12ft skiff dinghy centreboard discussion

Discussion in 'Hydrodynamics and Aerodynamics' started by Ben G, May 1, 2014.

  1. Mikko Brummer
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    Mikko Brummer Senior Member

    About the 1.9 multiplier for aspect ratio (the mirror effect of the hull & sea surface): I think that could be a bit optimistic. From my 470 example earlier I get, at a water speed of 2,8 m/s and leeway angle 5,0 deg:

    - centerboard lift 415 N
    - board area 0,380 sqm
    - board depth under the bottom (span) 0,935 m

    I'll let you do the math yourself, but I doubt that the multiplier will be as large as 1,9. And these numbers are without the free surface effect, I've yet to do those runs... I fear the efficiency will be even less then.
     
  2. Mikko Brummer
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    Mikko Brummer Senior Member

    Amendment to the last post: the actual area of the board under the bottom of the boat is 0,338 sqm, not 0,38 sqm - I took a part inside the trunk into my first number.
     
  3. Ben G
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    Ben G Junior Member

    Hmm, that gives multiplier of 1.23?!
    According to CL = 0.1*alpha/(1+2/AR); AR(eff) = 3.18 but actual AR (span^2 /A) is 2.58; CL = 0.307
     
  4. BlunderBus
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    BlunderBus Junior Member

    Planforms

    Hi Ben,
    I can't remember where I picked this up from but the article investigates plane form and heads in the direction of the same of the natural birds wing (like a hawk)

    Gliders like the Discus and DuoDiscus use something similar.
    Might a bit tricky with getting it in and out of a dagger board style centre case though.
    Food for thought anyhow.
    Hamish
     

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  5. Mikko Brummer
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    Mikko Brummer Senior Member

    Yes. That's very much in line with my 470 VPP, which uses a completely different (simple) method for side force calculation.
     
  6. SukiSolo
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    SukiSolo Senior Member

    Personally I quite like thinner chord sections, around 9 or 10 %. Maybe that is just me. I have played around with sections a reasonable ammount on a few designs of 12 through 14' boats. C/board or dagger board firstly needs enough lateral area for light air. Aspect ratio here seems to count. Your own craft is in the higher performance spectrum of which I have less first hand eexperience.

    By the time you hit Force 5 you can almost halve the lateral area and still go well upwind - at least that is my experience/impression. johnhazel is right, build a section close to that which you think is 'optimum' and be prepared to play.
    My own experimentation occurred in 12 and 14' classes sommewhat slower then 12' skifs. Nonetheless the requirement to work at slow (sub 2Kn speed) to average of plus 15Kn + on reaches was significant. I ended up using a timber cored foil and adjusting the chord centre a percent or two over a period of a couple of years until I could not improve. As it happens the section was a modified 63 type section. Absolutely critical is keeping the centre 'fatness' of chord correct as percentage and also tapering the foil downwarsd to get elasticity. I do not have reliable data for elasticity, but hey a mast yields as per gust so should the foil. I taper my own foils. All I can say is you know when they seem to equal the best in (my particular class) in the UK. Leading edge parabola MUST be consistent too, even if you change it.

    If I were you Ben, I would start around the 9 to 10 % chord thickness and play with a timber cored foi,l with taper and percentage. Your 12' skiffs go pretty quick upwind, so if continous plane my own inclination would be around 36.5% max chord thickness, along with variation on NACA 63 series section. Only real way is to try it! If you build for a little playing so much the better. I have ended up fattening the aft sections a little more than might be expected, but what works in the real world is often slightly and subtelty different from 'theory'! 505s' use around 38% aft so that might work for you.

    johnhazel is right - take your best guess and go for it but be prepared to be flexible. Ideally if you race against the same guys you have guage. The gains are small, upwind in my experience, a boat length or two at best. However that is enough to determine a race. I cannot measure downwind, partly because the hull itself has a sweet spot when it can outperform by 0,2 to 0.5 Kn in certain set conditions. My current sail is a little too slack on the leech for optimum in max boatspeed conditions but not far out. Rounding second in a 115 boat fleet on first beat in my 'amateur' boat...not all accident.

    Go for it, good thinking deserves success.
     
  7. Cheesy
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    Cheesy Senior Member

    hopefully you dont mind me polluting your thread too much but it is mostly very applicable to what I am trying to do... Build a new centreboard and rudder.

