Swing centerboard case design

Discussion in 'Sailboats' started by Jetboy, May 24, 2012.

  1. Jetboy
    Joined: Feb 2012
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    Jetboy Senior Member

    When designing a centerboard case for a swing down board, is there any benefit to having a square hard corner at the aft end of the case?

    I'm considering a design change in the boat I'm building to soften rear end of the centerboard case rather than have it make a sharp corner. It seems as though that should make for much less turbulence. Am I missing something here?

    This is a quicky drawing of what I was considering doing. The near side is my idea, as compared to the back side of a regular square case. It seems like this should considerably reduce drag, but I'm just not sure if it's worth effort to do or if I should try some sort of door mechanism to close behind.

    Keep in mind this is a pleasure boat, NOT a racer, but I do want it to sail well and be reasonably quick.
     

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  2. daiquiri
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    daiquiri Engineering and Design

    It seems a bit over the top to me. If you are worried about drag of the case, I think that rounding the lower aft edge will be sufficient. Even much less streamlining should work well.
    Perhaps this figure from Hoerner's "Fluid Dynamic Drag" might help you appreciate the difference between various configurations:

    Fig 22.gif

    If you look at the first three cases (with no protruding lip - the last three were meant for sheet-metal airplanes), you'll see that the optimum solution has a rear edge beveled down to 8°, and the corresponding difference in Cd is around 28%. I'm leaving you the task of calculating how much it means in terms of full-size drag, and how much would it actually affect the boat's performance.

    Cheers
     
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  3. CutOnce

    CutOnce Previous Member

    In performance dinghies the centerboard slot isn't left as an open rectangular slot. Usually the area surrounding the slot is recessed a millimeter or so, enough for a flange to cover a mylar gasket extending from each side. the mylar gaskets usually overlap slightly, so that the board will protrude with a little gasket material following the opening and closing off the slot. Behind the board the gasket meets and closes off the slot area from the trailing edge of the board to the end of the slot.

    The point is to reduce turbulence and keep the slot opening as closed as possible without impeding the board's ability to move up or down (or side to side in a gybing board's case).

    A quick visit to your local 5o5 fleet (or whatever performance dinghies are around) will be revealing.

    Your centerboard case mods would be completely unnecessary if you incorporated a slot gasket.

    --
    CutOnce
     
  4. Silver Raven
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    Silver Raven Senior Member

    Gooday 'J-B' If I get it right - your questions is; 'will there be any benifit, etc etc' - - answer is; Read what 'cut-once' (measure 6 times) has to say ! ! His suggestion is way easier & I would think much more efficient.

    BUT - your - operating program is ?? - "Keep in mind it is a pleasure boat" - so my question is - to you - What is your Pleasure ??? - - if it is to go quickly & efficiently - then by all means - go for 'Cut-once' overlapped mylar (been using it since 1965 & it works) but it will eventually get barnacles inside & you'll have to clean them off regularly or you'll scratch the c/b badly, increase wetted surface substantially & go slower - potentially. Which option do you wish - it's a personal choice. But if your slow cruising (anything under 8 kts average) then what's the problem ??? Enjoy your decission & your up-coming sailing. Caio, james
     
  5. Petros
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    Petros Senior Member

    none of this would be necessary at all if you switched to a dagger board design. Light, simple, easy to build, better performance, stronger, less of the boat hull structure cut open by the long centerboard slot (so hull is stronger and lighter), and way less drag, fewer parts to make, essentially no moving parts.

    I have owned and built both centerboard and dagger board dingys. Both are prone to damaging the hull on an impact with the board down, so I do not think there is any advantage to the centerboard.

    A dagger board keeps it simple, saves time, saves cost, saves weight, has better performance. What is not to like?
     
  6. Jetboy
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    Jetboy Senior Member

    The problem with the dagger board is that this boat has a cabin. So, I'd need a dagger board trunk that extends up through the roof of the cabin. That means that the trunk has to be something like 4 feet tall, and the dagger board would have to be more like 8 feet long to reach all the way through the cabin and still be liftable. Or I'll need to build some type of cable system to raise and lower it.

    I'm not sure that is any easier, and it cuts into the cabin space a lot more.

    Otherwise I like the dagger board idea.

    I also like the idea of a gasket. Seems easy enough to do.
     
  7. Petros
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    Petros Senior Member

    It does not need to be nearly that long, the dagger board just raises and lowers from inside the cabin. When down it takes up far less room in the cabin than the centerboard. When up you just remove and throw it into the V-berth or onto the seat. It can also be rigged with a simple down haul/up hall set up for raising it from the cockpit, no different than you rig for a centerboard.

    The only issue that in rough weather with the board part way up, sometimes a small amount of water leaks in along side the dagger board. When it is down the dagger board stop will seal it (use a rubber gasket on it), and when removed you can have a cover that folds over the top to keep splash out. Not a big deal.
     
