foil questions.

[merlinron]

" The Rules" state that an efficient foil can be had making the thickness 12% of the chord. this " rule" more or less coincides with what my print calls for in thickness of the bildge boards. the plan shows lines for contour, but, does not indicate whether the boards are symetrical or asymetrical (no section drawing). i would like to build asymetrical boards, for their efficiency.
now,..... does the 12% relate to a board that is symetrical at 12% of chord, indicating that an asymetrical board should (could) be only 6% of chord, or does it indicate 12% asymetricly, meaning that a symetrical board would actually be 24% of chord..... that seems awfully thick for a 16" wide board.

[Thorne]

Dunno -

These links might help -

http://www.boat-links.com/foilfaq.html

http://www.philsfoils.com/designTips.html

http://www.onemetre.net/Design/Fillets/Fillets.htm

http://www.solarnavigator.net/rudders.htm

[John E Hardiman]

No, an aysmetric board is not just a symetric board cut in half. What happens is that the thickness and cord dimensions remain the same, but the development line is curved (i.e. cambered)
Four-digit series
The simplest airfoils, described using a four-digit number in the following sequence:

One digit describing maximum camber as percentage of the chord.

One digit describing the distance of maximum camber from the airfoil leading edge in tens of percents of the chord.

Two digits describing maximum thickness of the airfoil as percent of the chord.

For example, the NACA 2412 airfoil has a maximum camber of 2% located 40% (0.4 chords) from the leading edge with a maximum thickness of 12% of the chord. Four-digit series airfoils by default have maximum thickness 30% of the chord (0.3 chords) from the leading edge.

The NACA 0015 airfoil is symmetrical, the 00 indicating that it has no camber. The 15 indicates that the airfoil has a 15% thickness to chord length ratio: it is 15% as thick as it is long.

IMHO though, do not use an asymetric board. While it looks good in theory, in the real world loss of AOA due to envrionmental conditions will make an asymetric board stall and wash out without significantly increasing lift.

[merlinron]

thorne,
awesome links... thanks. i've searched for info on other furums and always found some, but most of it went into hyper tech way too fast and flew right over my head. these were great and understanable, even for my small mind.
john,
so reguardless of symetry, % of chord remains the same and an asymetrical board in theory, will produce proportionatly more lift, with a small reduction for drag induced by relative thickness? the keels(there are two, the boat is a twin-swinging bilge board design) on my boat are set at 1 degree+/_( 20mm.offset front to back, of a 1200mm. long case) to the centerline of the hull and at right angles to the deadrise (with the boat upright, looking down the centerline, they are splayed out 7.5 deg.). i believe it's done that way to compesate for the loss of AOA as the hull heels and the centerline of the boat points slightly to leeward(?). as the hull is narrow aft with comparatively symetrical heeled waterline profiles, am i correct in thinking the AOA loss from heel is kept to a minimum(?) in comparison to a wider transomed boat and theoreticly the helm should remain fairly neutral as it heels?. with the above details, an asymetrical board should work, no?

[John Meachen]

Since we have had a posting from the extremely knowledgable Mr Hardiman,I would like to pose a question as a follow on.What,if any, advantage might be had by using a symmetrical foil with a slight toe-in?

[John E Hardiman]

so reguardless of symetry, % of chord remains the same and an asymetrical board in theory, will produce proportionatly more lift, with a small reduction for drag induced by relative thickness?

Don't fall into the "low drag/high lift" mind trap for sail boats. Lift and drag (including skin, form, and induced) of a foil are defined and developed perpendicular and parallel to the FLOW....Side force and driving force are defined and developed perpendicular and parallel to the HULL. A high lift/low drag asymetric foil opperating at it's designed AOA produces no driving force because the resultant vector points aft. If you increase tha AOA until you get driving force, you are way out of the optimum lift:drag area for most asymetric foils


the keels(there are two, the boat is a twin-swinging bilge board design) on my boat are set at 1 degree+/_( 20mm.offset front to back, of a 1200mm. long case) to the centerline of the hull and at right angles to the deadrise (with the boat upright, looking down the centerline, they are splayed out 7.5 deg.). i believe it's done that way to compesate for the loss of AOA as the hull heels and the centerline of the boat points slightly to leeward(?). as the hull is narrow aft with comparatively symetrical heeled waterline profiles, am i correct in thinking the AOA loss from heel is kept to a minimum(?) in comparison to a wider transomed boat and theoreticly the helm should remain fairly neutral as it heels?.

