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Guest Ketchup

... the only way a rudder would ever create lift forward at 10% is by the helmsman using it to steer the boat to its maximum speed. (Hard to do that with out a rudder) in all other cases it is drag... but a nessasary price of slow kit.

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Nope, I disagree again, so do the professional naval architects. Very good article here;

http://www.sponbergyachtdesign.com/keel%20and%20rudder%20design.pdf

 

Go thru to the rudder sections. Rudders produce substantial lift, and are critical to the boats performance (not just for steering :-) )...  

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Guest Ketchup

Don't agree with that article. But will say that the rudder can create lift by virtue that it controls the boats dircection and thus aids the keel and sail to gain their maximu lift.

 

With out it the boat would have no lift as it has nit control of it direction and sailing effecienty's.

 

Maybe that's what that article is referring to?...

 

I can also see that rudder will stop the forces of weather helm and transferring that force into forward force(by virtue of lateral resistance) but the rudder /foil in itself will not create much lift (that benefits boats speed or VMG)

 

And maybe thats where we are talking at crossed purposes!?

 

I am talking the foil and shpe itself you guys are talking the result of the foils use (helmsman control of the boat)

 

For sure without the rudder there would be no boat controll or direction and as such no forward force transfer.

Thus the rudder could be worth 100% of all forward and windward lift.

But as a foil going through the water. Nah!!!

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Don't agree with that article. But will say that the rudder can create lift by virtue that it controls the boats dircection and thus aids the keel and sail to gain their maximu lift.

Pretty bold, but OK. It could be that I am not explaining myself well.

Lets go back one step to address a comment you made re the aircraft. In normal flight. Firstly, the wing is giving lift only. No forward motion. Forward motion is being provided by the engines thrust. If you cut the engine, the only way the aircraft can stop from falling, is to drop the aircrafts attitude, or angle of attack and go into a glide. In normal flight, the Aircraft has a slight nose up attitude. Think of gravity as the force that would be normally applied to the Boat by the wind. If the aircraft had no forward momentum, the wing would be stalled and the plane would drop. But if the aircraft is till moving forward fast enough, the lift is stopping the aircraft from dropping like a rock. The combined forces act in such a way so as to cause the aircraft to move forward. OK, so hopefully you are following so far. The "attitude" of the aircraft IS NOT caused or created by the main wings. It is controlled by the tail wing. The back end of the aircraft has to be lifted to produce a nose down attitude. With the nose down attitude, the main wings can produce the lift vs the force of gravity and thus the overall affect is forward motion. But it takes effort to lift the tail. Cut the tail off (we will assume the aircraft is balanced with no tail) and the main wing will tilt up in relation to the lifting force and stall. So we can see by this that the affect of the tail lifting, is causing the main wing to remain flat in a forward flight attitude. It takes force to lift that tail. Exactly the same as the Sailboats rudder.

So back to the boat, the effect of the Rudder and the Keel, is to "lift" the boat. Hence my earlier comment that the boat is actually not pointing directly at where it is going. In fact, the nose is pointing down in relation to the actual direction.

Has that made better sense????

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A rudder does provide lift. Really obvious and exaggerated in something like a twelve footer. If it's heeling and you pull the tiller to bear away what happens? The stern lifts, the bow goes down.

 

Not so obvious on a bigger boat, but the same thing happens.

 

Also, the amount of lift relative to the big picture is affected by rig setup. Adding rake puts more sideways load on the blade, so to keep a straight course the whole thing gets angled to windward. The foil then effectively becomes asymmetric and generates lift to windward. On the downside it also causes drag as it's dragging sideways through the water.

 

This is why adjustable rigs are popular in lower powered development classes,they can load the helm to wherever they want upwind, then rake forward for downwind so the boat has perfect balance and minimal drag .

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Guest Ketchup

Pretty bold, but OK. It could be that I am not explaining myself well.

