A question for the engineers.Is there any reason for a fin to be tapered other than appearance? My reason for asking is that if a parallel sided fin is workable that a person could make one mould and pull two halves out of it thereby insuring a symmetrical fin.
Thanks
Don
Vancouver Island
Part 2 If I have a fin that is symmetrical and properly aligned, when it is going through the water it should have equal lift on both sides and some degree of drag. If I have a fin that is slightly assymmetrical but aligned so as to have equal lift on both sides will it have more drag than the first example?
Thanks
Don
Vancouver Island
Don,
Your fisrt question makes sense but I am going to re-phrase it slightly: is there an advantage to a tapered planform versus a rectangular planform?
As you know the fin generates lift much like an aircraft wing. It has been shown that aircraft wings with elliptical loading profiles have the lowest drag. Tapered planforms are closer to ellipses than rectangles are so in theory, the drag will be lower all other things being equal. You will also gain some advantage from a slightly lower surface area for the viscous drag. But these effects might be so small that you would never be able to measure them in practice.
Your second question does not make sense. A symmetrical fin going through the water will not generate lift unless it is going through the water at an angle of attack (leeway angle). The lift will not be “equal on both sides”. Rather, the windward surface will generate lower pressure than the leeward surface and the DIFFERENCE between those pressures is the lift of the fin (which will be pointed to windward).
Now if you have an asymmetrical fin, 2 things happen (one good and one bad).
First the good news: a cambered fin will generate lift at a lower (ideally 0) angle of attack. This means the boat will not have to travel through the water with a leeway angle. This means generally that the hull drag will be lower. But in addition to that, the drag of a cambered foil for the same lift as a symmetric foil is generally lower. So a cambered fin will result in less overall drag than a symmetric fin (assuming things like planform are constant)
Now the bad news: This will only work on one tack.
So unless you are building a boat that only needs to sail in one direction (such as a boat going after a speed record on a closed one direction course) cambered fins are not a viable option.
There is another possibility. You can come up with some clever way to change the camber of the fin from one direction to the other direction when you tack. Perhaps if the leading edge and trailing edge of the fin were rigid, but able to rotate and then you put some flexible material in between that would bend into a cambered shape in response to the pressure loading…
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Will Gorgen
Boy I have trouble explaining myself! I’m not planning on building an asymmetrical fin, but I would imagine that most homebuilt fins are not perfectly symmetrical and would have to be dragged through the water at a slight angle in order to duplicate a symmetrical fin. What I was after was- would an imperfect fin that that has to be dragged kind of sideways have more drag than a perfect fin that can just cut straight through the water. What I am working toward here comes from something you said. You said that a boat that needs any helm to go straight will be a slow boat(or words to that effect) That made me think that if the fin on my boat was not perfectly symmetrical then the boat would try to crab through the water and I would have to add a little rudder to correct that. So then I started thinking would it be sufficient to just turn the fin a little(permanently) so that I wouldn’t need any rudder to go straight(assuming the rig is balanced). That made me wonder if turning the fin a little would add more drag than having an even fin to start with.I hope that clears up my question. If not I’ll try harder.
Thanks
Don
Vancouver Island
Don,
First a bit of physics. What would happen you your boat if you did not have a fin. Suppose for a moment that you were able to connect the ballast to the boat with some magical string rather than the fin. Now try to sail that boat. What happens? Does it go forward or sideways? Well, the hull will provide a small amount of resistance, but basically the boat will go sideways (If you don’t believe me, borrow a small daggerboard dinghy like a Laser or a Sunfish and pull up the daggerboard while you are sailing to windward and watch what happens).
The wind is hitting the sails and trying to push them to the side. This is why the boat heel over (that side force is pushing the boat over). The part of the boat in the water - most importantly the keel fin - needs to resist this sideways force in order for the boat to move forward instead of sideways.
So the main job of the fin is to keep the boat from going sideways. How does it do that? It generates LIFT - just like a wing of an airplane. Only the LIFt is pointed “sideways” instead of “up”.
How do wings of airplanes generate lift? Well, you take a plate like surface (ignore for a moment the finer points of airfoil shaping) and you slice it through the air. A flat plate slicing straight through the air (no angle of attack) will not generate any lift. But take that same plate and slice through the air at a small angle and now you get some lift. If you take that same plate and now curve it (camber) then it will generate lift at zero angle of attack. So our plate like surface needs to either slice through the air at an angle of attack or it needs to be cambered or some combination of those two in order to generate lift.
Now for our little boats, the same thing happens. As the boat passes through the water, it must “crab” at a small angle (usually 2 - 5 degrees) in order for the fin to generate the lift required to keep the boat moving forward instead of sideways. This angle is called the leeway angle, but it is really just the angle of attack of the fin.
