Hull shape for ocean-going small autonomous boat

Discussion in 'Boat Design' started by andy47, Jun 7, 2017.

  1. FerroCementTho
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    FerroCementTho Junior Member

    You are correct in stating that placing an additional fin where you've indicated would probably fix your lead problem. However, the fin will add unnecessary appendage drag to correct a performance problem which would be corrected just by shifting your keel aft.
     
  2. andy47
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    andy47 Junior Member

    Thanks! So I understand that both options 1) and 2) would be correct, but 2) is more preferable because there is no additional fin (which means there is less drag and it also removes complexity). Btw I am not trying to argue, just trying to understand. If I move the keel like in the picture below, is it fine? The two keel areas divided by the red line would be equal.

    3.jpg
     
  3. FerroCementTho
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    FerroCementTho Junior Member

    Yes, however I suppose that depends on your definition of correct, but both would solve your lead problem.

    No worries, of course it's beneficial for you to understand these things before undertaking a project like this.

    Yes, moving the keel as you've indicated should be fine in terms of lead. However! Here is where it gets really fun. You have a swept back keel with a bulb-but-not-bulb on the bottom, which is where I assume the majority of your ballast will be, in order to lower your VCG as much as possible. If this is the case, be careful of your LCG. Most modern sailing boats either have a swept-back lead (as in un-leaded fuel) keel without a bulb or a (relatively) un-swept FRP keel with a lead bulb. In combining them into a swept-back FRP keel with a lead bulb, you have offset your keel's CLP from it's LCG, and so now you must be careful not to upset your boat's LCG in favor of properly placed CLP. Also, if you want to get really crazy about equalizing centers of lateral area (which may or may not be worth it), you should account for your rudder area's affect on your underwater CLP in the calculation of where to put your keel. A simple moment calculation will tell you how much your rudder will offset the placement of your keel. Note: accounting for the rudder's area will produce a forward-shift in your keel's placement, but it will definitely not be nearly as far forward as you originally had it.
     
  4. andy47
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    andy47 Junior Member

    I have been thinking about adjusting the boat's LCG by adding relatively small lead weights inside the hull as the boat's LCG will greatly depend on the electronics, especially batteries which is hard to estimate. I had to google all three-letter words :)

    Thanks for the suggestion about the rudder, I will take it into account.

    Now I have a question about the rudder design. I have seen boats with a keel like this (autonomous models), but all of these boats have the rudder on a "skeg". They have the keel's LCG just below the mast as in my faulty design, but with the skeg, they obtain the correct keel's CLP (and I wanted to replace this skeg with a fin). The advantage of the skeg is that it makes the rudder more robust, but there is one disadvantage for small model boats - a possibility of entanglement. Seaweed can enter the little gap between the rudder and the skeg and the rudder would get stuck. No matter how small the gap is, there is always a floating object in the ocean that can fit it. :)

    gap.jpg

    Entanglement or some kind of rudder failure is the main known reason why no autonomous boat has crossed the Atlantic yet. This is why I designed a simple rudder. However, I want to change it to a perfectly symmetrical, balanced rudder because this would save the servo from acting against the turbulent water attacking the rudder from side to side. I have never seen boats with a balanced rudder, does it have any disadvantage (besides that it's not optimal)?
     
    Last edited: Jun 9, 2017
  5. andy47
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    andy47 Junior Member

    I think I found the answer. The reason why rudders are semi-balanced and not balanced is stability (balanced rudders would require constant attention to the helm) which is not my case as the rudder will be controlled autonomously.
     
  6. kerosene
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    kerosene Senior Member

    I can relate to your eagerness and this topic is really interesting. I was playing with the idea years ago.

    But its my sincere suggestion that you pick up a book or two. You will get much better overall understanding. It will be a big task to create the boat - time spending with fundamentals is a small investment with big impact. And learning is fun.

    Of course you are learning now too but you are on the mercy of others more and the approach of going into details before good general idea is just far from best.

    Good luck.
     
  7. andy47
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    andy47 Junior Member

    I agree. Maybe I would grasp it faster if I build a small "ugly" model as a proof of concept. It will be built from foam and laser-cut plywood, so I can build it in 1-3 days. Then I will play with different configurations and see what works best.
     
  8. tspeer
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    tspeer Senior Member

    You're looking at the problem in the wrong plane. Look at your craft from above, and plot the force vectors in the horizontal plane. You'll see that the wingsail force is mostly toward the leeward side and somewhat forward, while the force on the keel is toward the windward side and somewhat aft. Lead is the moment arm between the two sideways components, and it causes a yawing moment. This yawing moment is opposed by the side force on the rudder. It's been found in practice that placing the center of effort of the sail rig a little behind the center of lateral resistance, balanced by a comparatively small force to windward from the rudder, is a good combination for both performance and dynamics.

    You really need to view a sailing craft in all three planes - longitudinal, transverse, and horizontal - to get an appreciation of how the forces and moments interrelate.

    There are some secondary issues that will affect your design, too. For example, with a swept keel like you've drawn, the loading will not be even along the length of the keel. Sweep will tend to load the bottom of the keel more than the top, but the lift will drop off to zero toward the end of the keel as well. So the center of lateral resistance will not be at the same location as the centroid of the area. Probably the simplest way to account for this would be with a vortex lattice code like AVL.

