Twin centreboards?

Discussion in 'Sailboats' started by Shifu, Dec 26, 2022.

  1. Shifu
    Joined: May 2021
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    Shifu Junior Member

    Can anyone point me at resources re boats with off-centre or twin centreboards? I am designing a 5 metre cruising dinghy and planning to use on or the other. Currently I am most interested in twin centreboards (i.e one board each side of the boat the cases fitting under the sides seats), thinking it will give the most flexibility. I want the boat to sail well on all points, but want a floor clear of a centrecase. Does anyone have any pointers or opinions on twin centreboards ?
     

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  2. alan craig
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    alan craig Senior Member

    Look for articles or designs by Philip Bolger, he had plenty with off-centreboards, twin daggerboards etc.
    ps. I don't know why I can't copy and paste a URL any more but youtube has a great video of scows with twin boards, just search "pathe racing scows".
     
  3. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    I have often thought twin centreboards were a great idea for small boats, for the reasons you mention.
    I have often wondered if you slant the boards outwards, you would get increased lateral resistance on the leeward board and be able to leave the windward boards up, for less forward resistance.

    One small centreboard boat I owned had hollow insides in the single fibreglass centreboard, with a small hole to allow water in. The water was intended to provide a little bit of extra ballast, but empty out when trailering.
    The other "feature", was that they omitted any downhaul on the pivoting board. This allowed the board to raise up when grounded, but the water pressure "locked" the board in place when sailing. The friction from the board and casing was enough to keep it in place.
     
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  4. bajansailor
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    bajansailor Marine Surveyor

  5. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    Look at John Wellsford's designs. He has drawn at least a couple with single offset boards. He is very good at his job and reportedly the offset board affects our OCD but not the boats' sailing ability.
     
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  6. Shifu
    Joined: May 2021
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    Shifu Junior Member

    Many thanks for all the responses everyone. Very helpful. If there are no obvious handling/sailing problems from twin centreboards then I hope to gain these advantages:
    1. Centrecases are always in the way especially when you have non-sailors aboard. With the cases moved to the side you get a clear floor with more space for camping, lounging, and ease of moving about the boat.
    2. Hopefully will increase options for adjusting the boat's balance.
    3. Use the windward board alone in heavy weather to reduce "tripping" forces.
    4. The open space between cases can more simply house a water ballast tank.
    5. The board slots are elevated above the keel line so impervious to picking up rocks and grit.

    I have thought about toe-ing the boards out a little to make them more vertical when heeled, (and it looks heaps better in drawings), but there is a builder's complexity penalty for that.

    Here's a 3D sketch of the latest version of the hull with some boards thrown in
    Lod = 5m
    Beam = 1.8m
    draft = 150mm
     

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  7. catahoula
    Joined: Mar 2020
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    catahoula Junior Member

    The melges M16 scow has had dual bilgeboards for....well, a lot longer than I've been alive. Those angle out away from the hull so they're more vertical on a heel. You should raise and lower them each tack though, so it is a bit more fuss to sail. But you could then make them asymmetrical foils. The boards on the m16 are just flat aluminum plate.

    Having flipped one of those in cold water and inconvenient distance from shore, I would also say in general to make sure your mast floats! 16 feet can be heavy enough that you need all the help you can get to get it back upright. Cool design though, looks great so far
     
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  8. Milehog
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    Milehog Clever Quip

    Keep the advantages of the KISS principle in mind.
    Unless the boards are small, you will suffer too much wetted area with both deployed.
    With only one deployed the other will be in the way of everything.
    Obviously added weight, materials, labor and complexities have a price as well.
     
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  9. rwatson
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    rwatson Senior Member

    Here is a more realistic design if you are using Centreboards, not Dagger boards.

    Some arrangement to streamline the hull slot could be useful
    TwinCentreboard.png

    PS The hull slots should be BELOW the waterline, to discourage cavitation. The boards will stall out of they are exposed above the waterline.
     
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  10. Shifu
    Joined: May 2021
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    Shifu Junior Member

    Yes the centreboard slots will need a flexible seal. I think we used to call it a "zap flap" back in the 80s?
    I have no doubts that the centrecases for this boat will be reasonably large projects in themselves, but once done that will be it. This is going to be my last boat, and she is meant as a complement to my other love, windsurfing.
     
  11. HJS
    Joined: Nov 2008
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    HJS Member

    Shown here is a folding centerboard used on boats up to 13.5 meters. Due to its special shape, only a small opening is needed in the bottom of the boat.
    If two centerboards are chosen, the total surface area must be the same as a single one. The wetted surface and friction is then aboat the same. The aspect ratio must be the same, thus the draft will be smaller as well as the heeling moment.

    upload_2022-12-28_12-16-6.jpeg
     
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  12. Shifu
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    Shifu Junior Member

    Wow! this is great!
     
  13. philSweet
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    philSweet Senior Member

    It works great in that sized boat. Opens the cockpit right up. Lot's of extra work to fit two cases, but well worth it in my opinion. I was very happy with the results for 25 years. Having sailed this thing for 25 years, I have a few opinions about how the boards, cases, and actuation should work. With a half-decked boat with a deck-stepped mast, the actuator needs to run up high on either the front or backside of the main frame that supports the mast. Otherwise it is in the way of things in the cockpit where the jib man wants to sit. The boards can protrude below the hull a few inches when fully retracted. This gives excellent trailering if the boards are just next to the trailer bunks.

    Nope. You only use one at a time. They have to be full sized. You don't ever want to have both down, unless maybe you are on a screaming run and are just dragging the tips. I ran a slightly cut-down Hobie 16 rig on this skiff with two trapezes and a custom gennaker, so I could really hang some canvas on it.

    [​IMG]

    Centerboard suggestionsSeeing as I have used only Daggerboards and Leeboards https://www.boatdesign.net/threads/centerboard-suggestionsseeing-as-i-have-used-only-daggerboards-and-leeboards.50689/#post-692627
    [​IMG]
    [​IMG]
     
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  14. sharpii2
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    sharpii2 Senior Member

    One advantage of keeping both boards down is better tracking ability. This means the boat will be less likely to change course, or it will change course slower.

    My first boat was a 10 ft scow that I built myself. It was supposed to have leeboards. I could never get to sail upwind with just onw leeboard. Maybe the area of the board was insufficient. Or the home-made sails werer too full. Or the sailor was too inexperienced.

    I ended up building a rack on each side of the boat which turned the two lee boards effectively into twin dagger boards. I also flattened the sails a bit. Then it sailed upwind. It also turned in, shall I say, a more stately manner. This I think is a good thing for a newbe sailor. That home-made boat taught three people how to sail before neglet and rot got it. This was with no sailing instructor in sight.

    The way you have the boards spaced looks good to me. They should be at least two chord lengths apart. Angling them, I think, will do more harm than good. Then you can really have only one down at a time.

    One should keep in mind that two smaller, twin boards do not equal one larger board of the same area. With biplanes, the second wing supposedly adds only 20% more lift in addition to the original wing, if it is the same size. I think water is different than air, even though it behaves in a similar manner. So maybe you can calculate the twin boards being about 75% as efficient a s single board with the same area and of proportions, instead of 60%.
     
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  15. HJS
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    HJS Member

    The comparison with biplanes is a bit misleading. The relative distance between the profiles differs markedly. The distance must be seen in relation to the chord of the profile. If the distance is at least twice the chord, the profiles have the same lifting force and drag as a single profile.
    JS
     
    Last edited: Jan 1, 2023
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