Assume for a moment that you had a perfectly symmetrical fin mounted perfrectly straight in the boat with the best airfoil section known to man. When you sail the boat, the boat will sail at a small angle to the direction you are pointing. This has nothing to do with how well or porrly you made the fin and has everything to do with the angle of attack required to generate lift.
Now, if you want your boat to go straight (with no leeway angle) then you are right, you can turn the fin so that it is pointed slightly toward the windward side of the boat. The problem with that idea is that when you tack that fin will now be pointed toward the leeward side of the boat. You could control this so that you turned it to one side or the other depending on which tack you are on (this is what the collective system on CBTF boats in fact does), but I think most classes rules would disallow this idea. But really there is nothing you can do to prevent the boat from sailing at a slight leeway angle. All boats do this. Graham Bantock’s National championship IOM does this. Full size america’s cup boats do this. All boats (with the exception of CBTF boats) do this. It is not a problem.
Just to clear up a few more points. When you apply rudder, you are not correcting the leeway angle. Leeway angle is the angle between the direction the boat is pointing and the STRAIGHT LINE that the boat is actually sailing. The boat will be perfectly happy sailing that STRAIGHT LINE as long as everything is in balance. Helm (which requires rudder input to correct) is the tendancy for the boat to TURN away from that straight line.
Imagine for a moment that you place your bare hull (no rig) in the pool. The Keel is on the boat, but you have taken the rudder off. The boat is in front of you pointed straight at you. Push the bow of the boat to the left. What happens? Does the boat slide smoothly to the left or does it turn? Of course we know that it turns. Now with the boat pointed straight at you again, grab the boat by the keel bolt (assuming this comes through the deck) and push the keel bolt to the left. What happens now? more than likely, the boat will slide to the left instead of turning, right?
Remember that sideways force from the sails? If that force is pushing sideways near the bow of the boat, it will cause the boat to turn. If the boat is moving forward, then you could counteract this turning with the rudder. Now if that sideways force of the boat is pushing against the boat right in the middle, then you would not need to use the rudder to keep the boat from turning. This is what we were talking about when we talked about balance…
Hope this helps clear things up…
Will Gorgen
I’ll try again. Lets go downwind this time. I am Joe Average builder and I have made a fin for my boat. Unfortunately this fin has a little more curve on one side than on the other. Now when my boat is going downwind (trying to go in a straight line) the fin is developing lift(on the side with more curve). This would probably make the boat try to go sideways and I would have to apply rudder to make the mark. Not a goog thing. Now in order to correct this I permanently turn my fin slightly to counter the lift. Now the boat goes straight with no added rudder. The question is “da-ta-ta-da”–Does turning the fin add more or less drag than adding a little rudder? I realize that the real answer is to get a good fin but that isn’t an option for some of us.
See if I managed to explain myself this time.
Thanks
Don
Vancouver Island
OK Don,
If one side of your fin has more curve than the other, then the midplane (an imaginary surface which is exactly halfway between the two surfaces) would have camber to it. So what you have created is a fin with a slight amount of camber. This means that the lift is going to be biased to one side. If the boat were travelling perfectly straight through the water then it would generate lift.
So using your example of sailing straight downwind (where no lateral resistance is needed), your boat will need to sail through the water at a slight crab angle (leeway angle) to cancel out the lift of your cambered fin. This angle is the “zero lift angle” of your cambered fin.
As far as neededing rudder input: The boat is still sailing in a straight line and there is no side force from the rudder needed to keep the boat sailing straight. The rudder will need to be deflected to the leeway angle so that it goes through the water straight, but it is still going straight through the water and not generating any lift. The only difference that you will notice is that the boat is going through the water at a slight angle compared to a boat with a perfectly symmetric fin.
Now lets angle your cambered fin slightly to one side so that the boat sails straight (no leeway angle) when sailing downwind. What happens to the drag of the fin? Nothing! The fin is still sailing through the water at the same angle it was before we angled the fin - the zero lift angle. What happens to the drag of the rudder? Nothing! The rudder still generates no lift and slices through the water at an angle of 0. What happens to the drag of the hull? Now the hull is sailing straight instead of at an angle. When the boat is sailing with a leeway angle, it usually creates more drag than when the boat is sailing straight. So the drag of the boat is most likely going to be lower if it is going through the water straight.
So the answer is that if you angle your fin slightly to the side then the drag of the WHOLE BOAT will be lower. But this is not because of the drag of the rudder needed to keep the boat from turning. And it is not due to ower drag from the fin. It is because of the reduced hull drag associated with the hull going through the water straight instead of at an angle…
Remember, the lift on the fin is not causing the boat to want to turn. Go back to the swimming pool example where you pulled the boat sideways by pushing on the keel bolt? The boat wants to slide sideways - not spin. It is when the sail force trying to push the boat one way is out of balance with the keel fin lift trying t push the boat the other way that the boat wants to turn. It is ony when these two forces are OUT OF BALANCE that the boat wants to turn and you need to use an additional SIDE FORCE from the rudder to keep the boat from turning.
