Skeg or Spade?

[K_V_B]

Originally Posted by Spindrift NH

About 6 months ago I saw a Tartan with a partial skeg hung rudder, the skeg broke off after hitting an object. There really wasn't much holding it on the hull, I believe 4 bolts, and the skeg was thin walled and hollow. I believe that even though the skeg broke off, the rudder was still attached. The owner was able to get hauled out and had a replacement skeg shipped and was able to make the repair.

I wonder how many skeg hung rudders are out there where it's the rudder supporting the skeg.

The whole discussion tastes of the stayed versus unstayed wing debate in the beginning of aviation... Van De Stadt already put spades on his designs in the fourties. I read an interview where he recalls his "missionary" work, convincing his fellow naval architects of the superiority of the fin keel and spade rudder (usually by beating them in a race), and how in his experience skews caused him more headaches than spades...



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[Juho]

Originally Posted by Stumble

Add in the next generation of daggerboard that not only provide lift to windward but also start lifting the hull out of the water (foiling) and we have defended right up to the event horizon of knowledge.

I have seen some quite weird daggerboard constructions on modern racing boats. Maybe all racing boats are soon expected fly above the water and forget all the hull speed nonsense . https://en.wikipedia.org/wiki/Sailing_hydrofoil

To generate lift Commanche uses two tacking daggerboards. Basically giant (well giant for a boat) asymmetric foils that don't have to deal with ballast loads at all and can be maximized for lift and minimal drag.

Maybe this separation of functions can really increase efficiency. The lift generating daggerboards can be very light and thin. And the RM generating ballast can be moved around wherever (not being tied to the keel).

She leaves the dock with two of them (port and starboard) and part of the tacking routine is to swap them (there is also some built in redundancy, since the port board can be used on the starboard side by flipping it up side down).

I'm sure we will see lots of evolution in the area of handling the daggerboards. (plenty of weird ideas flying inside my head already now )

Keep in mind though that there are real limitations in going to a fin design this extreme for a cruiser. First is simply that ideal shape changes as speed goes down, so if you can't maintain the same speed as a boat like this you need a different shape.

It is possible that many future cruisers will be somewhat faster than cruisers of today. If we don't care about safety, the difference could be small. If we want to have a heavy keel (for stability in storm), lots of payload, and a strong hull, we have some more limitations. Some keels/rudders could be folding keels/rudders to keep both depth and safety. But I guess also traditional (not speed optimized) heavy cruisers will remain popular.

As AP's get better at being able to drive to the boats maximum the shapes can be less forgiving than they were. Eventually it is possible to envision a boat that is faster at all times with the AP driving than with a human at the wheel.

Maybe there will be boats that can not sail without a computer controlling all the gadgets. For example the keel/ballast could be so light or so off-centre that the boat would fall if the computer would not constantly do its job.

See Daggerboard evaluation for an IMOCA 60 yacht : Owen Clarke Design - Yacht Design and Naval Architects for a fascinating description of the issues facing top flight designers with these boats.

Thanks for the link. Lots of interesting stuff there.

[Stumble]

Originally Posted by K_V_B

I wonder how many skeg hung rudders are out there where it's the rudder supporting the skeg.

The whole discussion tastes of the stayed versus unstayed wing debate in the beginning of aviation... Van De Stadt already put spades on his designs in the fourties. I read an interview where he recalls his "missionary" work, convincing his fellow naval architects of the superiority of the fin keel and spade rudder (usually by beating them in a race), and how in his experience skews caused him more headaches than spades...



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Bob Perry suggests that a good number of skegs are held on by the rudder not the other way around.

[model 10]

Originally Posted by K_V_B

The whole discussion tastes of the stayed versus unstayed wing debate in the beginning of aviation... Sent from my iPad using Cruisers Sailing Forum

Yeah and look at what has happened, fly by wire. Broken masts in the southern ocean will replaced by broken computers in the southern ocean. This makes me think of Hans Solo and the millennium falcon, we are about to hit the reef and the computer quits.

[leftbrainstuff]

Originally Posted by robert sailor

Spade rudders are the best rudder for steering a sailboat, bar none! They got a bad reputation from some cruisers because the boats they chose were not built all that well, especially the rudder but if the rudder is properly built it will be just as strong or even stronger than a skeg hung rudder. I have a partial skeg on our boat and like it but I know a well built spade is better.

That's a generalization that fails any engineering structural discussion.

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[jeepbluetj]

Originally Posted by Stumble

Bob Perry suggests that a good number of skegs are held on by the rudder not the other way around.

Yea, but really what does that guy know.......

[Dockhead]

Originally Posted by leftbrainstuff

That's a generalization that fails any engineering structural discussion.

