Asymmetric Daggerboards

I need some advice on two issues about asymmetric daggerboards: choice of foil shape and details of construction. Feel free to comment on either or both.

Some quick background. I have placed a few photos here: https://picasaweb.google.com/102913883250402531371/DaggerboardIssues#. I have a Macgregor 36 catamaran, and a few years ago I acquired a mast and Randy Smyth mainsail intended for a never finished “turbo” Macgregor project. The original rig was a 44 foot fixed mast and soft main, big genoa. The new rig is a 52 foot rotating mast (30 cm chord), square head fully battened main, 100% jib. Sail area went from about 550 sq. ft. to about 750 sq. ft., plus the 52 sq. feet of mast. Smyth specified moving the daggerboard trunks back about 4 feet. When I got the new rig on, I rigged up the leeboard style holders for the daggerboards shown in the photos. I sailed with the boards in various positions, and determined that Smyth was about right. We did one race like this, to everyone’s amusement. The leeward board held against the side, but the windward board went flying, skipping along the surface.

This new rig had a lot of power, and with the original boards, which only went about 3 feet below the hull, we had a lot of leeway. Then, in pretty quick order, we snapped off both boards in two different races. I was actually surprised to see that the 30 year old boards were hallow, but not surprised that the two wooden stringers were rotted. I started building new boards using carbon and foam, an NACA 0012 section that extends 6 feet below the hull. The first one worked great. I could go to windward with anyone, but it lasted less than a season. In a 70 mile upwind race, the wind picked up to 20 knots and we were doing about 12 to windward when we snapped the board off (stupid, I know, should have pulled it up).

So now I am going to build new daggerboards, and decided I really should make asymmetric boards. Now for the advice.

Foil shape. I thought there were some usual suspects, but when I went looking, I found lots of possible foil suggestions, including:

Malcom Tennant: NACA 64A413
Tom Speer: NACA 63-012, H 105, or H110
Martin Schoon: NACA 2412
Bernard Kohler: NACA 63(1)012
Dalquiri: NACA 63-412
Others on BoatDesign.Net: E387, FX 63-137, SD 7002, SD 7037, NACA 63-209, Selig S3021

Help! Does anyone have thoughts on a good foil? Assume it is for a boat with 33 feet waterline, 18 foot beam, 750 sq. feet of sail, about 4,500 pounds. Foil is 18 inches chord and 6 feet below the hull. The boat is currently rated about the same speed as an F31. While I consider myself a good sailor, I would like a foil that is fairly forgiving.

Second, construction. I would love advice on a structure and a lamination schedule. I probably spent $1,000 on materials for the last one, which lasted less than a full season. The board was the same dimensions as a Reynolds 33 board I borrowed: 1.5 feet by 9 feet. The Reynolds board, made of fiberglass, was 64 pounds. Mine was 48 pounds, including 18 pounds of carbon. It broke off at the hull, which had carbon skins almost 6 mm thick. Some rules of thumb I found said that would be enough, but apparently it wasn’t. For sheer webs, I had two basically I beams, each about 10 mm thick.

One naval architect (big ships) took a quick look at it and thought the failure was because the structure couldn’t withstand twisting, that the angle of attack caused the board to twist, which increased the AoA, which increased the pressure on the board, etc. until the board just twisted apart. I don’t know whether to believe that or not.

So, I am wondering whether the sheer web needs to be a box beam, as some have suggested, or some other structure. Secondly, if I make it out of biaxial and unidirectional fiberglass, what type of lamination schedule would be sufficient. I’m thinking about just getting a lot of glass, mostly uni, and making overweight, bulletproof boards. Maybe 50 pounds of glass.

Any and all comments, opinions, and suggestions welcome.

