new rudder needed?

Discussion in 'Sailboats' started by theob, Nov 23, 2014.

  1. theob
    Joined: Nov 2014
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    theob Junior Member

    My father built this very heavy and charming 14 foot sailing dinghy. I've done a lot of reglassing on the inside, painting, reconditioning...and she's looking great! excellent for fishing, but when it comes to under sail...a bit sluggish, to say the least.

    One simple improvement I think is to replace the sheet of aluminium rudder with something that has a little more...well... design!

    This is my first foray into foil/blade design, and I think the most pragmatic solution will be to sandwich some or all of the existing rudder blade, so that the raising and lowering is still easy enough without completely rebuilding the stock.

    If anyone had some basic tips on where to start the process, that would be a great help, thanks. the existing blade is 500 mm long, from where it comes out of the stock, width is 210 mm.
    photos attached- last one is of the internal centre board- a piece of plywood nicely shaped!

    cheers
    Theo Brandt
     

    Attached Files:

  2. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Save the bother of a fancy foil shaped rudder on that little puppy, as it will make no noticeable difference. On a technical level you can make an argument, but frankly that boat will never get going fast enough, to warrant the foil shaping trouble and the 1/20th of a knot difference in speed isn't going to be something you'd ever notice. Can you post more pictures? She's an interesting hunk of dinghy.
     
  3. theob
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    theob Junior Member

    thanks- I was hoping that it might improve her agility going about. In light breezes pushing the tiller over doesn't do much! Even a little more surface area might help? Having said that, not faffing about with making a foil sounds appealing! - mind you a nicely shaped bit of marine ply might look nicer... more pictures below.
     

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  4. souljour2000
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    souljour2000 Senior Member

    That is a nice-looking small craft...bigger and longer than it looks in the pics...If you have never done it...sit in some shallows near the beach with a light-med chop coming in and fill her to the gunnels with water and keep the engine (covered well,watertight if possible) and even another person of reasonable size in the boat with you....Sitting in 3 feet of water or less...see how she handles when full to the gunnels...and if she behaves well thus-filled, while you bail her back empty...If she still is more or less horizontal by the time you have bailed her half way, then I'd say she is nearly as safe as she is pretty.
     
  5. Canracer
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    Canracer Senior Member

    Sweet looking boat. Trouble in light wind? That's what the little outboard is good for. I am currently reshaping my rudder and centerboard. It takes a bunch of time and money, but that's the kind of thing I'm into. Also I already had the supplies and tools just sitting around my garage.

    Initially, I'd say that a rudder upgrade would have to include a new headstock. From your picture it appears that the stock is specifically designed for sheet metal.
     
  6. theob
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    theob Junior Member

    thanks guys- she has a ton of buoyancy, fore and aft and side thwarts all have glassed in air tight cavities (with bungs). Point taken regarding needing to change stock- that is why, if anything is to be done it would be sandwiching existing with two bits of shaped ply.
    I don't have the outboard when sailing- just stow some oars!
     
  7. WhiteDwarf
    Joined: Jun 2011
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    WhiteDwarf White Dwarf

    Foils for small boats

    Theo,

    PAR is correct regarding the speed improvement to your boat from a properly foiled rudder, but in terms of handling there could be a modest gain as a flat plate must produce more turbulence on the low pressure side when deflecting flow.

    Producing a nice foil is very easy, and a satisfying job.

    Yes, you will need a new rudder box as well, but that is little more than wood butchery.

    I hope the attached from Australian Amateur Boatbuilder helps...
     

    Attached Files:

  8. PAR
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    PAR Yacht Designer/Builder

    With a plate type of foil, on a boat like that, it'll stall fairly quickly if put hard over. You might try a new sailing technique when in light air, which is to ease the sheets a bit and bear off, just as you're coming to a tack. Once she's gained as much as she will, slowly but deliberately, put the helm down and bring her through the tack, with an eye toward not too much helm deflection, so the rudder can work all the way through.

    What I see often in these types of light air, low speed maneuvers is the helm is deflected way too much, way too fast, which kills what little speed you have, the rudder stalls at some point in the tack and you end up in irons or just a cumbersome maneuver.