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He said "for steering a sailboat" -- so I don't think he was talking about structure, but rather, about hydrodynamics. If that's what he meant, he's clearly right.

Concerning structure -- engineers don't recognize "the stronger the better", since there's no limit and there are always tradeoffs (at least weight, cost, etc., and in this case also hydrodynamics). Rather, engineers recognize "optimally strong" considering the forces which might need to be managed. I think that one of the key insights we can get from this discussion is that -- although a well-built full keel with barn door attached rudder is definitely going to be stronger than any kind of reasonably feasible spade rudder, the issue becomes trickier when you go to a fin keel and separated rudder and skeg, because of the structural difficulties of building a strong skeg.

A skeg which does the least damage to the hydrodynamic properties of the rudder must be thin and narrow, but if it's thin and narrow it will be very hard to attach to the hull in a strong way. If it's thick and long (like the Oyster rudders we've been shown), it will be easier to make strong, but the hydrodynamic damage is much greater.

Which means that many skeg rudders probably do almost nothing structurally (Bob Perry's comments about the rudder holding up the skeg) and probably don't have any real function other than to reassure potential buyers who have an irrational fear of spade rudders.

A partial skeg rudder probably makes more sense structurally -- I'm guessing. Less of a lever arm -- so it's like lowering the lower bearing (or adding a second lower bearing) to a spade rudder. It's called on to do less structurally than a full spade, and not more than this construction is capable of doing, but nonetheless should add a fair amount of strength to the whole construction, all other things being equal.

But all other things don't need to be equal -- you could simply increase the rudder shaft size a bit and get all the same amount of strength in a spade. And I guess that's why we start to see fewer and fewer even partial skegs, which were so popular 10 and 20 years ago, as almost all the main designers are now using spades.

[Dockhead]

Originally Posted by K_V_B

. . . The whole discussion tastes of the stayed versus unstayed wing debate in the beginning of aviation...

Yes, but I understand this, too. Cantilevers are instinctively unsatisfying to the human eye. There is something ugly about them -- strength created out of the brute, dumb application of more material, rather than by intelligently spreading and balancing the forces the way an arch does, or a suspension bridge, etc., constructions which are instinctively beautiful and satisfying to the eye.

The thing is, however, that sometimes there's no other way to accomplish the task. Ugly or not, cantilevers work perfectly well -- if they are properly designed. Airplane wings and sailboat appendages are places where you really need cantilevers, ugly or not.

[Sandero]

Cantilevers are not ugly... as much as counter intuitive. We know that a simply supported beam each support carries half the load... a cantilever means one carries the entire load. And there are other engineering issues such as supporting the cantilever connection.

So... in a rudder the rudder shaft is larger/stiffer for a non skeg and the support tube needs to longer and stiffer.

The performance of a balanced spade my be superior and in racing especially this is very crucial. I am unaware of actual performance comparisons of the same hull and rig between the two. I've mostly seen various keel types offered.. shoal, deep, wing and so on.

The full skeg appears to offer the benefit of strength, protecting the leading edge of the rudder.

Then of course one needs to consider lee and weather helm. A balanced helm is very close to CL I would think with the skeg and rudder acting together with little turbulence or cavitation. All of this fluid dynamics is way way above my pay grade. Intuitively I would feel more comfortable sacrificing a few hundredths of an knot for the apparent structural superiority of a rudder supported top and bottom and protected at the forward end.

[Dockhead]

Originally Posted by Sandero

Cantilevers are not ugly... as much as counter intuitive. We know that a simply supported beam each support carries half the load... a cantilever means one carries the entire load. And there are other engineering issues such as supporting the cantilever connection.

So... in a rudder the rudder shaft is larger/stiffer for a non skeg and the support tube needs to longer and stiffer.

The performance of a balanced spade my be superior and in racing especially this is very crucial. I am unaware of actual performance comparisons of the same hull and rig between the two. I've mostly seen various keel types offered.. shoal, deep, wing and so on.

The full skeg appears to offer the benefit of strength, protecting the leading edge of the rudder.

Then of course one needs to consider lee and weather helm. A balanced helm is very close to CL I would think with the skeg and rudder acting together with little turbulence or cavitation. All of this fluid dynamics is way way above my pay grade. Intuitively I would feel more comfortable sacrificing a few hundredths of an knot for the apparent structural superiority of a rudder supported top and bottom and protected at the forward end.

A lot of the points you raise have been discussed in the posts above -- the thread is worth reading in its entirety I think.

Two points summing up some of the discussion above:

1. A skeg rudder LOOKS like its "supported top and bottom", but that is only really the case with a full keel and barn door rudder. For fin keel boats, the skeg is just another cantilever, and one which is harder to make strong than the rudder itself. That is why leading designers do not consider skegs on fin keel boats to be a great structural advantage, even if of course they can add SOME strength.

