Catamaran daggerboard design and use

Discussion in 'Multihulls' started by Steve W, Aug 2, 2017.

  1. Steve W
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    Steve W Senior Member

    I am a bit confused over the pros and cons of different types of daggerboard setups. A lot of performance cats such as those designed by Tennant, Grainger etc use boards installed down the inside face of the hulls meaning that the board is vertical when the windward hull us just clear of the water whereas a lot of newer cats are installing them following the outside angle which to me seem less than optimal for a couple of reasons such as being more difficult to operate hanging out over the water usually requiring some kind of apparatus to raise them and also longer boards since you cant cantilever them down inside the trunk so , more weight, also it would seem to me that as the boat lifts a hull the board would be less vertical so provide less lateral resistance so maybe need more board deployed. I'm talking straight boards here, c foils would seem to be worse still at providing lift ( as in to weather, not upward lift) Using a straight board following the angle of the outboard face of the hull does not seem like enough angle to provide significant hydrodynamic lift to me to be worth the disadvantages ( hence c foils) but I could be wrong, just trying to understand the reasoning.

    Steve.
     
  2. Doug Lord
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    Doug Lord Flight Ready

  3. jorgepease
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    jorgepease Senior Member

    That was a good article!
     
  4. farjoe
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    farjoe Senior Member

    Are there any websites dedicated to the design and construction of homebuilt lifting foils for boats under 6m?
     
  5. Doug Lord
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    Doug Lord Flight Ready

    Don't know of any websites so far, but there is an excellent book: "Hydrofoils Design Build Fly" by Ray Vellinga and available on Amazon.
     
  6. farjoe
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    farjoe Senior Member

    Thanks I will look into that.
     
  7. catsketcher
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    catsketcher Senior Member

    Gday Steve

    One major reason is because of accomodation. Some early cats had vertical boards but these severely impacted the use of the hulls. So you have to go either outboard and cant one way or inboard and cant the other. From there it may come down to safety vs speed.

    For a non lifting board (all Tennats are non lifting) all the board does is provide a vertical force. In a racing boat that flies a hull and sails at a high angle an inward board will be more vertical when hull flying as well as easier to pull up and down. On top of this you don't have any worries about boards at the marina hanging over the side.

    Against this is safety. A little Aussie cat, the Seawind 24, had outboard boards that were heavily angled. Owners reported that when hit by a gust the boat would slide to leeward, lifting the leeward hull and pulling down the windward hull. The little Seawind was a pretty great boat. So a cruiser/racer designed by say Lock Crowther, that was not meant to fly a hull regularly would have the outboard boards.

    On my cat the boards are outboard and only have a modest cant. I have never flown a hull so it works pretty well.

    cheers

    Phil
     
  8. UpOnStands
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    UpOnStands Senior Member

    Also, more recent cat designs tend to have more vertical outboard hull sides. This can minimize some of the problems.
     
  9. Steve W
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    Steve W Senior Member

    Thanks guys, I used to own a Macgregor 36 cat that had a single vertical board in the port hull that you had to crawl past to get into the berth, It was a cantilevered board like Malcolm Tennants boats so the strong points were the bunk top and the hull bottom. On a cat with a single board vertical was probably a reasonable compromise but they went to 2 boards on later models. All beach cats are vertical (until the introduction of lifting foils) as they don't have accomodations to impact, although they are not cantilevered. It seems to me that unless you are getting significant hydrodynamic lift from the outboard boards they would be more of a pita as I would think they would need to be longer and need some kind of frame to raise them ( heavier). Even if you can push them down manually it can be precarious doing so, they can as Phil mentioned hang way out the side when raised and be less effective at preventing leeway as the boat heels and lifts a hull whereas the inboard style board gets more vertical so I would expect you would need less board deployed (less drag). So, what I'm curious about is if the lift generated by a board that is angled inward at, say, 5-8 degrees enough to be worthwhile given all the negatives in every other area in the real world ? I have a friend who bought a cat where the boards have been moved from inboard to outboard so someone thought it would be but who knows if it panned out. Grainger has some interesting thoughts on his website, he considers Mad Max to be one of the fastest cats in Aus and I see it still has its boards inboard. I'm sure the choice of inboard or outboard location at the design stage would depend a lot on the type of layout of the cat, ie, with a net and tube racing cat such as a GBE inboard is very convenient and simple whereas outboard would make more sense on a cat with a bridgedeck cabin but on a net and tube racing cat where efficiency is the only goal which way would you go?
    My other question relates to the actual use of the boards, how much to deploy on the various points of sail at different wind speeds. Obviously you would use more board upwind, less reaching and even less to none running but how much? I would guess that the more speed you have upwind the more lift you are generating so the less board you would need. It seems to me that you would want as little board as possible deployed for drag reduction but I don't know.
    Thanks for any insight,

