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Rudders. Square-edged? Or shaped?


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Most of the pics I've looked at here (of kit-build boats) seem to show rudders almost exactly as they were pressed out of the walnut sheet (ie, with square edges and no shaping).

I'd have thought that, in real (1:1, pre-fibreglass) life, the normal thing would have been to round off the leading and trailing edges.  Not a lot of work involved, surely?  I really don't imagine a 20th century trawler, or an 18/19th century schooner, would have been fitted with a seriously sculpted rudder, but would any decent shipwright have just put together a few thick planks and said 'yeah, that'll do!"?

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Do not forget, the direction was largely done by the trim of the sails.


 


The size of Victory´s rudder is mostly conform with the sources given in the literature. What is missing is the chamfer towards the aft edge and towards the bottom. Also it lacks a turbulence groove, like seen on the Shinx, Ambuscade and Barfleur (amaong many others) paintings and on chosen models.


http://collectionsonline.nmsi.ac.uk/detail.php?type=related&kv=65431&t=objects


http://collectionsonline.nmsi.ac.uk/detail.php?type=related&kv=65421&t=objects


http://collectionsonline.nmsi.ac.uk/detail.php?type=related&kv=65417&t=objects


 


http://collectionsonline.nmsi.ac.uk/detail.php?type=related&kv=31697&t=people


 


 


The original rudder of the St George 1785 was recovered lately from the 1811 wreckage and lokks like to have same shape. 


http://maritimearchaeologyprogramdenmark.wordpress.com/2009/04/22/recording-the-rudder-of-the-hms-st-george/


 "Das Ruder der St. George" (sorry, can´t find the english page)


 


And here comes one of my question: Did this one have a turbulence groove in the aft edge? Was anybody there? Has anybody have a closer look?


 


Cheers, Daniel


Edited by dafi
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Friends, I accept the wisdom of the cult of MSW!

Indeed, I've now looked at several pre-1900 rudder pics online (including that of the 1869 Cutty Sark) and I haven't yet seen one that was rounded off, let alone streamlined.

I admit I'm surprised though.  If the early shipbuilders had the sense to see that a lean, streamlined boat would travel faster than a fat, squared-off one, why didn't this intelligence carry through to the design of such underwater features as the rudder?

 

And, er, what's a turbulence groove?  Is is something caused by turbulence, or a design feature aimed at reducing turbulence?

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The effect of turbulences was very harmful to the rudder itself and the helmsman handling it.

 

If water finds an obstacle, it forms a circular turbulence behind it. On the ship, those occured alternating on both sides (*), forcing the rudder to tilt brutally to one side, where the pressure was less. This was great strain on the pintels, the rudder head, the tiller, the rudder gear and too to the helmsman.

 

The two ways to ease/eliminate this was to slim the rudder in the aft to get a smoother flow and avoid the turbulences (or at least put them more aft where they were not as effectively) or to introduce the turbulence groove, where a controlled counter-turbulence avoided the formation of the harmful turbulences.


Somewhere I found the hint, that it was either the slimming or the groove, but the paintings of the admirality models show the use of sliming and groove combinded, hence my question about the St. George´s rudder.

 

Turbulence groove nicely to be seen on the Jylland

http://www.flickr.com/photos/biedk/2704962696/lightbox/

http://500px.com/photo/5038922

 

Amicalement, Daniel

 

(*) nicely to be seen on the opening sequence of Startreck STNG or Voyager, where the ship flies into a nebula.

Edited by dafi
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The leading edge doesn't really need to be streamlined, does it?  It's got a large ship in front of it to break the adhesion.  At the speed they normally traveled, I don't think turbulance was a problem.

 

One model I am working on, Pinnace VIRGINIA, has references that indicate the training edge of the rudder is wider than the leading edge.   ...something about it would bite the water better and enhance turning.

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Hy Chuck,

 

the section of both - leading edge and sternpost - had usually triangular shape. This gave way to the movement of the rudder but eliminated a gap in between, that would have reduced effectivity.

 

Daniel

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The leading edge may have been (or may not have been) chamfered to allow a hard over rudder.   Depends on era and country.  As for the turbulance groove, I thought it was mainly in use by the French.  As I recall, the British tried it on a couple of ships but found no real advantage. 

