Dumb daggerboard question

Thread starter #1
OK - help me win a bet. There are three kinds of wood Sunfish daggerboards, the original
(full width with rounded bottom), the tapered (Shadow?) board and the spade shaped board with full width and flat, angled bottom. My question is: for the tapered board, which is the leading edge, the straight, plumb side or the tapered side?

A. Glos
Cazenovia, NY
 
#2
On page 29 of "Successful Sunfish Racing" by Derrick Fries, all three boards are shown in the correct sailing position. The Shadow board is shown with the straight long edge forward.

Isn't this in the Sunfish Bible?
 
#3
I'm not familiar with the older wood dagger boards, but, where applicable the thicker edge is the leading edge, and where the bottom edge is angled resulting in a long and a short vertical edge, the long edge is the leading edge. I have never seen a keel, centre board or dagger board that leads with the short edge.
 

Wayne

Member Emeritus
#4
OK - help me win a bet
This probably isn't going to help. . .

It’s a real can of worms. Different sailors and authors have conflicting opinions.

Both the 1972 AMF brochure and parts identifier sheet shows the Shadow Board leading with the short edge. As a matter of fact, all the manufacturer literature from Alcort to Vanguard depict all three wood boards leading with the short edge.

Derrick Fries photo caption is enigmatic because he says “The Edge” without explaining which edge, but goes on to explain the Old Board is differently oriented from the other two, long edge forward. To make things even more confusing, it’s hard to see which side is the long edge. My interpolation is it’s the right side in the photo…, making the other two designs facing short edge forward.

A diagram in The Sunfish Bible doesn’t help the issue when it shows all three wood boards facing short edge one way and the composite board oriented in the opposite direction.

No question the composite board goes long edge forward. It’s the only board that also has edges of different thicknesses and the long edge is also the thick edge so there’s absolutely no confusion there.

The Class racing rules from the late ‘80s picture a tapered edge in their cross section, however, as I recall the Old Board and Barrington Board came with equally rounded edges and it was up to the owner to taper them…, staying within the racing rules dimensions. Only the Shadow Board came with factory tapered edges, still these were symmetrical between the long and short edge.

The Sunfish Bible chapter, “Tuning The Boat”, shows how owners can shape the edges asymmetrically. The Bible also relates the short edge, long edge debate and comments different racers felt they found success with one or the other orientation, furthering the debate.

One thing that can be said, due to it’s swept shape, changing the Shadow Board direction changes the Center Of Lateral Resistance, counter point to shifting the sail plan’s Center of Effort by moving the boom’s position in the gooseneck clamp.
 

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#5
On page 29 of "Successful Sunfish Racing" by Derrick Fries, all three boards are shown in the correct sailing position. The Shadow board is shown with the straight long edge forward.

Isn't this in the Sunfish Bible?

I think you are correct Fred. But wasn't the board originally meant to face the other way? I think people discovered putting it in " backwards" worked better.

I would check my Bible but it is locked up at my mothers house and I don't have access to it.

Opps I should have read the other responses and not posted this since I added nothing to Fred and Wayne posts.

 
#6
Derrick Fries says that regatta results indicate that the Barrington board performs best with the swept back edge forward and the old board is best with the long edge forward. He makes no mention of the shadow board but since the shadow board also has a swept back edge wouldn't this also be the best forward edge? This contradicts the photo I mentioned in his book!
Back to square one!

Fred
 

Wayne

Member Emeritus
#7
Derrick Fries says that regatta results indicate that the Barrington board performs best with the swept back edge forward and the old board is best with the long edge forward. He makes no mention of the shadow board but since the shadow board also has a swept back edge wouldn't this also be the best forward edge? This contradicts the photo I mentioned in his book!
Back to square one!
My speculation is the Shadow board got such bad reviews from racers and recreationalists alike, and was superceded in short order by the Barrington Board, it wasn't worth mentioning beyond the above photo caption. No one I know who's sailed the other styles has ever tried to replace one in kind. They eagerly upgrade to either the Barrington or composite.

As far as which way to orient any of the wood boards goes..., provided no custom shaping has been done, it appears by the accounts given in The Sunfish Bible and Windward Leg over the decades, performance is as much personal superstition as anything else.

It would be great if someone would pick a day with a 10 - 12 mph steady breeze on a lake where water conditions remain pretty constant and make about 10 passes on a upwind downwind course with a Barrington Board oriented first one way and then the other. 20 passes in all with a third party for objective time keeping. That might settle the question in a somewhat more scientific manner than the gut feelings I've read.
 
#8
Alan Glos;131998 My question is: for the tapered board said:
Well...here is an explanation that'll make you win that bet ;). I am a pilot and have finished my qualifications to fly for an airline, just have to graduate college soon. I have taken many of classes on aerodynamics, which is basically hydrodynamics. Bernoulli's principle that applies to our explanation of how lift on a wing is created, was derived from his studies on pressure and flow in plumbing back in the day. Increase velocity, decrease pressure and vice versa, and engineers manipulate the curvature of the wing surfaces to create lift using this principle, among a few others.

