Methods for daggerboard crashworthiness

Discussion in 'Sailboats' started by sigurd, Jun 20, 2016.

  1. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    A thread for compiling and discussing solutions to safety when grounding a through-hull daggerboard.

    Reasons for having them in the first place (assumptions):
    1: Less induced drag, due to hull endplate and long span.
    2: Loss of lift plus ventilation of surface piercing leeboards.
    3: Shorter unsupported length per wetted length - ties in with
    4: Messy leeboard support is dragging when submersed
    5: Leeboards have a less than perfect hydrodynamic interaction with the hull side.
    6: Centerboards do kick up but don't have enough span without the slit being long and draggy.

    Please contribute with ideas, pics, examples of how to design for surviving crashing a dagger into something. I have a few concepts that have popped up over the years.

    1) Four bar pivot, built and depicted on this page by Reino Urala.

    2) The same could be done by linking the pull-up rope to the rope controlling the rake in an oversize trunk. These days rake controls abound on the J and C foils - never seen one linked to the pull up rope though.

    3) L (as viewed from the side) board: Simple pivot forward of the trunk, tspeer talked about it here. Illustrated in post #2 by HJS

    4) Sacrificial stuffing behind the board, same post as 3).

    5) Swing up with forward simple pivot and rollers, same page as 1)

    6) The 'tooth' concept - pretty much like when a tooth gets knocked out, it can come out of its socket without breaking. It'd be possible to make the trunk like that, but the release function might be compromised when the board was partially retracted.

    7)Build it strong enough (post #3)

    What sparked the thread was this quote from Shuttleworth regarding the Spectrum 42:

    So what kind of system is he on about?
     
  2. HJS
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    HJS Member

    I the attached file is a simple L-shaped variant presented which has been used on my boats up to 13.5 meters in length. The design has proved to be very reliable.

    js
     

    Attached Files:

  3. Steve Clark
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    Steve Clark Charged Particle

    Generally speaking, the grounding case for a daggerboard is the same as a fixed keel. and if the structure is robust enough, there should be no drastic consequences to normal groundings. This means that the hull exit needs to be suitably reinforced with structures which distribute and dissipate the impact loads into the hull, the top of the trunk also needs to be able top withstand the shock of the board head swinging into it.
    The blade itself needs to be strong enough at the trailing edge not to be wasted by minor impacts.
    All of these things are fairly easily achieved, and some shock absorbing structure certainly makes it easier, but I don't believe you have to go to exotic lengths to assure that the hull isn't ruptured during a grounding event. We have done this for a while and I really don't think it is the major problem everyone obsesses about.
    SHC
     
  4. sigurd
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    sigurd Pompuous Pangolin

    Good to know.
    I didn't actually mean survivability as in someone would die when the boat sank, any old flotation in the boat would prevent that, but minimizing damage to everything so as to not ruin the day with repairs and such.
    A replaceable cushion would do that just fine, unless one were grounding very often and got tired of building new ones.
    Or am I to understand that it's no problem to build the board and trunk strong enough that no damage occurs when you run into a rock at 20-40 knots?
    Maybe I'm watching too many cartoons and C-class regattas, but I'm picturing people flying through the air and landing somewhere sharp and then getting the boat in their head.
    Anyways, thanks for the input guys, I'm updating the OP to reflect it.
     
  5. tspeer
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    tspeer Senior Member

    Dick Newick used to build a crush space behind the daggerboard opening. You can see it in the plans for Rogue Wave. The head of the daggerboard is angled and there's a matching angle to the structural aft end of the box that allows the board to rotate through ~15 deg in the event of a collision. The wedge of crushable material between the case and the board opening could be replaced easily.

    Another approach I've seen illustrated was a forward swept box with a tapered board, so the board itself wasn't swept forward. The board had a somewhat low aspect ratio, but it could move up and aft in the event of a collision. Unfortunately, at the moment I can't locate the drawing where I saw it.

    How about crashworthiness for a J or L shaped board? There would be a large torsional load on the board as well as the rearward force.

    Does anyone have any recommendations for shock loads to the board itself?
     
  6. Steve Clark
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    Steve Clark Charged Particle

    Well mass and speed matter. What size boat are we talking about?
    I'm thinking that if you drive into a rock at 10 knots you should expect some damage but should be able to sail the boat home.
    On the ICs and small boats, we usually build slightly oversized trunks which are more or less rectangular. This allows us to change boards and board position without having to rebuild the trunk.
    In this we either install a cassette which fills the difference between the board and the trunk. Sometimes 2we just lightly secure this to the huill with a few screws which will pull out if you bump something hard enough. The board will then be kind of adrift in the trunk, but everything can be shoved back into place, a little G-flex overnight and you are good the next day. Otherwise, you nered to prevent the trailing edge from slicing aft into the hull. My record for this is two chord widths on an A Class catamaran.
    This can be accomplished by using a Delrin or UHMW filler block at the bottom. It fits in the square cornered trunk and has the airfoil profile cut on the inside. The surface area of the aft edge is usually enough to prevent the trailing edge from carving into the hull. You may ding up the trailing edge and the leading edge a lot, but the plastic just grunts and takes it.
    Making dagger boards that can be thoroughly abused is another fun topic, but with care you can build composite trailing edges that you could carry into battle as weapons.
    SHC
     
  7. sigurd
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    sigurd Pompuous Pangolin

    That's a couple of good questions, I'm just going to speculate, maybe someone else will have something more substantial to say.

    "How about crashworthiness for a J or L shaped board? "

    All of the 'swing up daggers' could jam, I imagine. Since it can hook things, maybe it'd be better to swing it out the bottom rather than up?
    The trunk is probably already somewhat strong against being exploded by the twisting board, being built for sideways resistance. Maybe hard to build a cushion for that, one that wouldn't crush under normal side loads.
    The nice thing is that we normally have pretty small rearward force (drag). The less we try to stop it going backwards, the less torque it would make too.


