Doubts about proper keel design

Discussion in 'Boat Design' started by andy47, Nov 6, 2017.

  1. andy47
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    andy47 Junior Member

    Hi all, I would like you to confirm if my approach of designing a proper rig is correct. I am designing a 2-meter autonomous boat that will sail across the Atlantic and I have put together a small-scale 65-cm model to test the concept and theory. Here's the picture of the model built for testing purposes:

    boat.JPG
    You can see it's propelled by a free-rotating wingsail that's adjusted by a “flap”. The center of the keel is aligned with the mast, i.e. the mast and the keel's CLP (Center of Lateral Plane) lie on the same vertical line. Note that the CE (Center of Effort) of a free-rotating wingsail is always (almost) aligned with the mast.

    The problem is, I have found out that this boat doesn't work very well in strong currents – it has a tendency to turn against the current. I assumed that maybe I should take the rudder into account when positioning the keel and this would also improve the rig.

    So I used a software to calculate the CLP of both the rudder and the keel:

    clp1.jpg
    The gray area is the lateral plane and the red dot is CLP. Here you see that the CLP is well behind the center of the keel, i.e. behind the mast. In the next picture, I extended the lateral plane in order to shift the CLP forward until it's aligned with the mast:

    clp2.jpg
    I have glued a piece of plywood in front of the keel to test the result:

    plywood.JPG
    Now, the boat is performing better in strong lateral currents. What I have learned is that I should move the keel a little to the bow in order to compensate the rudder's lateral resistance. However, I am still not very confident with the solution. The keel area seems too large and the boat doesn't look balanced to me at first glance, or at least it's not very common (I haven't seen it on hobby RC boats). When I posted a similar design a while ago on this forum, many of you have told me that the CLR is too much ahead of the CE (Center of Effort), resulting in a negative rig.

    What do you think? Should I take the rudder into account like I did? Is the keel in the last picture correct? Any comments are appreciated.

    By the way, here's a video of the boat sailing:

     
  2. gonzo
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    gonzo Senior Member

    The current will affect the course, but not the heading of the boat. If it is changing course, you have a different problem. You should research model sailboats since they have all those problems solved.
     
  3. rwatson
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    rwatson Senior Member

    Its also possible that the amas on each side "digging in" with wave and heel may be affecting the performance.

    I really cant think why you have three hulls with such a big keel. You are building in a lot of resistance for no reason.
     
  4. andy47
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    andy47 Junior Member

    Without the side floats, the boat may lean too much. It's critical for the correct wingsail operation (the "flap" must not enter the water). The idea is to make it like a trimaran. I have read that trimarans can regularly double monohull sailing speeds. That's why I think that the side floats should actually improve the speed. Shouldn't the benefit of lower angle of heel overcome the added resistance?
     
  5. rwatson
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    rwatson Senior Member

    As soon as you add the keel, the "double monohull sailing speeds" goes out the window.
    In those videos, you dont see anywhere near multihull speeds because of the large keel.

    Your term "lower angle of heel overcome the added resistance" is not even sensible. How does resistance reduce from lower angle of heel ? In a well designed monohull, resistance can actually decrease due to the immersed hull design.

    Have a look at a performance trimaran, and then have a look at your stubby little amas so close to the main hull, and realise that high performance multihull speeds are not going to happen on your boat.

    Once you give up on that idea, realise also that your attempt at preventing capsize is also doomed. Even worse, the amas will probably PREVENT the normal self righting abilities of a monohull.

    My "off the cuff" suggestion is to put some kind of float on the top of that mast, take any expensive electronics off, and then take it to your local beach and find some breaking waves. THAT is where you will find out about the crafts stability.
     
  6. andy47
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    andy47 Junior Member

    I mean that there are two things that will happen when I add the side floats:

    1) The added resistance of the floats will decrease the speed.
    2) The lower angle of heel will increase the speed because of greater vertical sail area.

    So these two factors fight each other and to me it's unclear which one is going to win.

    These are my thoughts - I can lower the angle of heel by using three options:

    1) Longer keel
    2) Heavier "bub"
    3) Side floats

    The highest speed would be obtained by finding a proper combination of these four numbers: keel length, bulb weight, float size and float distance. There is also a requirement that the boat must be self-righting (up to a 90-degree angle of heel, because the buoyant wingsail will help if capsized). Are you suggesting that I will get a faster boat by completely removing the side floats and increasing the keel length/weight at the same time? If I remove the side floats then I MUST increase the keel length/weight if I want to get the same heel angle at the same wind speed. By adding these floats, I can keep the keel smaller. Note that the heel angle is critical in my application - I have to be sure that the wingsail won't touch the water at high ocean winds. Then comes the speed optimization.
     
  7. rwatson
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    rwatson Senior Member

    "remove the side floats then I MUST increase the keel length/weight if I want to get the same heel angle at the same wind speed."

    That is the way to go.

    "There is also a requirement that the boat must be self-righting (up to a 90-degree angle of heel, because the buoyant wingsail will help if capsized). Are you suggesting that I will get a faster boat by completely removing the side floats and increasing the keel length/weight at the same time?"

    Yes. That is the only way to get the result you are after.
    Really, you need to aim for 180 degree recovery. You ARE going to get rolled. There is no way of avoiding it.
    If your model cant survive through breaking surf, its not going to survive the open ocean.

