Skeg or Spade?

[Sandero]

When a boat is properly trimmed the helm should be balanced... not lee helm... perhaps a bit of weather helm... In fact a balanced boat the helm should not be turning much at all and the boat should track a straight course. No?

[Stumble]

Originally Posted by DumnMad

Another point made earlier - upwind in heavy weather the spade will stall more readily.

Only partially true. A spade will stall at a lower angle, but at a higher turning force.

So a spade may stall at let's say 25 degrees where a skew will stall at 30 degrees. But a spade will generate more turning force at 25 degrees than a skeg will at 30.

But in heavy weather if you are throwing the rudder lock to lock you have serious other problems.

[Stumble]

Originally Posted by Jim Cate

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.

Jim

A spade rudder contributes between 10 and 15% of total drag. Where a skeg will increase that by an additional 30-35%.

So if a boat has a total drag of 100N, about 90N is from the hull and keep, and 10N is from the spade rudder. If you switch to a skeg then you would expect rudder drag to increase to about 13-13.5N, for a total boat drag to 103-103.5N. Or an absolute increase of about 3%.

This assumes a strait line, drag thru a turn is substantially higher.

[Lantau]

This is a great thread for anyone new to sailing/cruising to learn a fundamental law of sailing: sailors will always disagree!

Personally, I would never own a cruising boat without a skeg-protected rudder. I would go so far as to say (and will be attacked for it) that anyone heading offshore in a typical production boat with a spade rudder is either loopy or willfully unprepared. But that's just my opinion.

My skeg-hung rudder works beautifully and my boat tracks like it's on rails. That's the kind of 'balanced' rudder I want offshore. My autopilot and windvane steering like it, too.

[Jim Cate]

Originally Posted by Stumble

A spade rudder contributes between 10 and 15% of total drag. Where a skeg will increase that by an additional 30-35%.

So if a boat has a total drag of 100N, about 90N is from the hull and keep, and 10N is from the spade rudder. If you switch to a skeg then you would expect rudder drag to increase to about 13-13.5N, for a total boat drag to 103-103.5N. Or an absolute increase of about 3%.

This assumes a strait line, drag thru a turn is substantially higher.

Well, this is kinda what I expected, Greg, and I hope that DH sees your post. A 3% increment in total drag seems pretty inconsequential to me, and definitely not a "huge" difference. If racing, sure it matters. If cruising, even trying to make big miles to weather, it isn't a big deal IMO. And the numbers that you quote, are they also true as the boat approaches hull speed (and we're talking cruising boats here, boats that don't readily exceed hull speed, especially upwind)? I bet the drag numbers alter greatly as wave making absorbs more and more energy, and the drag increment for a skeg becomes a smaller and smaller factor in the total.

I'm not saying that one shouldn't use a spade, far from it. It is just that it does not make such a big difference that it should drive your choice of boat design on its own.

Jim

[TeddyDiver]

Originally Posted by Dockhead

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.

It's less efficient in L/D ratio but can produce the same lift with higher rudder angle as it was showed in the graphs in the Wolfson unit papers, so no more area is not a necessity. If you look at the polars to any sailboat at the point where the WMG to weather starts to decline, that's where the boats rudder angle becomes too big. On the other side of that point the angle should be still quite small not to produce considerable drag. Anything else, spade or skeg, would be an impediment, and in that case either the CE of the sails or the LR of the hull and keel is misplaced. More to towards reach and beam reach the side force part of the aerodymic force is reduced compared to close hauled and less lift is needed from the rudder. Then the rudder angle can be a bit to lee but there's still an angle of attack on the rudder producing weather helm. That is if the sail area and the wind is the same as close hauled but now it's ok to put some more canvas up until the sideforce of the sails comes up equal as in the close hauled scenario and so does the rudder angle.
Hope I explained my point clear enough, it's just my reasoning is quite geometric and it's a bit difficult to put it in words.

BR Teddy

[Dockhead]

Originally Posted by Jim Cate

Well, this is kinda what I expected, Greg, and I hope that DH sees your post. A 3% increment in total drag seems pretty inconsequential to me, and definitely not a "huge" difference. If racing, sure it matters. If cruising, even trying to make big miles to weather, it isn't a big deal IMO. And the numbers that you quote, are they also true as the boat approaches hull speed (and we're talking cruising boats here, boats that don't readily exceed hull speed, especially upwind)? I bet the drag numbers alter greatly as wave making absorbs more and more energy, and the drag increment for a skeg becomes a smaller and smaller factor in the total.