Without a set of lines I cannot make a good comment on your hull. Most modern, high speed twin keelers (Open 50's, 60's, 70's) have a "sailing centerline" that appears to toe-in WRT true centerline. This is caused by wide, hard bilged hull forms that are used by light, form stable, hulls. A hull with slack bilges and a narrow stern would not have nearly as pronounced a change in sailing centerline. The wild card are those wide hulls with firm bilges and narrow sterns. These hulls take on an asymetric foil shape when sailing heeled. Some work to weather very well, others were pigs.


with the above details, an asymetrical board should work, no?

It's not the thoeritical details, it's the environment... The orbital velocity of waves, depending on heading to the wave train, pitch, and hull response to variation in wind, can easily change the AOA +/- 10 degrees. Asymetric foils tend to be ill behaved with large changes in AOA, especially low AOA high lift/low drag shapes. If the water was as perfect in practice as it is in theory, then you would be correct, however, that is rarely the case and often a symetric "streamline" shape will do much better than a "lifting foil" in real conditions, especially if the trailing sections are shaped with some forethought.


What,if any, advantage might be had by using a symmetrical foil with a slight toe-in?

As I pointed out above, apparent toe-in is due to placing the foil on the apparent "sailing centerline". Or so I, the guy stumbling around in the dark about most things, think.

[Canoeyawl]

My understanding about a “slight toe-in” is that this automatically occurs as the hull makes leeway. The flow is not parallel to the hull.

[John E Hardiman]

My understanding about a “slight toe-in” is that this automatically occurs as the hull makes leeway. The flow is not parallel to the hull.

Do not confuse apparent AOA with toe in. Leeway gives the AOA to a foil placed straight along the "sailing centerline". The sailing centerline is the heading of the major axis of the submerged portion of the hull while in steady state sailing trim. Yeah it seems kind of circular in it's logic, but thats the way it is, sort of like determining sailing waterline length.

Toe in or toe out is measured from the cord axis of the foil to the sailing centerline. MOST hulls when heeled move the foil into a toe in configuration due to buoyancy in the aft quarters. This is a function of heel, not leeway.

[Canoeyawl]

A wrong choice of words… apparent angle of attack at first glance seems very similar to “toe-in” but I understand now that with an asymmetrical hull form when heeled you could have “toe-in” combined with AOA where a symmetrical form may not. Thank you for the clarification.
Are there advantages to a hull form that develops toe-in at the foil when heeled or is this detrimental to windward performance (would it have to be countered with more helm)? Would one choose a different foil shape based on the amount of toe expected?

[John E Hardiman]

Are there advantages to a hull form that develops toe-in at the foil when heeled or is this detrimental to windward performance (would it have to be countered with more helm)? Would one choose a different foil shape based on the amount of toe expected?

Generally, any heel is considered detrimental to sailing performance. However, it is a fact of the physics of sailing and you just have to deal with it. Some hull forms, mainly catamarans and hard chined small craft, are designed to allow the asymmetry of the hull to be the lifting foil but these are high power to displacement craft that can afford the induced drag losses.

While in theory you could design for toe in, you run into the same problems as with asymmetric foils, in that the AOA can change greatly. Many people have tried to sharpen the pencil to this point and use low drag sections, but I’ve not seen any definite improvement.

[David Geiss]

This is a fascinating thread.
I sail an 16.5' loa open canoe with a 44 sq lateen rig; leeboard suspended from the port gunwale, parallel to the keel line.

I steer with a paddle thus the leeboard is placed, fore/aft, just forward of the center of effort of the lateen. Paddle to leeward side of the canoe and needs to be levered off the gunwale to keep a point of sail.........letting the paddle go limp permits the canoe to head up.

Before the wind, the board comes up and the canoe is tilted to windward ...this induces a carve of the hull to leeward (in order to counteract the canoe's tendency to want to come to windward (even with the board up).

