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Old 08-06-2016, 02:23   #76
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Re: Skeg or Spade?

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Originally Posted by Dockhead View Post
I
This is another really big tradeoff, because for the rudder to be optimally effective, it should actually be as long as the keel -- as you see on most race boats. This is unacceptable on a cruising boat in my opinion, and you have to make a compromise on hydrodynamics here in favor of safety.
I just note that the requirements of racing boats and cruisers are different also in other areas than safety. Race boats may be planned for quick actions and minimal directional stability, while cruisers usually want good directional stability.

The narrow and deep keel and rudder of racing boats may have something to do with this. Keel needs to be deep also because of the ballast at its end, but a deep rudder is harder to explain. Maybe quick effortless turns are one thing. Maybe they want to have that turning momentum deep down (may help in beating). Maybe the water flow past a deep rudder is more stable in turns (and thereby more efficient). I should read a bit more about this.

My point is that it is possible that, even if we aim at maximum speed, also a short rudder (with equal surface area) could do the job as efficiently, or about as efficiently. I'm assuming that there is no need for fast manoeuvres, and that the rudder stays mostly straight (you should trim the sails so that this is the case).

(I note also that if one uses a wind vane, then the rudder might say straight all the time, and there would be no angle between the rudder and the skeg.)
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Old 08-06-2016, 03:39   #77
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Re: Skeg or Spade?

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Originally Posted by Juho View Post
I just note that the requirements of racing boats and cruisers are different also in other areas than safety. Race boats may be planned for quick actions and minimal directional stability, while cruisers usually want good directional stability.

The narrow and deep keel and rudder of racing boats may have something to do with this. Keel needs to be deep also because of the ballast at its end, but a deep rudder is harder to explain. Maybe quick effortless turns are one thing. Maybe they want to have that turning momentum deep down (may help in beating). Maybe the water flow past a deep rudder is more stable in turns (and thereby more efficient). I should read a bit more about this.

My point is that it is possible that, even if we aim at maximum speed, also a short rudder (with equal surface area) could do the job as efficiently, or about as efficiently. I'm assuming that there is no need for fast manoeuvres, and that the rudder stays mostly straight (you should trim the sails so that this is the case).

(I note also that if one uses a wind vane, then the rudder might say straight all the time, and there would be no angle between the rudder and the skeg.)
It has nothing to do with "quick turns". Deep rudder is for aspect ratio -- remember the rudder is a wing just like the keel and the sails. Highest lift vs drag is produced by long, thin foils. Think about the wings of gliders. Sailing upwind you have three wings working for you -- the sail plan, the keel, the rudder. The keel and rudder must generate effective lift (lift in excess of drag) in order to keep the lateral forces on the sail plan from pushing you off your course line -- in other words, make leeway.


If you don't care about going upwind, you won't care about this, and the rudder's job is different, and different shapes will work.

But for different situations besides going upwind, you need power from the rudder, however, so here the area of the rudder is important. And balancing the rudder makes it possible to produce power even from a big rudder, without requiring big forces from helmsman and autopilot.
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Old 08-06-2016, 05:26   #78
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Re: Skeg or Spade?

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Originally Posted by Dockhead View Post
It has nothing to do with "quick turns". Deep rudder is for aspect ratio -- remember the rudder is a wing just like the keel and the sails. Highest lift vs drag is produced by long, thin foils. Think about the wings of gliders. Sailing upwind you have three wings working for you -- the sail plan, the keel, the rudder. The keel and rudder must generate effective lift (lift in excess of drag) in order to keep the lateral forces on the sail plan from pushing you off your course line -- in other words, make leeway.


If you don't care about going upwind, you won't care about this, and the rudder's job is different, and different shapes will work.

But for different situations besides going upwind, you need power from the rudder, however, so here the area of the rudder is important. And balancing the rudder makes it possible to produce power even from a big rudder, without requiring big forces from helmsman and autopilot.
I'm sure racers want to make quick turns without losing speed, but I have no good idea how much that need influences rudder and keel design. Traditional long keels are obviously not good enough, but I don't know how much this influences their keel and rudder design (towards the deep short keel, deep short rudder approach).

