Full Keel Cross Section Question

Discussion in 'Boat Design' started by viking north, Apr 18, 2011.

  1. viking north
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    viking north VINLAND

    I am about to build the male mold for the shell of my 24in. deep full keel. The attachment point on the hull(root) is 9.5 in. wide for most of the hulls length. Many of the full keels i have seen on boats taper in width from top(root) to bottom.(cross section is wider at the top than at the bottom) Correct me if i'm out in left field here but doesn't that contribute to the flow being diverted(sliding) downward as it travels aft resulting in it seperating from the bottom edge of the keel creating a big vortex with it's resultant drag. Would it be better to have no change in width in the cross section or to make any change desired in a short taper up near the root and then maintaining a constant cross sectional width to the bottom.I can't seem to find any technical info on this. Thanks ---Geo.
     
  2. gonzo
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    gonzo Senior Member

    Have you tried an internet search? There is more debates about that subject that you can shake a stick at. I think you should stick to the original design. A full keel means the design is not for high performance anyway.
     
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    Remember the vortex you point at affects as drag abeam. Wider root reduces wetted surface and adds volume into the keel so you can use more ballast.
    BR Teddy

    ps. Marchaj in "Seaworthiness the Forgotten Factor" states (pg 263 fiq 138) that decreased section coeffiency Cx, in this case flat keel cross section, has slightly better roll damping characteristics compared to tapered keel cross section..
     
  4. viking north
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    viking north VINLAND

    Correct you are Gonzo on the many debates.Teddy read Marchaj cover to cover several times and again last night until 3 a.m., Page 55 discusses bilge shapes which affects the keel cross section but no mention of vortex or drag. His other book, Aero-Dynamics of sailing discusses flat plate characteristics, pages 236, 237 but no discussion on a flat plate of varying cross section as found on many full keels.In older soft bilge deep keel boats the cross section changed as a result of the hull shape. I have a med. to a hard bilge and have little to no ability to build in a foil shape(long keel) but the opportunity to control the cross section so exploring the pros and cons. I also have the opportunity to install an end plate to offset crossflow but have to dig up a little more info on how critical it is to have it run parallel with the waterline. While my math level limits complete understanding of the higher technical books ( the equasions can be mind boggling) i pretty well get the general conclusions from the diagrams and associated written discussions. Ok we have one pro-an increase in roll dampening. Have a few more books on order from the library and will check out more on the internet plus the great resorse here, a conclusion will evolve i'm sure.--Tnx. Geo
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    ;) It's day time here..
     
  6. cthippo
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    cthippo Senior Member

    This seems like it would be a fairly simple question to answer empirically. Couldn't you build a model and test the various keel forms and get an answer pretty quickly?
     
  7. viking north
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    viking north VINLAND

    The simpliest way is to give up and pass the job over to my designer but keels have always been my downfall and being stubborn and wanting to learn just good basic info on the subject I thread on. I thought there would have been alot of info on the basic full keel design since it has been around for so long but thus far it is elusive--Maybe it is so simple no one has writen about it, :) All the written word is taken up with the race horses while the lowely plow horse from which they all evolved is forgotten. However re kindled by Gonzo's post i went back into the net and did locate a boat design thread from 2004 which once i transulate into laymans terms looks like it might lead me in the right direction. Thanks -Geo.
     
  8. PAR
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    PAR Yacht Designer/Builder

    If you're that concerned about pressure bleed off, incorporate "Scheel" effects into the lower portion of the appendage. If your appendage if flat on the bottom and parallels the LWL, then you can simply attach a bit of metal to the bottom of the keel, that sticks out an 1" or less. This will help prevent bleed off and a really cheap way to slightly improve the keel's "bite".
     
  9. gonzo
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    gonzo Senior Member

    How much improvement does the Scheel keel have? Does it help only upwind?
     
  10. viking north
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    viking north VINLAND

    Ok, Par my good man you have answered possibly a two part question with one reply. STORY FIRST:On my second last build using a flat sided, vertically tapered , full keel, no cut out (Brewer bite) I was sailing amoung a pod of pilot whales . It was one of those rare days wind but no swell and crystal clear water to about 30ft. My young son leaning over the stern pulpit yells to me the whale is trying to bite the rope. I thinking , what the hell is he talking about, what rope so i looked back aft over the stern and sure enough there is about 30ft of what looks like 2in. samson braid trailing out behind and this pilot whale is snapping at it. Man i'm in trouble now, I've snagged a piece of rope around the prop and if that whale gets a good bite he'll pull the dam shaft clean out of the boat. I still can't believe my eyes so I take a closer look, yup you're right a bloody big trail of vortex bubbles coming off the bottom of the keel, looked like white rope.
    Is this a normal occurance with full keels, (Lightbulb)Oh my, after all these years just thought of something else which in my haste( weakness on keel design) blamed it on the keel but possibly it was really that 16in. three blade prop. Thank you DR. Par. it was good to lay on your couch :) (and I know what you're going to say, most likely it was the prop).
    Ok this babbling crazy man would like to humbly continue, Yes the end plate idea, it's something i've wanted to try for some time. So it should be parallel with the waterline--good i was thinking correct- have to think on that because i might have to ballast her 6 in. deeper in the stern to get cleaner water for the prop and that will most likely throw the whole parallel relationship off. Thanks again-now I think I should buy the beer--Geo.
     
  11. gonzo
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    gonzo Senior Member

    The vortex is something that happens with all keels.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Tip eddies or vortexes happen off the edge of anything, props, skegs, rudders, keels, etc.

    [​IMG]

    This image is off a Compact 35. The concept as Harry envisioned it, was to provide better windward ability on shoal draft vessels. He did this by concentrating the ballast low, in a bulb like protuberance at the bottom of the keel, plus incorporate an end plate. Early versions (early 1960's) show just a modest bulb and end plate as the image above suggests, but all extremes of this concept have been used. Many dozens of production models have used the Scheel keel.

    On your application Viking, you're probably married to the ballast and deadwood configuration you have without costly rework, but the end plate still can be employed.

    You can send the six pack of Bud long necks to my home address . . .
     
  13. viking north
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    viking north VINLAND

    Gonzo- once again i have to thank you for prodding me to search the net which i have done and came up with a gold mine of information which i seemed to have missed previous. All were threads right here on the forum oct. 2004, nov. 2004 and jan. 2006.
    Par you posted on the jan. 2006 thread along with all the other really good inputs. Thanks again all: thru the jigs and the reels the info comes flowing in---Geo.

    P.S. I'll deliver them personally,cold and to be really kind I'll help you drink a few. just to prevent a hangover :)
     
  14. viking north
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    viking north VINLAND

    Par I really would like to go with the end plate but if i have to ballast the stern deeper I end up with what the old timers call drag in the keel configuration. (deeper emerged aft than up front) This will put the end plate out of parallel with the surface of the water and the waterline say 6in. over 24ft(lwl). If this is not critical i'm gonna do it--Geo.

    P.S. Apparently according to the Jan. 2006 thread drag actually improves the flow characteristics of long keels so unknowingly by ballasting her down in the stern i might actually be improving the performance. It's been a good informative day.
     

  15. messabout
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    messabout Senior Member

    VK; A keel with some drag is not the same as a hull that is stern down. Keel drag and hull drag are not equivalents.
     
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