I always imagined that sailing to windward on the starboard tack (with the port gunwale heeled over to leeward) would be more efficient than the port tack, which has the port rail up and the leeboard "lily dipping". My experience is the other way around though, i.e., the boat feels more efficient to windward on the port tack which places the sail further from the foil (remembering that the foil is suspended from the port gunwale).

So here's the question (there had to be a question in here somewhere..........): I have heard sailors say that a wider board is good for light air and a "less wide" board is better for heavier air. Immersed length being kept constant and maintiaing the same NACA 0010 foil shape, is a board with a longer chord length better for light air and a foil with a shorter length better for heavier air and, if so, what's the easy explanation?

Thanks for your indulgence.

Best,
David

[John E Hardiman]

So here's the question (there had to be a question in here somewhere..........): I have heard sailors say that a wider board is good for light air and a "less wide" board is better for heavier air. Immersed length being kept constant and maintiaing the same NACA 0010 foil shape, is a board with a longer chord length better for light air and a foil with a shorter length better for heavier air and, if so, what's the easy explanation?


V^2.....that's the answer. Ok there are also some Renyolds number (Rn) effects in there,but for the most part it's V^2.

Lift force = 0.5*rho*Cl*A*V^2. With Cl being a function of AOA and aspect ratio, which in turn is a function of Cord and span. In stronger winds you could have the power to be going twice as fast. Therefore, you only need 1/4 the area to get the same lift force. Of course, leeward force could be twice as high so its not exacty a 1:2 reduction in area. Additionally, there is an optimum Rn for minimizing friction drag. Friction drag = 0.5*rho*Cf*A*V^2 where Cf is a function of Rn and Rn is a function of cord and V.

Realistically though, it sounds like your leeboard might be too far forward. In moderate conditions, the hull should balance and no steering with a paddle should be needed. Any steering force adds drag and loss of speed. (yeah, yeah, I know about the need to have a little weather helm as good practice, but a slight pull on a 1 degree to weather tiller is not hanging on to a paddle over the side for a few hours).

[David Geiss]

John:

I need to get up to speed on these formulae! I see that none of the criteria can be looked at individually......in bigger winds, I guess it's balancing the higher sailing velocities permitted by greater winds as against the increased windage operating on the hull.

Why is it that in light winds, the canoe seems to point higher efficiently but in higher winds, the most efficient point of sail seems to be a little more to leeward?

You are dead on with your comment that the leeboard may need to be moved aft. I was racing this past weekend and did very well overall (2nd out of 8)but I feel as if I could have derived more performance to windward.

In the ACA cruising class racing, one may not apply any forward paddle strokes following the commencement of a race, thus coming through the wind involves "scooching" forward in the boat and letting the paddle go limp. Too balanced of a helm and the boat does not come around or it comes around way too slowly...too much wetaher helm and one needs to lever that paddle all day, as you mention, in order to hold a course w/o coming up too much.

So, it's a delicate balance. As the canoe is now configured, I'm braking in effect while going to windward. I'm familiar with some of the rules of thumb for total area of immersed foils and am thinking that there must be a more precise way to determine proper size.

I mention this because too much immersed foil, i.e. a leeboard bigger than necessary, can't be good. So, I need to fiddle more with size and placement. Every season, it gets a little better.

Thanks for your insights!
Best,
David

[merlinron]

john,
thanks for the excellent explainations/advice. it,s an amazing can of worms one small question can open, especially to a neophite like myself. without having a "real" background of nautical education, one can easily be lead down the wrong road by one's self !!
i do believe that my boat might fall into that "wildcard" area of firm bilges, wide beam and narrow stern. 7'-2" beam, 10" of hull depth to the waterline, 2 chines, with the second chine right at the waterline just aft of amidships, waterline beam at that point 6'-9" and a 2'-9" wide transom, out of the water. still, i will definately re-consider using the asymetrical boards on your suggestion. this is supposed to be an easy going day sailor with overnight capabilities and not a hot rod, overall performance is guaged by how easy it is to have a nice sail, not necessarily just on how well it sails to wind( if questionably, the asymetrical boards will actually help). as stated earlier, this whole thread was generated by the fact that the plans don't show a section of the boards...it may very well be intended that they are symetrical.
you have straightened me out on one point, for sure. i had to think about it few minutes with my eyes closed ... as i thought earlier, heeling reduces AOA by the bow sliding off to windward. clarified by your excellent reply, the hull rolls just as a cone shaped cylinder would, and increases AOA. i'm just trying to put ten pounds of shyte in a 5 pound bag, appearently.

