Rudder Area Calculation

Discussion in 'Boat Design' started by ravenousrob, Oct 24, 2015.

  1. Oleboynow

    Oleboynow Previous Member

    not sure how that may work on a flat bottom, do you mean total water plane area?
  2. Oleboynow

    Oleboynow Previous Member

    well we are on same page
    I,ll make him a dwg, that is a nice wee boat, a bit longer with nicely raked stem, um, may do same in Corten
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    See picture

    Attached Files:

    • area.jpg
      File size:
      26.5 KB
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    As noted it is a "rough rule of thumb". No, profile area as noted.

    The poster gave next to no hard technical information at the beginning thus it is all one can say with regards to a starting point for a rudder - a first pass guesstimate. It was not until post number 13 (after the initial exchanges) that more information was provided to assist in clarifying such a rule of thumb.

    From the additional info provided though, PAR nailed it first time :D
  5. ravenousrob
    Joined: Oct 2015
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    ravenousrob New Member

    Hi Oleboynow, thanks for the reply and thanks for the offer for the drawing, however I do not have the ability to read DFX files and I have already machined the components for hanging my rudder. Again thanks for the information.

    Hi PAR,
    Thanks for the information, in my sketch I have continued the skeg above the prop, you recommend not doing this, could you explain this further? Will this cause excessive cavitation for example, or is it just that the prop will run to close to the skeg. I will heed your advice on balancing! Forgive my ignorance, but what lines would you like to see of the hull? Again thanks for the input it is greatly received.

  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Generally, you don't want a tight enclosure on a "captured" prop. The aperture should be generous and minimum tip clearances followed, if not offered more room. Some say 15% wheel diameter is the minimum, but frankly, the real minimum should be somewhat more than this, maybe as much as 15% of the radius.

    In your sketch, consider simply drawing a line horizontally to the top of the blade, for the rudder port. Next draw a vertical line from the hull down to the rudder heel fitting support. I understand the need for the heel fitting support, but the top of the blade, needs little more then a flat spot for the rudder port, so move this up as much as practical, so each tip has as much room to wash it's flow through the aperture as practical. This one thing will also help cut down on noise, beating into the steel hull shell too, as water gets bashed into this opening, while the tips swing through.

    Lastly, screw all the rounded corner stuff. Make the blade rectangular with tight corner radiuses. No, radiused trailing edge or blade bottom, just straight edges with 1" radius corners.
  7. Oleboynow

    Oleboynow Previous Member

    Sent you an email as I am interested in the build, but they never seem to (go)
    And your IM,s are not activated
    Brit lathes eh, had A DEAN SMITH GRACE man, best

    Toolroom lathe ever made. Light years ahead of it,stime weight abt 3500kg

  8. philSweet
    Joined: May 2008
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    philSweet Senior Member

    A bit off topic, but you mentioned an 11" prop. That is really, really small for this sort of boat. I'm usually the guy telling people not to worry so much about efficiency and accept the standard commercial offerings as acceptable and use a smaller prop. But in your case, I think you should be looking at 15" minimum, and 20" would be noticeably better, but more expensive. It's the sort of thing you need to plan for early on. You can play around with any of the several online prop calculators to get a feel for how things shake out. The issue is the low speed. A 20 hp outboard may have an 11" prop, but it is set up to run 20 hp at 25 knots, not 5.5 knots. To get 20 hp at 5.5 knot, you are looking at around 500 lbs thrust. That takes a larger wheel.
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