Shared Lift between rudder & Daggerboard

Discussion in 'Multihulls' started by arekisir, May 24, 2013.

  1. arekisir
    Joined: Jul 2010
    Posts: 26
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    arekisir Junior Member

    have been kicking round a few ideas relating to a simple 6.5m catamaran.
    One that I'm not sure has been explored is common sections used for rudder and dagger board on a catamaran.

    The advantages are making the same thing 4 times is easier than making 2 & 2.

    The platform of the foil would also be parallel making is easy to make, think extruded foil.

    If a conservative section say NACA 0011 with a cord of say 200 is used for both rudder and dagger board the following non conventional things would assist this compromise:

    Move dagger board forward and use the oversize rudders to help in lift as the board will be of smaller area than optimal and the rudder larger than normal.

    This concept was done by bill roberts (ARC 21) so it's not original thinking on my part.

    So my questions are:
    -When the rudder is loaded by moving the dagger board forward the helm will load up and the boat will be a pig to sail so the rudders have to be "balanced" Hobies used to rake the masts back and tuck the rudders under the boat to get the boat to point.
    I have heard numbers of around 20-22%% of the platform needs to be in front of the rotation axis of the rudder - is this correct for a 0011 section?

    -is their a clever way to balance a transom mount rudder. A parallel section will look pretty wrong "tucked" under enough to get 20-22% of the planform in front of the pivot.

    I hope this makes some sense

  2. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 351, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

  3. cavalier mk2
    Joined: Mar 2010
    Posts: 2,203
    Likes: 104, Points: 63, Legacy Rep: 214
    Location: Pacific NW North America

    cavalier mk2 Senior Member

    Many Dick Newick tris are made with this sort of set up with about that section shared by the rudder and daggerboard. 22% forward on the balancing seems pretty high though.
  4. CutOnce

    CutOnce Previous Member

    Most modern proas do this as well. More necessary with proas as the ends change on every shunt.

  5. arekisir
    Joined: Jul 2010
    Posts: 26
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    arekisir Junior Member

    22% forward does seem pretty high, I got the number from Gavin Lesur's book on "multihull seamanship" I just had a look for it but the kids look to have re-positioned the book some place so I cannot quote it right now.

    It explained that racing foils have lower leads (less than 20%) than cruising boats (up to 22%), it did not go into theory or reason so it is a bit hard to be conclusive.

    Quoting design foil - The Center-Of-Pressure (COP) is a point at one single angle-of-attack where the pitching moments are zero. Think of this as the place where you would push down with your finger on top of the airfoil and it wouldn't try to tilt in either direction (even while creating lift).

    Running the virtual wind tunnel in design foil the COP for a naca0011 in the appropriate renolyds numbers and angles of attack the COP is 26.4%

    So 26.4% appears the upper limit but the boat would have no helm feel at all and potentially have an unstable rudder. is this the tradeoff in loading the rudder - loosing helm feel and gaining instability?


  6. Petros
    Joined: Oct 2007
    Posts: 2,934
    Likes: 148, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    You do not have to "tuck" the rudder under the transom to get the balance you want. You just need to have 20 or 22 percent (or what every you choose) of the rudder ahead of the hinge line of the rudder. If you sweep the hinge line aft by mounting the upper pivot point forward of the lower one the hinge line is swept back, the rudder can be dead vertical. You can even sweep the rudder back, as long as the hinge line has more sweep than the rudder.

    Typically for the 4 digit foils their center of action is at about 25 to 26 percent, but as noted, not a good idea to go that far back because you will have no feel in the tiller. a little bit of weather helm seems a good safety feature in case you lose control of the rudder it would round up into the wind with the hinge line ahead of the center of action on the rudder. On an airfoil it is called the aerodynamic center, not sure what it would be called on a rudder. A "hydrodynamic" center?

    You will want the rudders shorter than the dagger boards so they have a lower aspect ratio than the dagger boards. this will make the rudders more stall resistant and always make sure you will stall the dagger boards before you stall the rudder.

    good luck
  7. catsketcher
    Joined: Mar 2006
    Posts: 1,315
    Likes: 165, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    A friend from years back had a rudder designed by a fab foil designer. We took it out and it over balanced. He got it redesigned and it still over balanced. In the end he got the builder to cut more off the leading edge so that it would work. So beware of numbers and computers when there is heaps of empirical data.

    I would just look at what has been done and copy that. Turbulent flow, rake, upsweep of flow all may impact the CP. My rudders have about 10 % in front of the pivot and that seems fine. Too little is better than too much.

    As for putting more load on the rudders - I wouldn't do it. You will need HUGE rudders to make it work - Here is why.

    You come out of a tack and all of your foils are close to stall. If your rudders stall before your boards then with the CE so far back the boat will weathercock in an instant and you are out of control and going no where. I sailed a cat that did this and it was unsafe.

    Think about planes. For safety the tailplane on a normal taildragger pushes down on the aircraft. This is so that if it stalls the plane will head nose down, generate more speed and not stall. Your set up will be the opposite and very prone to stalling. The more load you put on the rudder the worse the boat will tack. I like to think of it like John Letcher put in his book on self steering - rudder volume - the rudder area times the lever arm. To keep the same rudder volume you will need to have a much bigger rudder than if it was further aft.

    It is no hassle to build the rudders and boards different sizes. The amount of work saved is miniscule in the context of a boat build.


  8. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,021
    Likes: 248, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    There was a thread here a few years ago "How to dimension a sailing catamaran" that had a useful document for determining the loads on rudders and daggerboards.