[David Geiss]

Realizing I may have thrown the helm over hard on this thread, but hoping John or Merlinron might comment generally on how the thwartwise distance between the foil and the sail affects performance, the basis for the question being that a canoe with a single leeboard seems IMHO to sail more efficiently when the sail is on the side of the canoe opposite the leeboard.

I'm wondering on this also because in my near future there is a Melonseed or BB Sneakbox and one option I am keeping open is a single centerboard substantially off center (in order to keep the bilge open)

Thanks Gents!

[John E Hardiman]

Think of it this way, the sail is pulling/pushing the boat to leeward, the board is pulling/pushing the boat to windward. Now a hull has no natural restoring force (like gravity) in the lateral place. Place it on one heading and it will tend to stay that way, this is called neutral stability. (Note: there are a lot of other things going on here when the hull is moving, but I'm trying to keep it simple).

When the board is to windward, the system is more stable because the hull is balanced by the two forces both PULLING OUT on the hull which will rotate to take the natural line between the two. <----> like pulling on a rope. When the sail is to weather of the board, the forces are PUSHING IN, which is unstable and the hull wants to twist to a more stable configuration, just like pushing on a rope >----<.

Understand?

[Thorne]

**I** understand -- and that's a real accomplishment! Thanks for a very clear and concise explaination of a complex topic.

;0 )

This does seem to argue against off-center centerboards or daggerboards, despite their popularity for saving space. And against single leeboards, despite the $ and labor-saving disadvantages of building two and raising/lowering them on each tack.

[David Geiss]

Capiche!

Thanks very much John for "dumbing it down" for me a bit!

For a while I played with two leeboards bit found it to be a hassle.
Sounds as if one foil on the canoe is a compromise, but not a terrible one.

VTY,
David

[John E Hardiman]

The real question with an off-center board is how much drag you cause trying to keep the boat going in the direction you want it to go. As most vessels have some deadwood, this tends to keep the boat tracking at the expendature of some little drag (i.e. the lever arm to cause positive stability is long so the force/drag is small....thats why rudders are as far aft as possible). It is possible for canoes with no deadwood to "balance the pencil on the point", and this is were the ability to move/sweep the leeboard comes in handy

[David Geiss]

John:

I think I am understanding your point here.

The canoe I sail has no deadwood..........it's got an arched belly and plenty of tumblehome to the gunwales.

There are moments while sailing the canoe that the leeboard hums or cavitates slightly. Generally, raking the foil aft, even slightly eliminates it. This occurs generally when the foil remains plumb and I am sailing with the wind on my beam or aft of that.

The foil swings just above the gunwale on a stout hanger bolt (mounted at the end of a beefed up dedicated thwart.....www.homepage.mac.com/geisser/PhotoAlbum7.html)

I have seen some canoeists place the foil so that when it's plumb there is a generally neutral helm...then, when it's time to tack, the foil is raked forward thus placing the CLR fore enough to induce more helm to weather for the tack across the wind. Back on the other point of sail, the leeboard is returned to the plumb position.

Better might be to devise a means by which the foil can be moved fore and aft in its plumb position "on the fly". Have to put my thinking cap on for that one. That would permit foil placement depending upon load, degree of weather helm required, etc.

Thanks again for clarifying some of these issues. I'm biased maybe but have come to the conclusion that sailing a canoe with paddle steering is a worthy learning excercise. The lessons can be applied to sailing many other types of vessels.

Best,
David

[John E Hardiman]

John:

There are moments while sailing the canoe that the leeboard hums or cavitates slightly. Generally, raking the foil aft, even slightly eliminates it. This occurs generally when the foil remains plumb and I am sailing with the wind on my beam or aft of that.


Does it hum, or does it cavitate?

