Preliminary Rudder Area approximation

Discussion in 'Boat Design' started by Mitch1990, Oct 18, 2020.

  1. Mitch1990
    Joined: Feb 2020
    Posts: 71
    Likes: 0, Points: 6
    Location: Maine, USA

    Mitch1990 Junior Member

    Hi,

    I have been looking through the text I have by Molland regarding rudder design and I can't find any way to estimate rudder area for preliminary design? Molland states that ideally a course keeping and maneuvering assessment would be conducted although not normally possible during preliminary design. He goes on to say that the rudder area is normally estimated as a proportion of the lateral area with lateral area being the projected profile area.

    What is a good starting point for this percentage for a hard chined hull 18m LwL?


    Mitch
     
  2. sharpii2
    Joined: May 2004
    Posts: 1,955
    Likes: 137, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    I would start off with finding the profile (side view) area of the entire boat above the WL, including the sails.

    Then, I would multiply this by 1. 25 % (0.0125).

    This would give me the area of an ideal rudder, so it would just be an absolute minimum.

    Since no rudder is ideal, the profile length has to be taken into account. If this number is unusually large, the rudder is going to much more area. This is why the rudders on 19th century cat boats tended to be huge. The rudder was supposed to be useful with the centerboard fully retracted, so it had to be quite shallow. Thus, it had a very low aspect ratio. This made it quite inefficient. Greater blade area made up for this inefficiency--at the cost of much greater drag.

    More on this later.
     
  3. Mitch1990
    Joined: Feb 2020
    Posts: 71
    Likes: 0, Points: 6
    Location: Maine, USA

    Mitch1990 Junior Member

    Thanks Sharpii,

    The general consensus is to take some percentage of the profile area. Do you have a reference for this? I am thinking of using th DNV formula as a starting point, I haven't checked LR in detail but they don't appear to have a similar formula for SSC.
     
  4. TANSL
    Joined: Sep 2011
    Posts: 6,244
    Likes: 302, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    rudder design https://www.boatdesign.net/threads/rudder-design.14571/
     
  5. sharpii2
    Joined: May 2004
    Posts: 1,955
    Likes: 137, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    The number I gave you was part of a system I developed for my own use.

    The other part was an adjustment system based on the profile length of the rudder.

    The system works like this:

    1.) find the square root of 0.0125 times the (sail area plus
    the profile area).

    2.) Divide the profile length of the rudder by (5.0 times the product of step 1.))

    3.) Add 1.0 to this product.

    4.) Multiply the sum of the previous step by the product of the first step to get the blade area of the rudder.

    Using 5% of the underwater profile for rudder area will probably not give adequate rudder area for more modern sailboat designs. This is because the keels of these more modern designs have far less area than those of more traditional ones. Also, since the more modern hulls tend to be shallow, there is very little underwater hull profile as well.

    That system was probably developed when yachts first descended from sailing work boats, which usually had both deep hulls and long keels.

    Back then, there was often more hull profile underwater than there was above the water.
     
  6. TANSL
    Joined: Sep 2011
    Posts: 6,244
    Likes: 302, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Mitch1990's ship is likely not a sailboat.
     
  7. fredrosse
    Joined: Jan 2005
    Posts: 372
    Likes: 43, Points: 38, Legacy Rep: 56
    Location: Philadelphia PA

    fredrosse USACE Steam

    An observation from what little direct data I have, indicates widely variable rudder areas. For example, a 30 ft cabin cruiser, planing hull, with, in my opinion, a dreadfully small rudder area of about 0.7 square feet. Steering was OK at speed, but maneuvering difficult when docking.

    At the time I owned that boat, I was working for the USACE, dredging operations, and typical 34 ft workboats there had rudders about 3 square feet, about 4x what the pleasure boat had.

    So...what kind of boat are you interested in??
     
    TANSL likes this.

  8. DCockey
    Joined: Oct 2009
    Posts: 4,742
    Likes: 333, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Rudder Design for High Performance Boats, Donald L Blount and Dudley, Dawson, Professional BoatBuilder N078, August/September 2002 discusses designing rudders for powerboats. It includes a graph of rudder area as a function of displacement (up to 10 million lbs) and maximum speed (up to 50 knots). Back issues of Professional BoatBuilder can be downloaded for free at Professional BoatBuilder Back Issue Archive https://pbbackissues.advanced-pub.com/

    Performance by Design
    , Donald L Blount, 2014 has a 12 page appendix with similar information to the PBB article.
     
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.