Flat keel - what to do?

Discussion in 'Hydrodynamics and Aerodynamics' started by Greg Dunn, Apr 3, 2021.

  1. Greg Dunn
    Joined: Apr 2021
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    Location: Mersea Island, Essex, UK

    Greg Dunn Junior Member

    Hello, I'm a new member, and this is my first post. Although I am reticent to enter a scientific forum with zero background in naval architecture, I am a time-served customer of your product and have empirical experience of hydrodynamics.

    I am seeking the opinion of this community as to what to do with the flat keel section of my boat, a Yachting World Diamond. Should I:

    A. Leave it as is?
    B. Pad it out with closed cell foam using a NACA section?
    C. Buy a stock keel from another keelboat design?
    D. Do something else?

    Background

    I have owned this boat since 2005 and sailed a lot of miles in her, and raced against mixed fleets, so I know the boat well. The YachtingWorld Diamond was designed by Jack Holt in 1960 and is a double chined plywood keelboat of 30' (9.18m) length, but only 6'8" (2.0m) beam, and 4'4" (1.3m) draft, although my old girl has ended up at 4'10". The fin is 2" (50mm) marine ply, and class notes suggests it actually vibrates when twisting as the boat is planing.

    The problem is that the Diamond was designed to be sailed with four crew, two of which were on trapezes, and with the 'torso outside the fence' ban presumably being imposed after 1960, it is impossible to get the optimum crew weight to windward, and this anachronistic design doesn't seem to be allowed for under IRC formulation.

    However, I've digressed already. Whilst the design was revolutionary, and the hard chine has come very much back into fashion, the appendages were not, as there was an over-balanced tiny rudder too close behind a whopping fin keel with a flat section and a box section 500kg lead bulb.

    You can't see it in the pictures, but from the outset, I took the advice of Ian Wright in Brisbane, owner of legendary Saltash 11, and had a spade rudder manufactured by Phil's Foils, and mounted 30 inches further astern, and although broaching is the worst habit of the class design, my Black Diamond has never come close to losing control. Given that she sits 6" lower in the water, this is partly due to the fact her keel is at least 4" deeper than design, and she has a shedload of epoxy weight and outboard well structure, plus my belief that she was over-specced by the yard that built her, Coombes of Bosham, in 1962.

    So, back to the drawing board:

    A. Leaving as is. I have her sailed fifteen years and lifted a goodly amount of silverware, albeit it on what turned out to be a generous local handicap, until IRC royally shafted that (0.917, equivalent to a Sigma 33). However, I have a friend whose opinion I usually trust who reckons an aerofoil section won't make a ha'porth of difference, and possibly even make the old girl slower.

    B. It will be a nightmare replicating the leading edge angle to achieve the 30-35% chord, so I guess my question is whether an imperfect aerofoil shape would be an improvement on the flat board, and a supplementary question, probably the most critical of all, is which section should I use for a design capable of planing with two trapezes that I have never managed to get up on the plane?

    C. I did make one attempt to source a stock keel from a well-known manufacturer of keels, but they seemed reticent to supply 'out of class', so I don't know if there's some patent regulation or similar binding them to designers' rights.

    D. There's always something I haven't thought of, so I ask the community here for wheezes that might improve overall boat speed without the faff of B or the cost of C

    Many thanks in anticipation for your thoughts on this. I appreciate I will garner many, and probably opposing opinions on this, but all input will be appreciated. Apologies for the state of the underwater condition in the pictures, she is as hauled out in 2019, minus a horrible day's preparatory sanding on the starboard size yesterday, which prompted these thoughts......

    Kind regards
    Greg Dunn






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  2. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Welcome to the forum.
    With the aspect and thickness ratio's shown, I think your friend is about right. It looks the LE has a good (i.e. greater the 2:1) ellipse and the trailing edge is suitably tapered, so it will be an awful lot of cost and work for minimal gain to the foil lift coefficient. What you can do that will be 10 times more effective is to fair the very ugly intersections of the foil to the bulb and hull. You will get a significant increase in the L/D ratio, because I'm willing to bet that near on 15-20% of the total drag on the boat (yes, the whole boat) is due to those intersections.
     
