Rudder Area In Clr. Calculations ?

Discussion in 'Boat Design' started by viking north, Sep 17, 2011.

  1. viking north
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    viking north VINLAND

    I know this is a past debated subject but just in case there has been updated in engineering I throw it out there again. Some designers recommend including, some recommend not including the rudder when calculating the CLR. Since it is part of the underwater profile i'm inclined to included it or at least some percentage. What is the general consensis to do or not to do ? --Tnx.--Geo.
     
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  2. jehardiman
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    jehardiman Senior Member

    Depends on the hull form and which lead rule of thumb you are using, but generally the moveable area is not.
     
  3. viking north
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    viking north VINLAND

    Jehardiman--Sloop rigged (15% of DWL lead mast stepped on stn #4 of 10 of the DWL)- old style narrow beam to length --(8ft. on 28) double ender canoe stern -- full length 10 n. deep full keel on which is mounted a 9ft.long by 14in. deep modified 63 010 series foil / 63 010 skeg skeg off which is hung a 00 series rudder/shaft/quadrant combo. (This is a surfboat style/lifeboat conversion it's existing keel is 9in.wide by 5in. deep on which I built a 5 in. deep taper section to accomidate the foil and sheg.) Included is a rough to scale doodle drawing. Note the rudder will be deep and on the large side. Tnx.-Geo.
     

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  4. jehardiman
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    jehardiman Senior Member

    Don't include the moveable area, it is small (<10%).

    And, for what it is worth, in my opinion a 63xxx foil for a keel of those dimensions is a waste of time, money, and effort. 2:1 elliptical nose, parallel sides, 1:4 taper at the trailing edge will work just as well with such a low span to thickness and aspect ratio. Check out Hoerner FDL.
     
  5. DCockey
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    DCockey Senior Member

    Where did you get the 15% of DWL lead from? I assume you did some research and came up with that as a recommended lead. Did your sources include the rudder or not?
     
  6. viking north
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    viking north VINLAND

    DCockey: Got the 15% lead from several reference books including that used by an old boatbuilder I once knew-A Mr. Rosbourough of Rosbourgough Design and Yachts Fame.(Did a little study under him.) Skene's recommends 14 to 19% lead without the rudder area considered. Brewer (I think have to confirm) recommends including the rudder area--Several lesser known: some say yes some say no. Jehardiman--yes I know there's not much to be gained but it gave me an opportunity to study foils-basic foil design and building such. What better to practise on than one's own boat--No bitchin customers other than an old man talking to himself :)-- Tnx. Geo.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Ingore the rudder area if employing a spade, use 50% of the area if it's skeg or keel hung.

    Lead can be affected by several factors; hull shape, volume distribution at optimum heel angles, rig type and shape, appendage type and shape, etc., etc., etc. Knowing the little I do about this design, you should likely be on the 13% side of the coin in terms of lead. You don't have fat after quarters that's going to drive her bow down or leave a big quartering wave and the aspect ratio in both appendage and rig is low. I can only take guess about the "penetration" ability of it's entry and other aspects of the design.
     
  8. viking north
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    viking north VINLAND

    PAR thats my fault re the design specs I have all the info, the lines and the hydrostatics of the canoe hull but stubborn me wanted to do the keel work so the designer I am working with on an as need basis doesn't have all the info to produce her complete drawings keel/skeg/rudder included. He's a friend and I think deep down he thinks I'm some sort of mad scientist on this keel thing but too kind to say so. Even the locals (all boaters) hassle me " you're still working on the keel design--you'll never get that thing in the water". "Why didn't you just buy one" Somewhere in there is a message, but i'm having too much fun and too old to listen. Yes 50%, of the rudder area doing so I can be only 50% right or 50% wrong on such a small area it possiby will have little effect anyway :) OH and reduce the lead a little-- I get your drift there --she's has no hollow in her entry but not what i would say full either, and despite having a canoe stern her CB is just a few inches aft of midships so all in all fairley symetrical. Canoe hull weight is approx. 2500 to 3000 lb.(have to re weigh at this point of completion of stripping out) prismatic at 4000 lb is .5849 Tnx. Geo.
     