Humming is caused by several reasons, mostly either edge flutter from vortex shedding or turbulent flow over the skin. Shaping the section in subtle ways can improve these conditions, though in beam reaching or going down wind you may have too much board in the water and raking it aft will improve both conditions as well as add speed and reduce the tendency to yaw. (FWIW, I bet you did a really good job on the board and the traling edge is very thin, yes?)

Cavitation is caused by overloading the planform of the board or exceeding the critical speed for the section. Again, raking the board will reduct the apparent section thickness, but it points to a whole different solution required.

[David Geiss]

John:

You know I was going to use the word "hum", because that's what it does.....but I was trying to sound intelligent! Maybe I should dumb things down myself!

The trailing edges I do not make terribly thin.....say 1/8".

Before the wind, the board is genrally up and out of the water, or deployed roughly 10 degrees "south" of parallel to the gunwales. Leaving it deployed causes tendency to windward.

It's in reaching that the foil is sometimess left deployed more than it ought to be. A dagger case would let me modulate how much of the board is immersed but it's problematic in so far as the rig is concerned (i.e. boom).

Can you reccommend a decent treatise on the subject of foils......perhaps "Foils for Dummies" or something along those lines.
I enjoy tinkering but approaching all of this armed with better information would avoid some creative disasters.

I'm interested particularly in arriving at properly sized foils (immersed length and fore/aft widths for varying conditions) for various sail rigs. Also want to get a better handle on appropriate planforms and foil shapes for my hulls and rigs.

Thanks agian for your insights.

Best,
David

[Thorne]

David -

Are you saying that when reaching, you can't swing the centerboard up or down a few inches, due to it changing the balance of the rig? Seems to me that your rig is overly sensitive if it can't handle minor adjustments of the centerboard, but hey, I've been wrong before!

;0 )

Centerboards, in my limited understanding, can swing up and down to change the balance -- and are usually left fully down when reaching, swung up to 45 degrees or better when running downwind unless broaching is an issue.

If swinging the CB up and down doesn't give you what you need, perhaps a different foil design is called for?

The old "humpback" style centerboards provide more underwater resistance than a more modern high-aspect foil when swung down, as the board is significantly wider at the front/pivot point than further aft/down.

Here's a scribbled-on pic of the CB plan for a Chamberlain dory skiff from Mystic to illustrate the 'humpback' design -

http://www.luckhardt.com/cb-design1.jpg

[John E Hardiman]

Can you reccommend a decent treatise on the subject of foils......perhaps "Foils for Dummies" or something along those lines.
I enjoy tinkering but approaching all of this armed with better information would avoid some creative disasters.


For section shapes themselves, The Theory of Wing Sections by Abbott is the basic text. For foils in the real world, Hoerner's Fluid Dynamic Lift and Fluid Dynamic Drag are the standards (hard to find and expensive but worth it for the reduced data). For sailing specific, I wouild use Aero-hydrodynamics of Sailing by Marchaj.

As to the humming, if your trailing edge is cut square and ~1/8 thick, just leave it...your not going to get much more out of the board without major reshaping. We used to use a CB that hummed as a speed gauge....if it wasn't humming, you needed to trim the sails.:D

[Canoeyawl]

I always assumed that "humming" was a stall or approaching a stall, if an airplane wing did that it would fall out of the sky.

[Thorne]

From what little I've read, the trailing edge is supposed to in theory narrow to an infinitely thin/narrow edge -- which of course has to be trimmed in the real world.

So I'm guessing that the humming John is mentioning is produced by the various vortexes(vorti?) at the flat edge of the trailing edge causing the board to vibrate or hum??

[David Geiss]

No, it's not feeling like a stall...more as if too much leeboard immersed. As soon as I rake the leeboard aft, the humming completely disappears.

Alot of what my fellow canoeists are doing in terms of foil shape, size, is based upon tradition and rough formulae taking sail area and immersed section of the canoe.

Thorne, I like that sketch of the dory centerboard.........you have got me thinking about foils that are not rectangular. The Dutch I believe have traditionally used teardrop shaped leeboards which, for a given depth, give more lateral resistance than a rectangular leeboard.

