T Rudder designs for cruising catamarans

Discussion in 'Hydrodynamics and Aerodynamics' started by Dave_S, Oct 19, 2020.

  1. Dave_S
    Joined: Sep 2018
    Posts: 17
    Likes: 0, Points: 1
    Location: Brisbane Australia

    Dave_S Junior Member

    Looking at 0° AOA to steady state waterflow.

    The objective is to increase speed in swell on all points of sail particularly VMG upwind at the same time I don't want to loose any light wind performance.

    I want to reduce drag to minimum so to this end....

    Has anyone fitted the wing on a pivot to allow it to move freely with changes in waterflow direction when pitching creating minimal drag until the wing is locked in position when needed.
     
  2. patzefran
    Joined: Feb 2011
    Posts: 240
    Likes: 21, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    Unecessary complication, look at classic A cat, they don't loose any light wind performance !
     
  3. Dave_S
    Joined: Sep 2018
    Posts: 17
    Likes: 0, Points: 1
    Location: Brisbane Australia

    Dave_S Junior Member

    From your comment I'm reading in to this that if i choose a symmetrical wing and set it to 0* AOA I should have little drag.

    Where should I go for help on these ?
    1. How do I choose a profile, do I just look for a symmetrical with the lowest coefficient of drag and rely on the boats AOA when pitching to do the work.
    2. Which shape in plan view, there are boomerangs and elliptical, straight, straight with rounded tips etc... I have a CNC router so most 3D shapes are possible
    3. How much lift do I want, we can do 20kn but we are usually somewhere in the 8 to 16kn range.
     
  4. patzefran
    Joined: Feb 2011
    Posts: 240
    Likes: 21, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    IMHO :
    Non foilers use rudder's winglet only for pitch damping, so you need symmetrical profile at 0° Aoa relating to static pitch trim of the boat .
    NACA 00x profiles are good for the job .
    You don't need sophisticated plan shape as the influence on drag is marginal ( airplanes don't use ellipical shapes anymore !) .
    For damping, you don't need much lift, looks like classic A cat winglets are around 2% or 3% of rudder's area
     
    Dave_S likes this.
  5. CocoonCruisers
    Joined: Dec 2015
    Posts: 71
    Likes: 21, Points: 8, Legacy Rep: 10
    Location: Marseille & BuenosAires

    CocoonCruisers Junior Member

    (Hey Dave, we had talked about this on facebook)

    I think you need to mention the scale of your cat :) Schionning Waterline 1480 if i remember right ?
    If people compare with A-cats the answers might come out a few orders of magnitude off..

    And pitch damping was actually your main point for the project right ? Hoping to avoid the disruptions to the airflow over the rig ?

    Re /planform : the ideal would be roughly eliptical wingload distribution, otherwise you'll drag massive wingtip vortexes and loose some efficiency. But you can reach that goal either through the planform or through twist. (Dig up tSpeer 's posts on the subject if you want to figure it out for real !)

    Re /sections : have you seen airfoiltools.com ?
    (The only borderline useful curves in there will be the ones for Re = 1 000 000 (still too little) and Ncrit 5 (way too much). You can filter the others out to get a clearer picture in the comparisons. Remember the L/D figures are for 2D = infinite wing, so you'll reach only a fraction of that in 3D practice. Unfortunately the pressure distribution on the sections is not displayed. You'll need to keep an eye on that to avoid sections with large suction peaks that can lead to cavitation even at your rather reasonable speeds.)
     
    Last edited: Oct 21, 2020
    Dave_S likes this.
  6. patzefran
    Joined: Feb 2011
    Posts: 240
    Likes: 21, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    Answer : Agree, I just gave an order of magnitude of A cat classic winglet surface. A quick computation of the condition to have the same adimensional pitch damping coeff
    for 2 different boat is approx S2/S1=L1*I2/L2*I1, where S is the winglet area, L the waterline length and I the Inertia pitch moment. Assuming I~L^4 ,
    S2/S1=(L2/L1)^3 , for L1=5.5 m (A cat) and L2 = 14.5 m (cruiser) this give a ratio = 18
    And pitch damping was actually your main point for the project right ? Hoping to avoid the disruptions to the airflow over the rig ?

    Answer : See Hoerner, fluid dynamic drag or Theory of wing sections Abbott and Doenhoff , elliptical planforms are theoretical optimum , but Straight tapered wings with rounded tips are within 4% of elliptical induced dragfor same aspect ratio and taper ratio from 0.3 to 0.5
     
    CocoonCruisers likes this.
  7. Doug Halsey
    Joined: Feb 2007
    Posts: 462
    Likes: 128, Points: 53, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    This isn't the whole story.

    It's true that a wing with nonelliptical planform can be twisted to make the loading elliptical, but the twist required to do that varies with the amount of lift. The rate of change of the loading with the lift depends solely on the planform.

    If your base setting has zero lift, then you shouldn't use twist.
     
    CocoonCruisers likes this.

  8. patzefran
    Joined: Feb 2011
    Posts: 240
    Likes: 21, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    I apologize, moment of inertia dimension is strictly L^5 wich gives an estimate S2/S1~(l2/L1)^4 =48 with L1=5.5 m and L2 =14.5 m
     
Loading...
Similar Threads
  1. Dave_S
    Replies:
    2
    Views:
    100
  2. 23feet
    Replies:
    2
    Views:
    233
  3. WildCard
    Replies:
    20
    Views:
    1,644
  4. Howlandwoodworks
    Replies:
    9
    Views:
    974
  5. KeithO
    Replies:
    6
    Views:
    1,088
  6. motorbike
    Replies:
    9
    Views:
    997
  7. Wally2
    Replies:
    3
    Views:
    1,023
  8. motorbike
    Replies:
    19
    Views:
    3,789
  9. Kim Klaka
    Replies:
    25
    Views:
    3,356
  10. dpaws
    Replies:
    5
    Views:
    1,797
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.