    "get a faster boat by completely removing the side floats and increasing the keel "
    ... AND redesigning the hull completely. You need a much more classical shape for optimum performance. You don't need to totally emulate the shape of large yachts, as being a much smaller boat, the hydrodynamic forces act a bit differently, but your current hull shape is quite inefficient. Best look into the shapes and configurations of competitive model boats. You probably should have started with a secondhand competitive model boat hull. eg News | Frank Russell Design http://www.frankrusselldesign.com/news/
     
  8. TANSL
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    TANSL Senior Member

    Andy47, you can do many things but the normal thing would be to not use two things that try to solve the same problem. Do some calculation to determine which of all possible solutions is most effective and discard all others. I'm sure that using only one of the possibilities you can get what you want but it is not enough to make videos, it is necessary to do some calculation to avoid the blind test-error process, which is always very frustrating. Good luck.
     
  9. rwatson
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    rwatson Senior Member

    This idea is a bit more "way-out", and only if you are feeling experimental. The rig you have looks potentially fragile in heavy weather, with all the bits and pieces and counterbalances hanging off it.

    You may get some advantage from a radical re-design.

    Like this photo. They are Savonius Rotor sails, that might be less problematical than a wingsail.
    For larger boats, the vibration can be a problem
    SavRotor.JPG

    here is another variation, that discusses the pros and cons of the three main types of spinning sails.

    http://blog.modernmechanix.com/mags/PopularScience/7-1933/rotor_ship.jpg
     
  10. andy47
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    andy47 Junior Member

    Thanks, I will post the CAD design of the 2-meter hull when it's ready. I will have to tweak the "standard" competitive shape a little, because the solar panels on top of the hull are fighting for space. I want to add as many solar panels as I can while keeping the bow hydrodynamic and narrow, so it's a little trade-off.
     
  11. andy47
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    andy47 Junior Member

    Do you mean mechanically fragile or does it have another meaning? The whole thing will be made from carbon fiber + Kevlar and all the rods you see will be carbon fiber tubes which I am unable to break using my hands, legs and weight.
     
  12. rwatson
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    rwatson Senior Member

    Its good that you have made it really robust. I presume the wing is sealed from water too.

    The individual parts may be strong, I guess without seeing it myself, its the hinges and joints that looked a bit vulnerable, not just to breakage, but debris like seaweed, old plastic etc.

    It might just be my perception without seeing it "in the flesh".

    I would definitely get the boat to roll a few times in the surf for the ultimate test.

    Lots of fun :)
     
  13. andy47
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    andy47 Junior Member

    This will be my priority to hide everything that can catch seaweed if possible. Also the keel will be swept-back and the bulb won't stick out in the front.
     
  14. Earl Boebert
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    Earl Boebert Senior Member

    I have a couple of suggestions based on playing around with model yacht designs.

    First, I would select a successful free-sailing model yacht design from the pre-radio control era and save design time by just building it. These boats were designed, often empirically through many revisions, to sail in a "balanced" fashion, that is, with decoupling of the roll and yaw axes. This means that they will track through gusts. One design that I know from experience has this property is Ted Houk's "Rip Tide" from 1949. I can tell you that when the one I built gets hit by a gust it just heels and jumps forward. Scaled up to 2 meters my not-t0-be-trusted back of the envelope calculation gives about 50 lb/23 kg which intuitively seems about right for an ocean going vessel. I would use the time saved to concentrate on refining my control system.

    For seaweed/trash resistance I would replace the fin and bulb of the original design with a "seal flipper" keel such as that shown for Gus Lassel's "Sun Kiss," with maybe even more leading edge rake. I would test this by towing a simple float+keel combination through patches of junk (too easy to find these days, alas). I would also increase the rake on the rudder and have a long skeg to resist fouling. If you have a properly balanced design, and (as I understand your intentions) the boat is not going to maneuver much, so the rudder will act essentially as a trim tab.

    I would do my testing on a full-size prototype and not spend time on a 65cm version. I design, build and sail 65cm boats (I am the US class secretary for the RG65 class) and scale effect makes them much different from larger models. In a nutshell, they have a higher power to weight ratio than the boat you are contemplating and operate in a much different Reynolds number domain. There are just too many confounding factors in trying to go from a 65cm design space to a 2m one.

    I would make my first prototype so that I could adjust both the rig position and the keel position. My goal would be to get the hull balance as perfect as I could make it, so the boat would track by itself on the desired point of sail. There is no effective theoretical way of doing this to my knowledge, so the last bit of optimization has to be done by experimentation.

    Best of luck on your endeavors, it sounds like a really great project. Keep us informed.

    Cheers,

    Earl

    RiptideSmall.jpg

    SunKissSmall.jpg

    PS: I have full size plans of "Rip Tide" in electronic form. PM me if you want one.
     

  15. rwatson
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    rwatson Senior Member

    These excellent points resonate with me also.

    In my mind, I would be inclined to go for even more traditional keel shape (see attached) , purely for performance and trash collection reduction.

    The problem is that the high aspect, bulb keels are designed for rapid maneuvering in a fleet, and also require quite high hull speeds to take advantage of windward performance increases.

    None of this seems to fit the role of a long distance voyager, where hull fouling and small hull size almost cancel out the high aspect keel design advantages.

    OldHullSmall.png OldHullSmall.png
     
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