2. The hydrodynamic question is not indeed a question of "turbulence and cavitation". The rudder (like propellers, airplane wings, and your keel) works by producing lift. It's a wing. This is most crucially important when sailing upwind, when lift from rudder and keel is what allows the boat to go upwind instead of being blown down to leeward. So keel and rudder are wings, which is why they are usually carefully shaped as NACA airfoils. The rudder by the way needs to be angled to leeward in order to produce the optimum amount of lift, since it is a symmetrical foil and generates lift only on the basis of having some angle of attack. So going upwind you need a bit of weather helm, not too much, to allow the rudder to be at the optimum angle of attack.


The issue with efficient rudders is not so much speed, as it is power, resistance to stalling, and effective lift (lift minus drag) when going upwind. Speed is a side effect. Naturally cruising boats not sailed much upwind will not care that much about it. Even such cruising boats do need power in the rudder, in order to resist broaching, but that can be achieved by sheer size (like it is on my boat), at the expense of some speed.


So naturally there are different tradeoffs, and different people will have different priorities. But in my opinion thin skegs like on the old Moodys and many French and American boats of the 80's and 90's are really pointless, adding little if any strength (as Bob Perry said "the rudder holds up the skeg") while hurting performance a lot. So in my opinion, if you are willing to give up performance (pointing ability more than sheer speed), the way to go is either a full keel with barn door rudder, which is really superior strength wise to any fin keel setup, or at least go the Oyster way with a really massively strong skeg.

In any case, though, the performance difference between good and bad rudders is not measured in "hundredths of a knot". It is measured rather in VMG to windward which you can achieve, and in many cases this will be whole knots. The rudder is really important for sailing upwind.

[Sandero]

Originally Posted by Dockhead

A lot of the points you raise have been discussed in the posts above -- the thread is worth reading in its entirety I think.

Two points summing up some of the discussion above:

1. A skeg rudder LOOKS like its "supported top and bottom", but that is only really the case with a full keel and barn door rudder. For fin keel boats, the skeg is just another cantilever, and one which is harder to make strong than the rudder itself. That is why leading designers do not consider skegs on fin keel boats to be a great structural advantage, even if of course they can add SOME strength.

2. The hydrodynamic question is not indeed a question of "turbulence and cavitation". The rudder (like propellers, airplane wings, and your keel) works by producing lift. It's a wing. This is most crucially important when sailing upwind, when lift from rudder and keel is what allows the boat to go upwind instead of being blown down to leeward. So keel and rudder are wings, which is why they are usually carefully shaped as NACA airfoils. The rudder by the way needs to be angled to leeward in order to produce the optimum amount of lift, since it is a symmetrical foil and generates lift only on the basis of having some angle of attack. So going upwind you need a bit of weather helm, not too much, to allow the rudder to be at the optimum angle of attack.


The issue with efficient rudders is not so much speed, as it is power, resistance to stalling, and effective lift (lift minus drag) when going upwind. Speed is a side effect. Naturally cruising boats not sailed much upwind will not care that much about it. Even such cruising boats do need power in the rudder, in order to resist broaching, but that can be achieved by sheer size (like it is on my boat), at the expense of some speed.


So naturally there are different tradeoffs, and different people will have different priorities. But in my opinion thin skegs like on the old Moodys and many French and American boats of the 80's and 90's are really pointless, adding little if any strength (as Bob Perry said "the rudder holds up the skeg") while hurting performance a lot. So in my opinion, if you are willing to give up performance (pointing ability more than sheer speed), the way to go is either a full keel with barn door rudder, which is really superior strength wise to any fin keel setup, or at least go the Oyster way with a really massively strong skeg.

In any case, though, the performance difference between good and bad rudders is not measured in "hundredths of a knot". It is measured rather in VMG to windward which you can achieve, and in many cases this will be whole knots. The rudder is really important for sailing upwind.

Lot's of interesting ideas and mostly opinion here with little examples and calculations.

VMG IS a speed calculation of course. Either you sail faster toward a mark or you sail high and perhaps faster with a tack closer to it. I'd like to see the number so we can assess the real differnces. I don't dispute that different foils and rudders have different performances. I would just like to see real world number comparisons.

Perry's comment has some currency... for surely the steel in a rudder post is stiffer than the GRP of a skeg. But that hardly the rudder supporting the skeg. Yes the are both together a cantilever..... but the rudder part is NOT it is simply supported by the skeg's lower hinge and the rudder tube.

I don't want to be splitting hairs... I want to see what the difference means between the same boats... one with a skeg and the other with a spade... sailing to a windward mark 20 miles away for example. What does this mean real world practicality. Will one boat reach the mark 1 min sooner, 10 minutes, 30 minutes, 1 hr?