    On your wingsail, it would be better to raise the counterweight to the level of the top tail boom instead of the bottom one. When the craft rolls (or the wingsail flutters), the top of the wing will experience a different translational acceleration than the bottom. Since you're a physicist, consider the effect of Ixz on the rotation dynamics of the wingsail.
     
  9. tspeer
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    tspeer Senior Member

    I wouldn't worry about debris getting into the gap between the rudder and a skeg. Seaweed will wrap around the leading edge of the skeg and slide down toward the end before dropping off. If you have an all-moving spade rudder, there is a gap between the root of the rudder and the hull, and seaweed sliding along the hull is likely to lodge in that gap.

    There are ways to get the benefits of both hydrodynamic balance of the rudder and the robustness of the skeg. A horn balance is a portion of a control surface that projects forward of the hinge line in order to reduce the hinge moment. An ordinary horn balance would be subject to the same fouling issue as the spade rudder, with seaweed getting caught in the crack between the horn and the skeg. A shielded horn balance extends the skeg forward of the horn to prevent this, as shown in the second figure:
    [​IMG]
    Provided your steady-state rudder deflection was small, I don't think there would be too much danger of fouling with a shielded horn balance like the one shown.

    There are other means of providing reduced hinge moments, or of reducing the servo power requirements, albeit at the expense of more complexity. You might consider a servo tab to reduce the hinge moment. This could be done in conjunction with the horn balance, or without it. You can adjust the amount of balance through the relative lengths of the horns on the tab and skeg, which determine how much tab deflection is obtained for a given rudder deflection. A servo tab will make the rudder a little less effective, so you might need to increase the rudder area somewhat. It is also essential that the freeplay in the hinges and tab controls be kept to an absolute minimum.
    [​IMG]
    Finally, there are ways of combining direct force from the servo with tab motion to effect control of the surface with reduced power. In the figure below, the first configuration is a pure servo tab, which moves the rudder entirely through the tab forces. The second is a spring tab, in which part of the rudder hinge moment is controlled directly by the servo and part of the rudder hinge moment is controlled through the tab. The third configuration is a geared spring tab.

    https://history.nasa.gov/monograph12/figure6.2.jpg

    There are, of course, many possible variations on how these concepts are implemented. For example, instead of the coiled spring shown in the spring tab, the free link could be connected to the rudder using a torsion rod along the hinge axis.
     
  10. upchurchmr
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    upchurchmr Senior Member

    Saying that the sailwing will be at 75 degrees while a normal sail is at 45 degrees just illustrates your ignorance about sailing.
    You need to learn to talk about the angle of the boat to the wind.
    Tacking - about 45 degrees.
    Reaching 90 degrees + or -
    Running - 180 degrees.
    These are really rough definitions, but each broad category has different requirements for the sail angle of attack to the wind.
    Soft sail or wing sail (of any kind) both are set in the broad range of 10 degrees to the centerline of the boat while tacking or going upwind as well as possible.
    Check out the current America's Cup racing to see the best that is being done on boats that require constant intelligent (human) control.

    You need to get a basic sailing book along with the basic yacht design book and start over.

    What kind of a physicist? Particle physics or astronomical or what?
     
  11. andy47
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    andy47 Junior Member

    The sailwing will be freely rotating in the direction of the wind +/- some small angle adjusted by the tail. I don't think you will find it in yacht books or any (basic) sailing book. It's sailing of the future. There is no tacking at all.
     
  12. upchurchmr
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    upchurchmr Senior Member

    Its been done before in full size, and it doesn't change the basics of how sails work.

    You don't understand what tacking is - your ignorance is pitiful.

    How close to the wind do you think this thing is going to sail? Body of the boat to true wind angel going as close to the wind direction as possible.
     
  13. andy47
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    andy47 Junior Member

    You are right that seaweed will wrap around the leading edge when the boat is moving forward, but I am worried more about periods when the boat will be completely out of control. The boat is designed to survive a crazy storm, even a hurricane. Sometimes it will fly, sometimes it will go underwater. I can't rely on the water dynamics here.
    Regarding the gap between the hull and the ordinary rudder (without skeg), I am still thinking about it. Would the rudder really get stuck because of seaweed in that gap? Not sure. That shielded horn balance is really interesting. I am going to investigate it further.
     
    Last edited: Jun 11, 2017
  14. andy47
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    andy47 Junior Member

    Sorry about my ignorance, but you ignore my concept. There are two worst wind directions: upwind and downwind. I guess the boat will be fastest when the wind direction is port or starboard. I think so because the wings of an airplane are lifting it vertically - perpendicular to the "wind".
     

  15. andy47
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    andy47 Junior Member

    Good suggestion. But first, I will try it with the center of effort of the sail exactly above the keel's CLP, because I don't know how "little behind" it should be.

    Wow, I am shaking my head that I didn't get it sooner. You are right that the force acting on the keel is not exactly at the center. I have designed the keel like this because of the inside aluminum skeleton that holds the keel and the mast, and there must be enough place for electronics under the hatch. It brings challenges, but the good thing about this keel is that I can modify it later (just the keel) if I really screw this up.

    I wanted to keep the center of gravity of the sail low, but this changed my mind. I will make one more fitting for the counterweight at the level as you suggest and I will try both levels during testing.
     
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