I'm not saying that one shouldn't use a spade, far from it. It is just that it does not make such a big difference that it should drive your choice of boat design on its own.

Jim

Jim, it's an excellent question you asked, which puts everything into perspective.

The 3% difference is exactly why, earlier, I ventured to say that when sailing off the wind, cruisers won't notice the difference between a spade and a skeg. 3% of drag will affect your speed, don't get me wrong -- it will be important to a racer. But in my opinion, not for most cruisers, maybe not even for me. So while Greg is correct, but I don't think I was wrong, either.

But as Greg said, the difference is far greater when you have rudder angle on, going upwind. The full skeg rudder acts as a brake when it's turned, and remember it has to be turned much further to get the same lift. The spade rudder is a smooth foil which requires less rudder angle and maintains a smooth flow of water over it. You are right that it would be good to put some numbers to it -- maybe Greg has some. But the difference is very great. The key to getting upwind is speed and leeway, which are related to each other. You have to go fast and make as little drag as possible, making enough lift to keep your boat moving upwind and not being blown off to leeward. The skeg rudder is not a good tool for this job compared to a spade.

[Dockhead]

Originally Posted by TeddyDiver

It's less efficient in L/D ratio but can produce the same lift with higher rudder angle as it was showed in the graphs in the Wolfson unit papers, so no more area is not a necessity. If you look at the polars to any sailboat at the point where the WMG to weather starts to decline, that's where the boats rudder angle becomes too big. On the other side of that point the angle should be still quite small not to produce considerable drag. Anything else, spade or skeg, would be an impediment, and in that case either the CE of the sails or the LR of the hull and keel is misplaced. More to towards reach and beam reach the side force part of the aerodymic force is reduced compared to close hauled and less lift is needed from the rudder. Then the rudder angle can be a bit to lee but there's still an angle of attack on the rudder producing weather helm. That is if the sail area and the wind is the same as close hauled but now it's ok to put some more canvas up until the sideforce of the sails comes up equal as in the close hauled scenario and so does the rudder angle.
Hope I explained my point clear enough, it's just my reasoning is quite geometric and it's a bit difficult to put it in words.

BR Teddy

Your point is very clear, and in my opinion these are important points. You are certainly right that if the sail plan is imbalanced or the keel misplaced, resulting in excessive weather helm, this will slow you down, as the high turning force required to keep the boat from rounding up is deducted from driving force, and besides that, any rudder called on to produce a lot of turning force will also be producing a lot of drag. The only thing I disagree with you about is desirable rudder angle when going upwind. With the rudder at 0 angle, the only difference between skeg and spade, assuming both are good foil shapes, is aspect ratio -- can't argue with you there. But 0 rudder angle, a neutral helm, upwind makes for a very, very slow boat. You must have some weather helm and a few degrees of rudder angle to go fast upwind -- every racer knows that. The rudder is much smaller than the keel and has to be tweaked to balance the lift of the keel.

Also remember that the rudder does actually do steering, besides producing lift, and whenever the rudder is to lee, it's killing lift. So you want the whole range of small steering corrections to be on the weather side, so the average position needs to be somewhere to weather. Another reason why you need a little weather helm.

[TeddyDiver]

Originally Posted by Dockhead

....The only thing I disagree with you about is desirable rudder angle when going upwind. With the rudder at 0 angle, the only difference between skeg and spade, assuming both are good foil shapes, is aspect ratio -- can't argue with you there. But 0 rudder angle, a neutral helm, upwind makes for a very, very slow boat. You must have some weather helm and a few degrees of rudder angle to go fast upwind -- every racer knows that. The rudder is much smaller than the keel and has to be tweaked to balance the lift of the keel.

Also remember that the rudder does actually do steering, besides producing lift, and whenever the rudder is to lee, it's killing lift. So you want the whole range of small steering corrections to be on the weather side, so the average position needs to be somewhere to weather. Another reason why you need a little weather helm.

At 0 angle it's not neutral helm. Let's say leeway being 7deg, then the angle of attack of the rudder is that very same 7deg and has weather helm (assuming it's not totally balanced) and if you release the helm the rudder turns along the water flow and the boat will turn to wind. On seaway for steering it's mostly needed only smaller rudder angles, let's here assume + - 5 deg, then the rudder is all the time producing lift to weather, in this case the angle of attack varies between 2 to 12 deg and have weather helm accordingly.
If you want to have few degrees more rudder angle going straight in same circumstances, lets say 3 deg, then the angle of attack would vary from 5 to 15 degrees and I quarentee you at 15 degrees there's a lot of drag..

