Daggerboards on a cruising cat

Discussion in 'Multihulls' started by rapscallion, Aug 19, 2017.

  1. rapscallion
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    rapscallion Senior Member

    I'm looking at cats in the 40' range for cruising, and I'm wondering how important the dagger boards are to the fixed keel. I prefer a Spartan interior and a light boat for cruising as performance is important. If I wanted to sail at monohulls speeds, I would buy a monohull.
     
  2. Angélique
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    Angélique aka Angel (only by name)

    The need of appendages for leeway resistance depends on the chosen hull form. Wharram Tiki's for example don't have any boards nor LAR keels.

    If a Cat needs appendages for leeway resistance, usually the faster hull forms, then the designer/builder can opt for LAR keels or daggerboards.

    LAR keels are less trouble to build and no trouble at all to operate. But both LAR keels are deployed all the time, so they give more wetted surface, hence more resistance and so lower speed. And they give an increased fixed draft which is also there when drying out or gunkholing.

    Daggerboards are more effective in providing leeway resistance, in ratio to the forward resistance they give themselves, but are more work (costs) to build, to maintain, and to operate (systems and handling), and both daggerboard casings are occupying some space somewhere.

    So, what is best for you depends all on your own priorities in the mentioned areas.

    - - - - - - - - - - - - - - - - - - - - - - - -
    P.S. - - LAR = Low Aspect Ratio
     
    Last edited: Aug 19, 2017
  3. Steve W
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    Steve W Senior Member

    The vast majority of cruising cats out there will have LAR keels so your choices are going to be much smaller if you want daggerboards. Most cruising cats less than stellar performance probably has more to do with carrying too much stuff more than the choice of appendages. If you can resist the urge to fill up all that space and set the boat up to sail efficiently you will probably do ok with keels. Obviously daggers will be better and the trunks usually don't impact the interior too much as they usually follow the hull side.
     
  4. Angélique
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    Angélique aka Angel (only by name)

    We don't know yet if the Raps wants to buy or build, for a build there's often a choice between LAR keels or daggerboards, or this can be discussed with the designer.
     
  5. Angélique
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    Angélique aka Angel (only by name)

    Raps, what's up with your Rocket Girl G-32 Janet C . . ?
     
  6. rob denney
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    rob denney Senior Member

    Dagger boards will make a noticable speed difference, improve your upwind performance and enable the shallow draft which is one of the advantages of a cat. LAR keels also add to the chances of tripping in big breaking seas.

    Daggers are an accident looking for a place to happen in respect of hitting the bottom or floating debris. They are usually thickness limited due to their short chord so tend to break if they are not treated properly. Hence, a better solution for a cruiser is a kick up board like the Tornado and Iroquois cats have. This is still a compromise as you have a long slot which should be sealed when the board is down (either a quadrant shaped board or a folding closure), a pivot pin which is hard to access/maintain and a hard to reach fouling site. But at least you don't slice a hole in the hull when you hit something at 20 knots.

    An option which eliminates all these problems is a single, kick up daggerboard under the bridgedeck. It requires a bit of thought but is easily supported, kicks up in a collision, has no drag when raised, takes no internal space and is lighter than 2 cases and the beefing up they require.

    There are better solutions, but they don't work on a cat. ;-)
     
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  7. catsketcher
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    catsketcher Senior Member

    If you want to sail fast - get a daggerboard cat. The choice of daggerboards vs keels is but one of the myriad choices designers and builders face but it is a good signal as to the intent of the design. A builder who wants a daggerboard will usually build a lighter boat with less interior and have a more performance orientated bias when choosing all of the other parts of the design. A minikeel cat signals a less performance bias.

    Making daggerboards good for cruising can be done. For most cruising time the boards will be raised - motoring and deep downwind. So if you run over a floating line the rope will run along the hull and then kick up the rudders - no hassle - less than if you have hard mounted rudders and diesel legs. I have accidentally run over fish trap lines about four times. Only once did I get the rope around the daggerboard and that was a pain.