I hope I am helping to clear this up for you…
Will Gorgen
O.K. I think we are on the same page now.If we have a cambered foil and angle it so it produces no lift would it have more drag than a symmetrical foil of the same dimensions?
Separate but related question. You said that the drag would come from the hull and that begs the question In a rudderless boat does the hull control the direction or does the keel? I imagine it’s a little of both but is one dominate?
Thanks
Don
Vancouver Island
We have to be a little careful here. Drag is made up of three very different mechanisms: Form drag, Viscous drag (related to surface friction and turbulence) and induced drag. Form drag is related to pushing the water out of the way as you pass through it. Induced drag is a function of total lift and planform shape. Niether of these change between a symmetric and cambered wing (assuming the same wing thickness, chord, span and planform shape).
The only portion of the drag that changes with a cambered wing is the viscous drag. Every airfoil section has a certain amount of viscous drag for a given angle of attack. Many airfoils have a low drag region near some angle of attack where the flow is nice and laminar. As the the angle of attack changes from this point (up or down) the drag will slowly increase. Then at some point, the pressure field around the airfoil will cause the boundary layer to transition to turbulent. This causes a large increase in drag. As you change the angle of attack further, the portion of the airfoil covered by the turbulent boundary layer will steadily increase which causes the drag to rise rather strongly. Then you reach seperation or stall wherethe drag gets really big.
A symetric airfoil will have the lowest drag at an angle of attack of 0. Or saying it another way, a symmetrical foil will have the lowest drag when it is producing no lift. As you add lift you add drag.
The best way to think about cambered foils is that they are “biased” toward a certain amount of lift. A cambered wing will have its lowest drag at some positive amount of lift. The “drag bucket” for the viscous drag will be shifted so that it is centered (lowest lift region) around some positive lift.
So a cambered foil will have lower drag than its symmetric counterpart at some positive amount of lift. This is why cambered wings are the norm on airplanes.
At 0 lift, the cambered foil will probably have more drag than the symmetric foil.
But generally cambered foils are better in terms of drag than symmetric foils. The problem with using a cambered foil in sailboats is that you need to generate lift in both directions - on both tacks. So on one tack you would have lower drag. On the other tack you will have higher drag. And downwind you (no lift) will have higher drag.
Generally (when the rules allow) a boat that uses bilge boards will have cambered foils. Take a look at the dagger foils on Open60 They only put those down on one side or the other when the boat is heeling that way. Those foils tend to be cambered. Windsurfers that are used to set speed records by sailing in one direction will have cambered fins. So cambered foils are good when you only use them to generate lift in one direction.
For your keel fin, you will pay a small drag penalty for your cambered fin. You will have less drag on one tack than a symmetric foil, more on the other tack than a symmetric foil and more down wind than a symmetric foil. If your camber is a very slight amount, you will probably never notice. The drag will be so small that it will only be worth about one bad tack per leg. Unless it is really noticable, you are probably alright.
Now for your second question about rudderless boats:
The way you steer a rudderless boat is by changing the balance of the sails. If you want the boat to head up, you ease the jib and/or trim the mainsail. You would do the opposite if you wanted to head down. For example, with a windsurfer where there is no rudder, you steer the board by rocking the sail forward to head down and back to head up (unless you are full on planing at which point you steer it like a waterski).
If you are on the boat, you can also use crew weight to help steer. When the boat heels, it tends to want to head up, so by using your crew weight to heel the boat you can cause it to head up and flatten the heel to get it to head down. Generally a lot of body english is required to get the boat to tack (think roll tacking).
It’s all about balance of forces…
Will Gorgen
I didn’t mean “how” do you turn a rudderless boat I meant “what” turns the boat. Like if I had a boat that insisted on turning when I was just pushing it across a swimming pool what would I look for. What I was planning in my quest was to pull(I don’t think I can consistantly push straight)my rudderless boat across a swimming pool to see if it had any tendancy to turn or go sideways. I thought if I used a cordless drill with a drum in it to pull the boat it would give me a constant speed and direction. I would turn the drill off when the boat was moving well and then see what direction it went. I would then make adjustments until the boat continued straight and true. Then I could install the rudder an use the same technique to make sure that the boat would go straight hands off. I have fiddled with the trim button so much and got nowhere(the boat always seems faster on one tack) but the winds at my pond are so shifty its proved very tough to get a happy medium. If I could satisfy myself that the boat would go straight on its own then I could move on to more productive stuff. By the way thanks very much for sticking with me. You have been very helpful with this and many other treads.
Thanks
Don
Vancouver Island
I see what you are getting at.
