Dinghy Rudder Design

Discussion in 'Sailboats' started by BobBill, Dec 2, 2009.

  1. BobBill
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    BobBill Senior Member

    Is there a formula for dinghy rudder design?

    What are the differences between say a barn door rudder, a blade and a balanced foil, other than the obvious shapes?

    If one wished to covert say from a barn door rudder to a balanced design, are there tables or formula to refer to?
  2. Ad Hoc
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    Ad Hoc Naval Architect

    For a dinghy, pretty much anything will do. I wouldn't worry about formulae etc, since the speeds you're going and the size of your boat doesn't warrant an "analysis".

    Basic rule of thumb though, take 2% of your profile underwater area, and this is the size, area, of rudder you'll need. But since your boat is so small, simple trial and error will suffice, if you not sure.
  3. PAR
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    PAR Yacht Designer/Builder

    Nothing personal, but if you have to ask, you're probably not qualified to make this change without some direction. Since you've asked, post a picture of what you got and some general dimensions. Better yet would be the year, make and model of your boat. If I don't have line drawings on it and basic spec's I'm sure someone here does and a new rudder design recommended to suit your needs.

    There are several different types of rudders that can be employed on small craft. All have good and bad issues related to them, which is a common theme with decision making about yacht design features. Rudders tend to serve the general purpose of the design. A racer will attempt to get the least amount of drag with the most efficient shapes and the lightest structure for the loads expected. A shoal cruiser may have no need for this level of expense, draft, complication, contrivance or efficiency, so a barndoor could be installed, though this would usually be seen on an era piece, like a cat boat. Typically, small craft have to address shoal requirements, so the rudder may have a kickup feature, which can be engineered in a few different ways as well.
  4. CT 249
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    CT 249 Senior Member

    Rudder design is critical for speed and design in many dinghies. If you get the wrong design, the boat can be almost unsailable in some conditions.

    A barn door rudder can make a high performance boat basically impossible to sail; try sailing a fast cat with the rudders up to give the barn-door effect. I don't think the 2% rule comes close on dinghies. One of my dinghies (Int Canoe) sails beautifully with an under-hull blade about 12" x 7'; the Tasar has a similar lateral area in all other respect but sails beautifully with a rudder that is (at a guess) 3' x 10'. The Laser is somewhere in between and has a heavy helm, even when sailed well enough to win titles, because it is raked whereas the Tasar and Canoe rudders are vertical and have the rudder post effectively about 30mm and 40mm behind the leading edge.

    However, they vary so much you're better off just checking out good comparable boats.
  5. BobBill
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    BobBill Senior Member

    Rudder Design

    Thank-you all for your time and advice.

    (Par, I am neither professional designer nor builder, but I have a few turbo-ed keelboats, dinghies, bikes and cars under my belt. All with improved performance. I am qualified, as are all of us, to experiment. Poly and epoxy are my mainstay house repair tools (boats too) even duct tape. I do not know all the answers, ergo this thread. Nothing personal.)

    The boat is a 1973 Kite, long defunct class, unfortunately.

    It is 11'7" LOA, 10'10" or 11' LWL, 5' beam, 200#s all up. It will carry 90 square feet of sail (original was 78 sq ft on wood flex Finn-like mast, not practical to replace or refurbish. Replaced with Force 5 spars/sail @91 sf.) Portsmouth was 100.9.

    (Also removing styro per Par's advice for which I am grateful.)

    To the point: Original boat came with rigid barn door rudder. I am replacing with a kick-up Laser II rudder.

    This rudder will be altered to be balanced, and, if necessary lengthened.

    I am also going to rebuild bow to be perpendicular.

    I will try to post old brochure that shows boat and maybe the rudder...will post pics of original rudder and the new one that needs rework.

    I am doing this because I like to experiment and breathe new life into abandoned (sail)boats.

    The rudder change to a kick-up is a purely practical decision, but experimenting with aspect etc cannot hurt.

    CT 249's advice coincides with my thoughts, and why I bought the Tasar/Laser rudder assembly. Seems comparable enough to begin some changes. Ad Hoc's 2% will be a good comparison mark.