Anyways, the Sunfish's daggerboard is shaped like a wing, with both sides having the same curvature (neutral stability) This is not really the same for your generic aircraft...usually you see this design on aerobatic aircraft. Not really creating lifting forces to one side or the other (ideal for a daggerboard shape). You manipulate lift by changing the angle of attack on these types of wings (pitching the aircraft up/down)

You want the straight edge forward with the tapered edge facing towards the stern. The idea here is you create the smallest aerodynamic footprint so to speak. Ever see those giant vortex's that come off the wingtips of airliners? They create drag, and lots of it, especially when the aircraft is heavily loaded, slow, and in a clean configuration. That wing is working hard to keep the aircraft in the air. They are so strong and create so much disturbance to the air that the Boeing 757 has an FAA mandated minimum 5 mile following distance ATC must follow for separation of aircraft (due to it's particular wing design). They have been known to roll small airplanes who get too close! Nothing like going for a barrel roll 200 feet above the ground when you didn't induce it!

Point being...you can reduce the size of these vorticies by creating a smooth entry into the water and a smooth exit, which is what the tapered edge does. The tapered edge is similiar to the elliptical wing the WWII British Spitfire used and the modern kitplane Silence twister uses. Aerodynamically they are almost 100% efficient, in theory anyway. So if you want to go fast, straight edge forward. You will reduce your drag more than putting it in backwards, which would just leave you with a larger disturbance off that nice squared edge on the tip. :) Hope ya win that bet!

BC
 
#9
Here's a link that may help show how the shape affects turbulant flow. Winner for low drag seems to be the elipitcal shape. Airfoil shape gets really complicated.

Strange to say, leaving the trailing edge of a centerboard square produces the lowest drag. The square edge produces a vortex that the water flows around. When we flew RC planes years ago, going from a rounded edge to a square edge on the alerions increased the roll rate an increadable amount.

Worst shape for trailing edge is rounded. I got the profile backwards when I made my rudder. As soon as the SF got up to speed the tiller shook like the stick shaker on an aircraft. Doh!

If you Google: NACA AIRFOILS SAILBOAT you'll get an idea of what a profile should look like. I'm not sure it really matters anymore to SF racers since they use the premade foil.


http://www.dauntless-soft.com/PRODUCTS/Freebies/Library/books/FLT/Chapter17/WingPlanform.htm
 
#10
Ever see those giant vortex's that come off the wingtips of airliners? They create drag, and lots of it, especially when the aircraft is heavily loaded, slow, and in a clean configuration...

BC
Wanted to correct my statement. The larger the vorticies, the more lift that is being produced, thus more induced drag...drr.
 
#11
Here's a link that may help show how the shape affects turbulant flow. Winner for low drag seems to be the elipitcal shape. Airfoil shape gets really complicated.

--->You're talking about two different things here. Elliptical is the overall wing shape, viewed from top down. the shape of your airfoil is your lateral cross-section, it's not a wing shape type as viewed from above.

Strange to say, leaving the trailing edge of a centerboard square produces the lowest drag. The square edge produces a vortex that the water flows around. When we flew RC planes years ago, going from a rounded edge to a square edge on the alerions increased the roll rate an increadable amount.

--->Perspective 1: i'm curious where you found that a square trailing edge is the best idea for lowest drag. The vortex is something you don't necessarily want, unless I am not taking something into consideration for hydrodynamics, which I would like to find out :). From my 7 years of flying and 170K education at ERAU and UND, my understanding is the vortex is where the two layers of flow between the upper and lower surfaces of the shape meet. The pressure difference, with the bottom layer of flow being of a higher pressure relative to the upper layer, causes it to twirl around the top and make the vortice (which in this case may not apply due to the mirrored curvature of both sides of the daggerboard) It's an effect that robs the wing or daggerboard in this case, of useful surface area or effectiveness on the tip.

For example: If you fly an airplane closer to the ground, the bottom layer of flow cannot flow around to the top of the wing, thus reduced vorticies, so you can fly an airplane close to the ground below Vr (rotation/lift-off speed) but above Vs (stall speed), and if you try to maintain that airspeed above roughly one wingspan above the ground, there is a good chance you will stall and settle back onto the runway...this is because the wing loses some effective surface area on the tip due to these vorticies, among other factors involved. Your induced drag (by product of lift) goes down within one wingspan and increases the higher you go above that. This is called ground effect and is a great example of how vorticies rob your airfoil of performance.

--->Perspective 2: Now it is possible that having it in with the tapered edge forward could be beneficial. There still will be a more abrupt re-attachment of flow on the square edge of the tip, which could cause some turbulent flow and create the vorticy you speak of. But the fact that the tip is tapered and the entire edge has a more of a sharp entry than the straight edge, it may be better for a less disturbed entry and more speed. The thicker the airfoil, the more air molecules pack up on the leading edge, even more so with the straight edge. This is called a stagnation point. But when you have a taper or sweep in the wing, the air molecules roll off the edge towards the tip so to speak, thus the aircraft is allowed to travel at a faster airspeed than the airfoil "thinks" it is going. That is how engineers are able to trick physics so wings do not have to be ungodly heavy and overly re-inforced to support the loads in flight. Though, tapered and swept wings need a higher airspeed than a straight edge wing of similiar size to maintain flight.