    Maintaining boat momentum and steerage is key, so watch the helm angle and commit to the turn with an eased helm, using smooth a motion.

    If you want to make a wooden rudder, consider strip planking it, rather than using big hunks of solid wood. Even if edge glued, these tend to warp, while strip planked don't have enough internal stresses associated with them to develop any significant stresses.

    [​IMG]

    This is a strip planked foil blank, glued up and getting shaped.

    [​IMG]

    Check out this site.

    http://pdracer.info/building/foilsandrudderca.html#
     
  9. theob
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    theob Junior Member

    thanks everyone- plenty of food for thought
     
  10. SukiSolo
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    SukiSolo Senior Member

    One other thing to note is the c/board. It may be a little small, leading to a lot of leeway. Maybe ask someone to have a look whilst she is sailing upwind especially in light air and see is she is drifting off. Increasing the size or improving the shape makes a big difference in this area, probably more than the rudder alone.

    The sail looks very small, I would have expected at least 8 sq m of area and this may be part of the speed problem. Another thing is - balance, it may be that the mast is too far forward an needs moving or raking back to get a reasonably balanced helm where she will tack more easily. She should have light weather helm when going upwind, if she is neutral, rake/move the mast aft.

    A simple rudder would be a fixed blade laminated as PAR has already outlined. A lot faster and easier to make than a lifting one with matching housing, more reliable too.
     
  11. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    I agree that changing the rudder section into an airfoil is not a priority here, considering the fact that it will work mostly at low speeds. In this speed range a flat-plate rudder section will work just as well as an airfoil would.

    However, there is one simple modification you can do in order to sensibly improve the stall and post-stall characteristics of your rudder. Adding a series of leading-edge serrations is a proven method for increasing the stall resistance, the post-stall lift of a flat plate rudder, without affecting the drag.

    Check the attached files, where you can see the geometry of the serrations which was tested by Selig et al., and where they should be placed on the rudder blade.

    Cheers.
     

    Attached Files:

  12. Skyak
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    Skyak Senior Member

    A very interesting suggestion. I have heard of leading edge 'protuberances' improving stall characteristics but I have never seen it quantified. The down side is that it is a bunch of metal cutting (not fun work) and the result will be a real weed grabber.

    My offering would be to tack a bullet profile shaped leading edge on to the aluminum plate. This is the easiest improvement and has little on no downside. The one note, only one bolt hole can be tight the others are slots for movement of the wood or plastic relative to aluminum. The profile would just be the nose of a typical NACA section reaching tangent to the plate. This nose adds a little area ahead of the pivot and greatly improves the stall characteristics of the rudder foil.

    Your boat looks attractive and strong. I concur that the sail looks rather small and suspect that a big part of your trouble might be plain lack of speed. I am not a fan of motors on such boats and would suggest a sculling oar would be a fine addition for maneuvering with the sail up.
     
  13. theob
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    theob Junior Member

    thanks Daiquari- this would prove to be an excellent fish scaler as well I expect!
    luckily it is all coastal sailing- no weed to worry about.
    Skyak, I am having a few problems visualising your remedy- if you happened to have a reference image hany.
    agreed in terms of sail area- the boom is somewhat over length however. The foresail is worse, being about the size of a large towel! And I never mix sail with motor...one for adventures, one for fishing...
    cheers
    Theo
     
  14. PAR
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    PAR Yacht Designer/Builder

    The Humpback leading edge has been tested to some degree and the results are what you'd expect. At certain speeds these can be advantageous, but the shape needs more then just a profile, they need to be 3D, so they can act as vortice generators. Getting this shape right will vary across the speed range, not to mention is quite difficult.
     

  15. Canracer
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    Canracer Senior Member

    You can order a foam core from a specialty shop. They will CNC mill some HDPU into a nice foil shape. If you tell them you want it in two halves, they should be fine with that. Glue the foam onto each side of your plate and cover everything with a couple layers of fiberglass. There you have it; a nicely shaped rudder. I'm not sure what type of epoxy will work on aluminum, but I'm guessing that something is possible.
     
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