    Anyway I have run into a few problems, after looking through
    http://airfoiltools.com/compare/index
    It looks very much like there is almost no difference in the "good" turbulent sections around 10%

    The problem I have come up against now is that the class I am making the boards for is a Sunburst, which is an old (slow) design (plywood hard chine) 3.5m two person hiking boat with a pretty modest sail plan.

    The problem I have is that our combined mass is somewhat more than ideal for the class, requiring 549N board force as opposed to the min crew mass of around 384N (again board force). The board area is restricted to 0.294m2 (in a 0.3 * 0.98m box) giving a Cl of around 0.7 compared to 0.49...... Maybe this is why the ideal crew mass is what it is.

    Now a rectangular board is not going to be the best drag wise but will the increased drag from the board be out weighed by reducing the lee way angle (and assumed reduction in drag) of the hull by X degrees?

    Would I also be right in thinking that the larger cord at the tip would reduce the possibility of tip stall?

    Finally to what degree can the rudder be loaded if at all?
     
  8. SukiSolo
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    SukiSolo Senior Member

    Re your Sunburst. I once sailed a Nat 12 which pointed but went sideways. Originally designed for daggerboard this one had a conventional case. The problem was lack of area. By changing the elliptical board to a new near tapered rectanglular shape which I built, an extra 100 X 100mm was added. It was the only way to get away with using the existing case. Had I had the tools at the time to change the case as well it would have been even greater in area and a better planform.

    It transformed upwind performance with no perceptible offwind, though there must have been a bit more drag. With that Sunburst being a Mini GP14 you would most likely be best to maximise the foil plan. If you taper the foil vertically you should be able to minimise drag when raised offwind, if it pivots.
     
  9. Mikko Brummer
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    Mikko Brummer Senior Member

    At the risk of digressing a little from Ben's original thread, here's a representation of the effective depth for a racing-cruising type of keelboat. This is from a thesis on VPPs, based on the Delft/DHSYS method for predicting the effective span (depth, draft) of the keel.

    As you can see, the efficiency depends a lot on Fn (hull wavemaking), but also on heel. At zero heel, the effective draft never reaches 1, which would be the physical draft of the boat. It's at its best at very slow speeds (when the free surface effects are small), and worse at high speeds, as one could expect. At very slow speeds, and at a moderate heel angle of about 15 degrees the effective draft is at its most, nearly 1,7 times the physical draft. Why heel influences so rapidly so positively the efficiency of the keel beats me - this would be an interesting study for simulation, to gain more understanding. Probably it has to do with the hull generating side force in unison with the keel, when heeled a little.

    These numbers are naturally only for this type of boat, and they are not measured figures, but rather extrapolation from Delft tests (with too small models?).
     

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  10. SukiSolo
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    SukiSolo Senior Member

    Maybe the keel is generating a vertical lift element despite being symmetrical section?
     
  11. Ben G
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    Ben G Junior Member

    All good.. this got me thinking. The leeway angle probably doesn't alter the lift-drag of the board much until it's highly loaded.. The induced drag is dependent on the effective span.. fattening the tip will move the downwash deeper into the water and so the effective span will be slightly higher than an elliptical planform.
    But the main effect of fattening the tip will be unloading the board at the root (near the hull). So you'll have less leeway..
    A benefit of unloading the board at the root is it won't create as much wave drag.
    It would be interesting to compare (Miko?!) the drag of the hull + foil with a rectangular planform fin to a hull with an elliptical fin. Due to less wave-making, I bet there would be less of a differnce than lifting line theory would suggest :)

    -A larger chord tip will be less likely to stall, yes.

    -Also, I find loading up the rudder can work well on the 12'. Gives more height.. for a variety of reasons. Some say to keep raking your mast until you can no longer tack.. then straighten just a little!
    As the main de-powers in heavy wind, the jib loads up more and starts to neutralise the helm- so for us it pays to set the boat up with weather helm in moderate breeze.

    Another important point I've been consideering, is that people consider the design of the board to suit steady state upwind conditions. Actually you're better off designing the board to get you away from the start line first! If you can win the start you can control the race with only equal speed.
    So if my board work well at low speed, high lift, I can slow the boat down more while maintaining control, keep in the front row, then accelerate fast.
    If my board is poor at low speed, you have to come in late and fast to the start, ie pot luck with higher possibilities of a second row start.
    This could be a major difference. The handling characteristics of a 12 probably amplify this compared to many boats, though.