Glider wings are a good example of the benefits of long foils. But I'm not sure the same laws apply as strongly in rudders. Rudders need to have lift in both directions, depending on the direction of the wind, and they are therefore symmetrical (unlike glider wings). I don't know how much keel and rudder should be used just as "simple direct walls" opposing drag, and how much they should be designed to generate additional lift (special profile as in glider wings). Again I'm unable to estimate what the impact of these differences is, and how much you would lose top speed with a somewhat shorter (and safer) rudder, and how much you can use the keel instead to achieve similar results (in principle rudder could be there just to assist in turns).
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Old 08-06-2016, 05:45   #79
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Re: Skeg or Spade?

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Originally Posted by Juho View Post
I'm sure racers want to make quick turns without losing speed, but I have no good idea how much that need influences rudder and keel design. Traditional long keels are obviously not good enough, but I don't know how much this influences their keel and rudder design (towards the deep short keel, deep short rudder approach).



Glider wings are a good example of the benefits of long foils. But I'm not sure the same laws apply as strongly in rudders. Rudders need to have lift in both directions, depending on the direction of the wind, and they are therefore symmetrical (unlike glider wings). I don't know how much keel and rudder should be used just as "simple direct walls" opposing drag, and how much they should be designed to generate additional lift (special profile as in glider wings). Again I'm unable to estimate what the impact of these differences is, and how much you would lose top speed with a somewhat shorter (and safer) rudder, and how much you can use the keel instead to achieve similar results (in principle rudder could be there just to assist in turns).

This would be correct if rudders only functioned to turn the boat. But upwind particularly rudders also function to provide lift in addition to maintaining course. It's why racers and cruisers who care about performance tune their rig to have just a small touch of weather helm rather than neutral or lee helm when sailing upwind. That provides key lift to counteract leeway. So a low aspect rudder or attached rudder is much less effective at this, and a skeg rudder is also less efficient since among other reasons if the rudder is a 1-3 degrees canted to leeward upwind, the skeg can't be.


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Old 08-06-2016, 05:45   #80
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Re: Skeg or Spade?

Quote:
Originally Posted by Juho View Post
I'm sure racers want to make quick turns without losing speed, but I have no good idea how much that need influences rudder and keel design. Traditional long keels are obviously not good enough, but I don't know how much this influences their keel and rudder design (towards the deep short keel, deep short rudder approach).

Glider wings are a good example of the benefits of long foils. But I'm not sure the same laws apply as strongly in rudders. Rudders need to have lift in both directions, depending on the direction of the wind, and they are therefore symmetrical (unlike glider wings). I don't know how much keel and rudder should be used just as "simple direct walls" opposing drag, and how much they should be designed to generate additional lift (special profile as in glider wings). Again I'm unable to estimate what the impact of these differences is, and how much you would lose top speed with a somewhat shorter (and safer) rudder, and how much you can use the keel instead to achieve similar results (in principle rudder could be there just to assist in turns).
Again, the purpose of high aspect rudders is not "quick turns". It's maximum lift with minimum drag while going upwind. The lift of keel and rudder is what keeps the boat moving upwind rather than being pushed off to leeward.


They work slightly differently from glider wings because they don't have an airfoil shape. Water is incompressible, first of all, and secondly they have to work both ways so need to be symmetrical. But wings and foils also generate lift from angle of attack, and that's how keels and rudders work. Many of the same rules apply, including the significance of aspect ratio.


For "quick turns", the shape of the keel is more important than the shape of the rudder. The longer the keel, the more resistance to turning. Likewise with a skeg, which can't be steered. There is a tradeoff here too of course -- a boat which is easier to turn, will not track as well. That's another reason why don't use rapier-thin keels with torpedos on cruising boats, even though this type of keel is more efficient upwind.

Another factor is the chord thickness of the foils, but that's getting a little beyond the topic. Both keels and rudder can stall, just like an airplane wing. Racers can risk a higher propensity to stall, than cruisers can, so we have keels and rudders which are more forgiving, even though they are somewhat less efficient.
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Old 08-06-2016, 05:51   #81
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Re: Skeg or Spade?

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Originally Posted by malbert73 View Post
This would be correct if rudders only functioned to turn the boat. But upwind particularly rudders also function to provide lift in addition to maintaining course. It's why racers and cruisers who care about performance tune their rig to have just a small touch of weather helm rather than neutral or lee helm when sailing upwind. That provides key lift to counteract leeway. So a low aspect rudder or attached rudder is much less effective at this, and a skeg rudder is also less efficient since among other reasons if the rudder is a 1-3 degrees canted to leeward upwind, the skeg can't be.