  8. Jetboy
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    Jetboy Senior Member

    I think the trunk would have to be all the way to the top of the cabin. The distance from cabin to bottom of hull is roughly 4'6". So if the board is going to go say 34 inches past the hull, it will need to be around 48" long. Since the trunk will need to be at least 12" high, I think the daggerboard will have to be removed out the top of the cabin.

    That's not the end of the world, just takes up a big space in the cabin to have a trunk running all the way through. It would be easy enough to have a small access lid on the top of the cabin to remove if necessary, but 99% of the time it would just live in the trunk and not be removed. I'd be sure to put a pin in at the top so it can't drop when trailering. I'd also want a positive forced up and down. Probably from lines out the top of the cabin for easy access.

    My other big fear is thin water sailing, but I can probably manage that by being careful. I hate the idea of hitting something without an engineered failure point, but I guess that's something I'd have to live with if I go that route.
     
  9. PAR
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    PAR Yacht Designer/Builder

    I disagree in that both centerboard and daggerboard configurations experience similar damage in strikes. Most centerboards will bounce over a bottom strike, while a dagger board will rock back and usually bash it's trailing edge into the case. You might have some leading edge damage to a centerboard, but it will be considerably less then that of trailing edge mashed into the case. Secondly, a dagger strike brings the boat to a halt, while a strike with a centerboard will slow her, but once raised or the board bounces over, you're good to continue, while the dagger needs to be pried out of it's case or smacked with a hammer, just to free it assuming you're not teeter tottering on it.

    I also agree in that a dagger isn't as suitable except in very small craft, as the centerboard case profile and foot print can be worked into the cabin furniture, with a sealed top, while the vertical dagger case is more inconvenient to accommodation arrangements.

    Granted the dagger has advantages in preformance and foot print in small craft, the disadvantages in this poster's cruiser, suggest the preformance enhancements don't out weigh the disadvantages within the accommodation desires.
     
  10. daiquiri
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    daiquiri Engineering and Design

    Yes, mylar gaskets do have a potential in reducing drag if properly installed. But I have seen so many realizations which are hydrodynamically plain awful, so much that it becomes hard to believe they give any drag reduction at all. For example, the ones which use aluminum plates or rims to fasten the gasket around the slot. A glued gasket would be much better in that sense then, though more difficult to remove when the time comes for the replacement.

    Slot strips are a good compromise, imho, like this product by Hawk Marine: http://www.hawkmarineproducts.com/slot.htm

    I wonder if there is any comparative test around of the boat performance with and without the gasket?
     
  11. PAR
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    PAR Yacht Designer/Builder

    The preformance differences will be measured in fractions of a knot in all but zephyr wind strengths. I've used tapered rubber (picture big windshield wiper blades) strips, let into the lower corner of the slot on some boats. Works well, makes a good seal and keeps water from spitting out the lanyard hole. Mostly this is frivolous stuff, considering the gains provided. If sailing identical boats in very light winds, yep, the covered slot will have a slight advantage, but after this, a measurable difference, but not one that couldn't be made up by trash talking to the other skippers and making them screw up a tack.
     
  12. daiquiri
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    daiquiri Engineering and Design

    ROTFL! :D :D :D
     
  13. Jetboy
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    Jetboy Senior Member

    I've run my current boat - a 17 foot daysailer with a swing keel a Venture 17 - aground many times in one of the lakes I sail on. It's a rocky bottom and I just leave the pin out. It's nice to know that when I hit the rocks I can just crank in the keel a bit and either keep going or turn back for deeper water. It's a slow boat though. Hitting the same rocks at 10kn is going to be more of a problem than at 3kn.


    I like the idea of a slot strip. With the right material it could work nicely - especially if it's replaceable every couple seasons, or every season if necessary. I think I'm going to build a recessed lip for a slot strip and go with a swing centerboard. If I don't like it I can always build a plug and change it to a daggerboard a lot easier than retrofitting a dagger board into a swing centerboard.
     
  14. brian eiland
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    brian eiland Senior Member

    Prindle Cat centerboard arrangement

    You might consider looking up the design as utilized on the Prindle 18-2 and 19 cats. It was a pivoting centerboard with a somewhat wide circular trailing edge such that it filled up its slot whether all way down or partially retracted.

    If you are talking of putting this CB on a ballasted monohull that stays in the water then I would have further suggestions as to how to construct the trunk with a removable UHMWPE liner to fit into a larger rectangular trunk in the vessel that can be cleaned out of marine fouling. I'm working on a similar design for a cruising monohull's daggerboard trunk.
     

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  15. bahala
    Joined: May 2012
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    bahala New Member

    Swing keel

    Hello all you smarter people then me, I have a Vic Carpenter 31' sailboat with a swing keel when i bought the boat third hand the cable for the swing keel was cut off at about 18" and the pipe it was attached to was cut off at a foot. I have had the keel down on the hard and up it works! i can not find were the cable attaches to the swing keel and i can't figure out how to control the raise and lower I have seen a c and c corvette swing keel set up but need some help?
     
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