    Steve.
     
  10. groper
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    groper Senior Member

    to go upwind its pretty much more board the better. practical size limits the ultimate size.

    As to cant - the more vertical the better for upwind performance. some race boats cant them inwards so they are vertical once flying hull - not applicable to cruising boats. A good cruising boat still needs good upwind performance tho, and they normally travel with little heel - hence the vertical boards are ideal for performance but they ruin the interior space - which is also important for cruising :). So the best compromise on a cruising boat would be whichever cant was less - inward or out, and keep the board running down the topsides so you don't have a trunk in the middle of the hull and maximise interior utility. Really big cats however - can have a trunk in the center and you still have enough room in the hull to fit full size accommodation spaces on both sides of it - this doesn't apply to smaller boats tho.

    For racing it can go either way depending on whether the boat is designed to float or fly. Some vertical lift may increase speed - such as the C boards used on the racing trimarans, however you loose some leeway due to the angle. The racing tris offset this loss by including the centreboard in addition to the C boards in the amas.

    If the race boat is a floater only - then inward cant is likely best so the board is vertical providing maximum lift upwind whilst the boat is heeled and flying a hull. This concept is proven as the fastest smallish catamaran in Australia right now is setup like this (40fter). Round the bouys racing is usually won on the upwind performance - less so on the downwind and reach legs.

    Assymetric boards is another consideration - again is depends on the intended usage. For racing maximum lift to drag ratio is paramount - hence better to use assymetric boards 1 at a time and size them accordingly. For cruising - most people cant be bothered rasing and lowering on every tack and the total performance not so important to most people. Hence they normally use symmetric boards both down at once. More area in the water but and not as good lift to drag ratio - set and forget when tacking upwind...

    Horses for courses steve - no simple answers I'm afraid :)
     

  11. Steve W
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    Steve W Senior Member

    Hi Groper, thanks for the insight, pretty much my thoughts also. On cruising cats you either have low aspect keels or either kickup centerboards or if it is more performance oriented daggerboards which, as you say, usually follow the outside angle of the hull topside because of how they fit with accomodations rather than any sailing related issue. However with net and tube type racing cats you get to choose whether they angle in or out at the design stage based on performance alone. To my way of thinking angling out such as most of Tennant and Grainger cats have them place for two reasons, easier to raise and lower so they can be cantilevered down into the trunk so they can be considerably shorter for the same draft so , lighter. Daggers are rarely very light so any savings in length is worthwhile plus you don't need any additional structures for raising them which is also a weight saving. The other more obvious advantage is of course the board is close to vertical when the windward hull is just out of the water. I would guess that with them angled inward ( underwater) you would need to deploy more board for leeway resistance (more drag). I have however seen cats switch from outboard angled boards to inward angled which leads to my confusion, I totally understand the desire for hydrodynamic lift and on tris its a no brainer as, as you say, they have a daggerboard in the main hull for upwind work and also the boards in the amas can, and are angled at a much greater angle than can be achieved on a cat so provide real lift, even with simple straight boards. I just question how much lift a board in a cat angled at maybe 5-10 degrees give vs the disadvantages.
    As far as asymetrical boards go they only make sense on actively sailed cats, ie racing cats and I think would be necessary on cats with inward angled boards. In the US we have Gemini cruising cats with asymetric centerboards that require you to go down into one hull and crank a winch handle to raise one and then cross over to the other lower the other. Not going to happen imho on a cruising boat.

    Steve.
     
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