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  3. Rumars
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    Rumars Senior Member

    Option D, custom keel. This gentleman has done it, you can see the keel and its construction in the pictures. https://bstubbing.wixsite.com/slipperybeaver
    The keel was designed by Dibley Yacht Design ... The Dibley Advantage http://www.dibleymarine.com/ wich means you can simply ask them how much they want for the plans, making the "custom" into a "off the shelf" business.
    One big advantage is loosing at least 100kg, without loosing rightning moment. The disadvantage is of course additional draft, but if this is critical, a lifting arrangement can be used.
    The actual keel construction is no big deal, and you already have the lead to cast a new bulb. If your boats structure was overspecd maybe you don't even need to reinforce the floors.
     
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  4. jehardiman
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    jehardiman Senior Member

    Yeah, increasing the aspect ratio will improve the lift coefficient, at the cost of draft and and perhaps wetted surface and keel root loads.
    That is the tradeoff. You would have to work the shapes and math to ensure you don't (in order of priority):
    1) decrease the existing stability (i.e. you need to preserve, or hopefully increase, existing BG couple even if B (volume and location) or G (weight and location) moves and/or changes a little)
    (EDIT: Forgot this ... 1.5) change the center of lateral resistance )
    2) increase the existing frontal area
    3) increase the existing surface area
    4) increase root bending moment (both knockdown static and maximum dynamic)
    5) increase draft
    FWIW, I really think they (Dibley) could have done much better with the root and bulb intersections (see IMG_8903). If you are going to spend that much money, time, and effort you should grab the low hanging fruit.
     
  5. Greg Dunn
    Joined: Apr 2021
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    Location: Mersea Island, Essex, UK

    Greg Dunn Junior Member

    Thanks so much for your counsel on this, I'm going to run with it for this year (should have done it years ago). I'm sourcing some 2" arris rail to nail into the joints between hull and fin, and bulb and fin, and then fair with filler, concentrating on maximum efficiency for the leading edge from the forefoot. As another good friend has said, just the self confidence one has done the right thing is worth a quarter of a knot. I'll get back to you later in the season with the outcome of the fairing.
     
  6. Greg Dunn
    Joined: Apr 2021
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    Location: Mersea Island, Essex, UK

    Greg Dunn Junior Member


    That you, Rumars, I hadn't known of the, ahem, Slippery Beaver upgrade, very impressive. I'll get in touch with them to find out how the mods have affected performance. The mast looks about as tall as mine, maybe more, and certainly outside class. This is my long term objective, to get a custom keel, but for this year, I'll settle for making her underwater profile prettier.
     
  7. Greg Dunn
    Joined: Apr 2021
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    Location: Mersea Island, Essex, UK

    Greg Dunn Junior Member

    Thanks for the extra input, Jehardiman, I'll come back to the font before I select my custom option.
     
  8. Rumars
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    Rumars Senior Member

    Forget the arris rail, buy some XPS, glue all over the joint, shape with sandpaper, paint.
     
  9. wet feet
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    wet feet Senior Member

    I think you are heading in the right direction.If you focus on removing blemishes on the leading edge and fairing the sharp corners it will have to be an improvement.The easy way to fillet the corners is an epoxy/microballoon mix and it may take two goes to get a decent surface.Then give the entire bottom a good sanding to get rid of all the brushmarks from the old antifouling.Best done wet for the safety of all around and if you re-coat with something like VC17M it can be sanded to a nice satin sheen before launching and you will definitely have reduced skin friction.
    A new custom keel is quite an undertaking,not only from the aspect of making an accurate casting pattern but also because you have the challenge of getting the bolt positions in the new keel to line up with the holes in the boat.It goes up a notch if you switch to a higher aspect ratio keel since the potentially higher loads would be imposed on a structure for which they might exceed the comfortable limits.At which point said keel and existing hull might go their separate ways.None of which takes any account of the cost of the exercise.
     
  10. Greg Dunn
    Joined: Apr 2021
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    Location: Mersea Island, Essex, UK

    Greg Dunn Junior Member

    Thank you for your reply, Wet Feet, and sorry to come back to you so late after your reply.

    Things have moved on, bottom sanded smooth and awaiting fairing of the keel:hull joint, but a new idea has dawned - CNC cut plywood cheeks to be attached, the aft section to be faired to the line and the leading edge faired to the chosen ellipse - this would provide the benefit of an aerofoil section with the minimum work for (I hope) a reasonable cost.