Lets go back one step to address a comment you made re the aircraft. In normal flight. Firstly, the wing is giving lift only. No forward motion. Forward motion is being provided by the engines thrust. If you cut the engine, the only way the aircraft can stop from falling, is to drop the aircrafts attitude, or angle of attack and go into a glide. In normal flight, the Aircraft has a slight nose up attitude. Think of gravity as the force that would be normally applied to the Boat by the wind. If the aircraft had no forward momentum, the wing would be stalled and the plane would drop. But if the aircraft is till moving forward fast enough, the lift is stopping the aircraft from dropping like a rock. The combined forces act in such a way so as to cause the aircraft to move forward. OK, so hopefully you are following so far. The "attitude" of the aircraft IS NOT caused or created by the main wings. It is controlled by the tail wing. The back end of the aircraft has to be lifted to produce a nose down attitude. With the nose down attitude, the main wings can produce the lift vs the force of gravity and thus the overall affect is forward motion. But it takes effort to lift the tail. Cut the tail off (we will assume the aircraft is balanced with no tail) and the main wing will tilt up in relation to the lifting force and stall. So we can see by this that the affect of the tail lifting, is causing the main wing to remain flat in a forward flight attitude. It takes force to lift that tail. Exactly the same as the Sailboats rudder.

So back to the boat, the effect of the Rudder and the Keel, is to "lift" the boat. Hence my earlier comment that the boat is actually not pointing directly at where it is going. In fact, the nose is pointing down in relation to the actual direction.

Has that made better sense????

Yip i see where you are coming from!

 

I am just talking the rudder as a foil going through the water not as a price of equipment attached to a boat and in relation to boat control.

 

Sure by virtue of its lateral resistance at the back of the boat and its function to control direction, it stops the back of the boat from spinning out and as such provides forward momentum but the rudder foil itself is shaped for minimal stall "wide range of angle of attack" control ahead of lift. (In a slow keel boat)

 

Actually a better example of your reasoning would be A windsurfer. In this case the rudder is a fixed fin and you steer by rake of the mast (angle of the Wing)

 

Rake it back and it turns to windward, rake it forward and it goes downwind.

 

But take the little fin away and it cannot go any where! As the stern will be pushed around with no control.

 

But there is an exception, take the fin away, and with the right hull shape (chined hull shape) , you can sail it (albeit with less control) by using lift of the hull shape.

 

You ever sailed a laser witH no rudder?...

 

Also, a bit like the tail rotor (PR noe hey uses a train wing) on a helicopte.

 

On the tail rudder thing wheels. Yes the tail flaps (horizontal ones and not the verticle Rudder) will provide lift an keep the plane flying level by providing lift. But the verticle rudder will not (other than some yaw reduction maybe?.

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I think you are getting it, but you still have a couple of small points blocking the way. You have to realize that all forces acting on a Hull combine to lift the boat forward. The boat is not pushed forward until the wind becomes aft. Once the wind becomes aft, the boat is pushed and the rudder/Keel serve to act as a directional blade only, rather than a wing. It is only as the wind moves forward that the forces act in such away that the boat "lifts" through the water.
The ability of a rudder to steer is part and parcel with it's lift. It is all about how much force is applied by rudder angle, as to whether you sail on a straight course or are turning. The cord, or foil shape is what gives the foil it's efficiency of lift. Also, when the rudder is straight, eg water forces are equal on both sides, the water force is still producing lift. it is just that the force is equal both sides and cancels out any want of direction. By turning the rudder a fraction, the wing shape causes a longer path for water to flow and the forces become unequal an result in the foil "lifting" to that direction.
Using the 930 as an example, the fact the thing is on it's side, and they are most certainly light footed (KM has taken me sailing on hi
    All points of a design is a matter of compromise. If you talk to any designer, every single design is compromised with the overall intention of the boat. Even when you get into the pure bred race boats, a designers top question to the skipper is, where do you want the best performance to be. Upwind or down wind. The two are very different things and require very different key points to a design. AC racing of old with mono hulls is a good example of this, where the boat is very much designed to the known conditions at the time they want the best performance from the Boat. So there are times were we see Boats with two very different strengths. One maybe perform better up wind and another better down wind. Down wind is all about the least possible drag and tend to be wider hulls and skip across the surface, where as an upwind boat tends to be narrower and all parts in the water are adding to its forward momentum.
So in saying that, choosing any appendage down in the water is just one of the many compromises. You have three points of any blade. It's length, it's width and it's shape. Each has advantages and disadvantages and it is up to the designer to work out what the best all round solution is.
  
