Twin rudder issue

Discussion in 'Boat Design' started by Michail, Sep 16, 2011.

  1. Michail
    Joined: Aug 2011
    Posts: 61
    Likes: 1, Points: 0, Legacy Rep: 15
    Location: Chile

    Michail Junior Member

    I started building "creatively" a sailboat based on the design of souriceau.

    I have two questions:

    First, is there any real advantage in having two rudders? The idea of the original design was to use two rudder fins so that the boat can sit on them when the tide is low. I found this an excellent solution, because the usual way to do it was to use two keels, but sailboats with two keels are much less efficient compared to single keel.

    And if you have two fins, then obviously you would have to have two rudders, so it was a natural decision to have them in the original design.

    However, apart from this, is there any other advantage? Can the twin rudder design be changed to a single rudder design?

    Second. Since I am constructing the boat in fiberglass, I decided that making strong fins and attaching them to hull is not a trivial matter, and since I do not really need this feature of boat sitting level in low tide, I dumped the idea of using two fins.

    If I eliminate the fins, I would have to increase the area of the rudders to keep the same lateral section, and since the fins account for 30 % of the area versus 70 % rudder, I would have to increase the length of the rudders (extend them aft), since having 30 % in front of the axis and 70 % behind would make the rudders too "soft"
     

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  2. Stephen Ditmore
    Joined: Jun 2001
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    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member

    I'm thinking a single rudder should be deeper than dual rudders need to be. This would be especially true if your boat's wide stern causes it to trim forward as it heels, lifting the top part of a centerline rudder clear of the water's surface. Some raceboats with single rudders have found it advantageous to move the rudder forward in order to keep it immersed.

    Concerning keels, I too have heard that low aspect dual keels are not efficient, but I've heard that high aspect dual keels, where each is smaller in area than a single keel would be (but just as deep) seem to do pretty well.

    If you have draft restrictions I'd say stick with the twin rudders. The issue is building the skegs strong enough? Oh, I get it, you want to eliminate the skegs. What about going with deeper, higher aspect rudders, and if necessary having them kick up or slide up/down? Also, with twin rudders I don't think losing some lateral area would be a terrible thing as long as you use a stall resistant "wide bucket" section - probably NACA 0012.
     
  3. Michail
    Joined: Aug 2011
    Posts: 61
    Likes: 1, Points: 0, Legacy Rep: 15
    Location: Chile

    Michail Junior Member

    I do not care about letting the boat to sit level, so I do not want to spend efforts on thinking about the fins and their attachment.

    However, your other point about heeling and a single rudder with wide stern seems to be a point well taken...

    The stern of the original design is really very wide, in fact, it is about 98 % wide compared to maximum beam. (stern about 2.15 m., beam 2.2 m, LOA 4.75 m.). Since I will keep the contours of the boat, then it seems that it would be safest to stick with the twin rudder...
     
  4. tunnels

    tunnels Previous Member

    The way i see it you are trying to reinvent the wheel .:eek:
    Twin rudders are there for the reasons discribed !! :p
    The twin rudder fins you call them are also there for a reason just you havent realized why yet !!!. :eek:
    You are playing with something that has already been designed that way for a reason !,you need to get your head round it !!:?:
     
  5. Lister

    Lister Previous Member

    This is a very dangerous initiative.
    How many boat did you built berore this one?
    If none, keep the "creativity" to something else, and buy the Souriceau plans and follow them to the letter.
    Lister
     
  6. tunnels

    tunnels Previous Member

    I totally agree !!! part time 25 cent redesigneers are the people that go to sea and end up having to be rescued because something failed !!. :confused:
     
  7. CatBuilder

    CatBuilder Previous Member

    Just make sure the Center of Effort of your new rudder is precisely where the COE of the pair of designed rudders would be. Don't throw the balance off.
     
  8. tunnels

    tunnels Previous Member

    So in other words stick to the plans:D
     
  9. CatBuilder

    CatBuilder Previous Member

    :D

    Sort of. I was thinking he could change over to just one rudder, but he has to use the same foil shape and account for the center of effort.

    When I saw he wanted to extend the rudder aft, I got worried. :D
     

  10. tunnels

    tunnels Previous Member

    If I eliminate the fins, I would have to increase the area of the rudders to keep the same lateral section, and since the fins account for 30 % of the area versus 70 % rudder, I would have to increase the length of the rudders (extend them aft), since having 30 % in front of the axis and 70 % behind would make the rudders too "soft"

    But they dont have 30%+70% !
    The 30% is the non moving fins as you call them ,so its not really a part of the rudder blade as such !!
    You had been draw it all out full sized to get your head around what you are playing with .
    All sounds like a path to a giant sized cock up looming to me !!:eek:
     
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