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Ed Zachary!

If we didn't have weather helm, we would have to invent it The rudder won't be providing lift if it's not turned a few degrees to windward -- that's what makes the lift-generating angle of attack.

Likewise the keel needs a a bit of leeway to present the right angle of attack to generate lift. A boat with no leeway at all is impossible for this reason.


When good sailors try to get their boats going well to windward, this is what they are playing with, and why nearly all of them hand steer while settling the boat into her "groove" -- to feel what the rudder is doing -- to feel when it starts to draw.
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Old 08-06-2016, 06:05   #82
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Re: Skeg or Spade?

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Originally Posted by malbert73 View Post
This would be correct if rudders only functioned to turn the boat. But upwind particularly rudders also function to provide lift in addition to maintaining course. It's why racers and cruisers who care about performance tune their rig to have just a small touch of weather helm rather than neutral or lee helm when sailing upwind. That provides key lift to counteract leeway. So a low aspect rudder or attached rudder is much less effective at this, and a skeg rudder is also less efficient since among other reasons if the rudder is a 1-3 degrees canted to leeward upwind, the skeg can't be.


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That makes sense. Having some weather helm makes the rudder somewhat asymmetric and create some lift (with a rudder that has some appropriate profile). A dagger like rudder can have a stronger profile than a wide one. The keel will have a slightly different angle, so different rules apply. Skeg obviously limits the "profiling" possibilities, but maybe one can do some of these tricks behind a skeg too (a fixed skeg makes the form asymmetric, and that might be interesting).
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Old 08-06-2016, 06:09   #83
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Re: Skeg or Spade?

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. . . Skeg obviously limits the "profiling" possibilities, but maybe one can do some of these tricks behind a skeg too (a fixed skeg makes the form asymmetric, and that might be interesting).
Skeg doesn't do anything interesting hydrodynamically. On the contrary, it screws everything up. It forces the water to make a sudden change of direction where the skeg and rudder are at different angles -- increasing drag and reducing lift. The water stream is already separated when it reaches the rudder, which reduces the effect.

It also prevents balancing the rudder.

The first thing is not really important except upwind, but the second thing is.
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Old 08-06-2016, 06:30   #84
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Re: Skeg or Spade?

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Skeg doesn't do anything interesting hydrodynamically. On the contrary, it screws everything up. It forces the water to make a sudden change of direction where the skeg and rudder are at different angles -- increasing drag and reducing lift. The water stream is already separated when it reaches the rudder, which reduces the effect.

It also prevents balancing the rudder.

The first thing is not really important except upwind, but the second thing is.
I was thinking that a symmetric skeg, followed by a "drop profile" rudder, turned slightly to one side, can be said to resemble an asymmetric airplane wing. In this situation one side of the skeg+rudder would be straight, and the other somewhat convex (i.e. no clear angle on either side where the skeg meets the rudder).
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Old 08-06-2016, 06:31   #85
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Re: Skeg or Spade?

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Originally Posted by Dockhead View Post
The water stream is already separated when it reaches the rudder, which reduces the effect.
More so with a low aspect rudder and with a single rudder close the keel. Thus twin rudders are more efficient when sailing. Motoring is quite the opposite, then you want a low aspect rudder closer the screw. One of the trade offs in cruising again.
But regarding skeg rudder I disagree some. With low angle of attacks if well designed a skeg hung rudder works as assymetrical foil with less drag and more lift than a symmetrical spade can ever do. Thou such rudders are quite rare..

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Old 08-06-2016, 09:45   #86
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Re: Skeg or Spade?

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Again, the purpose of high aspect rudders is not "quick turns". It's maximum lift with minimum drag while going upwind. The lift of keel and rudder is what keeps the boat moving upwind rather than being pushed off to leeward.


They work slightly differently from glider wings because they don't have an airfoil shape. Water is incompressible, first of all, and secondly they have to work both ways so need to be symmetrical. But wings and foils also generate lift from angle of attack, and that's how keels and rudders work. Many of the same rules apply, including the significance of aspect ratio.