The humpback seems to operate on a similar principle.
Maybe one approach on a canoe would be to have two leeboards both mounted to the same gunwale, the fore board being bigger relative to that aft (around the backrest thwart). The canoe could be set up for a balanced helm with both boards deployed and I could raise the aft board (handy in any event from where I am steering with the paddle) thus creating windward helm for coming through the wind.

Before the wind, perhpas use the aft board slightly deployed with the fore board up..........reaching, a combination of the two.

I guess Fenger thought all of this through on Yakaboo a long time ago but he lost the paddle along the way and used centerboards in lieu of leeboards.

Best,
David

[JimConlin]

I hear that the humming is caused by vortices at the trailing edge fluttering between sides. Some of the multihull folks have found that it's markedly reduced by shaping the trailing edge not at 90 degrees, but at about sixty degrees. With that asymmetry, there's no ambiguiity about which side the small vortex caused by the non-zero-thickness trailing edge will form on.

[Thorne]

Interesting! Quite a thought, to trim the trailing edge more to one side than the other to avoid noise...

;0 )

As for the Dutch and their leeboards, the shape can be regarded different ways. The teardrop shape may have been more influenced by other factors than 17th C foil design ...

I suspect that shape was developed to make freeing the board from the mud and sand easier, to avoid tangling lines when raised to the rail, etc.

As for more or less than a rectangular foil -- that depends on whether you are measuring the top or the bottom of the teardrop, don't it.

Here's a 17th C painting of the Dutch fleet saluting the Royal Barge -- note the leeboard on the ship firing the shot --

http://www.wga.hu/art/c/capelle/saluting.jpg

[John E Hardiman]

From what little I've read, the trailing edge is supposed to in theory narrow to an infinitely thin/narrow edge -- which of course has to be trimmed in the real world.

So I'm guessing that the humming John is mentioning is produced by the various vortexes(vorti?) at the flat edge of the trailing edge causing the board to vibrate or hum??


Some of the multihull folks have found that it's markedly reduced by shaping the trailing edge not at 90 degrees, but at about sixty degrees. With that asymmetry, there's no ambiguiity about which side the small vortex caused by the non-zero-thickness trailing edge will form on.

1) The trailing edge should never be taken to a knife edge (only works in theory, but then again on props only 0.10" thick having a cutoff thickness of 2% means the edge is 0.002" so same-o-same-o to your hand ;) ), this will cause the pressure side flow to roll over onto the suction side and cause loss of lift and erratic stalling.

2) Yes, the humming is caused by vortex shedding; and yes, you can control the amplitude of the shed vortex by shaping of the trailing edge. Without getting into too much detail, the shed vortex size is determined by the Stroul frequency of the trailing edge, a function of cutoff thickness, speed, and Renyolds number. By manipulation of the enclosed angle of the section and boundary layer (the shaped trailing edge Jim Conlin mentioned) you can determine what sized vortex you want to "bind" to the trailing edge and make it behave well. Hoerner devotes some length to this topic.

3) You can never eliminate this trailing edge vortex, only control it. If the board is gently "humming", this means that you have a nice, small, high frequency (well over 200 Hz), stable vortex, which is what you want. You do not want a large, low frequency, vortex that alternately washes out each side of the foil and causes increased drag. I have seen shedding so bad that a dagger board was slammed back and forth in the trunk at about 4 Hz.

4) Finally, remember that the board is also a tuning fork and amplifier. Often you can "hear" an approaching motorboat prop in the board long before you see it or hear the motor exhaust.

[John E Hardiman]

Thorne, I like that sketch of the dory centerboard.........you have got me thinking about foils that are not rectangular. The Dutch I believe have traditionally used teardrop shaped leeboards which, for a given depth, give more lateral resistance than a rectangular leeboard.



The best foils are not rectangular because a rectangular planform does not give elipitical loading, which causes too much tip losses and drag. The Dutch, or the other hand, used more rounded shapes for the reasons Thorne described, the big thing being a rounded tip was less prone to damage.

For a symmetric foil at fast sailboat speeds it is my opinion (which is the only thing a NA has to sell) that the best all round foils will have an aspect ratio of 3 to 5, a thickness of 12 to 18 %, a taper ratio of ~50% and a sweep of -5 to -7 degrees, a 2% cutoff trailing edge, and a section shape I keep very close to my chest. :D .