     
  12. SukiSolo
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    SukiSolo Senior Member

    Ben G can you actually measure any leeway?. A lot of deep foiled boats seem to have a minimal ammount of it. I have tried to guage it on some classes by tracking them with a safety boat, using sight lines and other ways of visually guaging it. Very hard with some boats to see any at all. Others with short small foils go sideways like crazy, try a Cadet.... Even asked some guys with gps if they could measure it (Int Moths, and Merlins) and no they could not, maybe not the right tool tough.

    Re your skiff board, again a little more gut but also supported by own experiment is to have the front leading edge parabola a little fatter than maybe full optimum. That seems to help establish flow very fast on take off from sitting on the start line. Although generally the foils I've built will only go to max displacement (some will just plane) upwind, that appears from my empirical testing to make the most difference. It seems to be a little borne out by the relative success of quite blunt front sections like the 0012 - too blunt for your skiff. So that is pretty much where I've got to so far. Yet to play with a gybing board in terms of messing with shape (only built one) but might go there again, but very small angle of attack.

    As it is not too hard to change the front parabola in terms of hours, I would be prepared to build a foil and play with it later if it does not perform. It's only going to be a bit of epoxy and balloons, maybe a little glass. I frequently repair wooden cored glassed foils (a lot of well made but damaged Milanes ones) and often plane off the leading 10mm replace with a hard timber (say sapele) re epoxy and re glass in. They blend in fine and polish up well. Making a leading edge female template for the sandpaper is the key to keeping good symmetry on the leading edge, occassionally a profile guage.

    Re increasing the Sunburst foil. The extra area makes a bigger real world difference than any minor extra drag. The less leeway from bigger foil automatically reduces drag. Better to have slightly too large a foil than a too small a one, ideally a smaller daggerboard when really windy, but pivoting boards work by raking aft and moving the clr.
     
  13. wet feet
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    wet feet Senior Member


    Frank Bethwaite made the same observation in his book.I suppose we shouldn't be too surprised that classes which are limited to a section based on a sheet of ply,cut to a planform and then bevelled for a very limited distance all round are less efficient upwind.
     
  14. Ben G
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    Ben G Junior Member

    I had a look through this paper earlier (see post 11) - importantly the root chord is constrained by the program inputs, and the final root chord is on this boundary, resulting in the planform becoming 'pinched' at the root. Imo, the output looks pretty - as to how much it's influenced by the program inputs, I've no idea.

    Sukisolo - I have observed a lot of leeway in some conditions. In light conditions (with our big rig) we cannot pinch. We have a long bowsprit for that rig (4m) and the drips into the water, off the end of it, pass under the wings by the time it's amidships. Angle works out to be 6-7 degrees. Not necessarily slow but it occasionally limits tactical options.

     

  15. SukiSolo
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    SukiSolo Senior Member

    Ben G, can anything pinch in really light air? It seems that there is stalling around the mast/sail as much as below the water in that initial very low speed mode. However in the bottom end Force 2 range say 4-5 mph wind speed you should be able to as quite a few classes can - all in displacement mode. Often on the sea, waves or swell make you steer up and around and the sheets are following to maintain speed a case of free-er is much faster.

    It is possible that for your big rig the foil is too small, like my Nat 12 example before, that may have been theoretically OK, but in the real world needed more area. I note that the 505s' have different area boards, just seems to bear out the theory and practice of the faster you go the smaller the foil area.

    I am sometimes a little wary of just going deeper to get a higher aspect ratio, it can be overdone. Right at the bottom end of the speed scale, those Access 2.3 dinghies have rudders that are too short front to back IMHO. Also had this on a 24' Sportboat (planes well), even the naval architect said so but the client 'had to have it'. You can stall the rudder on sharp turns, too easily.
    I would estimate a good Nat 12 would have a jib sheet angle of 7 degrees and a leeway angle of zero (ie unmeasurable) to one or two degrees in 3 to 5 mph wind. C/board tip will be 1200mm under the keel and the rudder is of course contributing. Yes, a slow boat compared to your skiff but very efficient for its small size and sail area. Generally they don't use gybing boards.

    Must say I'd like a ride on your skiff, looks pretty good!. Closest I've tried so far is an RS100 and that was quite quick and fun.

    If you can splice on say another 100mm depth to your existing light air board you could try it out. The best thing about Bethwaites book (HPS) was do not be afraid to try things out. Certainly do the 'work' ie research etc but don't be frightened - it is all a learning process. How else do we find out? You've designed and built your own very fine skiff and that is brilliant, don't let a little foil experiment get in the way! I'm still optimising a hull from about 3 years ago, more rig stuff than anything else, so it can take time to work through it.
     
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