For "quick turns", the shape of the keel is more important than the shape of the rudder. The longer the keel, the more resistance to turning. Likewise with a skeg, which can't be steered. There is a tradeoff here too of course -- a boat which is easier to turn, will not track as well. That's another reason why don't use rapier-thin keels with torpedos on cruising boats, even though this type of keel is more efficient upwind.

Another factor is the chord thickness of the foils, but that's getting a little beyond the topic. Both keels and rudder can stall, just like an airplane wing. Racers can risk a higher propensity to stall, than cruisers can, so we have keels and rudders which are more forgiving, even though they are somewhat less efficient.
A bit more abut the keel. I used earlier once word "drag" when I meant "drift". I'll talk a bit more about drift vs. lift.

A fixed keel points always forward and is symmetric. It is not designed to create (asymmetric aeroplane wing like) lift on either side. If the boat goes slightly sideways, then we should maybe call than movement drift (i.e. not moving straight forward because of the wind).

A keel is basically similar, but as discussed, its direction may differ slightly from the direction of the keel. If its profile is like a drop or something similar, it may create lift on one side (it in a way becomes an asymmetric foil).

If the profile of the rudder is straight (like a piece of plywood), then I don't see much use in driving it sideways (with some weather helm) (for best possible upwind speed, VMG), because that might create unnecessary drag. Maybe racers know better what the profile of a race boat rudder is like (and if the idea is to make its angle such that the other side of a drop profile rudder is about straight).

I note also that if we try to create similar effect with both keel and rudder (both having a drop profile), then it would make sense to keep the rudder straight, and let the drift create the angled flow. Here I assumed that keel and rudder have exactly similar profile and size. If they are different, some small rudder angle could be ideal. Racers maybe know more about the profile of the keel of a racing boat and its possible use in this way.

If one wants to create (asymmetric foil based) lift with a keel, the easiest approach could be an additional asymmetrical daggerboard. Again I don't know if asymmetric daggerboards are used. I would guess yes.
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Old 08-06-2016, 11:11   #87
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Re: Skeg or Spade?

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A bit more abut the keel. I used earlier once word "drag" when I meant "drift". I'll talk a bit more about drift vs. lift.

A fixed keel points always forward and is symmetric. It is not designed to create (asymmetric aeroplane wing like) lift on either side. If the boat goes slightly sideways, then we should maybe call than movement drift (i.e. not moving straight forward because of the wind).

A keel is basically similar, but as discussed, its direction may differ slightly from the direction of the keel. If its profile is like a drop or something similar, it may create lift on one side (it in a way becomes an asymmetric foil).

If the profile of the rudder is straight (like a piece of plywood), then I don't see much use in driving it sideways (with some weather helm) (for best possible upwind speed, VMG), because that might create unnecessary drag. Maybe racers know better what the profile of a race boat rudder is like (and if the idea is to make its angle such that the other side of a drop profile rudder is about straight).

I note also that if we try to create similar effect with both keel and rudder (both having a drop profile), then it would make sense to keep the rudder straight, and let the drift create the angled flow. Here I assumed that keel and rudder have exactly similar profile and size. If they are different, some small rudder angle could be ideal. Racers maybe know more about the profile of the keel of a racing boat and its possible use in this way.

If one wants to create (asymmetric foil based) lift with a keel, the easiest approach could be an additional asymmetrical daggerboard. Again I don't know if asymmetric daggerboards are used. I would guess yes.
Yes -- daggerboards are used for this purpose.

But you are wrong about keels not generating lift. I explained how they do it in one of the posts above. If they did not generate lift, you could not sail upwind.

When you sail upwind, your COG will deviate from your heading -- did you ever notice? The difference is the angle of attack of the keel and is the angular component of leeway. A boat not well trimmed or which is badly configured for sailing upwind will have a lot of leeway. That's for two reasons -- 1. if the keel is not "flying" well, it will require a larger angle of attack to generate any lift; and 2. inadequate lift will increase sideways displacement (automatically increasing the angle of attack). Both of these things increase leeway and screw your VMG to windward.

That's why a good sailor, trying to get the boat going well upwind, will turn off the pilot, take the wheel in his hands, head off, build up speed, then gently tweak the boat higher and higher, heading off again if he starts to lose speed, then tweaking up again, until he gets the boat in her "groove" with all the boat's wings (sail plan, keel, rudder) "flying" and generating lift -- or "drawing" if you prefer an analogy to sails.