Yes, for the pulling-the-boat-across-the-pool-with-the-drill-winch exercise it is likely that the boat’s tendancy to turn will be dictated by the interaction between the fin and the hull. If the keel generates lift when the hull is moving straight, that will cause the boat to pull to one side which will look like an arcing turn if the boat is moving forward. another way to look at it is that the fin wants to go straight which causes the hull to move throught the water at an angle which results in an asymmetric drag wanting to turn the boat.
I agree with the idea of pulling the boat but I am a bit leary about letting it coast. This might not be stable. Think of it like trying to throw a dart tail first. It is unstable and wants to flip around. I think most hull shaped are setup with a fairly fine entry and forward rocker which will create more drag forward of the fin than behind it (just like the flights on a dart create more drag forward of the CG of the dart) which will tend to make the boat want to flip around. By pulling it, you establish stability. I would try pulling it all the way. If the string is long enough, you should still see a tendancy to turn one way or the other.
By the way, you should do those tests at full race ready displacement. Add a little weight on deck to account for the missing rig. The boat will behave differently when it is sitting on its waterline than when it is floating high…
Sounds like you have a good idea of how to trim out your hull and keel fin system. Let us know how it turns out.
Will Gorgen
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Don
… if I had a boat that insisted on turning when I was just pushing it across a swimming pool what would I look for<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Hi Don
Nice, non-straightforward questions!
IMHO, you are looking for the hull being slightly heeled, due to the keel not being at 90 degrees to the hull’s design waterplane. A rudderless hull turns when heeled. The turning moment due to heel is amazingly powerful, and I’m guessing is far stronger than any modest asymmetry in the fin.
As Will explains, the likely effect of an asymmetrical fin (all other things being equal) would be to have lower leeway on one tack and higher leeway on the other when on the wind. On the run, the crabbing action of the hull might make you think the boat was pointing off its course and might make you think the boat was therefore “turning”. My guess is that the turning moment is actually very slight and the boat is just, ah, crabbing. Of course, a little rudder is needed to keep the boat on course in the same way a little steering is needed to keep a badly repaired accident-damaged sub-frame twisted car on the road while the whole thing crabs along.
Lester Gilbert
http://www.iomclass.org/
http://www.onemetre.net/
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>
Nice, non-straightforward questions!<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Sorry! When I started this thread I thought I knew what I wanted to know. As it turned out I knew what I wanted to know but wasn’t that sure about how to phrase it. You’d never know my mother was a English teacher, would you?
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>IMHO, you are looking for the hull being slightly heeled, due to the keel not being at 90 degrees to the hull’s design waterplane. A rudderless hull turns when heeled. The turning moment due to heel is amazingly powerful, and I’m guessing is far stronger than any modest asymmetry in the fin.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Good point! This is the problem when one is a devout builder and not wealthy enough to own a CNC mill. Hand fabricating can introduce so many minor(and major) errors that it’s hard to pick out the problems. This is especially tough with boats when there’s hardly a straight line in the whole thing. Now I have to try and figure a way to determine if indeed there is 90 degrees between the keel and the waterline. I’m going to try my towing idea though,if nothing else it should tell me if something is wrong.
Thanks
Don
Vancouver Island
Just another side to this.
In the 1970’s there was a Dick Carter One tonner down in NZ with a pivoting keel so it could be pivoted to windward to elimimate the lieway compoment. It worked quite well, until one race with the kite up the mechanism gave way and the keel swung around to right angles. I believe there where some teeth marks in the back end of the cabin.
JohnB,
I remember that incident. There was a writeup in one of the magazines about that incident.
In many intercollegiate dinghies, the centerboard trunk is secured to the hull by a set of ribs that line up with the trailing edge of the centerboard when it is all the way down. The forward edge of the trunk is quite loose and allowed to flex. So when the board is loaded, it will tend to twist around the trailing edge which will cause it to flex to a positive angle of attack. Same idea…
Don,
Greg posted a link to his keel alignment jig on the US1M site some time back. Here is that link:
http://members.tripod.com/~ghmyc_1m/photos_triplecrown_gv.html
Something like that might help you with your alignment…
Will Gorgen
JohnB,
That was the Wai Aniwa wasn’t it? What a beautiful looking boat.
The IOR rule sure produced some “dogs” in terms of sailing behaviour, but goodness me some of them were pretty.
Sorry - completely off topic I know.
Muzza
Yes, I was looking at that yesterday. When I installed my keel trunk I made it about 1/8" thicker(athwartships) than my keel. When it was time to install the keel I shimmed the bottom 1/4" with 1/16" ply on either side. That way if I do discover a misallignment I can file and/or build up the shims as needed. You can never make anything too adjustable in the prototype stage and for me everthing is a prototype.
Thanks
Don
Vancouver Island