Inboard leeboards?

Discussion in 'Boat Design' started by W9GFO, Jan 23, 2017.

  1. W9GFO
    Joined: Dec 2014
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    W9GFO Senior Member

    I'm building a 20ft 2/3 person boat that will occasionally need a centerboard/daggerboard/leeboard. Most of the time it will be rowed.

    A daggerboard would be simplest but I think I much prefer something that will pivot out of the way when it strikes something. So that leaves me with a centerboard that would require a long slot along the bottom which, most of the time, will be a source of drag when it is least welcome.

    Lee boards would work, but frankly, I don't like them. However, if they are slightly inboard with the slots just above the waterline then it seems I can get all the benefits that I am after.

    I doubt that this is a novel idea, so can anyone think of some examples where this has been used before? Other constructive comments are welcome too.
  2. alan craig
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    alan craig Senior Member

    Your inboard leeboards will need a slot to pivot into to work as required, and that slot will probably be immersed as the boat heels when sailing. I can see some advantages: each in/leeboard can be profiled to be more efficient on one tack and can even be given a few degrees angle of attack, like a leeboard or a wing, so could be more efficient than a centreboard. The leeboard cases could be built into the front of a seat or bouyancy tank and leave cockpit space free for camping etc.

    The downside is it's twice as much work as a centreboard, and if the boat doesn't heel enough to immerse the upper end of the leeboard, to allow the hull to act as an end plate, any efficiency gains will be lost. And you have to raise/lower each leeboard every time you tack.
  3. tom28571
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    tom28571 Senior Member

    Many of us have had daggerboards and bilgeboards (in daggerboard form) on boats for a long time and have not had serious problems with hitting things. Sure, it happens but the results are not disastrous unless the operator makes a minor inconvenience into a serious event. The simplicity of a daggerboard along with minor perormance advantage makes them attractive.

    While I usually prefer a centerboard for shallow water, in small dinghys, a simple daggerboard is a good choice.
  4. Rurudyne
    Joined: Mar 2014
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    Rurudyne Senior Member

    Some time ago I came across an idea for centerboard construction that may interest you.

    The goal was to produce a centerboard with a lower height profile in the hull, which may help with your "inboard leeboards" by allowing them to be concealed under cabinetry etc and take up less space in the hull.

    Normally centerboards pivot on a pin located near the bottom of their case. The idea I came across was to instead create a virtual pivot point below the hull by using a semicircular slot and pins about that virtual pivot point.

    Though more complex, this seemed like a potential way to reduce the size of the centerboard case.

    My own interest in the idea is more for adaptions to help prevent a very shallow draft cruiser being blown about by the wind than for sailing craft, using a single off-center centerboard under galley countertop.
  5. W9GFO
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    W9GFO Senior Member

    I don't mind the extra drag when under wind power, the boat will be pulled by a single point - heeling moment will be negligible and could be overcome by the crew shifting position. I expect that the windward board would be the favorable one for trimming purposes. So the hull will act as an endplate on the high pressure side of the board - but the low pressure side would probably be easily aerated. It's not ideal but I think adding a few extra inches of length should make up for that loss. Is that a reasonable assumption?

    The only boat I have sailed with a daggerboard is a Sunfish, decades ago and I was frequently creating furrows in the bottom. This boat will need to be easily beached and I would prefer to be able to use it in the shallows without worrying about breakage. Area of operation will be the inside passage to Alaska.

    I do like the simplicity of a daggerboard but there is an additional problem with it. Due to the design of the boat the daggerboard would have to be mounted quite far forward - right through the center of the already small cabin. That's because it will be pulled by a single point, the "sail" introduce zero torque to the hull. To make it sail with a slight weather helm means the board has to be forward of the line of the pull. Also, I would have to devise a way to deploy/retract it without accessing the cabin - which is not impossible, but then it is in the way of other stuff on the deck (the thing that is pulling - which must be free to swing side to side).

    Still, I have not completely ruled out daggerboards.

    I have had similar ideas, but the main reason I was looking to the inboard leeboards was to get rid of the long opening on the bottom of the boat which most of the time would only be a source of drag when rowing.