The other element of this whole picture of course is speed -- the amount of lift you get out of keel and rudder is related to boat speed, so if you can't sail fast, you can't sail high [remember that one]. That also gives larger boats with longer waterline an advantage upwind, because it takes less power relative to the size of the boat to drive a larger boat at any given boat speed. Or stated a different way, at 80% of hull speed, a larger boat will be sailing faster and generating more lift and having less leeway, than a smaller boat is, all other things being equal. "Speed is life", in sailing upwind, just as it is in flying.


This combination of factors is how I beat a race-trimmed Beneteau First 40 with carbon sails in a long (about 20 miles) upwind tacking duel a couple of years ago in the Stockholm archipelago, despite having dirty hull, bagged out old sails on their last legs, and a huge wheel-steered dinghy hanging off davits.

We has sailing much higher than I was, possibly as much as 10 degrees. I sailed low and fast as hell. My boat speed was much higher than his -- maybe 8.5 or 9 knots to his 7 or 8, and my leeway was that much less despite the baggy sails and everything else. So he would tack inside me every time, but I would pass him on the outside every time, and ended up at the narrow gap between islands ahead of him. He was not happy.

He should have beaten me and, no doubt, would have, had he footed off a bit and gotten his keel flying better.


As to "profiles" of keel and rudder -- different profiles create different hydrodynamic effects, but the most important ones are aspect ratio and angle of attack. Keels and rudders all generate lift the same way -- by attacking the water at an angle. You don't need to keep the rudder straight to let the keel attack the water at an angle -- the angle of keel to water flow is inevitably generated by leeway -- by the sideways displacement of the boat. The wind needs to be blowing the boat off her course line before the keel will start working.

Meanwhile, the rudder will be at some degrees off straight ahead in order to counter weather helm, and those several degrees produce more lift. Actually it's that very same lift which prevents the boat from rounding up. When the boat's in her groove, these position will be quite stable.


Materials on aerodynamics are useful here, for example:

https://en.wikipedia.org/wiki/Angle_of_attack

"The lift coefficient of a fixed-wing aircraft varies with angle of attack. Increasing angle of attack is associated with increasing lift coefficient up to the maximum lift coefficient, after which lift coefficient decreases."

Note that as I wrote in one of the previous posts, angle of attack is an independent source of lift, separate from lift generated by pressure differentials due to camber. One of the errors which threw you off above was assuming that a symmetrical wing (a wing without camber) can't generate lift -- this is not true. A cambered foil will improve lift vs. drag, and will generate lift even at 0 angle of attack, but even a flat sheet of plywood can generate lift, if you hold it at an angle to the wind.

See: https://en.wikipedia.org/wiki/Camber_(aerodynamics)


Airfoil Types

Symmetrical Airfoil

The symmetrical airfoil is distinguished by having identical
upper and lower surfaces. [Figure 2-11] The mean camber
line and chord line are the same on a symmetrical airfoil,
and it produces no lift at zero AOA. Most light helicopters
incorporate symmetrical airfoils in the main rotor blades.

Click image for larger version

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https://www.faa.gov/regulations_poli...a/hfh_ch02.pdf


Here is another wing which has no camber -- it is absolutely symmetrical just like our keels:


Click image for larger version

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Symmetrical airfoils are commonly used for aerobatic planes and helicopter rotor blades. Lift is produced exclusively by angle of attack (also called "incidence")

Some other information:

"Airfoil sections are of two basic types, symmetrical and nonsymmetrical.

"Symmetrical airfoils have identical upper and lower surfaces. They are suited to rotary-wing applications because they have almost no center of pressure travel. Travel remains relatively constant under varying angles of attack, affording the best lift-drag ratios for the full range of velocities from rotor blade root to tip. However, the symmetrical airfoil produces less lift than a nonsymmetrical airfoil and also has relatively undesirable stall characteristics. The helicopter blade (airfoil) must adapt to a wide range of airspeeds and angles of attack during each revolution of the rotor. The symmetrical airfoil delivers acceptable performance under those alternating conditions. Other benefits are lower cost and ease of construction as comparedto the nonsymmetrical airfoil.