So this could mean having the daggerboard in backwards could mean some low speed upwind issues? Maybe? Not sure..Anyways...basically, it is possibly that there will be less resistance and more speed inserting the daggerboard, tapered edge first. Viewed from the bottom of the tip, the daggerboard has a similar shape to the "swede-form" hulls that fast kayaks use. that is a sharp, but gradual entry with a fast but smooth exit. I used to own a twin kevlar kayak that had that shape, it was a rocket and could glide in smooth water with very little effort. I think we all know that the best shape for this daggerboard would be a elliptical one. Maybe i'll have to build one out of fiberglass and try it...:D<----

---> as far as the roll rate on your R/C models...are you talking about the aileron being part of the wing tip as well? as in half the span of the trailing edge? because in that case, you are just increasing the surface area and effective surface area on the aileron by deflecting more of it into the relative wind. Then again I haven't seen the thing, so i'm taking a best guess. <---


Anyways...thats my perspective...I think that there is some testing in order this weekend!! :D

BC
 
#12
A point to consider...possibly...just thinking out loud here.....if by putting the daggerboard in backwards, I wonder if you end up moving the center of pressure/effort, that is acting on the daggerboard to provide lateral resistance going upwind, further aft a few inches, causing the arm (distance between the daggerboard and rudder) to be shorter, and decrease rudder effectiveness? hmmm...

BC
 
#13
OK guys, my mind is doing flips and barrel rolls with all the physics discussion but I'm getting the message that the straight edge was intended to be the leading edge which answers the original bet question. However, I also get the message that in real sailing conditions of waves, heeling, etc. the boat performance may be better with the board in "backwards". Fries' book says the same thing when he speaks of actual regatta performance.

Fred
 

Wayne

Member Emeritus
#14
A point to consider...possibly...just thinking out loud here.....if by putting the daggerboard in backwards, I wonder if you end up moving the center of pressure/effort, that is acting on the daggerboard to provide lateral resistance going upwind, further aft a few inches, causing the arm (distance between the daggerboard and rudder) to be shorter, and decrease rudder effectiveness? hmmm...
Right, that's what I was referring to in my earlier post.

The Shadow design presents a paradox since neither edge is perpendicular. So should it be swept back as logic and aesthetics both speak to. The physics say swept forward moves the CLR ahead (the same as moving the boom (CE) back) thereby introducing more weather helm which translates into increased rudder drag. So, my vote is swept back.



OK guys, my mind is doing flips and barrel rolls with all the physics discussion but I'm getting the message that the straight edge was intended to be the leading edge which answers the original bet question. However, I also get the message that in real sailing conditions of waves, heeling, etc. the boat performance may be better with the board in "backwards". Fries' book says the same thing when he speaks of actual regatta performance.
When it comes to the Shadow design my take is the "straighter" edge should be the leading edge. And I do not think reversing the direction of this design would find any improved performance.

I think the confusion comes when controversy about the orientation of the other two boards is extrapolated to the Shadow design. With the other two designs you clearly have perpendicular edges and it's the directional positioning of the bottom curve or angle cut that's in question. With my Barrington board I went with the hydrodynamic logic and faced the longer edge forward. The waters only get muddied when you look at all the spec sheet pictures and articles written over the years that depict just the opposite.

. . . perhaps "only The Shadow knows"
 

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#15
The vortex off a square trailing edge is really simple. The Mythbusters did the same thing with their "Truck tailgate open, Truck tailgate closed" episode. Leaving the tailgate closed put a vortex bubble in the bed of the truck which air coming over the top of the truck flowed over. Seems counter intuitive but it does reduce the drag. On a daggerboard you'll get a small vortex bubble directly behind the trailing edge. Water flows around the bubble when it rejoins from both sides of the daggerboard. I've seen some pictures on the web of people sanding a flat trailing edge in a racing foil.

The shape of the daggerboard and the shapes of the wing relate when you load up the daggerboard to the point it starts to 'stall' or produce turbulant flow. Symmetrical airfoils stall out as well as other types.

Square off the trailing edge of the ailerons on your RC plane. I first this on a "Whitney Brick", that plane was fast and rolled like crazy. Tried the same thing on a "Ugly Stick" and got the same improvement.
 

U-123

"Less judgment than wit is more sail than ballast"
#16
Just to let you all know, in the Shadow daggerboard--at least on my boat [1974]-- you really don't have a choice. The daggerboard slips down the daggerboard sleeve tapered side forward and board leading back-- like in the manual/advertisement pictures and drawings--easily and effortlessly. On the other hand if I try to slip the board tapered side facing rear---- it slips in with much difficulty and will not even go all the way through without having to force it. It's the same orientation as the rudder-- and that can only go one way.
 

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