    I have just enough room under the floor for a regular centerboard - but that slot... It may be that I am overestimating the drag penalty of a long slot on the keel, but I can imagine cursing it with every stroke on the long trip to Alaska.
  6. tom28571
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    tom28571 Senior Member

    The first thing is to quit calling a bilge board a leeboard. A leeboard is always mounted outside of the hull. If its inside the hull, it either a centerboard or a bilgeboard. Many centerboards are mounted fairly far off center and this can make more room in a small cabin with a swinging type board under the edge of a berth.
  7. W9GFO
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    W9GFO Senior Member

    Are there bilge boards that pierce the hull above the waterline?
  8. philSweet
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    philSweet Senior Member

    My 16' sailing dingy had twin bilgeboards built into the seat transoms, which saved a bit of structure. They had a slot in the top of the seat. I have a 20' version designed that pushes the front of the bilgeboard case forward under the foredeck and gives the board housing extra height and a slot in the foredeck for actuation. I like them in this size range. They keep the center of the boat open enough to use as a camp cruiser or cram it full of lobster traps; and the boards run just inside the trailer bunks, keeping the boat centered on the trailer. It is also handy to be able to tripod the boat in the yard.
  9. messabout
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    messabout Senior Member

    The centerboard or dagger board slot is not a serious drag maker if you use a simple "gasket" which consists of a pair of rubber or other resilient strips that meet at the middle of the slot. Gaskets of that kind are common on many small sailing boats.

    One of the downsides of a board case is that in certain terrain they can get jammed up with small pebbles or other junk. The gasket also helps, but does not entirely eliminate, the problem. Of course this is a potential problem only when you beach the boat.....which seems that you might, if you camp out along the way.
  10. W9GFO
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    W9GFO Senior Member

    I think you are right, that in practice the drag would not be significant enough to matter with a gasket. And yes, running it up on the beach is an almost certainty. In this case the gasket may be a detriment as it will retain the sand and pebbles that are forced past the gap.

    Is it correct to call what I have proposed "bilge boards"? I've tried to find a very precise definition of "bilge" to no avail. I have always understood the bilge to be completely below the resting waterline. The boards I have in mind exit the boat above the waterline, not through the bilge so I feel they are more similar to lee boards than to bilge boards. And from what I can find, "lee boards" are named as such not because they are on the outside of a vessel, but because the board on the lee side is used to prevent lee-way.
  11. rustybarge
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    rustybarge Cheetah 25' Powercat.

  12. HJS
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    HJS Member

    Here I show an example of how it can be done. This design I have applied to boats up to 13.5 meters. There is a minimal opening in the bottom and can additionally be folded up.


    Attached Files:

    Angélique likes this.
  13. tspeer
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    tspeer Senior Member

    They're called bilgeboards. The inland lake scows have them - notice the slot for the windward board in the hull of this boat:
    On a scow you don't say, "Ready about," you say, "Board down," before you tack.

    One nice thing about bilgeboards is the board trunks can be built into the sides of the cockpit, which results in a nice unobstructed area.

    There is some exposed slot when the board is down. The scows minimize this drag by using a thin metal board.
  14. Angélique
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    Location: Belgium ⇄ The Netherlands

    Angélique aka Angel (only by name)

    - Seahorse Naval Architects English - ---> - 48 m Bilge Board Schooner English

    - Seahorse Naval Architects Dutch - - ---> - 48 m Kimzwaard Schoener Dutch

    I don't know in what stage this design is, and I don't think there's one build yet, but it looks interesting though ! - :cool:

    Note: there's a translation mistake on the linked English page, they mean: ‘‘48 m Bilge Board Schooner’’

    | - Dutch : English - | - Kimzwaard = Bilge Board - | - Steekzwaard = Dagger Board - | - (scharnierend) Midzwaard = (pivoting) Centerboard - | - Zijzwaard = Leeboard - |

    ( above: usually with the omission of the portion in parentheses )​

    There are pivoting bilge boards in the below drawings, and not vertical sliding dagger boards.

    The Dutch text is a correct representation of what is shown in the drawings.

    Edit suggestion for the below English quote: ‘‘dagger boards’’ is meant to be ‘‘bilge boards’’ in this case, also I would have said ‘‘pivoting’’ instead of ‘‘swinging’’.

    Last edited: Feb 28, 2017

  15. Doug Lord
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    Doug Lord Flight Ready

    This is a 23' foiler scow whose foils exit the hull just above the waterline:

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