Larry Forgy

 

G'day Larry - Hay cobber - trust you'll have a great 2012.
You'll get lots of very informative answers in here - I'm sure. Many of them might be way to far over my head.
You might care to give some thought to the - pressure created per sq " or per sq ' at the hull exit point as compared to the tip of the blade. There is much pressure added - with little or no gain - due to the shape of the c/b being the same foil shape at the top as at the bottom. Much pressure & thus better efficiency can be gained if the foil is a graduated foil from the hull exit point down to the tip of the foil.

Nice boat, nice pics - looks quick. Quick produces some foil problems. Good section (at the top) - 0012 - might think about - much thinner section c/w max cord at 48% aft - at the bottom. Then it will (ooops - might) sail higher without stalling, not want to twist & not want to 'lead' the top of the boar & unnecessarily load-up the twist pressure at the hull exit point. The turbulation at the hull/board exit point is only a very small fraction of the problem compared to over 1/2 the c/b that's not working efficiently especially at speed.
I'm sure there will be many in here that will not agree however many highly qualified yacht designers have gone to a lot of effort to prove the effectiveness of the above c/b & rudder shape. To each his own. PM me if you wish or just go with the flow (pun intended). I'm not in here to get into an arguement with people, I'm just trying to share & help. Ciao, & have a great day, james

 

Hey Larry,

In asking questions you've given me a head start with a few questions of my own. I'm quite familiar with National Advisory Committee for Aeronautics airfoil profiles as my ultralight uses a NACA 23015 airfoil. It never occurred to me that airfoils could also be used for rudder and keel shapes too.

The Polish yacht plans I have show a simplistic rounding on the leading edge and a taper to the training edge with a consistent thickness. Simple and effective I'm sure, but modern profiles might yield a better performing boat.

Thanks for the head start, I begin reviewing the airfoil profiles you listed.

 

The most important factors in the board design are the planform area and depth. If the boundary layer is turbulent for the whole chord, there is negligible difference in profile drag between section shapes of the same thickness. (You can see that for yourself in XFOIL by setting the transition point to, say, 0.025 chord.) The amount of laminar flow that can be practically obtained in water is debatable. You need to know the operating conditions of the foil to design a laminar flow section that has its low drag range in the lift range you're actually using.

Given your history of breaking boards, I think it's important to go for a somewhat thicker section that will provide more strength and stiffness.

It's possible the board is breaking because it is twisting under load in a way that loads the tip compared to the root. This can be countered by adding 45/45 laminates for more torsional stiffness. Raking the board aft, or applying the 0 deg laminates at an angle that sweeps forward somewhat from root to tip instead of being purely 0 deg, will cause the board to twist as it bends in a direction that will unload the tip and shift the load toward the hull for less bending moment.

 

Your breaking boards were because (obviously) the I beam area at the case exit, high load point failed, from compression/tension and twisting. Once the I beam, went the board was finished. But there should be no problems with a boat of your size, albeit a powerfully rigged one. I've had no foil/dagger problems with a similar sized multihull, although it is much lighter weight than your Macgregor. Why not go back to a combination of old/new technology; forget the hollow boards, they're always suspect, make them solid, the full length I beam (actually becomes a solid box) of the best timber, then heavily reinforce by routing out the hard point, exit area for a metre or so and fill with more carbon; the rest of the board can be a light wood with sealing box weave or diagonals of unidirectional carbon laminates (wood doesn't crush the same as foam, has a memory too, which is useful). Sounds old school ... but it works. Also the weight will be less than you think in comparison to exceedingly heavy laminates of solid carbon over foam. Build one in wood/carbon and one in pure exotics - and test them out - you've got little to lose.
Any of the 12-15% asymmetric foil shapes listed will work fine. Go for elliptical tips, imo, they help reduce twisting moments.

 

G'morn Gary. I still owe you a 1000 apologies, but you're a big boy now & that can wait. "They are 'given' though!!!!