    Attached Files:

    Last edited: Dec 9, 2009
  6. gggGuest
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    gggGuest ...

    Think your single/double quotes were in a bit of a mess Chris... I've put in feet and inches...
    I'd have put the typical IC rudder maybe a tad smaller than 7in chord. They really are minute: quite amazes me that they work!

    I reckon an L2 rudder is going to be in the right ballpark. I'd leave it out of the box until I'd tried it: balanced rudders are very unusual on dinghies of that size and have some potential downsides. Just make sure the leading edge is vertical/parallel with the axis of the fittings.
  7. BobBill
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    BobBill Senior Member

    Rudder Numbers

    gggGuest, mirrors my thoughts. I politely refrained from commenting on the measurement typos.

    Yes, I plan to try the rudder first. I needed the head anyway to create a kick-up tail and the balance part is pure experiment.

    I have a prelim sketch for it to be "balanced" to only about 30% or 7 inches below keel, then tapered to current shape and may lengthen to 4 inches deeper (longer).

    Will see how first test goes, as the sail lift will have a lot to do with it.

    May even mess with the dagger board a bit to create more lift, but that will be second to last step, after the bow tipping, if I decide to go that route.

    Fun stuff. Been messing with boats for 50 years and learning new stuff all the time.

    Many thanks to all who took the time to add a comment and advice!

    Will keep posting results under Turbo-Kite, FWIW!
    Last edited: Dec 6, 2009
  8. Munter
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    Munter Amateur

    I'd say you've got a reasonable chance of success in swapping the tasar blade for the traditional one. If rotated at the correct angle, the center of pressure of the tasar blade will be closer to the pintles and should give less helm. Don't over rotate it though or you may lose all feeling in the helm.

    I recently put a B14 blade on the back of a sailing canoe that had always had unpleasant weather helm from the significanly raked traditional style rudder. The new blade was a pleasure and seemed to improve performance a little.
  9. BobBill
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    BobBill Senior Member


    That is what I thought.

    When the two rudders are side-by-side, they are the same height; however, the Laser II is a true foil. the barn door is flat, with slightly tapered leading and trailing edges. It really is not very efficient; more blunt force, at least to my uninformed eyes.

    I believe the new foil will be much more efficient with less working surface, and the balanced configuration should also ease feel and help eliminate the terrible weather helm the door rudder produced. I have to agree with the previous observation that I too found the narrow blades on Laser II and Tasars puzzling, but the science seems to be in the foil shape and aspect.

    The only added surface, I have decided, will be the leading edge portion to secure a balanced foil, at least right now, and may taper it a bit to the bottom edge.

    The entire top of the lead edge will be underwater on plane downwind, so that should reduce problems in that area.

    When heeling to weather, normal effects are expected due to foil being hung behind and not under hull.

    I planned to use wood for the new leading edge, maybe mahogany, clear, and fixed it to current foil via SS fasteners or wood dowels, epoxied in place, then finished with glass cloth and fair it it to match current leading edge, maybe a hair finer edge.
    Last edited: Dec 7, 2009
  10. C 249
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    C 249 Junior Member

    I'm not sure whether the L2 rudder is exactly the same as the Tasar one, but it looks pretty close. In that case, there's no way I'd mess with it. On the Tasar, the amount of balance is just right to make a delightfully light helm that still has great feel.

    If you wanted to add more balance, changing the rake of the foil in the rudder box would be a lot easier.

    Sorry about the typos earlier, and Jim's estimate of the typical IC rudder size is no doubt more accurate than mine.

  11. BobBill
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    BobBill Senior Member

    Rudder Improvement

    No problem with the info from before. I understand.

    I bought the rudder assy as I was really looking for a head to change to kick-up blade. Now that I have the complete assy, looks to be best option to change over.

    I am guessing the different hull shape and weight (added 30#) will require a bit more rudder surface, but I do not plan to touch the blade until I sail it first. but, if it needs a tweak, will be ready to proceed. the alteration will be easy enough.

    I might get lucky and it will do fine as is.
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