    So the question is, which aerofoil section should I use for such an easily driven hull? I have tried to read Theory of Wing Sections by Abbott and Von Doenhoff, and am semi-seduced by the 6% series, but have read round the net that I should perhaps be looking above 10%. I also read that designers abandoned the NACA '00' series before the old king died, in favour of 63/64/65 sections, but the IP seems fairly closely guarded.

    So I come back to the forum to seek opinions on which section to go for. If it is 6%, where should the chord be, as between the 00 and 65 NACA sections it seems to vary from 30% to 50%?

    One bit of information that may be germane is that the hull and spars weight is 1440kg/3170lbs (the design weight was 1050kg, but the glass sheathing, wet well and bulkheads added the extra 300kg)
     
  11. wet feet
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    wet feet Senior Member

    I tried to read Abbott and von Doenhoff too and in the end just used it for the coordinates of foil sections,which were not easily obtained in pre-internet days.I went with the 63 series at 6% thickness because that was as thick as I could fit in a centreboard case or alternatively if I went thicker I would have had to sacrifice a lot of lateral area to maintain the proportions.The other advantage I had over your situation was that in a dinghy it is fairly simple to achieve the high quality surface that a laminar flow section needs.I seriously doubt that any boat left afloat will be able to do the same.I know of a number of dry sailed boats that do maintain a very good bottom finish and if you can put up with the inconvenience of constant launching and recovery that may be a way forward.If you dig into the book there is a reference to test foil sections being painted and then having the surfaces rubbed down with 320 grit paper-just imagine that amount of work over the whole keel surface.... There is also the matter of the sailing characteristics as a 63 series won't be performing at it's best immediately after a tack as it needs a little distance for the flow to stabilise.Maybe not a problem in open water with tacks half a mile or more apart,but in a river it does make a difference.

    There is an alternative source of foil information at https://m-selig.ae.illinois.edu/uiuc_lsat/Low-Speed-Airfoil-Data-V1.pdf .It focuses on low speed foil usage and in aeronautical terms that is the range in which boats are operating.It actually makes it a bit difficult to get useful information as the range we are typically in falls off the bottom of the scale.I have used a couple of foil analysis programs as an aid and they do show useful trends,but not numbers that I would be too dependent on. XFLR5 is free and you might enjoy playing with it at no cost and the slightly more friendly Profili costs very little from Stefano Duranti and gives results for several sorts of comparison that can be produced in html format.

    I have doubts that you would easily achieve a totally true foil section although it could be done with a fair bit of work.Symmetry an a blemish free surface will count for a lot.Be aware that some plywood will curl up if you machine large volumes off one side and getting a large block accurately located and soundly bonded to an old surface will be a challenge.Its late April now and by the time all the study and manufacture has been done,it could easily be June.For this year,you might do more sailing if you fair and sand the existing keel and make the alteration a project for next winter.
     
  12. Greg Dunn
    Joined: Apr 2021
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    Location: Mersea Island, Essex, UK

    Greg Dunn Junior Member

    Thanks again, and you have swayed me towards a winter project. The main reason is that I won't launch until early July, in the Hamble, for the RTIR, so she'll go in clean and faired, and with a couple of other regattas later in the year, I should be able to gauge the success of the minor mods. I actually heard today from a former YW Diamond owner who did fit a foil section keel years ago, and the upwind performance was significantly improved. I also intend to experiment with dry sailing Black Diamond this season, as I have a launching trolley and space in a boatyard.

    Regarding the recovery from tacking, I have found with the relatively long fin keel that a wider arc of turn usually gains more ground to windward than a crash tack, as that does seem to kill momentum.
     
  13. Howlandwoodworks
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    Howlandwoodworks Member

  14. Rumars
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    Rumars Senior Member

    If you go the way of fairing with applied cheeks, use XPS foam not plywood. It's cheaper, lighter and machines faster. You can hot wire the form yourself if you like. A light layer of glass (under 200gr) to offer a good base for the fairing and you are good to go.
    I would use a NACA 0012 profile.
     

  15. jehardiman
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    jehardiman Senior Member

    Greg, with the the aspect ratio and sweep you have on that foil, I would not use something as thick as 12%. Not only would it add drag, but it would also add buoyancy low which will reduce the ability to carry sail...all other things being equal. Have a look at this old thread on how to shape narrow keels and root fillets.
    https://www.boatdesign.net/threads/long-cruising-keel-foil-shapes.5280/
     
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