By the way, sure you can steer without a rudder. But you will never win a race. If you could gain a significant advantage not having the appendage in the water, everyone would be racing with no rudder. There is always a compromise, but over all, there is also a gain.

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What you Guys are forgetting is the a sailboat creates leeway, some more than others. The result of this leeway combined with the pressure of the sails is that there is a low and high pressure side to the keel and rudder so with a nice shape the keel and rudder will want to pull the boat into the low pressure, not really creating a lot of lift but certainly negating the leeway.

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Guest Ketchup

Exactly willow. The keel produces no lift until it reaches the speed it is designed to be efficient at. what it does do is negate the sideways push of the sail (leeway) and transfer this force into forward momentum.

 

Once moving at speed the lift generated by the keel and to a lesser extent the rudder is very little. its just the reduction in leeway that drives the boat forward.

 

As said before these sideways forces in the sails would push the boat sideways with no keel and rudder but with just a keel you stop the sideways force and convert it into forward force. But! This cannot happen as the stern would be pushed around and the boat would head into the wind with no comtrol. Thus the rudder stops this and controls the direction.

 

Therefore forward drive is maximized and this is deemed to be lift. But really its a big reduction in leeway.

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None are so blind than those that do not want to hear.

But OK, I do want to educate here, and so don;t take the following as belittling. You do actually have the answer, you just need to see it.
Exactly willow. The keel produces no lift until it reaches the speed it is designed to be efficient at. what it does do is negate the sideways push of the sail (leeway) and transfer this force into forward momentum.
Think very clearly about what you just wrote there Ketchup. Question yourself and then answer us as to why and how the keel negates sideways push. Question and then answer us as to why the combined forces equate to forward momentum.
For both Willow and Ketchup, Question and then answer us as to what leeway creating a "low pressure zone" actually is. What does low pressure on one side and high pressure on the other side of a surface end up doing??


Once moving at speed the lift generated by the keel and to a lesser extent the rudder is very little. its just the reduction in leeway that drives the boat forward.
And once again, the reduction in lee way is caused by what?? What is the keel doing and how, to stop leeway movement.

As said before these sideways forces in the sails would push the boat sideways with no keel and rudder but with just a keel you stop the sideways force and convert it into forward force. But! This cannot happen as the stern would be pushed around and the boat would head into the wind with no comtrol. Thus the rudder stops this and controls the direction.
No argument there, that is exactly what happens. Why does the Rudder do that. In what direction is the wind wanting to push the boat and in what direction does the force from the water flowing over the rudder want to take the rudder ?? Think very very carefully about this and you will see that this force is called lift. Don't make the mistake of thinking that because the boat may not be up to full speed, the rudder is not generating any lift. Even at slow speeds, there is lift, it's just not at full efficiency yet. That efficiency is made by the profile and size of the rudder. Large for slow speed, smaller for faster speed.

Therefore forward drive is maximized and this is deemed to be lift. But really its a big reduction in leeway.
Dude, I really wonder of you are arguing the point just because you want to argue the point. Think very clearly about what you have just said in that statement.
Hint: There are many books available to the beginner about how a sailboat works. Several here could suggest many excellent books and articles. I think IT already has suggested reading one with his link he posted earlier. Go have a really good read. But if you come back with a statement like you disagree with the literature, then you better provide a pretty damn good scientific reason as to why, because all of the Sailing world will be wanting to know why the theory has been wrong all these years and it may mean the designers have it all wrong and a whole new generation of Hulls maybe  created.
 

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Basically the appendages sail through the water.

 

As an aside after damaging our good rudder on a Wednesday night race we had to show up with our old one last week. It was terrible, we couldn't get the same height upwind, if we sheeted in hard as the load went on the rudder the flow would detach and stall I could see it happening due to the transom hung rudder. Down wind with no load on the rudder the boat was possibly a little quicker. There is certainly a lot to good rudder design whether you have 1 or 2 isn't as important as good design IMHO.

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Guest Ketchup

Here what you are saying wheels but how does a aerofoil shape have any lift at the moment before it moves or has any water/air movement flowing over it?

 

At that moment it is just providing lateral resistance.