Let's see some numbers!

[noelex 77]

Originally Posted by Sandero

and protected at the forward end.

But does the skeg "protect" the forward end of the rudder or make it more vulnerable?

If the skeg is damaged say from collision with debris the rudder will not function normally. In fact, if the area of the bottom bearing is pushed out of alignment the rudder is likely to be immovable.

By contrast, the bottom (the most vulnerable part) of a spade rudder can be bent, damaged or even broken off and the rudder will still rotate. It will not function normally, but is likely to still be usable.

I do have a lot of empathy for the view that many spade rudders are marginal from a strength point of view. Modern yachts are more lightly constructed than the older models and rudder strength is one area where the compromises are more obvious, but I think a strong spade rudder can be constructed.

[valhalla360]

Originally Posted by Sandero

Lot's of interesting ideas and mostly opinion here with little examples and calculations.

VMG IS a speed calculation of course. Either you sail faster toward a mark or you sail high and perhaps faster with a tack closer to it. I'd like to see the number so we can assess the real differnces. I don't dispute that different foils and rudders have different performances. I would just like to see real world number comparisons.

Perry's comment has some currency... for surely the steel in a rudder post is stiffer than the GRP of a skeg. But that hardly the rudder supporting the skeg. Yes the are both together a cantilever..... but the rudder part is NOT it is simply supported by the skeg's lower hinge and the rudder tube.

I don't want to be splitting hairs... I want to see what the difference means between the same boats... one with a skeg and the other with a spade... sailing to a windward mark 20 miles away for example. What does this mean real world practicality. Will one boat reach the mark 1 min sooner, 10 minutes, 30 minutes, 1 hr?

Let's see some numbers!

The difficulty in providing examples is boats with skeg hung rudders typically have much different design criteria compared to spade rudders. Also, it will vary based on wind and wave conditions. In light air, I would expect a modern fin keel with spade rudder to outrun an old full keel with barn door rudder design by a good margin upwind and downwind. Skeg hung is part way in between. As the wind builds, the drag is less of an issue and the advantage is less. Add big waves and things change yet again. I don't think anyone is saying skeg hung is bad just not as good for performance. As far as strength, it absolutely is a cantilever design with all it's inherent weaknesses. With proper design and construction those weaknesses can be overcome with either type of rudder. - If the skeg was a monolithic piece that penetrated the hull and was supported a foot or two above the hull bottom, it would be easier to build a stronger skeg hung but many just meet the hull bottom and then expect the hull skin to support the cantilever. - I've yet to come across a spade rudder that doesn't have an upper bearing well above the hull bottom. As someone else pointed out, take a broomstick and hold it at the end with one hand and try to hold it upright while going thru the water. Then do the same experiment with two hands a foot apart holding it. The only mistake them made was that the example more closely represents a spade rudder design.

[Dockhead]

Originally Posted by noelex 77

But does the skeg "protect" the forward end of the rudder or make it more vulnerable?

If the skeg is damaged say from collision with debris the rudder will not function normally. In fact, if the area of the bottom bearing is pushed out of alignment the rudder is likely to be immovable.

By contrast, the bottom (the most vulnerable part) of a spade rudder can be bent, damaged or even broken off and the rudder will still rotate. It will not function normally, but is likely to still be usable.

I do have a lot of empathy for the view that many spade rudders are marginal from a strength point of view. Modern yachts are more lightly constructed than the older models and rudder strength is one area where the compromises are more obvious, but I think a strong spade rudder can be constructed.

That's what Dashew says -- he says that skeg rudders are MORE vulnerable to damage than spade rudders are, because they jam when any damage occurs.

Another vulnerability of skeg rudders, which spades don't have, is ripping the skeg out of the hull skin. An old Moody sank like that in a relatively mild grounding some years ago. Recent skeg rudder skegs are usually just glassed into the hull skin because it's so hard (or impossible) to tie them into the hull structure. When tweaked, these can tear out and create a hole.

[Dockhead]

Originally Posted by Sandero

I don't want to be splitting hairs... I want to see what the difference means between the same boats... one with a skeg and the other with a spade... sailing to a windward mark 20 miles away for example. What does this mean real world practicality. Will one boat reach the mark 1 min sooner, 10 minutes, 30 minutes, 1 hr?

Let's see some numbers!

There are no numbers as far as I know, because you just don't see the same boat built with both types of rudder. But if you want to settle it by racing for money, two similar boats one with a skeg and the other with a spade, I would be game

The difference is very clear on the racing course, where no racing boat has been designed with a skeg rudder in at least 30 years. The hydrodynamic superiority of spade rudders is not controversial, and the difference is not minor ("hundreths of a knot").