    I would not get rid of daggers and go for something else. I have ropes of kevlar unis wrapped around the case so that even if my boards hit something they will not break the hull. Also I cut the 300mm bottom off the board and glued it back it back on, so if I run aground it will break off the bottom of the board. Nicely moulded cases are the go - mine are totally silent and this reduces my stress on board. Any clanking or banging gets me worried.

    If you go daggerboards I would recommend raising motors and rudders. With modern 4 stroke outboards you can power quite large cats with outboards. If you put in sail drive with daggerboards you get the worst of both world as you can't easily sit on the bottom which is a great feature of cats.

    cheers

    Phil
     
  8. rob denney
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    rob denney Senior Member

    Good points as usual, but running aground at speed is less likely than bumping into something that is floating below or near the surface. Impacting a log or large sea life at 15+ knots on a reach may or may not tear the kevlar rope off your hull as the board is forced into the wedge in the case. It will certainly destroy your board and make it impossible to raise without drying the boat out and/or chopping the board into pieces.
    As the boat decelerates to near zero in a second or so, anyone standing or facing forward is going to get thrown into the first solid object in the direction of travel, which won't be pretty. Probably won't do the rig much good either.

    You can live with the slim but real chance that this will happen, or opt for a simpler, cheaper solution in which it won't. How slim depends on where you sail ; PNW, North Atlantic and Transpac it is worryingly high.

    Another consideration is weed and plastic bags. These can be removed from a centre mounted foil with a stick while the boat is at full speed, without lifting the foil. Lifting a loaded daggerboard at speed will require you to either stop or steer a serious S course to unload it. Then repeat the process to get the offender off the rudder.
     
  9. DennisRB
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    DennisRB Senior Member

    You only need one daggerboard for daggerboard performance. It has to be bigger than if you had two but it still ends up being cheaper and easier and only takes space in one hull. John Shuttleworth has a performance analysis comparing one dagger to two and found little to no performance increase with two.
     
  10. DennisRB
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    DennisRB Senior Member

  11. Angélique
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    Angélique aka Angel (only by name)

    But to have vertical lift on the lee hull as below you'll need a dagger in each hull . . .

    Seahorse Magazine ---> San Diego-based maxi and superyacht design maestros Reichel/Pugh have entered the fast-growing world of the large performance multihull *

    Reichel / Pugh ---> The R/P 45 ft Offshore Performance Catamaran * ---> It Had To Happen *

    * 3 × the same article, just some spares in case one gets offline.

    [​IMG]

    ‘‘ . . . . C-shaped daggerboards provide leeway resistance as well as some vertical lift to reduce virtual displacement – also ensuring the bows stay up when the boat is being pressed hard downwind. . . . . ’’

    [​IMG]

    [​IMG]

    For a performance version I would free her from the burden of the two diesel saildrives (weight and drag), and give her two lifting high thrust outboards right away, placed in the bridge deck against the hulls, just fore or aft of the companionways, whichever place would give the best overall weight distribution, this would also free some valuable interior space in each hull.

    P.S. - - When used as an all-round cruiser, she'll need lifting rudders as well, I believe . . :cool:
     
    Last edited: Aug 25, 2017
  12. DennisRB
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    DennisRB Senior Member

    Yeah sure. Boards that produce lift require two. But how "high performance" is the intended boat really going to be? Seems to be reaching for the stars. The OP specifically wants a cruising boat.
     
  13. Angélique
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    Angélique aka Angel (only by name)

    How much more expensive would the C-shaped daggers in post #11 be, compared to straight ones . . ?

    Including the casings of course, and let's say of the shelf if that was possible, and self build . . :cool:
     
  14. DennisRB
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    DennisRB Senior Member

    I have no idea but would not be surprised if the cost was over 3 times as much as one straight dagger. PS that RP45 is my new dream boat.
     
    Last edited: Aug 26, 2017

  15. iskip
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    iskip Junior Member

    My daggerbords give me a other kind to use: Beaching ! I go very close on the beach, stay good (maximum 1 Meter) in the water and can clean my unterwater ship, very easy. My 44ft cat is in 30min cleaned and is ready for a faster sailing.
     
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