"Nonsymmetrical (cambered) airfoils may have a wide variety of upper and lower surface designs. The advantages of the nonsymmetrical airfoil are increased lift-drag ratios and more desirable stall characteristics. Nonsymmetrical airfoils were not used in earlier helicopters because the center of pressure location moved too much when angle of attack was changed. When center of pressure moves, a twisting force is exerted on the rotor blades. Rotor system components had to be designed that would withstand the twisting force. Recent design processes and new materials used to manufacture rotor systems have partially overcome the problems associated with use of nonsymmetrical airfoils
."
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Old 08-06-2016, 12:27   #88
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Re: Skeg or Spade?

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But you are wrong about keels not generating lift. I explained how they do it in one of the posts above. If they did not generate lift, you could not sail upwind.
This is just a question of terminology. I used terms "lift" and "drift" as in "rockets lift" and "parachutes only oppose fall/drift". If you try to drive a boat forward but it goes 5 left, I called that "drift". If you say that you plan to drive 5 more left, but keep the bow pointing 5 more to the right, you could call that also "lift" that compensates the tendency to drift. I was driving the boat forward but drifted. You seemed to drive the boat slightly sideways to generate lift.

Quote:

As to "profiles" of keel and rudder -- different profiles create different hydrodynamic effects, but the most important ones are aspect ratio and angle of attack. Keels and rudders all generate lift the same way -- by attacking the water at an angle.
Yes, that's the basic "plywood profile" case. I tried to address the difference to asymmetric foils (as in glider wings) in the previous posts.

Quote:

"The lift coefficient of a fixed-wing aircraft varies with angle of attack. Increasing angle of attack is associated with increasing lift coefficient up to the maximum lift coefficient, after which lift coefficient decreases."
Aeroplane wings are asymmetric foils. Their design and use are a bit different than with symmetric foils (e.g. best angle of attack is different on different sides).

Quote:

One of the errors which threw you off above was assuming that a symmetrical wing (a wing without camber) can't generate lift -- this is not true.
If driven slightly sideways, it of course creates a sideways / lifting force. I addressed the asymmetric foil case (as the lift of a glider wing) and the possibility to provide similar asymmetric lift using e.g. a drop profile rudder.
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Old 08-06-2016, 12:31   #89
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Re: Skeg or Spade?

Juho,

I didn't see anything that Dockhead wrote that is wrong, but to vastly simply and point out what I think is the confusion.

Keel are obviously symetrical side to side. Not because they have to be or that doing so is ideal on a given tack. But because it is equally important that a boat be able to sail upwind on port and starboard tack. So the keel is designed to be equally good on either tack, this the symetric shape.

The way a symetric keel generates lift is due to the direction of the water flowing over it. Water does not flow parallel to the centerline of the boat, it actually flows at a slight angle, called the leeway angle. It is the asymmetric flow over the symetric keel that generates aerodynamic lift.

Theoretically a boat could be designed with an asymmetric keel, and it would significantly out point a boat one one tack, but be disasterous on the other. This is why canting keel boats and fast multihulls have tacking daggerboards. They actually swap one asymmetric board for the other when tacking. But this is far to complicated for a cruising boat to mess with so let's ignore it from here on out.

So we are left with a boat that generates lift from the keel by converting leeway into lift. When trying to deign this system the other major factors that need to be taken into account are drag (which always hurts) and speed (more speed = more drag, AND changes the ideal shape of the foil).

So in order to reduce drag there are some simple things that play a part. The first is skin friction, the less surface area of the foil the less friction there is and the better the boat performs. Great, so we want as small a surface area as possible.

Second because there is more drag at edges than along the fin (tip vortexes and slippage) we want the fore and aft length of the keel to be as short as possible.

Third is shape. A hundred years or so ago a series of foil shapes were tested called NACA foils. These basically define the best shape of a symetric keel at any given speed. The faster the boat the narrower the chord, and the further back max chord is. I can't even begin to explain how this works, but there are tables and charts, and it's very authorative with a lot of explanation of you look it up.

Given these we now know that we want as small an area as possible, with as short a fore and aft length as possible.

Now deriving how much surface area you need is complicated, because the faster you go the less area you need. But this is far to complicated for me, so let's just assume we need 1% of the sail area as surface area (which is actually pretty reasonable).

