Should we re-design or just rebuild my rudder?

Discussion in 'Boat Design' started by ArAKern, Feb 1, 2018.

  1. ArAKern
    Joined: Feb 2018
    Posts: 9
    Likes: 0, Points: 1
    Location: Southampton UK

    ArAKern Junior Member

    Hello et-all

    My rudder has failed the tangs have sheared from the shaft as I have the opportunity should I try and redesign the rudder to improve things?
    The boat is a Macwester 30' yacht with bilge plates and a centre skeg/keel they are known to suffer from a "heavy helm" (other owners have rebuilt there rudders is improve things but no design details are available)

    rudder section (645x470).jpg https://drive.google.com/open?id=14nihYxeznCC4lxy_NiOhzzigYXbpPdf1
    The things I could address seen in the photo are
    1, rudder shaft angle is steep, (self centres? due to gravity rudder was heavy 30Kg+ no shaft)
    2, There is little to no balance area to the rudder
    3, The shape of the rudder (improve aspect ratio?)
    4, Move rudder away from the prop.
    but are these actually issues?

    I deicide to do some investigation and measured 25 yachts of similar size with skeg & keel hung rudders the first thing I noticed that on all of them the length was longer than the chord, lowest was 1.4x up to 3.2x the Macwester rudder is 0.65 ie the chord is longer than the length.
    I also looked at surface area, range was 0.32m2 up to 0.63m2. I have see there is a rule of thumb between sail area and rudder surface area should be 2% but none of the rudders I measured achieved this the range is from 0.9% to 1.76%.

    Is there any other rules of thumb I could apply in designing a "basic" new rudder? balance area 10-20% surface area, chord to length, surface area to underwater profile....

    I did loosely measure up this ideal which would give me some balance area decrease the rake of the stem but also decrease surface area but increase the aspect ratio.
    Rudder edit (1024x734).jpg https://drive.google.com/open?id=1jX9EDZ6UUTH2tAR7YRuhAk-GN6iWFAE0

    any thoughts advise most welcome
     
  2. MikeJohns
    Joined: Aug 2004
    Posts: 3,173
    Likes: 198, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Yes You'll ease the load on the helm a bit with your suggested alteration. But so would wheel steering. The environment these type of boats are moored in can be very hard of the steering gear as they cycle through the tides.
     
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