 

Once the forward movement is started then the lift is generated. And at what speed if the sideways push negated BY the keel and the forward lift/momentum generated

 

At the moment the sails fill and generate a sideways force on the boat and keel, the boats will initially (and fora short while) have more side ways movement than forward (this will differ for each boat and keel)

 

Then at a certain speed the forward momentum will be greater than the sideways momentum as lift is generated of the keel.

 

I think the key thing here is... the type, size and shape of the keel determines the amount of forward momentum versus the sideways leeway And thus lift. The rudder is there to direct thee forces and stop the stern spinning out to leeward or windward (depending on balance and rake. Or weather helm or lee helm)

 

Some poorly designed keels will produce bugger all lift. Yet some will generation huge lift.

 

EG a H28 versus a Y88 or a Ross 930 maverick.

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Guest Ketchup

Heresna interesting scenario...

 

Take a Hobie 16. It has no centreboards and "average" rudders and, with a lot of rake, they sail up wind very well. Nit with great height but pretty good VMG.

 

They use Hull shape and form to create lateral resistance which converts the sails forces into forward momentum.

 

What would happen if you put centre boards on one? Would they point higher for the same speed?

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What you Guys are forgetting is the a sailboat creates leeway, some more than others. The result of this leeway combined with the pressure of the sails is that there is a low and high pressure side to the keel and rudder so with a nice shape the keel and rudder will want to pull the boat into the low pressure, not really creating a lot of lift but certainly negating the leeway.

Willow, Wheels and I are not forgetting leeway at all. Leeway provides the angle of attack for the keel, no angle of attack, no lift (generated by the LP, upwind side, as you said). Keels and rudders are necessarily symmetric foils - they must work on either tack, so must be symmetric for best performance. Both the keel and the rudder, being symmetric means they need an angle of attack to work - hence no lift downwind when leeway is minimal, and quite a lot of lift upwind where the keel has leeway, and the rudder has leeway and helm angle to provide lift. As most boats require a little helm to prevent rounding up, the rudder has a larger angle of attack than the keel, and proportional to size, in some cases generates more lift than the keel (not more in total, as it smaller - just more per m2).

 

In Ketchup's post re Hobie 16's. They use Asymmetric hulls to provide lateral resistance via lift. This is merely a way to make them a beach cat - shallow draught. If it was more effective than centreboards, that is what the real race cats would use. It's not. 

 

Keels and rudders work just the same as your sails - aerofoils in a fluid/gas stream.

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Guest Ketchup

Its amazing watching the AC72 at speed and foiling upwund just how much foil was actually in the water in the Verticle plane as opposed to the horizontal wing.

 

It really proves that the faster you go the smaller the foil you need.

 

Which begs the question... what size foil is optimum for a boat that does 0.1 - to say 8 knots uowind?

 

Obviously the faster you go the smaller the foil but what would you design for? 5 knots uowind?(lets say your optimum upwind speed) and sacrifice above and below that or a general compromise?

 

As such the importance of a trim tab on the back of the keel cannot be underestimated

 

It allows less chord for more lift.... or does it? :-)

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True Ketchup, and a great feat of engineering! (AC70)

 

An example of the issue of foil size v speed that pretty much everyone has seen is that of a jet's wing. As the plane takes off or lands, the wing chord length (wing width) is extended to provide the lift requirements at the lower speeds. It is retracted once the speed is increased.

 

You cannot say what size foil is optimum for a vessel just by speed. MUCH more data is required, incl weight of vessel, available power, hull shape, immersed, foil assisted, or fully foiling  etc

 

I'm not entirely sure how the trim tab on the back of the keel on some race boats works - if straight ahead, it does (artificially, when recessed into the trailing edge) increase chord length, and I understand gives the keel more lift when set correctly, but I have not seen a flow diagram. I'd expect that it  is used to direct the flow as it exits the keel, but here we are really getting into the realm of the naval architect, and supposition is useless!!

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Guest Ketchup

Yarp! Agree.

 

I sailed a race boat in San Fran a few years back and the tab was used to "bend" the water around the back edge of the keel to increase the " pressure zone" and thus give lift to windward.

