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Old 24-06-2016, 06:46   #256
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Re: Skeg or Spade?

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Originally Posted by Dockhead View Post
What you say about the limits of aspect ratio with skeg rudders is true, but if you will read any of the basic information on hydrodynamics, you will see that this is not the only difference in the hydrodynamics between skeg and spade rudders. They produce far more drag, to generate a given amount of lift. Drag is the enemy of going upwind. This is not controversial among boat designers.
Why talk about difference of the two when the other doesn't excist in real world?
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Old 24-06-2016, 06:59   #257
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Re: Skeg or Spade?

I feel like this entire discussion is just turning in circles. The same arguments have been made over and over again. But to sum up.

1) rudder strength is a function of how strong it is designed to be. It is trivially easy to design either a spade or skeg to a given strength.
2) skeg rudders of a given strength are more expensive and more difficult to build
3) skegs are less hydrodynamicly efficient than spades
4) skegs stall at a higher angle of attack (you can turn them further), but generate less side force at a given angle of attack (you have to turn them further).
5) skegs will stall at a lower induced side force than spades

Dockhead is wrong above when he said the differences don't matter when reaching. A skeg will always have more drag than a spade, even when centered because of the increased wetted surface. So even in a strait ahead tank tow test the skeg is worse. But every time you steal the boat even a little the extra drag becomes even more pronounced.

How much worse is this drag, well a recent paper I found was actually testing twin spades versus a single spade, but also included a skeg as a control. In this test the result was that a skeg developed 1/3 more drag for a given side force than a spade does. And that the twin spade has 1/3 the drag of the single spade.

Rewritten it looks like...

1x - double spade
3x - single spade
4x - skeg

So why the huge benefit from the double spade? Because the windward rudder is out of the water, and the double spades can be much smaller for three reasons. 1) they operate in clean water un disturbed by the flow from the keel, and 2) they are verticle when the boat is heeled so are much more efficient 3) the higher efficiency from the above two reasons allows a much smaller rudder, with less wetted surface.
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Old 24-06-2016, 07:51   #258
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Re: Skeg or Spade?

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. . . Dockhead is wrong above when he said the differences don't matter when reaching. A skeg will always have more drag than a spade, even when centered because of the increased wetted surface. So even in a strait ahead tank tow test the skeg is worse. But every time you steal the boat even a little the extra drag becomes even more pronounced. . . .
OK, this is true, but it's a small difference, probably too small to matter to a cruiser, compared to the difference going upwind, which will matter very much to cruisers who care about going upwind.
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Old 24-06-2016, 08:31   #259
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Re: Skeg or Spade?

"Unlike the case with full keels, I don't see any real benefit of skeg rudders. You can make a spade rudder just as strong as any skeg rudder, as we discussed in a lot of detail above. It's just a question of design. Hydrodynamically the spade is far superior." Dockhead

Just because it's been discussed does not mean we all agree. This is beginning to sound much like the global warming religion debate. The science is not settled. My real world experiences support my favoring the skeg/rudder design over the spade, for strength and protection. You can't sight and compare crappy skeg designs and proclaim all skeg designs to be be inferior to spade design as a settled science.
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Old 24-06-2016, 08:48   #260
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Re: Skeg or Spade?

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"Unlike the case with full keels, I don't see any real benefit of skeg rudders. You can make a spade rudder just as strong as any skeg rudder, as we discussed in a lot of detail above. It's just a question of design. Hydrodynamically the spade is far superior." Dockhead

Just because it's been discussed does not mean we all agree. This is beginning to sound much like the global warming religion debate. The science is not settled. My real world experiences support my favoring the skeg/rudder design over the spade, for strength and protection. You can't sight and compare crappy skeg designs and proclaim all skeg designs to be be inferior to spade design as a settled science.
This is a much different discussion from global warming. Global warming is little more than SWAG (Scientific Wild Ass Guesses). Rudder design is based on well understood physics and engineering principals.

The only example in the thread so far where skegs come out ahead is if you have a solidly built skeg against a weakly designed spade which is a red herring discussion as it can easily be flipped to a weakly designed skeg. In every other case, most importantly when both styles designed to the same criteria, the spade comes out ahead. This is the physics.

If we've missed a situation where skegs come out ahead (ASSUMING THE SAME DESIGN STANDARDS), please share it with us.
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Old 24-06-2016, 10:14   #261
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Re: Skeg or Spade?