  9. PAR
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    PAR Yacht Designer/Builder

    That also doesn't appear to have enough rudder area and the skeg is way bigger then it needs to be. The "brewer bite" thing you're attempting should be abandoned, in favor of a clean skeg, maybe with a bridge at the shaft only, but no lower. Also how much higher is the bottom of the skeg then the keel? A couple of inches is all you need and can afford with shoal draft appendages.
     
  10. viking north
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    viking north VINLAND

    Know what you're saying -- re the skeg --I'm kind of locked into a comprimise on that one as the shaft tube is FRP and glassed in over the top of where i would need to place some substancial keel bolts to make a shorter horizontal length skeg good and strong in case of grounding in a surf situation. I can still play aroung with it and I have another fastening idea but might be stuck with that Brewer Bite :). The rudder I just drew in without any thought of correct size other than it's general shape and being as deep as possible for those following seas thus the need to add dept. in the skeg . The entrance to my bay "Shag Bay" has two islands and a diagional reef plus it splits at the opening into another bay, "Blind Bay" that also has a diagional reef. Theres lots of deep water in the main channels (up to 90ft.) but when leaving or entering on an outgoing tide(3.5kts) with a strong on shore wind --hang on to your hat -- steep 10 to 15 ft. cross seas--with lots a foam in their teeth. ( we call it coming thru the washing machine)90% of the guys turn back or head for another bay --I like my long keels-big, deep, and strong rudders thus the extended shoe with a bearing cup. :)

    2:00 a.m. goodnight all off to the bunk ---Geo.
     
  11. Tad
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    Tad Boat Designer

    Rudder size is important for reliable handling under sail.....that is at low to very low speed. If you intend to always run the engine it's less of an issue.

    In a sailing vessel rudder area should be between 1 and 2% of sail area, if your sail area (main & 100% fore triangle) is 400 sq. ft. then the rudder should be close to 6 sq. ft. minimum.

    I understand your desire to keep the rudder heel above the keel bottom but I would really go for more depth to the rudder.
     
  12. viking north
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    viking north VINLAND

    Thanks TAD my sail area 196main-154 jib(fore triangle) total 350. I think I'll still base it on 400sq.ft. and go with the 6 sq. ft. of rudder as you suggested. I'm pushing on the dept of the skeg as much as i can,- it's extended shoe is the base for the rudders bottom bearing. Also fighting at reducing it's horizontal length as suggested by PAR but man i'm struggling with keel bolts in that area with the existing shaft tube already glassed in-Just don't want to cut it out it is so well done. I have a couple of different approaches to try yet but yes get the rudder deep. Thanks --Geo.
     
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  13. MikeJohns
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    MikeJohns Senior Member

    Make the rudder larger, The trailing edge would be better straight too, not curved. Also the prop and aperture breaks the articulated foil (skeg rudder combination) and the skeg below the rudder is too short vertically and the skeg too long to contribute since you'll get mostly spanwise flow in that region as it sits. So the skeg is not so useful as you have it.

    There's no advantage using low drag bucket foils here. JEH gave good advice before. Just adopt something easy to fabricate with parallel sides (body) and taper the TE and get a parabolic LE .
     
  14. viking north
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    viking north VINLAND

    Mike it's another game of trade offs, i'm stuck with the the fact this is a conversion and have to live with an as built existing hull, allbeit not a bad hull form. As prev. mentioned it's a case of having to go with a horizontally longer skeg in order to install some substancial keel bolts within it's structure. (future photos of the build will better show this) However the good advise given is definately a driving force here and i have come up with a modification involving a stainless base plate and vertical webbs along with existing in place fasteners that will allow me to reduce the skegs length a little. The end result will actually be stronger-- all the better for rudder support. Big rudder--Big rudder -- that I can handle-- Tnx. Geo.

    P.S. While i know i'm not gaining alot, I'm having fun with the foil shapes, cost and time is not a factor here (finally) resulting in i've become Oh no! Oh No! my daddy always told me play around with those and you'll become a FOIL FREAK :p
     

  15. xlr8
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    xlr8 Junior Member

    There is a simple method to calculate rudder area:
    Minimum A= 0.003 x s x h /L + 0.05 sqm
    Max A = 0.004 x s x h / L + 0.15 sqm

    s= sail area sqm h= height of fore triangle from DWL m L= horizontal dist from top of fore triangle to area CG of rudder
     
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