 

the designer (from the S and S deign team but cant remember the name) did all the calcs and gave the settings(angle) at different boat speeds.

 

As the boat was a one off we never really knew what difference it made but felt the VMG was good upwind for the boat (but compared to what) and being a upwind "beast of a design " it was always going outpoint everything we sailed against anyway.

 

the ultimate upwind machine that never really progressed, but Bruce Farr said it was a real advance in upwind "straight line sailing", was the twin keel NZL20.

 

That puppy had real moments of brilliance but i wonder why farr never advanced that concept more?

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Ketchup, a lot of what you are now saying I have already said in my many posts. But a couple of key points to consider may help.
Firstly, I am referring to a Boat underway, not statioinary. Thus water flowing over the Keel rudder.
I am referring to a boat heading to windward.
Remember I said the Keel develops 10% of the lift. Not 50%. But 10% is still a lot when it comes to control.

The Keel and rudder does not move the boat foreward. It is a combination of forces on angles of resistance, that result in the boat being moved foreward. Think of how iceskates work in allowing a skater to move faster than they can run, yet leg movement is slower than if they were runing. In actual fact, a sailboat is actually " pinched" forward. A result of the effort of force applied in one angle and the effort applied by Keel and rudder in the the opposite angle, but both angles are actually a V shape, not too opposing parrellel lines.
So taking this all back somewhat. The forces generated by keel and rudder are not just sideways. Remember the hull is over on it's side somewhat. A 930 is "lightfooted" in that it heels over easily. So for point of discussion, lets just say the boat is on a 20deg heel. The Keel is producing that sideways force and this point is why it is critical to understand lift rather than just a resistance to leeway. The Keel is "lifting" with that lifting force being at that 20deg angle. NJot 90deg, which would be purely horizontal and not 0 deg which is vertical. It's 20deg. Now think for a moment that the boat is stationary and you are able to stand oin the bottom and place your hands on the Hull and push at that angle of 20deg. What will happen. The Boat will move in two directions. Sidways and slightly upwards. The weight of the boat stops the thing from lifting out of the water. In other words, Gravity is one force being applied straight down. The result is that your pushing force and gravity result in the boat moving more sideways away from your push and up slightly in result of your pushing force, but the stronger force of gravity stops you lifting the boat out of the water.
Did that mak sense?? I really would rather you or anyone say, no it didn't make sense please explain better, rather than argue points that are well established by People far cleverer than me.
Once again, back to KM's 930. Ross is no fool with the design. It's a wonderful boat and a really good design back in it's day. The Hull shape, the Keel shape and size and the Rudder shape and size were all important to the design as a package. KM has changed that design a little and of course now in more recent times, materials, knowledge and experience have lead to positive innovations and modifications.
So where was I orrigianally coming from in my very first Post. Well the Hull has a specific shape. Hull shape is important. It is designed to act also as a wing and give balance in regards to particular directions of force. My knowledge goes no further than that, to understand more, we need a designer like nzrat to chime in. The keel is creating a force in another direction, that has that angle of 20deg, lifting the Hull slightly and taking it sidways. The rudder is also creating a force in the same angle of direction. But the amount of attack dermines how much force and thus the ability to steer is created. But it is very important to understand, that to keep the forward/aft attitude of the Boat at the correct level, the rudder must be at that 20degrees of angle. The rudder causes the aft end to lift ever so slightly and the nose to push down ever so slightly. The curve of the Hull is thus balanced by this attiude. If the curvature of the Hull was not important, then we would have striaght sides and a heck of a lot more room in the boat. The Hull shape is important to the best point of sail. Normally narrower hulls will point higher.
So once agqain, in relation to KM's 930, having two rudders and thus ine will always be pointing straight down, the force generated will be horizointal only. There will be no ever so slight lifting force on the Aft end. There will be less balance to the Hull. Even porposing issues can be created. Every single aspect of shape sitting in the water has an affect on the Hulls performance and a really good designer will include all those points in the design.
Righty

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By the way, if you ever get to see thos AC cats coming directly toward yoiu or directly away from you, you will see this huge angle of the Hull slipping sideways. They are sailing with an increadible amount of sideways slip, due to so little sitting in the water giving that lateral resistance.

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