack. For this larger rudder angles were probably required.

My understanding is that missing stays, described here as "coming to a full stop," was generally to be avoided in a square rigger. There was risk of damage to the rig as the vessel is aback, and risk to the rudder as she backs down. Neither the rig nor the rudder is designed for sailing backwards. Where missing stays was likely, a vessel would more often wear about instead of attempting to tack. Fair winds!