Canting Keel Trunk Within a Trunk Idea

Discussion in 'Sailboats' started by Doug Lord, Sep 28, 2006.

  1. Doug Lord

    Doug Lord Guest

    I'm designing an experimental boat for myself- nearly 18'LOA. The boat will use a canting keel-in reality a canting daggerboard- with about 150lb. of lead at the bottom. The boat will be trailerable so I've come up with this idea: The daggerboard will fit within a pivoting trunk that is, essentially, pinned at the bottom of a larger "V"-shaped trunk. The larger trunk will be an integral part of the boat attached at both the deck and bottom.This will make it as leak proof as humanly possible and also add strength to the structure.At the top the "V" trunk will be wide enough to allow the pivoting trunk+board to angle up to 25-30° to each side. At the bottom it will be slightly wider than the board+pivoting trunk. A gasket will be used to cover this gap to smooth the flow without any regard to sealing the inside of the "V" trunk. The trunk will automatically drain the small amount of water that gets inside as soon as the boat moves.Lines controlling the board +pivoting trunk will run on deck to the other part of the movable ballast system-the parts of which are powered by an electric motor or just manually.There may be a very light cover over this whole assembly. To retract the keel for trailering a block and tackle would be attached to the board,a pin removed and the whole thing pulled up.
    This boat is highly experimental and requires that every ounce carried pays it's way-hence the canting system.The whole canting assembly will be easily removed. The idea of the keel in the first place is to make the boat self righting but it only provides a relatively small percentage of overall righting moment. The "V" trunk seems like a simple and effective way to make this installation.
    Does anyone see any part of this that doesn't sound too well thought out?
    I'd appreciate almost any comments to help me think this out before I lock it in in the final design.
     
  2. usa2
    Joined: Jan 2005
    Posts: 538
    Likes: 1, Points: 0, Legacy Rep: 11
    Location: Maine

    usa2 Senior Member

    It should work. Ive built a 45" radio controlled boat that has almost exactly the same type of canting "V" trunk you describe. The key is have it so you can completely fill it up, and still have the boat floating high. If it has too much volume, it may fill up unexpectedly and swamp the boat. And there is always the risk of submerging it, so having it fillable as part of the design should get around any problems created by that.
     
  3. raw
    Joined: Jul 2006
    Posts: 133
    Likes: 5, Points: 0, Legacy Rep: 59
    Location: Oz

    raw Senior Member

    yep, a canting keel on a small boat will not pay its way in the righing moment department when a human sitting on the rail will do much more. KISS principle would be better.

    Other than that I'm sure the mechanics of it would work. just design for it to be flooded and not compromise the boat.

    When are you starting to build........the most important question that should be answered.
     
  4. Doug Lord

    Doug Lord Guest

    canting trunk

    Thanks 2 and raw. The boat is a "sit in" keelboat with on-deck sliding ballast that develops the major RM. I just didn't want the self righting ballast sitting there doing nothing except in an emergency. This way upwind at a 15° angle of heel it contributes around 470 ft.lb of RM.
    I'm on a waiting list for a shop but I'd say the boat wouldn't be done for a year or so.
    When floating static there is only about a gallon of water that gets in the "V" trunk. The trunk dimensions and the cant angle are limited by the crews legs. One of the reasons for doing it this way was to keep any mechanism/moving parts outside and away from the crew.
    I'll have to look at the effect the "V" trunk will have from a 90° knockdown on righting but I wouldn't think very much.
    Thanks for the comments...
    ======================
    2, did your model drain the "V" trunk when moving? Any pix?
     
    Last edited by a moderator: Sep 28, 2006
  5. bhnautika
    Joined: Feb 2006
    Posts: 852
    Likes: 57, Points: 38, Legacy Rep: 571
    Location: australia

    bhnautika Senior Member

    Doug Is this like your idea. It’s for a canting dagger board, which also had to be removable for the boat to go on a trailer .I have cut away and removed some parts to get a better look.
     

    Attached Files:

  6. Raggi_Thor
    Joined: Jan 2004
    Posts: 2,457
    Likes: 64, Points: 0, Legacy Rep: 711
    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    I think Raw is right, that's why the Backman 21 has a canting keel (and a forward centerboard) with 500lbs in the bulb, while the Backman 18 has a very simple vertical drop keel with 150lbs bulb.

    The problem is that on a small boat with a small bulb, the canting mechanisme, the loss of lift or the addition of an extra forward centerboard (for lift), the extra weight of gear, the extra drag around the keel/bottom flange (or whatever you call it :)), all this costs more than the small benefit from the extra rightening moment.

    It's probaly better to use the extra time and money making the boat as light and slick as possible.
     
  7. Raggi_Thor
    Joined: Jan 2004
    Posts: 2,457
    Likes: 64, Points: 0, Legacy Rep: 711
    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    If you cant the keel you need some additional lift to windward.
    You can add an extra centerboard or fins on the bulb.
    See the pictures from www.backmanboats.com

    On the 18 footer KISS is best :)
    On the 21'er an extra centerboard is worthwile,
    on the 31'er hydralics an extra fins...
     