How about he uses - back to back box sections (down the center) of whatever he chooses? AS carbon is - without doubt very stiff - it is also quite brittle. Try using some 'kevlar' in the laminate - maybe? As you've said - wood is good - & highly underated - in todays high tech world - of everyone trying to re-invent the wheel? Are you in agreement with 'drifting' the foil aft at the bottom - in order to decrease the load at the bottom & also stop cavitation of the foil? Ciao, james

 

Airgun, silver aussie cobra, forget about any apologies.
Are you suggesting a carbon box beam with a central carbon wall? - yes, that would work very well too. But I like a solid core, just me. Because you get a box with carbon on all sides anyway, plus that little extra wood memory. I know, I know, a box is only as good as the outside laminates.
The elliptical tip reduces twisting a little and makes a cleaner exit for water flow, reduces vortexes, just imo, but it won't reduce cavitation, if it occurred; that depends on board rake, because ventilation begins up at waterline. Cheers.

 

This is not a new problem, and is basically born out of the same problem as everyone else who has "decided to build a dagger/centre board/keel". Although, to be fair, you have done a lot more work. The problem is, that you have three stages, and you don't know all the inputs. Fluid Dynamics, Load Cases and Structural Design.

Fluid dynamics is the big one for you. You need to know enough about the foil to determine how big to make your main spar. A quick back of the envelope sum for total lift gives:
.5*1025*5^2*1.2*(1.82*0.45) = 12.5 KN (which would lift 1.2 Tonnes if you mounted it horizontally)

That's a pretty significant force, however, as you rightly found out, there are torsional issues as well. So you need to understand how the "pitching moment" of the foil changes with Cl (or AOA). Thankfully, until you get near stall it will tend to try to reduce the angle of attack (nose down pitching moment at positive Cl). Once stalled, it may change sign (ie. nose up pitching moment at positive Cl) and cause you structural problems (divergent flutter). Any change in planform will affect these characteristics, and can be beneficial (or not) to the stresses (though sweep back will increase the beneficial pitching moment, as long as you don't overdo it). You'll have to do some research and logical thinking to decide on what is best for you.

In applying the load cases, you need to think about how the fluid loads are to be applied. They are actually pressure forces which need to be applied to each skin, but you can only apply them as a distributed load and a torsion, for beam theory. Also consider grounding loads, and whether these will be an issue, and what you want to happen if you do ground the boat (do you want sacrificial boards?). Here you should also consider any point loads on the foils: notably the lifting eye. Does it have to lift the foil when it has load on? if so what's the friction?

So, with all this done, you can start on the structural design. And you should now be able to size your spar correctly with suitable spar-caps and web(s). You should be able to design a suitable D-box or other structure to take torsion, and you should be able to get some idea of the required skin thickness (which will be considerably greater at the leading edge). It is at this stage that you should decide on your laminate schedules, but apply the same logic as before. Read around the subject.

Just remember that opinion is not fact. If you are not sure, ask or search for it.

Hope this helps,

Tim B.

 

Actually, that's a bit debatable, probably to the extent that they don't, dependant on planform etc. Don't confuse elliptic loading with elliptic planform. Since this is a dagger board, the planform will be very much dictated by the retracted draught, so, I would expect predominantly parallel with perhaps some slight taper over the last few feet, into a nice parabolic tip would be a reasonable solution. However, that will be more of a "styling" question than anything else in this case. A bit of thought about the tip-shape will gain a little bit of performance though.

You are right about cavitation, though, Gary. Planform will have no effect, only changing the section shape will make a difference. However, since the board will probably ventilate at the waterline before it cavitates I wouldn't worry too much.

Cheers,

Tim B.