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I cannot comment on the turbulence/hydrodynamic issue, but  I can say with confidence that rudder angle seldom exceeded 25 degrees each way in sailing ships. There was no real necessity to angle off the stern post in most cases. Even if the rudder could be turned further when 'hard over', drag would be a greater issue than steering.

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Strange ...

 

I always followed the situation described in Goodwins Construction and Fitting page 131 who gives an angle of 60° on the post in 1720 and 90° after that date. The braces and pintles he gives for the rudder support that.

 

In arming and fitting the braces look like a flat post ...

 

XXXDAn

Edited by dafi
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  • 4 weeks later...

Druxey, you are correct. I have not studied hydrodynamics but have experience on modern day sloops.

If you turn the rudder over too hard, too fast you will lose speed. Without headway, the vessel becomes harder to steer.

I believe that with the rudder hard over, water gets trapped in front of the rudder, creating an obstacle, cushions and diverts the water that would have come in contact with the rudder blade, making the rudder much less efficient.

Traveling at 3 to 5 knots, a small difference in rudder angle makes a huge difference in the vessels' direction, due to the density of water.

With the length of waterline on a three masted vessel, they would be capable of much higher speeds, making the helm much more responsive to

changes in the rudder's position.

Just a couple of cents worth.

Bryan

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I am just looking at my drawings from the NMM as I am getting towards the stern detail of my Pegasus and I looked again when I saw this query.  No real help but they do appear to be square edged ats far as it goes.

 

The kit accurately follows the shape of the rudder in the drawings - but I have no idea what the "horn shaped" proruberance on the aft of the rudder is.    Attachment of the steering lines - seems to be too low?  Can anyone enlighten me ??

 

post-905-0-00297500-1367097646_thumb.jpg

Edited by SpyGlass
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Hello Spy,

 

Those  shapes are called hances which are a fancy name for steps where the rudder is reduced in width as it rises upwards. Hances are also to be found say on the Quarter deck capping rails where the level drops towards the Gangway. Differences in shapes of the hances is as far as I know purely a decorative feature.

 

B.E.

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Hi Mark, yes I understand, that is the Lower hance, but I've not seen any reference to it having a practical purpose in any of my reference books.

 

The Spectacle plate to which the rudder pendants attach is fitted just below the hance, it is not shown on the plan.

 

Regards,

 

B.E.

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There is no way on earth ( I think) that this would be there just for decoration surely.

If nothing else it would be b***y nuisance when shipping or unshipping the rudder.  ( or is it a stop for a handling strop for the rudder when inverted)

It lies just above the water line too low for rudder control I think..

 

Qiuick look doesnt show it on other plans or models of the era - perhaps if no one knows I shall give the NMM curator a ring .

Edited by SpyGlass
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OK, my only experience of boats was dinghy sailing back in the 1950s.  Then, the rudder extended below the main body of the vessel, so the effect wouldn't have been the same as on a larger, commercial-or-naval vessel.  I can see that 18th-century thinking might not have developed far enough for sophisticated fluid flow patterns to have been studied and/or followed up.

 

I accept the evidence you've all put forward (for rudders simply being flat slabs of timber slung behind the keel).  So OK, I've resisted the temptation to taper the rudder on my little Mere Nostrum, even though it's a 20th-century boat.

 

.

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Sorry, but the lower hance ('protuberance') is merely decorative: it has no practical function other than that the convex curve sheds water off the batten that runs up the aft side of the rudder.

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Yes I was going to say the same.   This is just a stylistic decoration.  A hold out from earlier times where the rudder was even more decorative.  In fact,  they often had additional carving etc on them.  Later on this feature seems to be minimized further and then finally it disappears.

 

1830

 

large.jpg

 

1845

 

large.jpg

 

Chuck

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I came across this in a free ebook.

 

http://books.google.co.uk/books?id=pjcDAAAAQAAJ&pg=PA183&lpg=PA183&dq=rudder+hances&source=bl&ots=RIg8romngM&sig=rjmht5w4UnRffdpUPcVQ_yFRu-0&hl=en&sa=X&ei=9ieEUdiZNKmm4gSe3oCABw&ved=0CFwQ6AEwCA#v=onepage&q=rudder%20hances&f=false

 

I think it may address the original question since it seems to say the fore and aft of the rudder were different sizes.

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