    I have used this recently on a project where the builder wants to add daggerboards to his cruising catamaran design. An earlier version of this document (in the first post) had the split between the daggerboards and rudders as 70/30 respectively. However, a later version (post #26) changed that split to 67/33 respectively.

    As for the centerline of the rudder stock in relation to the chord: Most designers will use a position of between 15% and 18% of the chord. This is because the center of pressure typically occurs along the quarter chord line (25% chord) of the rudder blade, and you want enough moment arm between the turning axis and the center of pressure to make the rudder balance well to the hand on the tiller (feel) and self-center if you let go of the tiller. The 15% to 18% position does this nicely, and this is independent of the foil section shape.

    I hope that helps.

  9. ImaginaryNumber
    Joined: May 2009
    Posts: 436
    Likes: 59, Points: 28, Legacy Rep: 399
    Location: USA

    ImaginaryNumber Imaginary Member

    Or maybe you could use two tandem daggerboards, plus rudder, like Dick Newick's trimaran Vaka Fanāua. Two Four Six for the price of one?

  10. catsketcher
    Joined: Mar 2006
    Posts: 1,315
    Likes: 165, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    I think the tandem daggerboards are used to help with tradewind sailing. Trimming them allows you to get the boat to self steer. Dick did that way back in 1972 with Three Cheers.

    I still don't like the idea of loading the rudder much at all. When you come out of a tack the first thing that loads up is the leech of the main. So you have to ease this out a lot with a boat that can't load its rudders up. On a small boat this is fine but on a larger one it gets hard quick tacking. I would do what Farrier does and give the boat almost neutral helm. Farriers are lovely to sail and can sail without a rudder if needed - that's a good safety point. I sailed a mono that had a loaded rudder and it lost its rudder. It was a pig to get back to the coast. Only 3 of us could steer it.
  11. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 175, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    I agree with Catsketcher.

    The rudder and daggerboard do different things. The board should work best at about a 5deg angle of attack. The rudder has to work up to 30deg or so.

    The centre of effort of a foil moves forward as speed increases. So in lightwinds you may not have an overbalanced rudder, but it can suddenly overbalance when you accelerate. You don't want that - like Catsketcher I've been there done that. So 20% of area forward of the pivot is the sensible maximum. If you have a daggerboard style rudder then I agree the amount of forward rudder angle does look weird, but it works

    Normally you'd expect the rudder area to be about half the board area. Don't go more than a 3:1 aspect ratio as it is too easy to stall out after a tack or in big waves.

    Richard Woods of Woods Designs
  12. arekisir
    Joined: Jul 2010
    Posts: 26
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    arekisir Junior Member

    I am humbled by the insightful appraisal.

    I have attached a profile view of the boat I am sketching.
    The CE is the combined mast and sail.

    If the rudder has 20% of the area forward of the pivot & with this sail & foil setup, will this boat:

    -Sail upwind well without huge leeway
    -Track well (or self steer) on all points of sail

    Not all these points will be met but I would like to understand the tradeoffs.


    Attached Files:

    Last edited: May 29, 2013
  13. catsketcher
    Joined: Mar 2006
    Posts: 1,315
    Likes: 165, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member


    Now that is interesting - you got windage.

    In all the discussions I see on multis and CE/CLR stuff no one talks about the hulls windage which I find strange. Your boat may have the CE between the boards CLR and the rudders but it will probably have neutral or even lee helm.

    Consider this - a mono heels a lot and this creates a torque that twists the boat up into the wind. So designers fiddle with a thing called lead. Now you already know that foils don't actually lift from their middle but for the purposes of design we think they do - so we fudge and call this lead. But mono keels are usually (unless they are new designs) have keels with a long chord so the CLR of the keel can be off by a large amount. So really we don't know.

    Then you design a cat which has its rig causing windage drag and lift and a hull above water that is all drag. The CLR of your hull drag (which will be considerable) will be in the middle of the hull's silhouette. You have to factor this into your other CLR/CE calculations.

    My larger cat has a little bit of lee helm in light winds. She shouldn't at all but she has lovely high bows that I am pretty sure are the reason. According to sail theory she should have weather helm. The CE is behind the daggers, especially when I put the staysail up instead of the genoa but she doesn't listen to theory. I think the hull windage is such an issue as cats are very light, have a huge above water to below water silhouette ratio and have highly loaded foils. (I could get rid of the helm with some rake but she tacks so well I am loathe to fiddle too much)

    So you may find that this design with its high windage very hard to compute.

    Then again - the foils are way too small. My 6m cruiser cat has a single centreboard, 500mm chord and 1.1 meters span. The 7 metre version has same chord and 1.2 metres span. Your 200mm chord boards may stall easily. Go the big foils to give security - who cares about going .05 knot slower at top speed when you can manouvre beautifully with lots of grip on the road. I would double the chord on the CB and get it at least 1m span. That boat has lots of windage before you even get to the rig. She gonna need grip and lowly loaded foils to cope.


  14. daiquiri
    Joined: May 2004
    Posts: 5,371
    Likes: 258, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Hi Arekisir,
    What is the maximum speed you expect to obtain from your boat?

  15. arekisir
    Joined: Jul 2010
    Posts: 26
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    arekisir Junior Member

    What is the maximum speed you expect to obtain from your boat?

    I do not have the skillset or experience to produce polars but am hopeing for the following:

    -Max speed upwind (speed across water) 8-10 knots
    -Max speed off the breeze (speed across water) 14-15Knots

    Downwind it will struggle without extras (kite)

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.