 

Good points, Tim. My thing about elliptical tip shape: compare a cantilevered 1.5 metre dagger, say, with a chopped tip, same, there is less area at tip on the elliptic shape to twist? Also, yes, it is far more stylish (and has connotations of Spitfire). Blunt chopping looks kinda crude, even if it has a parabolic end. Gut feeling and eyeballing, (not scientific) the latter is draggy.
The problem of when sailing downwind, lifting an elliptic tip dagger up so the tip is in the case, and which produces a horrible slot just like a centreboard design; the answer is, don't lift it so far, leave the elliptical shaped area exposed, sweet, minimal drag and balanced steering. The only time you lift a board right out is when you beach.
One point about asymmetric foil shape on a cat: that windward one has always to be lifted, (elliptical tip showing of course) otherwise they fight each other - might be better to go symmetrical if you want less crewing work.

 

The moments depend on the axis used. Convention is moment about the quarter chord. For an un-cambered (symmetric section) the quarter-chord moment will be close to zero until stall is approached. For a cambered (assymetric) section at low Mach number or a section in water (essentially zero Mach number) the quarter-chord moment will usually be nose-down and close to constant until stall is approached.

For a location other than the quarter-chord the moment will be different and will vary more strongly with angle of attack. Move the location of the axis rearward and an additional nose up moment component will be added equal to the lift x axis distance aft of quarter-chord. This additional moment will increase with increasing angle of attack since lift will increases with increasing angle of attack. At some angle of attack if the axis is far enough aft the total moment will become nose-up and then continue to increase if the angle of attack increases.

For divergence the moment which matters is the moment about the elastic axis, not the quarter chord moment. The elastic axis is the location where force normal to the foil would cause it to deflect in the normal direct but not twist. The amount of twist will be equal to the moment about the elastic axis divided by the torsional stiffness. If the elastic axis is sufficiently far behind aft of the quarter-chord then an increase in the angle of attack will result in the moment becoming more nose-up which in turn will increase the angle of attack. Whether an equilibrium is reached before structural failure depends on how fast lift increase with angle of attack which is proportional to the speed squared, how far aft of the quarter-chord the elastic axis is (closer to the quarter chord is better) and the torsional stiffness (stiffer is better).

Bottom line - to avoid divergence which can cause structural failure make sure the foil has sufficient torsional stiffness and if possible locate the elastic axis as close to the quarter chord as possible.

 

Hi guys. TomS, TimB & Gary - about Larry's request/problem that we are all trying to assist him in finding a way to start to 'get-it-better'.

Wish I could draw on a computer - but I don't know how to do that. Also wish I still had both the Wing-masted 'B' class (Mystrey) & the twin 'A' classes (My Way & Nine Lives) but I don't DARN !! So here goes - the old-fashioned way - with words.

All of these things we (this team over here in OZ) have learned the hard way & designed, built, & they even worked - golly gosh. Lots of help from Locki, Bob Millar, Peter Cole & Harold Stevenson, Peter Blaxland, Max Press, Bill Hollier, a few 'Poms', a Dutchman & others.

Let's take a c/b for an 'A' class - just as an example. We made a c/b for 'Nine Lives' - Brian Leverton sailed it - into first place in the Australian champs - 1/ Brian was good, 2/ boat was fast, stiff & light, 3/ rig was faster (not now to discuss that) - - Tacked on a dime, higher to windward & faster, faster on a reach & jibed on another dime. - Put boat to bed for the winter (yea I no we don't have one but) Improvements over winter - Reduce area of boards by 50% (along with the rudders (& by 50% as well). -

Spring comes. boat back in water - results of change in area & shape & foil section - - boat faster on all points of sail - tacked faster, very little stall-out (which Brian could see from out-on-the-wire) & then Brian went on to win the world champs.