BR Teddy

[noelex 77]

An interesting article here on someone who spent £150,000 to modify his Swan:
How to remodel a Swan 44 to improve performance - Sailing Today

A chunk of the cost was changing the skeg rudder for essentially a spade rudder.

"The surface area of the rudder has doubled, but the loss of the skeg means the overall wetted surface hasn’t increased. Under the original S&S design, the skeg served little purpose, as it didn’t support the rudder. Nonetheless, forces in the new spade rudder will be significantly higher, so the bearings and the rudder stock had to be beefed up as well."

[Dockhead]

Originally Posted by TeddyDiver

At 0 angle it's not neutral helm. Let's say leeway being 7deg, then the angle of attack of the rudder is that very same 7deg and has weather helm (assuming it's not totally balanced) and if you release the helm the rudder turns along the water flow and the boat will turn to wind. On seaway for steering it's mostly needed only smaller rudder angles, let's here assume + - 5 deg, then the rudder is all the time producing lift to weather, in this case the angle of attack varies between 2 to 12 deg and have weather helm accordingly.
If you want to have few degrees more rudder angle going straight in same circumstances, lets say 3 deg, then the angle of attack would vary from 5 to 15 degrees and I quarentee you at 15 degrees there's a lot of drag..

BR Teddy

Your point that the rudder can produce lift even at 0 rudder angle, and that there are not fundamental differences between skegs and spades (other than inherent differences in wetted surface and aspect ratio) at 0 rudder angle, is important and right.

But "neutral helm" is not helm held to lee several degrees. This is a mistake in terminology. "Neutral helm" is when the boat has no tendency to either round up or fall off, and will go straight with 0 rudder angle.

So let's get this straight (so to speak ).

Your other point -- that the real angle of attack of the rudder is rudder angle PLUS leeway, is also true. But what this means is even greater advantage of spade rudders, which require less rudder angle to generate the same force.

As an interesting aside -- the reason why aspect ratio improves the lift vs. drag of any wing is because there is less downwash (sidewash), the higher the aspect ratio, which increases the effective angle of attack, allowing the wing to generate more lift with less real angle of attack. Skegs have the opposite effect, increasing downwash and reducing the effective angle of attack. This requires more and more rudder angle to do the same job. As Ted Brewer said, this reduces the tendency to stall for a given rudder angle, but this is not an advantage in reality, because a given rudder angle produces entirely different amounts of force, on a skeg vs spade rudder. And this difference is even greater when the spade has a higher aspect ratio, which is almost inevitably will. So you see there are several factors all combining to increase drag on a skeg rudder, and the real practical difference on a sailboat going upwind is very large and very noticeable.

[Stumble]

Originally Posted by Jim Cate

Well, this is kinda what I expected, Greg, and I hope that DH sees your post. A 3% increment in total drag seems pretty inconsequential to me, and definitely not a "huge" difference. If racing, sure it matters. If cruising, even trying to make big miles to weather, it isn't a big deal IMO. And the numbers that you quote, are they also true as the boat approaches hull speed (and we're talking cruising boats here, boats that don't readily exceed hull speed, especially upwind)? I bet the drag numbers alter greatly as wave making absorbs more and more energy, and the drag increment for a skeg becomes a smaller and smaller factor in the total.

I'm not saying that one shouldn't use a spade, far from it. It is just that it does not make such a big difference that it should drive your choice of boat design on its own.

Jim

Keep in kind that a 3% increase in drag is not directly equatable to a 3% decrease in speed. It is far more. First more drag does mean sailing slower, but the effect is more pronounced as speeds get higher. since the force needed to go faster raises exponentially the effect of added drag is also exponential (I think).

Secondly the added drag will also reduce pointing ability. How much? No idea I am well outside my depth here. But I would guess a few percent.

Combine these two, and while beating you are likely looking at sailing 2-3 degrees lower, with a 5% reduction in speed. The vmg to a windward point is thus probably around 10%. Or more, since you have to combine the vector losses of both sailing slower, and at a lower angle.

Downwind the loss is going to primarily in drag not pointing, so figure the 3% drag loss as about 5% speed loss.

Does this matter? I guess it depends on how much sailing performance matters to you. But this is in the same order of magnitude gain in switching from blown out Dacron rags to 3di composite sails.


