drag differences between daggerboards and centerboards

Discussion in 'Sailboats' started by CutOnce, Oct 27, 2010.

  1. CutOnce

    CutOnce Previous Member

    All:

    I'm in the process of refining my design brief for building a low cost cartoppable home build. More or less, this fits the "Low Cost Design Contest" criteria.

    Given this will be a car topper, and a boat friendly to spur of the moment sailing anywhere, I'm wondering about the real world drag differences between a well build daggerboard slot and a well built centerboard trunk aperture. If using a centerboard, I'd certainly install mylar or whatever overlapping gaskets to seal the slot.

    Some of the places I want to use this boat have rock shores, reefs and beaches, so a daggerboard will require serious attention to avoid major hull damage.

    My concern is how much of a performance penalty will a kick up pivoting centerboard place on the design? Is there any studies published? Any design concepts worth hearing about on how to minimize drag?

    Thanks in advance for your help.

    --
    CutOnce
     
  2. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    First of all, it is quite simple to build a center board identical to a dagger board in underwater shape. In that sense, it's the dagger board that can't always mimic the center board, but a center board can always mimic a dagger board. Of course it could be argued that a dagger board could be built that was so wide that it had the same underwater shape as a center board. Fort all practical purposes however, nobody would ever do that.
    If performance is desired, in other words, always go for the foil that's parallel fore and aft and slender and deep, like a typical dagger board. But because the dagger board design is prone to damage from grounding, allow the board to pivot.
    Build a dagger if you want to save weight or to simplify the construction or to give more cockpit room but not to attain a specific shape.
    Many shoal draft boats nest a pivoting board inside the ballast keel and it's just as skinny as a dagger, which allows the boat's interior to be unencumbered by a centerboard trunk. All that's needed is a pendant, which can be positioned to work well with the interior design.
    If a good amount of ballast is required low down, a center board can be large enough in area to carry that weight while a dagger or drop keel would need a bulb at the bottom, which won't allow removal of the board from above, and won't allow the bulb to enter the slot, meaning beaching is problematic.
    Regarding the open slot turbulence, the centerboard can have a pretty hydrodynamic set of "lips" made of rubber or plastic, etc., which can easily be maintained or replaced on a boat that is small enough to flip over on the beach.
     
  3. CutOnce

    CutOnce Previous Member

    Appreciate the fast response.

    Since I'm designing the boat myself, I've got free reign to play with slot length on a centerboard - allowing a high aspect ratio board like current I-14 daggerboards.

    What I was wondering is how bad the induced drag and turbulence would be on a 4' long underwater slot versus the same type of board in a daggerboard slot only 10 inches long?

    First thoughts would be to make this like a Contender centerboard.

    --
    CutOnce
     
  4. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    There are those here who might shed some light on that subject. I would assume you are comparing no slot to an open slot without a closure. Maybe someone who's studied this very question could comment. And I'd assume a well designed closure would be similar to no slot at all.
     
  5. jehardiman
    Joined: Aug 2004
    Posts: 2,763
    Likes: 357, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    The extreamly small difference in drag between a close fitting daggerboard slot and a long faired centerboard slot is lost in the weeds in actual practice. I doubt you could measure it on a real boat. The bailers will have much more drag effect.

    On the flip side, the daggerboard slot will have more drag than the centerboard slot when the board is up.
     
  6. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    There's is measurable drag, but boat speed would be hardly affected, especially in a light, easily motivated hull form. You might lose a 20th of a knot under some conditions with the centerboard, but the lack of damage when you encounter a rock or the bottom is considerable.

    Weight and interior space considerations are the usual motivations to use a daggerboard. Most dagger installations are lighter and have a smaller foot print.
     
  7. cardsinplay
    Joined: Oct 2010
    Posts: 330
    Likes: 3, Points: 0, Legacy Rep: -74
    Location: Camp Plasma

    cardsinplay da Vinci Group

    Write a letter to Mike Leneman at Multimarine in SoCal. He's been exploring this very issue on an otherwise matched pair of VPLP designed M23 trimarans which he distributes here in the US.

    Truth is, the factory centerboard version is slower than the daggerboard boat. It becomes readily apparent when they are sailed together.

    Your issues should be decided by the utility you seek with this boat you are drawing, since it is unlikely that you'll get the chance to A/B your boat with another of the same design having the different board type. That makes the performance/drag argument somewhat moot and shifts it all over to the utility function.