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Originally Posted by valhalla360 View Post
This is a much different discussion from global warming. Global warming is little more than SWAG (Scientific Wild Ass Guesses). Rudder design is based on well understood physics and engineering principals.

The only example in the thread so far where skegs come out ahead is if you have a solidly built skeg against a weakly designed spade which is a red herring discussion as it can easily be flipped to a weakly designed skeg. In every other case, most importantly when both styles designed to the same criteria, the spade comes out ahead. This is the physics.

If we've missed a situation where skegs come out ahead (ASSUMING THE SAME DESIGN STANDARDS), please share it with us.
Must I keep posting the same picture? Well, here it is again.
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Old 24-06-2016, 10:36   #262
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Re: Skeg or Spade?

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Must I keep posting the same picture? Well, here it is again.
That's a lovely picture of rudder but fails to include an explanation of how it is superior to a spade rudder...so I guess you will have to keep posting it.
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Old 24-06-2016, 11:17   #263
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Re: Skeg or Spade?

Quote:
Originally Posted by Kenomac View Post
"Unlike the case with full keels, I don't see any real benefit of skeg rudders. You can make a spade rudder just as strong as any skeg rudder, as we discussed in a lot of detail above. It's just a question of design. Hydrodynamically the spade is far superior." Dockhead

Just because it's been discussed does not mean we all agree. This is beginning to sound much like the global warming religion debate. The science is not settled. My real world experiences support my favoring the skeg/rudder design over the spade, for strength and protection. You can't sight and compare crappy skeg designs and proclaim all skeg designs to be be inferior to spade design as a settled science.
I don't think you really understood what I said. Definitely both skeg and spade rudders can be badly made. I'm sorry to say, but probably a majority of both types are badly made.

It's not logical to say that you "favor skeg/rudder . . for strength" -- because the strength is not determined by whether it's a spade or a skeg -- it's determined by the design spec. Protection is the same thing -- a spade rudder which is just as strong as some given skeg, will not need any "protection" from a skeg which is no stronger than it is. It's all a question of the specification it's designed to.

As to the hydrodynamics -- this is objective, and there is no controversy among designers about this. There is not a single serious study showing that skeg rudders are equal to spade rudders in hydrodynamics. This is not like global warming at all. Unlike global warming, which is an incredibly complicated phenomenon which no one really understands, and which has been horrendously politicized, the hydrodynamics of rudders is aerodynamics 101 -- simple, uncontroversial stuff, where straightforward theoretical principles are confirmed by lots of testing.

So I'm not really sure what you're arguing. There's nothing fatally wrong with a skeg rudder on a cruising boat where performance is not overly important. A hugely overbuilt skeg rudder, like the rudder on your Oyster, is something I fundamentally like -- a hugely overbuilt rudder behind a hugely overbuilt skeg -- this is nice to have on a cruising boat. Remember I hit an uncharted rock in the Baltic year before last, so I value strength in a very personal way. Would I give up the upwind performance it would cost to have a rudder like that? I wouldn't, personally. But does it suck? Absolutely not.

But don't make the mistake of thinking that your own rudder is some indication of what all skeg rudders are -- it is not. Your rudder is strong because it was designed to be that strong, and was built superbly, not because of what type it is.
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Old 24-06-2016, 12:31   #264
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Re: Skeg or Spade?

Just to remind that with well trimmed sails with a well designed rudder you should't need to turn the rudder much at all, mostly hold it in place to counter lee helm. If you must turn the wheel one or other way constantly there's something wrong. Presuming sails are tiptop then the rudder produces either too much or too little lift. A spade or a skeg rudder having the same foil shape are equal when there's zero helm. Cruising sailboats have quite narrow speed variation, little less or at hull speed so the lift of the rudder should depend only of the leeway. Planing racers are different in that regard and so are their rudders So the lift/drag discussion is bit unrelated for cruising displacement speeds IMHO

BR Teddy
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Old 24-06-2016, 12:41   #265
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Re: Skeg or Spade?

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Just to remind that with well trimmed sails with a well designed rudder you should't need to turn the rudder much at all, mostly hold it in place to counter lee helm. If you must turn the wheel one or other way constantly there's something wrong. Presuming sails are tiptop then the rudder produces either too much or too little lift. A spade or a skeg rudder having the same foil shape are equal when there's zero helm. Cruising sailboats have quite narrow speed variation, little less or at hull speed so the lift of the rudder should depend only of the leeway. Planing racers are different in that regard and so are their rudders So the lift/drag discussion is bit unrelated for cruising displacement speeds IMHO

BR Teddy
But you do need that little bit all the time to provide some lift. If you leave the rudder straight ahead, you will have a large amount of leeway.