So our ideal keel will have 1% of the surface area of our sail area, and be as short (for and aft) as possible, and as deep as possible. In real life there are limits however, first is material properties. We can simply cannot build a keel stuff enough to match what a designer would like to have, so the keel is restricted by the material properties. Second we don't want to go any deeper than X for practical applications. Third, unless there is enough volume in the keel we will be forced to add a bulb which we probably won't want in a cruiser for practical reasons.

So the ideal keel will be 1) as deep as allowed, as short (for and aft) as possible, and 3) thick enough to contain the volume of lead needed for ballast with a shape that minimizes drag and maximizes lift at the expected boat speeds.


Bt the way everything about the keel applies equally to rudders. Except that rudders don't have to worry about ballast volume. And that rudders can change their angle of attack to the water stream. Ideally in a race boat you want to have zero helm, meaning the rudder is pointed strait ahead, which reduces drag to a minimum.

BUT there are some practical downsides to such a situation, the less helm you have the easier it is to stall the rudder. Extremely good drivers can control this, bad ones or even good ones that aren't paying attention cannot. Secondly there is a safety factor, ideally if the helm is released you want the boat to point into the wind a little bit of weather healm allows for this, a truely neutral helm will just keep going strait forever.

On crewed high end race boats it is not unusual to change the rig tune to induce wether helm for deliveries, then tune it back out for racing. The more weather helm (up to a point) the more forgiving a boat is.

Since everything I wrote about keels is equally true for rudders (except as discussed) it is easy to see why long thin keels matched to long thin rudders have evolved. They reduce drag, increase lift, and allow for higher speeds. On high end maxi racers we are now litterly at the extreme of what material science allows. Boats like Commanche cannot have a longer, thinner, keel because engineers don't have the materials to build one or the knowledge of how to do so yet.


So why do skegs suck? Simple, they redirect the water flow around them. Basically the water flowing into the rudder is no longer sliding across the hull, it is now moving parallel to the hull. This means that the rudder cannot derive lift (honestly it just derives far less) without being turned. The skeg effectively permanently stalls part of the rudder. Thus robbing the boat of lift, and adding drag.
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Old 08-06-2016, 12:51   #90
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Re: Skeg or Spade?

Quote:
Originally Posted by Juho View Post
This is just a question of terminology. I used terms "lift" and "drift" as in "rockets lift" and "parachutes only oppose fall/drift". If you try to drive a boat forward but it goes 5 left, I called that "drift". If you say that you plan to drive 5 more left, but keep the bow pointing 5 more to the right, you could call that also "lift" that compensates the tendency to drift. I was driving the boat forward but drifted. You seemed to drive the boat slightly sideways to generate lift.



Yes, that's the basic "plywood profile" case. I tried to address the difference to asymmetric foils (as in glider wings) in the previous posts.



Aeroplane wings are asymmetric foils. Their design and use are a bit different than with symmetric foils (e.g. best angle of attack is different on different sides).



If driven slightly sideways, it of course creates a sideways / lifting force. I addressed the asymmetric foil case (as the lift of a glider wing) and the possibility to provide similar asymmetric lift using e.g. a drop profile rudder.
There's quite a lot of confusion in this post, and making up your own terminology really makes it worse. Confused language leads to confused thought. If you're interested in this stuff, you should read a good book on it.

"Lift" is the right term, and using the right term is important, because it is exactly the same concept. There is nothing "drifting" about it. Just like an airplane wing, which holds up the craft because of the force generated, a keel holds the boat to windward, because of exactly the same force generated in exactly the same way. And there is no such thing as "asymmetric lift". If the total vector of forces generated is to windward (or "up", in the case of an airplane), it's "lift", period. The vectors total one way or another.

And to say that "aeroplane wings are asymmetric foils" is false. Many are, but not all.

As I wrote, most aerobatic planes have symmetrical foils -- for exactly the same reason why our keels are symmetrical foils -- because they have to work in both directions.

Most rotary wing aircraft have symmetrical wings.

Jet airliner wings are symmetrical towards the wingtips, to reduce propensity to stall.

Many supersonic aircraft have symmetrical airfoils.

As I wrote, both cambered (assymetrical) and symmetrical airfoils are used to generate lift in airplanes and helicopters.
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