Tips on Dagger Board Trunk Construction?

Discussion in 'Boatbuilding' started by CatBuilder, Dec 21, 2011.

  1. Yobarnacle
    Joined: Nov 2011
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    Yobarnacle Senior Member holding true course

    At first the foam sounded good, but then you couldn't inspect for stress cracks. Maybe building a removable cofferdam around the trunk, tall enough to be above waterline, would accomplish the same, safety wise, in case of a broken trunk.
     
  2. CatBuilder

    CatBuilder Previous Member

    Most of our boats (Kurt Hughes) have 4 separate watertight compartments in each hull:

    1) Bow area
    2) Large locker coming back from bow to forward stateroom (size a combo head/shower room on a typical boat)
    3) Main cabin, running amidships, where board trunk is
    4) Separate engine room

    All 4 of those are behind watertight bulkheads that go keel to deck.

    I don't want to try it, but I'm pretty sure you could cut a hole right in the bottom of the boat and you won't sink - provided you keep the weight off the boat.

    Kurt has a few pictures of his boats with big holes in the hull. One from a jet ski going through one and another from a whale going through.

    These boats (if kept light) are designed to float and sail even when holed.

    The design philosophy here is to do what you can for crashes (impact absorbing foam block aft of dagger board, 16 layers of glass on the trunk, dagger board that can absorb impact itself, etc...) then, keep the weight off so if you do get holed somehow, you can still navigate with some water in the bilge.
     
  3. rberrey
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    rberrey Senior Member

    My plans call for a wear shoe at the bottem of the case. I think it is just a buildup of a microballon mix molded to within 1/8" of the dagger board shape. Rick
     
  4. CatBuilder

    CatBuilder Previous Member

    The only thing about that is you're going to have to make sure your Delrin piece is sufficiently attached to the boat to take the full load of the boat's sails all loaded up (in leeway prevention).

    The bottom (as designed) of each trunk is tapered so the core goes away. There is a 3/4" ply (or foam) doubler then attached to the hull inside, as well as another couple layers of biaxial pad (inside and out).

    All this is held in with layer upon layer (10 layers) of biax around the hole, solid glass around the hole, glassed cove joints inside, several laminations of triaxial on the lower, aft part of the trunk (behind the crash block), etc...

    Your Delrin has to take the same loads that all of that glass takes, as the board is point loaded where it leaves the hull.

    It can be done, but it's got to be very strong in order to keep from having to replace it.

    Here's a picture of what happens when a board impacts a trunk - in this case a Maine Cat that doesn't even have crash blocks... just solid laminate to hit...
     

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  5. rberrey
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    rberrey Senior Member

    I also plan to use a 4 lb pour foam in the lower portion of my boards and lighter layup. A 16 lb foam and heaver layup in the upper section of my boards. I would reather have my boards shear if they dont pop up than break my case. If the case breaks then it should break near the hull connection. Beef up the connection , keep the board close fitted to the case, build in a top strut for the case to keep the top of the dagger board from going aft durring impact, and let the board break. Rick
     
  6. Yobarnacle
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    Yobarnacle Senior Member holding true course

    Sounds like you and the designer have thought it through pretty well. Scale is an important consideration. That's a big boat. I don't know what ratio inertia scales up at. But inertia gets huge as boats get larger. An example: A super tanker starts slowing down 3 days before arriving destination. During these three days she is over running her revs. By that i mean the ship is traveling faster than the propeller would push it.
    Lightness is important because it reduces inertial forces in collision. At the same time, overbuilt might be good.
     
  7. CatBuilder

    CatBuilder Previous Member

    That's a good plan, Rick, since the board's load goes up proportionally the more you deploy it.

    If you have an inch of board out, your lighter laminate (which will be good for absorbing crashes) will be plenty to support that tiny force.

    As you start putting more board out, that's when you need a heavier laminate.

    Makes good sense to me.
     
  8. CatBuilder

    CatBuilder Previous Member

    Yeah, this guy strikes a balance between the two. I can clearly see you have plenty of sea time. You'd be impressed with the plans. All the stuff you normally worry about, he addresses in the plans.

    I chose this designer because I already knew what boat I needed (features wise) and didn't want to try to design my own. His designs were as close to my ideal as I could find. I'm only changing a few cosmetic and layout items from the design and will come out with what I consider the perfect catamaran.

    He strikes a balance between lightness (very important for these boats in general) and well built. He does this by paying very close attention to both weight and fiber orientation.

    My boat, the one you saw being built in this thread, has a weight, if built correctly (no adding extra crap) is 11,900lbs (6 tons). I can load it up to a full waterline displacement of 18,292lbs (9 tons). For a proven ocean crosser (sister ships cruise the world), she's fairly light.

    However, inertia (mass x velocity) does scale up on this boat since she does 20 knots under sail in a good gust. A little more V to make up for the missing M.

    There are also kick up rudders in case you whack one of those on something. They just pop up instead of breaking or bending the shaft. Add outboards to this (my personal setup) and you have yourself a very reliable underwater profile, safe from most bumps and bruises.

    Ok, time to shut up. :D Going on too much about how much I like the design. :)
     
  9. Charly
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    Charly Senior Member

    Hi Rick, How will you fair it into the inside of the trunk? will you be able to get a squeege down in there through an access port, or do you build up the shoe first and then install it?
     
  10. Steve W
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    Steve W Senior Member

    I had a Macgregor 36 cat and the daggerboard trunk(single board in the port hull) was just solid glass (no core)and quite light,but the bottom was massively tabbed in and no provision made for any kind of crush zone, it just relied on strong construction and it indeed showed no damage at the aft end at all, the top of the board was pushed about 3ft down into the trunk when sailing so that the board could be shorter (lighter) and so on impact the load was taken by the back of the trunk/hull interface and the top by the berth top/trunk front interface.
    Steve.
     
  11. rberrey
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    rberrey Senior Member

    I,ll look at it a little closer Charly after Christmas, I,ll pull my plans out. I had a bit of out patient surgery that turned inpatient, so I,m not quite up and about yet. Rick
     
  12. Yobarnacle
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    Yobarnacle Senior Member holding true course

    Seatime? A lifetime of it. It's approaching renewal time of my masters ticket again. The new one will be issue 8, good for 5 more years. The question is, will I be good for yet 5 more years?
     
  13. CatBuilder

    CatBuilder Previous Member

    Ah... just renewed my 100T Masters this year as well. Watch out when you go in - aside from the TWIC stuff, you get a little passport looking thing instead of the paper now. Things sure are changing.

    You have about double the time I have, since I'm a younger guy. Sounds like I was getting born about the time you got your first issue. :D

    I could tell though. You were asking all the right questions and thinking about all the right stuff.
     
  14. Yobarnacle
    Joined: Nov 2011
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    Yobarnacle Senior Member holding true course

    If you want to check out my project, the thread Alternative Motorsailer Rig, has a few photos and some concept drawings. She's 40 years old but sound hull
     

  15. Charly
    Joined: Dec 2009
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    Charly Senior Member

    Well I hope your Christmas is merry and bright, and here's to a speedy recovery.
     
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