The problem with the item in red is that you can't create rudders that have the same foil shape while countering leeway. You can create a skeg that is pretty much as good on one tack but once you switch tacks, it will be much worse.

As previously mentioned, with airplanes a skeg hung rudder isn't trying to create port or starboard lift during normal flight so lift isn't an issue and the elevators always produce upward lift during normal flight, so the skeg can be optimized to provide upward lift.
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Old 24-06-2016, 13:03   #266
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Re: Skeg or Spade?

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But you do need that little bit all the time to provide some lift. If you leave the rudder straight ahead, you will have a large amount of leeway.

The problem with the item in red is that you can't create rudders that have the same foil shape while countering leeway. You can create a skeg that is pretty much as good on one tack but once you switch tacks, it will be much worse.

As previously mentioned, with airplanes a skeg hung rudder isn't trying to create port or starboard lift during normal flight so lift isn't an issue and the elevators always produce upward lift during normal flight, so the skeg can be optimized to provide upward lift.
You don't, think about keel, it produces lift without turning it. It's the leeway creating the angle of attack. So does work also rudders. If it's not enough at zero helm then it's too small or too thin. Turning the rudder creates more lift with the cost of even more drag so the rudder works best in line with the keel. You should ideally turn the helm only to alter course or to counter wave action and such.

BR Teddy
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Old 24-06-2016, 14:13   #267
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Re: Skeg or Spade?

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Originally Posted by TeddyDiver View Post
Just to remind that with well trimmed sails with a well designed rudder you should't need to turn the rudder much at all, mostly hold it in place to counter lee helm. If you must turn the wheel one or other way constantly there's something wrong. Presuming sails are tiptop then the rudder produces either too much or too little lift. A spade or a skeg rudder having the same foil shape are equal when there's zero helm. Cruising sailboats have quite narrow speed variation, little less or at hull speed so the lift of the rudder should depend only of the leeway. Planing racers are different in that regard and so are their rudders So the lift/drag discussion is bit unrelated for cruising displacement speeds IMHO

BR Teddy
You need a few degrees of rudder angle when sailing upwind, even with perfectly balanced sails. That's why all sailboat rigs are set up to provide some weather helm (NOT lee helm). You are right that leeway alone will give an angle of attack to both helm and rudder, but the rudder is very small compared to the keel and needs more angle of attack to balance the lift produced by the keel. So a sailboat sailing upwind with maximum efficiency will have several degrees of rudder angle held steadily (NOT "turn the wheel this way and that constantly").

Lastly, remember that lift vs drag is only one part of the picture.

The other part of the picture is wetted surface, which is drag, in absolute terms, as opposed to lift vs drag. A skeg rudder has to be larger to produce the same turning force, or lift, because only part of the surface is movable, and that part of the surface which is movable, is less efficient.
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Old 24-06-2016, 15:02   #268
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Re: Skeg or Spade?

Another point made earlier - upwind in heavy weather the spade will stall more readily.
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Old 24-06-2016, 16:02   #269
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Re: Skeg or Spade?

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Must I keep posting the same picture? Well, here it is again.
Ok, so Ken, what happens to the bottom of that fine looking skeg if it is driven onto rocks. I realise it has a big metal shoe, but at some point something is going to break eventually. So if the bottom of that skeg breaks off is it separate from the hull, fully watertight? Or does the boat now sink?

How was it built, split mold? In that case how were the two halfs joined together? Very hard to do quality glass work deep down the bottom of a skeg like that.

I am sure that skeg is fine and strong enough, but not all skegs are. Just having a skeg is no guarantee that the rudder is strong enough.
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Old 24-06-2016, 16:41   #270
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Re: Skeg or Spade?

DH, you keep on harping on the additional drag of a skeg vs a spade rudder of the same steering capability, and I reckon that is generally true. But the question that comes to mind is how great is that increment compared to the total drag of the hull moving through the water? I suspect that it is a fairly small fraction of the total. As I understand things, at low speeds the dominant drag is skin friction, and at high speeds it is wavemaking. The parasitic drag from appendages, while certainly a factor, I believe to be small by comparison. If this is in fact true, then your repeated statement that the upwind performance difference between skeg and spade is huge is questionable. I'm surely not very knowledgeable in this field, so do take this as a query and an attempt to put some scaling on the figures.

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