    There are ways to decently reduce the drag signature from a centerboard, get good upwind numbers and have a near, hassle-free beaching machine. You'll have to think it all through in order to incorporate the solutions.
     
  8. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 140, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    dagger board is simple, light, easy to build, less to maintain, and in practice the has less drag if tight fitting at the opening. All reasons to use it on a light weight roof top sailor.

    There is I think a false argument that a swinging center board is more damage resistant, in my experience from owning both types on small boats, the swinging center board is just as prone to damaging the hull and the board as the dagger board, and the centerboard tends to need more maintenance, and requires more extensive repairs. If you drag or strike a center board or a dagger board, both are likely to need some kind of repair. I have found the dagger board less troublesome. I will only build boats with dagger boards now. Unless it has a lot of weight (counterbalance), in my opinion, the dagger board is a better choice for simplicity, weight, and performance.
     
  9. Paul B

    Paul B Previous Member

    Of course this was also proven more than 25 years ago when the I14 class converted from CBs to Daggerboards. Some older boats were converted and were MUCH faster than their ex-sisterships.
     
  10. cardsinplay
    Joined: Oct 2010
    Posts: 330
    Likes: 3, Points: 0, Legacy Rep: -74
    Location: Camp Plasma

    cardsinplay da Vinci Group


    Perhaps a more effective centerboard design is the answer for this opinion? I have owned dozens of different beach cats and without fail, when a daggerboard equipped boat slams into an obstacle at speed, it creates big problems with hull and daggerboard trunk integrity.

    The centerboard boats (Prindles and Tornados) have handled the same impact in stride, the board rotated up in its trunk and only the smallest of leading edge snackling remained. Once past the obstacle, the bungee tensioned board slid back down in place and the boat sailed on.
     
  11. Doug Lord
    Joined: May 2009
    Posts: 16,653
    Likes: 323, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ======================
    Hey "cards" has Chris Ostlind returned to the forum-just curious?
     
  12. CutOnce

    CutOnce Previous Member

    Thanks for the answers. I was kind of expecting the real world differential to be minimal with a well-implemented gasket.

    There won't be autobailers (self bailing cockpit through transom) so that isn't an issue.

    There are distinct on-the-water advantages to a centerboard, other than damage mitigation - being able to adjust the CLR aft is nice, as well as adjusting the area of lateral resistance. Also helps when you rake back the rig in wind.

    Daggerboards are generally left down all the time, except when reducing area in heavy winds by pulling up the board a couple inches.

    It's really worthwhile to have all of you folks to bounce the issues off - thanks!

    --
    CutOnce
     
  13. CutOnce

    CutOnce Previous Member

    Care to expand on this?

    --
    CutOnce
     
  14. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I too will agree with the minor damage results with a centerboard, compared to a bottom strike with a dagger. The dagger will rotate to a degree, if the upper leading edge is angled aft, like I do with my designs (and as many others do).

    If a dagger strikes the bottom, it rotates aft as much as it can, then the trailing edge mashes itself into the back king post of the case. The result in a light strike is a damaged trailing edge. If the strike is hard, the case is usually damaged as well as the board. I've personally ripped them clean out of the boat. I've repaired this type of damage dozens of times.

    If a centerboard strikes the bottom, it's usually not a big deal and just retracts, maybe with minor leading edge damage, though if reinforced, not very often. In a heavy strike, the pin, it's bushing, leading edge damage, etc. are possible, but the case stays in the boat. Now, I have also seen centerboards that have dug into the bottom, because folks had them locked down with tackle and then sideways forces, or the boat backed up which did lots of damage to the case and board, but this is a user issue, not a functional flaw of the concept.

    The I14 has gone though years of development and the older, non-converted boats did get beaten, but the weight of the centerboard and it case, plus other "improvements" were the major contributors to the speed differences. If two identical boats, one a dagger, the other a centerboard, both optimized, (streamlined/sealed slots, well fitted boards, similar sectional shapes, etc.) There would be only the weight difference to note except in ghosting conditions, where very slight drag gains could be advantageous.
     

  15. Paul B

    Paul B Previous Member

    Incorrect.

    Read what I said above. Pre-rule-change boats did convert and raced against their ex-sisterships, with the only difference being the CB configuration vs the daggerboard configuration. Similar setups and preparation and sailing skill. The weights were the same due to min weight rule.

    The CB boats were not anywhere near as fast as the daggerboard conversions. Even with a gasket you are dragging along a lot of water in the CB trunk and it is surging.
     
Loading...
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.