Pivoted Daggerboard

Discussion in 'Sailboats' started by ancient kayaker, Apr 3, 2012.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    Has anyone tried this method of retracting a daggerboard? The board retracts neatly under the foredeck, does not interfere with the main boom or vang, and allows the daggerboard trunk (thick line) to have a closed top that might accommodate a rowing seat. If held by a bungie or similar arrangement, if it struck an underwater obstacle the board would retract without damage.

    The slot is longer than a daggerboard normally requires but is still shorter than a centerboard. The retraction linkage would have to be developed, but should be no more of a challenge than a typical centerboard.
     

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  2. fcfc
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    fcfc Senior Member

    Something similar has already been done. (not exactly a daggerboard, but a curved keel). Boat name Mopelia (about 26ft). The keel does not pivot. Just lift in curved tracks.
     

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  3. messabout
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    messabout Senior Member

    Always thinking, eh Terry? Keep up the good work. I think that Robb White did something like this on some of his boats. I do not remember the details, except that his board was curved like a scimitar.

    The scheme that you have drawn offers some trim possibilities but for a small boat it hardly seems worth the complication....and weight ...and longer, spitting, slot unless the board was curved in which case the housing would need to be higher. Whites' board housings had open tops. I suspect that it would need some rollers or something to help with the retract, especially when it encountered an underwater obstruction. We need not dismiss the idea, just yet, because it may have some other,yet to be discovered, merit that is worth the trouble.
     
  4. Petros
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    Petros Senior Member

    how is that going to work with the mast step location? looks like the risk of something jamming or fowling is going to be high, but it is an interesting idea. Might it be possible to make the slot smaller by having it raise straight up a bit first, and than slide forward?

    Personally I like KISS, and will stick with a simple dagger board. I have owned both dagger board and swing keel/centerboard sailboats, I see no advantage to the swing keel.
     
  5. kvsgkvng
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    kvsgkvng Senior Member

    Great idea but needs more detailing

    I think the idea is great, but as shown, it will damage the trunk as the daggerboard is more stiff than the trunk plate. I attached self-explanatory sketch to illustrate it. Perhaps providing rounded ends for both parts, the trunk cover and the daggerboard, this situation could be avoided.
     

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  6. kvsgkvng
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    kvsgkvng Senior Member

    The schematic of this yacht has the same vulnerable feature which I highlighted in my previous post. This keel will be damaged very badly, if it hits hard ground. As the retraction and storage scheme it is very well designed.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    FCFC - that’s neat. I thought it might have been done before.

    Gene - Good points, what might be worse is getting the daggerboard out if something goes wrong. Needs a foredeck hatch. Or it can drop through the bottom, but we can guess where that would lead, especially as it would probably need a bit of ballast to work smoothly.

    Petros - Unless you have to have symmetry the mast step can go one side of center and the trunk on the other.

    Kvsgkvng - This daggerboard can pivot forward on hitting something, so I don’t expect damage where you have indicated, although I agree the corners should be rounded.



    Thanks for the comments folks! A lot of the Ontario Highland lakes are full of boulders lurking just below the water, not close enough to affect the waves but shoal enough to catch a board, and not often buoyed. The most irritating thing about a daggerboard for me is interfering with the vang with the board mostly raised for downwind, it makes it difficult to quickly stow the rig for rowing under bridges, and then there’s the water splashing up the slot when rowing. But on the other hand it’s simple and easy to build .
     
  8. PAR
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    PAR Yacht Designer/Builder

    Daggerboards commonly have this bottom strike issue. My dagger boards design "relieve" the trailing edge, so it'll kick back up into the case to a degree (see the Dace plans Terry). It's also common to reinforce the trailing edge for this eventuality.

    The sketch above shows a healthy slot forward of the deployed board, which can't have particularly good benefits on flow at it's root.

    How about a pneumatic board?
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    - ? Inflatable?
     
  10. PAR
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    PAR Yacht Designer/Builder

    Many years ago, I devised a pneumatic daggerboard. It fit in a very small case, below a thwart, with a sealed lid. The actual board was made from 1/8" plywood "hoops", several of which nested inside each other. The foilish shaped hoops where attached to each other with a rubber sleeve, which proved a real pain to vulcanize closed. It functioned with a foot pump and a Schrader valve. The smallest section would pop out, dragging the next, etc. until deployed, which took a fair bit of foot stomping, but I was young then.

    It was a pretty Mickey Mouse setup, but did work to a degree. Leaks were a constant problem. If I had to do it again, I'd use a different approach, but it could be a possibility. In recent years I have made pneumatic sail battens that work well and I think my engineering skills could handle the assorted issues of a pneumatic daggerboard now. Some thing to think about. Maybe a water ballast arrangement, using a telescoping daggerboard as it filled? Come on, get inventive . . .
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Ah, ha! Now I understand. To quote ALF (the Alien Life Form in the 70's TV program) when he was examining a human flush toilet, "interesting concept" :) . . . reminds me of the sectional bronze centerboards that were quite popular many decades ago.

    Teasing aside, I must confess I had a similar idea for a pneumatically deployed telescopic wingsail but I didn't get as far as building one.

    I think the telescopic daggerboard idea has merit, retracting can be as simple as a cord to raise it. Instead of air pressure, deployment could use a simple weight, if the device flooded to eliminate buoyancy. Telescopic pneumatic actuators are also available that would be better sealed than a non-circular foil could possibly be, but as you discovered providing the compressed air source would be a challenge in a sailboat. Of course there's really no need to release the pressure, a small air reservoir could be used and the lifting cord could simply push the air in the cylinder back into the reservoir when retracting. From my own engineering experience I know that all these pneumatic components are available, but they're not cheap, nor are they particularly light.

    It's a pity you didn't have an opportunity to take a video of the experiment. The "sound effects" from the underwater air leaks must have been, to say the least, evocative of those bathtub moments we have all had at one time or other :eek:
     
  12. gggGuest
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    gggGuest ...

    It was widely used on the Cherub class in the UK back in the late 60s and early 70s. It required the use of deck stepped masts and a doubled mast support under the mast so that the foil could run through. None have been built for decades - replaced by conventional daggerboards.
     
  13. kvsgkvng
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    kvsgkvng Senior Member

    yet another folding daggerboard

    I was somewhat captivated with the idea and here is my take on a folding daggerboard. It should work. So far I couldn't find a drawback except that the thickness of the gaggerboard will be still under the hull. To minimize this effect, the daggerboard should be as narrow as practical. To reduce the footpring a small niche could be carved along the center of the hull (just a thought). The latch mechanism below the hull could be shrouded to minimize any drag. This daggerboard could be fitted inside any existing trunk without any modifications.
     

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  14. gggGuest
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    gggGuest ...

    If I understand your drawing correctly I fear that your pivot plate will need to be made out of unobtanium rather than stainless steel in order to get sufficient resistance to bending.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Not to mention it's basically a shoal fin with a board, possably tripling the draft of the vessel, board up, compared to a dagger or centerboard fully retracted. It also seems a pretty convoluted and complicated contrivance, when a simple pivot bolt would do the same thing.
     
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