    "Does anyone have any recommendations for shock loads to the board itself?"

    I'd be interested to know. I imagine some people build it so that the leading edge is not structural, but rather foam/wood and glass? something that gives a little and spreads the shock a little?
    With dyneema and a very resilient core (corecell i.e.) you can get springback from small dings to at least near original shape. Don't put it in the outer layer, or everything behind it will be broken while the dyneema looks fine. Some of the high end windsurfing boards are made like that, besides I've done some laminate testing with hammer and sharp objects: The stuff is nigh indestructible - there is no comparison to carbon or glass.

    Going to read the article now, it has Cheers in it!


    Steve Clark, I wouldn't narrow the subject to any specific size, but in my case the biggest boat I have planned to build some time is a 9m proa, which might use a dagger type thing. Foiling in the arctic for instance, with ice and big swimming mammals everywhere - lots of things to hit. But I guess your question goes to whether the boat is light enough to jump out of the way or stop, before serious damage, maybe? About maximum 600kg in that hull, probably, plus any injured polar bears? (j/k)
    Great details about how you build the boxes. Could you maybe say something about how to make a weapon-grade trailing edge?
     
  8. serow
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    serow Junior Member

    It seems to me that the dagger board should break before it destroys the boat on anything other than an out and out racer where weight is critical. This should not be difficult to achieve so discussions about boat damage should not arise.
    Designing a board that has a pivot below hull level and snaps back at a certain load should be possible,like a dagger board/centerboard hybrid. Obviously there will be high stresses and some compromise on streamlining at the hinge. This must have been done. Angling the front so it rides up the obstruction is more likely to work in theory than practice, I should have thought.
     
  9. HJS
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    HJS Member

    The attached picture shows a variant of the L-shaped centerboard. It rotates around + in the lines plan , far beneath the boat. Four boats with this solution has been sailing for over twenty years without having been questioned.

    JS
     

    Attached Files:

  10. sigurd
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    sigurd Pompuous Pangolin

    I disagree. I would much rather repair the boat than build a new foil. My foils should be made by a lot of expensive carbon, and the shape cut by CNC. Whereas the boat shape is much less critical and can be repaired easily in comparison.
     
  11. serow
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    serow Junior Member

    Well each to his own..

    I have a mental picture of you drifting in your life raft clutching a pristine foil, smiling from ear to ear and muttering, 'phew, close shave, nearly lost this'.

    In the distance the last remnants of your stricken craft slowly sink without trace.
     
  12. Petros
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    Petros Senior Member

    I have had a swing up keel do a lot of damage to the trunk even though the down haul held rather than give way first, we were making a tack in a narrow channel and struck a submerged rock, I suspect there was a lot of sideways force on the board and hull.

    I am not sure just swinging a board back in a slot will solve the problem, you are assuming the board will always be pushed straight back. Sailing close hauled there is a lot of side load on the dagger board box and board itself, some side slip and a very large force is transmitted sideways into the hull and box.

    None of the proposed solutions would protect you from that kind of collision. It might help with strikes where the force is straight back, but all other types of impacts will likely still cause extensive damage.

    I like the 4 bar link set up, but you need to have a larger than desirable slot in the hull. I also like the simple crush-able insert. this idea can be used to soften side impacts as well, so that is a simple solution that can work in any type of strike.

    of course just lowering the initial impact force may not save the dagger board if you are caught sideways on a sand bar in a river, but will help with impact loads.
     
  13. sharpii2
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    sharpii2 Senior Member

    Another idea to consider is to have just to trailing edge of the board extent to the top of the trunk. The leading edge would then end at the base of the trunk. The whole top front corner, from the forward base of the trunk to the aft top of the trunk would be missing.

    The top aft corner of this board could attach to a rope or a powerful shock cord, which will reeve over a rounded corer on the aft edge of the trunk, down to the base of the trunk, inside the boat.

    If the board strikes something, the top corner will swing forward, pulling on this rope or shock cord. Stretching this rope or shock cord will diminish the shock load, much like sacrificial crush piece would.

    All of the boats I've ever sailed in have had boards that have been pretty much like after thoughts, with just rounded leading and trailing edges. Some had tapered trailing edges. All of them worked fine.

    If you're going to go to all the trouble of having a near perfect fluid dynamic board, you might as well do the same for the sail, if not the hull too. Otherwise, you may be just doing a lot of work and spending a lot of $$$ for very modest improvements in performance.
     
  14. rob denney
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    rob denney Senior Member

    The best, perhaps only way to make a daggerboard case crashproof on a decent size/speed boat is to eliminate it. This has the added benefit of removing a large amount of the structure required to withstand the crash and the drag from a case which is not a perfect fit, which is most of them, above dinghy size.

    Shock absorbent material is a poor solution. It has to be removed and replaced after each collision and the leading edge of the foil is likely to be damaged.

    A better question, in my opinion, is how to eliminate the case. Lee boards are a solution for multihulls and slab sided monos. The only structure they require is 3 lines to hold them in place. A fairing on the hull or the board and perhaps a fence to stop air entrainment would smooth out the flow. At some stage the weight saving overcomes the surface piercing drawbacks. At all stages, the peace of mind is worth having.

    It will be interesting to see if any boats in the Race to Alaska suffer from damage to their daggerboards in an area renowned for the number of floating logs.
     

  15. gggGuest
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    gggGuest ...

    Well I dunno, I can (and have) pick the foil up, take it home, and rebuild the trailing edge with the aid of some carbon and parcel tape covered ply formers. And do the whole job in the warm and under cover. A hull repair is rather more inconvenient.
     
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