Bridgedeck centreboard why don't they work???

Discussion in 'Multihulls' started by valery gaulin, Jan 10, 2017.

  1. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Another point of view by Phillip Berman

    http://www.multihullcompany.com/Article/Daggerboards_vs._Keels
     
  2. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    An f-41 with keels,....
    [​IMG]

    I'm not excited about the extra draft of this idea..
     
  3. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    the Virtues of Shallow Draft

    Rather than mini-keels, I might just have to go backe to the boards in the hulls idea if I found the central board idea unworkable. Why would you ask?? I love shallow draft capabilities,...I mean really shallow draft.

    I imported the first Fountain/Pajot 37' catamaran to the US in 1986. It had two kick-up centerboards, one in either hull.

    With the boards and rudders kicked up I drew 19 INCHES! of water. I once took that boat on a trip thru the backside of Cape Hatteras
    [​IMG]
     

    Attached Files:

  4. valery gaulin
    Joined: Jan 2017
    Posts: 215
    Likes: 20, Points: 18, Legacy Rep: 7
    Location: Montréal

    valery gaulin Senior Member

    @Brian Elland: I really love the shallow possiblities of a Catamaran.

    I think that central boat idea is for sure possible with some compromise.

    I like the compromise of having steel cable, like stays, to solidify the entire board and prevent flex without too much penalty.

    This is my opinion anyway.
     
  5. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 303, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    To what extent do details in the development of the boundary layer matter? Was your section designed to create a leading edge separation bubble, and if so, how might ventilation differ for sections that are designed to avoid leading edge separation bubbles?

    Can you elaborate more about the mechanism for the seal at the water's surface? I can understand how the ambient atmospheric pressure exists all along the free surface, making the pressure constant, but the flow along the surface also experiences acceleration. It's not obvious to me why there's a large difference in the chordwise pressure gradient in the near surface region.

    Did your models have low aspect ratios because of the limitations of the water tunnel? Most practical designs would have foils that extended much deeper compared to their chords. Do you have any plans to explore larger spans?
     
  6. valery gaulin
    Joined: Jan 2017
    Posts: 215
    Likes: 20, Points: 18, Legacy Rep: 7
    Location: Montréal

    valery gaulin Senior Member

    I think that if flow at the leading edge goes from the surface of the water to the bottom of the board you get ventillation therefore flow seperation. Inversing the flow from bottom of the board to the surface of the water at the leading edge by inversing the rake, forward rake, solve ventilation or flow seperation.

    Kind of similar to tip vortex of a wing. Forward sweep the wing of a plane and I think you loose alot of the tip vortex.
     
  7. valery gaulin
    Joined: Jan 2017
    Posts: 215
    Likes: 20, Points: 18, Legacy Rep: 7
    Location: Montréal

    valery gaulin Senior Member

    Kind of like this simple picture
     

    Attached Files:

  8. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    hopefully cmharwood can find the time to reply. must be a busy guy.
     
  9. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    BTW, just in case there are some readers unaware of a particular feature on the forum, I'll submit this info....

    If you see a "QUOTE=name >>", those two little >> will lead you back to the very posting where that quote came from.
     
  10. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Fairing for my Twin Centerboards

    I think I could put up with a fairing something like this for my twin nacelle mounted centerboards.....
     

    Attached Files:

  11. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Center Structure on Team NZ

    Take a look at that upper portion of that center 'pod' structure on Team NZ's new boat,....at time frame begining 2:08

    More on TNZ's new boat: https://www.youtube.com/watch?v=4SVT44tmWFw

    ...not that we would build a cruising cat like that, but we might try to emulate that central spine.
     
  12. Mulkari
    Joined: Jan 2012
    Posts: 44
    Likes: 11, Points: 8, Legacy Rep: 10
    Location: Latvia

    Mulkari Junior Member

    It would be very interesting to have centerboard vs mini keels comparision. Daggerboards on a race corse would certainly outtperform both, but when cruising then ease of use, safety during collision with some random stuff and shallow draft also are important factors. Since cruisers generally try to sail more on a downwind routes few degree of lost windward capability is not that important. Downwind centerboard boat all other things being equal certainly would outsail mini keel boat simply because board would be raised while keels are permanently fixed and dragging all the time.
     
  13. Ilan Voyager
    Joined: May 2004
    Posts: 1,292
    Likes: 225, Points: 63, Legacy Rep: 758
    Location: Cancun Mexico

    Ilan Voyager Senior Member

    For cruising boats, with no intention of high performance the simplest good solution is the best.
    No need of a heavy centerboard and all its possible problems as the system is nicely complex for a very hypothetic result. I prefer to use all this weight for more fuel. and the cost for first rate 3 speed winches, and a a track with a chariot you can push with one hand under 6 tons of tension on the main sail.
    Centerboards have the inconveniences of a big case full of water taking lots of place in the boat and a mechanism for the board susceptible of problems. Lots of maintenance also.
    Minikeels are an hydrodynamic horror and are excellent for breaking a hull in case of hitting some reef or rock in the same way the keel of a monohull breaks its hull.

    You have also the very good solution shape, first designed bu Rodney March (he "committed" so many good designs) in 1976 for the DART 18 beach catamaran.
    This design has been copied on dozens of designs small and big catas with always good results. The performances are very decent even good. I remember a pair if sisters, no more than 220 pounds together, with their improved Dart 18, who were a pain in the a** for all the machos in the regattas in light and medium wind. Their Dart was astonishing fast.
    There is nothing to break, and if hitting something, the hull simply climbs and slides on the obstacle.
    On big catas that gives plenty of room for the tanks. As always there are cons; more work making the hull, more wetted surface, a bit heavier.
    If I want a no worries beach or cruising cata, I take this design immediately.
    Pics SL 16 beach cat designed by Yves Loday 2012

    But a well designed daggerboard is the best with no doubt.
    With some brain juice added there are very few problems. A case with a crash box and a "fuse design" of the board, some teflon strips and all that works smoothly and surely.
    With a small inclination the intrusion inside the hull is minimal, and the boards can be made cheaply and efficiently in cedar, glass and epoxy. We used to make three for the price of two for the symmetric profiles, and we sold plenty.
    A well designed wood glass board can stand lots of abuse, it's very hard to break laterally as they are very resilient, and in case of frontal hit a "fuse" design with the crash box will take care of the occurrence.
     

    Attached Files:

  14. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Not a bad idea there Ilan.

    Back when I was the east coast distributor for Prindle and NACRA catamarans (Chesapeake Catamarans), there was a new man who came in and took over NACRA. He asked me what might gain him a bigger market share in the beach catamaran market. I suggested he might make a 'daggerboard-less' model, perhaps utilizing something like the Dart skeg. They did, and the first one was the NACRA 5.0, followed closely by the 5.7.
     

  15. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    CS hull form

    The Dart 'skeg' brings up another interesting possibility. :idea:
    How about a variation on Malcolm Tennant's CS hull form?? I had suggested such a hull form for my Dynarig motorsailer.

    http://www.runningtideyachts.com/dynarig/Tennant_Hull_V_ChainDrive.php

    Another subject thread on the CS hull form's potential use for a sailing vessel
    http://www.boatdesign.net/forums/motorsailers/cs-hull-sailing-performance-35387.html
     
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.