    Attached Files:

  8. Crag Cay
    Joined: May 2006
    Posts: 643
    Likes: 49, Points: 0, Legacy Rep: 607
    Location: UK

    Crag Cay Senior Member

    Raw is right. Weight on the rail is more effective, especially in very light small boats. But Doug's plan is to design a 'sit in' boat so telling him to 'sit out' is irrelevant.

    The 2.4M class get very miffed when people tell them to dump the lead, flatten their buttocks, get some hiking racks and plane! It's not the game they want to play.

    So try the board in a box. It was frequently used in the early days of canting keels in all sorts of boats. Just bear in mind the buoyancy requirements of the boat when your experiment inevitably exceeds your 'design's stability envelope'.

    [​IMG]
     

    Attached Files:

    • 2.4.jpg
      2.4.jpg
      File size:
      14.4 KB
      Views:
      2,551
  9. Raggi_Thor
    Joined: Jan 2004
    Posts: 2,457
    Likes: 64, Points: 0, Legacy Rep: 711
    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    Yes, but he plans to have on deck moveable ballast as a replacement for crew on the rail.
     
  10. Doug Lord

    Doug Lord Guest

    canting trunk

    Thank you all very much for your comments. The total RM is about 1800ft.lb.'s of which 470 will come from the canting board upwind at a 15° angle of heel. This hull is about 18' long and 1.67' wide at the waterline-zero form stability for all practical purposes so the 26% RM from the keel will make a difference. Almost the same as a 160lb. crew sitting 3' to weather.
    This boat is designed to be able to foil off the wind with the board locked on the cl; upwind the
    foils will provide the extra lateral resistance required by the loss of area when the board cants(with a change in flap angle). There are problems because the board mechanics have to be disengaged from the Ballast Wing off the wind so the board can be locked.I'm willing to put up with some annoyance to make the tests I need to make. This is an experimental boat where three main things are being tested: 1) the practicality of the sliding on-deck ballast /wing system, 2) my pet project of proving that a self righting keelboat CAN foil albeit only off the wind in this case, 3) Is this boat REALLY selfrighting? Also, I imagine that testing this stuff with 171 sq.ft SA upwind and 250 offwind
    singlehanded while sitting down inside the boat will be loads of fun-and maybe a bit scary in a fun sort of way.
    Back to the "V" trunk: it seems to me that this trunk is only slightly more area than a standard trunk(due to the "V"). The trunk that pivots will be all carbon and probably more a frame than an actual trunk-it really doesn't need sides, for instance. And the pivot will be something like a solid carbon rod inside a delrin(?) bushing-very light. I will get a friend who is a naval architect help me engineer that part of the structure and generally review other areas of concern. So for the modest gain in RM on such a narrow boat I'm still leaning toward the canting board assuming that it can be built as lightly as I think it can.
    I appreciate your thoughts on this thing very much!
    -----------------------
    bhnautika-that looks just like what I had in mind except that this trunk will go from the bottom to the deck. Thanks for the illustration! PS- is your trunk designed to drain as the boat moves or to "carry" a small amount of water at all times?
    ----------
    Raggi, thank you for the illustrations as well; I think wings on the bulb make a lot of sense for canting keels-saves having to have another trunk/fixed/rotating fin. I came up with a solution that allows the wings to be retracted off the wind-but not on this boat.I appeciate your effort!
     
  11. Crag Cay
    Joined: May 2006
    Posts: 643
    Likes: 49, Points: 0, Legacy Rep: 607
    Location: UK

    Crag Cay Senior Member

    "....... but he plans to have on deck moveable ballast as a replacement for crew on the rail....."

    Good point, well put. Sorry.
     
  12. Raggi_Thor
    Joined: Jan 2004
    Posts: 2,457
    Likes: 64, Points: 0, Legacy Rep: 711
    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    I waited for that one, "foil" I mean :)
     
  13. Doug Lord

    Doug Lord Guest

    foil

    Hey, I can't help myself! "foil", I mean.....
    --------
    Ragnar, are the foils you showed on a canting keel bulb adjustable while sailing?
     
  14. raw
    Joined: Jul 2006
    Posts: 133
    Likes: 5, Points: 0, Legacy Rep: 59
    Location: Oz

    raw Senior Member

    So now it's not just an 18 ft retractable canter, it's also a sit inboard on deck ballast assisted folier. Shaking my head now....should've known better.
     

  15. bhnautika
    Joined: Feb 2006
    Posts: 852
    Likes: 57, Points: 38, Legacy Rep: 571
    Location: australia

    bhnautika Senior Member

    Doug In this case we expected some water to remain in the dagger board case .The tolerances around the hull and fairing were pretty close so the water would not be sloshing in and out. I did think at the time a little water would stop the case from sucking in air; I never got around to checking.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.