Let us take a rectangle of say 3' deep x 1' - fore & aft x 1 1/4" to 1 1/2" thick - using a foil something like NASA 0012 - now we then cut off the front of the rectangle by 17* (* = degrees) from the bottom up - & 11* cut of at the trailing edge. The bottom was squared off - - sorry Gary - we'd tried 'Spitfire' tips - for Locke's sake but they proved to not be as effecient - at least to us. They could be seen to be cavitating - from out on the wire. Good place to study foils in water, me thinks. (we used the same place 'out-on-the-wire' to see the boards & rudders on 'Sundancer')

The max thickness of the board - at the hull was aprox 28* back from the front. However at the bottom the board was 6" fore & aft, max thickness was 48* aft of leading edge & 1/2" thick max. The taper was in line with no fancy knuckles or hard-points or bent * - just plain straight-line simple. Observations - other than the obvious - ie. same boat, same sail/mast/boom/sheeting system, same one person on board - same shore crew (me) showed us that the c/b's & rudders didn't cavitate until at 35* - which is just stupid - (if a boat wont tack or manouver at 28* angle max then the entire system wants putting in the bin & start all over again -wrong shape & is way to far 'over' helmed.

I guess some important matters have to be remembered here. 1/ most importantly - this is all IMHO - very much so, 2/ I was in the FRP industry - playing with 1/4, 1/2, 3/4 & 1 ton monos, I was building 'Australis' cats & a few 'Brecken cats, 16' & 18' skiffs, Farr 36's & modifing many yachts to get better results. Oh & occasionally I got to go home - pick-up my boys & take them racing (bit one-minded back then - still), 3/ I was already importing carbon fibre from Dr Fisher in England into OZ - direct from his lab. So the foils were made of imported 'A' grade prime Canadian cedar (I'm an ex-pat Canuck), back to back 1" thick - glued with epoxy, recessed carbon fibre & covered with 25 oz Admirality cloth - up to 3 layers - 4
@ the hull exit point, all in one piece using heat cured, double catalyst system, 68* solids Iso polyester. (OH I forgot - "Wheels" in 'crew.org' said - you don't need to heat cure polyester - that's a stupid waste of time). Our 'Barcol' readings were above 86% (usually around 92%) & they were then left to air cure for 14 days, after being heat-cured @ 80*c for 72 hrs. Never broke a board - c/b's or rudders in 15 years of playing with such thin boards.

Gary, what I was thinking - 2 back to back - box sections out of Canadian Cedar (very long fibers) wrapped with 2 layers 280 grm kevlar in epoxy resing - might have been a reasonable - strong-back - then carbon, kevlar- carbon skins. What you think??

Gee guys - sorry I can't draw on computer - & I guess your all now sorry also. Anyway - I'll go away now & stop bothering everyone with such matters. What with my 85% dyslexia - I hope I didn't make to many mistakes & apologise for taking up so much room & time.

Ciao for now (thank Gawd you'll all be saying) from down-under, james aka 'jj-geri-hat-trick' 10 times regards for a FAB 2012 to everyone. jj

 

Very innaressing, Silver Canuck, you tapered the DAGGERboards (they're not cbs, cobra) fore and aft, but mostly for'ard, to a small square tip - which is a lot different than a chopped equal chord at exit, as at tip, dagger. Yeah, well, foil shape is more art and "that feels nice" - many schools of thought.
Also Canadian cedar sounds very much like Port Orford cedar (Chamaeycyparus Lawsoniana) which is marketed here in Auckland, rated as the best timber for weight/strength. Kevlar is okay in tension but weak on compression - but combined with the cedar/carbon, must have been okay - as from your experience.
Curious about your cavitation with Lock's preferred elliptical tips. More details please. Which Sundancer was that? The one I knew was David Barker's 40 foot cat which was built in (polyester) in Sydney and then sailed home to Takapuna beach, Auckland.

 

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Cut that out.. your posts are great-more-more-more!

 

G'day Doug & Gary - Tnx Doug - I sure don't deserve the comment - but at 72 in Feb - I'll take all I can get - THANKS

OFF TOPIC - cause I want to climb back up the ladder & pick another 40ks of fresh Lychees BUT

Banque Populaire is doing 32.7 with 1349 Miles(kts?) to go WOW that blinken thing can sure - eat up the miles quickly. Ciao, james