The above is my conjecture btw. I make no claims to having the engineering background to reduce this stuff to the math. The drag numbers however come directly from a paper so those I stand by.

[Dockhead]

Originally Posted by Stumble

Keep in kind that a 3% increase in drag is not directly equatable to a 3% decrease in speed. It is far more. First more drag does mean sailing slower, but the effect is more pronounced as speeds get higher. since the force needed to go faster raises exponentially the effect of added drag is also exponential (I think).

Secondly the added drag will also reduce pointing ability. How much? No idea I am well outside my depth here. But I would guess a few percent.

Combine these two, and while beating you are likely looking at sailing 2-3 degrees lower, with a 5% reduction in speed. The vmg to a windward point is thus probably around 10%. Or more, since you have to combine the vector losses of both sailing slower, and at a lower angle.

Downwind the loss is going to primarily in drag not pointing, so figure the 3% drag loss as about 5% speed loss.

Does this matter? I guess it depends on how much sailing performance matters to you. But this is in the same order of magnitude gain in switching from blown out Dacron rags to 3di composite sails.


The above is my conjecture btw. I make no claims to having the engineering background to reduce this stuff to the math. The drag numbers however come directly from a paper so those I stand by.

You are understating the difference when sailing upwind. When the rudder is 0, the only difference is wetted surface and maybe form of the foil. But sailing upwind when you need to have the rudder cranked over the difference is enormous. No kind of 3%. For similar rudders, one spade and one skeg, it takes approximately double the drag (and double the rudder angle), for the skeg rudder to produce the same amount of lift. I linked the research on this somewhere above.


As to blown-out dacron vs laminate sails -- I completely agree with this. That's actually a very apt comparison, because the laminate sails really come into their own with the wind ahead of the beam, and don't make so much difference with the wind abaft the beam. What that means is that cruisers who are happy with 10 year old Dacron sails, will not get much out of spade rudders. And that's fine. We all sail in different ways.

[TeddyDiver]

Originally Posted by Dockhead

Your point that the rudder can produce lift even at 0 rudder angle, and that there are not fundamental differences between skegs and spades (other than inherent differences in wetted surface and aspect ratio) at 0 rudder angle, is important and right.

But "neutral helm" is not helm held to lee several degrees. This is a mistake in terminology. "Neutral helm" is when the boat has no tendency to either round up or fall off, and will go straight with 0 rudder angle.

So let's get this straight (so to speak ).

Your other point -- that the real angle of attack of the rudder is rudder angle PLUS leeway, is also true. But what this means is even greater advantage of spade rudders, which require less rudder angle to generate the same force.

As an interesting aside -- the reason why aspect ratio improves the lift vs. drag of any wing is because there is less downwash (sidewash), the higher the aspect ratio, which increases the effective angle of attack, allowing the wing to generate more lift with less real angle of attack. Skegs have the opposite effect, increasing downwash and reducing the effective angle of attack. This requires more and more rudder angle to do the same job. As Ted Brewer said, this reduces the tendency to stall for a given rudder angle, but this is not an advantage in reality, because a given rudder angle produces entirely different amounts of force, on a skeg vs spade rudder. And this difference is even greater when the spade has a higher aspect ratio, which is almost inevitably will. So you see there are several factors all combining to increase drag on a skeg rudder, and the real practical difference on a sailboat going upwind is very large and very noticeable.

The skeg/spade discussion is relevant only for boats with lower aspect ratio rudders. Why some boats use them can have reasons like limited draft requirement and long keel. But the boat as a whole has been designed to be balanced, hull form, the rig, sails, keel and rudder. There's no point to put a high aspect rudder to a boat which is not designed to have one. High aspect there's only spade so there's nothing to debate of..
Downwash is byproduct of lift. Induced drag is created by the tip vortices.

BR Teddy

Ps. Lifting line theory! Bound vortex and Free vortex.

[Dockhead]

Originally Posted by TeddyDiver

The skeg/spade discussion is relevant only for boats with lower aspect ratio rudders. Why some boats use them can have reasons like limited draft requirement and long keel. But the boat as a whole has been designed to be balanced, hull form, the rig, sails, keel and rudder. There's no point to put a high aspect rudder to a boat which is not designed to have one. High aspect there's only spade so there's nothing to debate of..


BR Teddy.

Yes!!

Very perceptive, putting it all into perspective.


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