Hull shape for ocean-going small autonomous boat

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

  1. andy47
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    andy47 Junior Member

    I am building a 2-meter autonomous sailboat that pursues to cross the Atlantic Ocean. I am deciding between two hull shapes that need to be optimized for speed. Stability is not an issue as the boat will be balanced with a long keel. There will be solar panels of known dimensions mounted on top of the hull, so I need to keep the minimum width.

    Type A – inspired by an aircraft carrier:

    boatA2.jpg

    boatA1.jpg

    Type B – a typical boat:

    boatB2.jpg

    The advantage of hull type A is that the underwater profile is narrow and I think this would make the boat faster. However, I have some doubts. The difference between my boat and an aircraft career is in size. The height of my boat is only 20 cm while the ocean waves are typically much bigger: about 1-5 meters. The water will constantly attack the concave surface of the hull from below, so I am wondering if this shape isn't actually making the boat slower. A picture is more than 100 words:

    water1.jpg

    water2.jpg

    Another advantage of the hull type A is a slightly reduced weight, because the hull will be filled with a rigid foam.

    What do you think? Would you recommend the “aircraft career” (type A) or the standard shape (type B)?

    By the way, this is how the boat will look like (here it's designed with hull type A):

    boat.jpg
    boat2.jpg
     
  2. kerosene
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    kerosene Senior Member

    I recomnend you read some boat design books.
    Read nature of boats 1st. Then maybe some kayak related material. Also rc sailing community would be good source for checking what works on small scale.

    Of the cross sections the simpler will have less skin friction. But you need to think other views too. You want skinny ends though this will make the bow dive more.
    Your keel looks massive, the cross section is way larger than the sail. Why is that? International 1 meter scale sailboats use drop shaped bulb and way skinnier keel. I would think that they are near optimal being a competition class. Look at those boat shapes too.
     
  3. upchurchmr
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    upchurchmr Senior Member

    Forget the aircraft carrier. That shape has nothing to do with speed at sea. And it has huge power source.
    Look at current sailboats.
    Your keel weight has a ridiculous shape, that will slow you down a lots.
    There is no value in showing waves on this boat like you are. The waves will be huge compared to what you show.
    There has been a recent thread either here or on Woodenboat.com about the same subject - have you researched?
     
  4. FerroCementTho
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    FerroCementTho Junior Member

    Ditch your keel and go with a tall aspect-ratio fin keel with a bulb on the end instead.

    If you're going to be designing a sailing vessel, no matter how small, you also need to have a pretty in-depth understanding of what "lead" is, and what it'll do to your sailing dynamics (that's lead as in follow-the-leader, not lead as in lead ballast). Right now you have negative lead, meaning your keel CE is forward of your sail CE. This is no good. I assume you've displaced the mast and keel longitudinally from each other because they both have motor mechanisms to control them (your keel appears to be canting, unless I'm mistaken). Try and get them closer together, and at the very least get the keel CE aft of the sail CE.

    The fittings you have on the bottom of your hull are a resistance nightmare. I realize these probably aren't the final fittings, but definitely try and go with a clamshell fitting or something more low-profile to keep resistance down in way of thru-hulls.

    Maybe you can share with us the main factors influencing your design decisions, whether it be speed, distance traveled, or something else.
     
  5. andy47
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    andy47 Junior Member

    Regarding the keel, it has been designed for rough ocean conditions and a possibility of entanglement. A skinny keel with a bulb is not suitable for this mission. I will try to improve the weight shape, but definitely not like a bulb sticking on both sides as seen on RC boats.

    Regarding the waves, I care about the waves that affect the speed of the boat. The 5-meter waves with long periods don't provide additional resistance related to the hull shape.

    Good point about the skin friction. Biofouling would even increase it. I decided to stay with the typical shape.

    I will try to improve the fittings on the bottom, but I can't completely get rid of the fitting behind the keel, because it's holding the mast. But I can add a hydrodynamic shield around it, just behind the keel.

    Regarding the negative keel, I realized that it's not seen in the pictures that the keel is not vertical. The weight at the end of the keel is exactly below the mast (the mast is the "fitting" that you see behind the keel). Not sure if this is what you mean. Btw, there will be also one more fin between the keel and the rudder to equalize the keel area on both sides of the mast.

    Thanks for your valuable comments.
     
  6. FerroCementTho
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    FerroCementTho Junior Member

    What I'm getting at is that your center of effort diagram will look like what I've attached labeled "Negative Lead Example," while it should look like the one labeled "Positive Lead Example." If you're adding another fin aft of the mast in order to equalize keel area fore/aft of the mast, you're really just adding unnecessary appendage drag in order to have the boat perform as it should, when you could just arrange your centers of effort in order to have a proper amount of lead and save that appendage drag.

    Your design seems to have a pretty low aspect-ratio rig. Have you considered the possibility that in ocean conditions, with 5m waves of any period, such a low rig will actually be dwarfed by the height of the wave, effectively becalming the boat every time it dips into a wave trough? Sure it won't be a total absence of wind, but it will be a very different sailing dynamic once the height of the swells go beyond the height of your rig above waterline.
     

    Attached Files:

  7. upchurchmr
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    upchurchmr Senior Member

    Very nice explanation of "lead" between the air center of effort and the water center of effort.

    And "positive lead" is absolutely critical to proper sailing. Perhaps not quite so much, though.
     
  8. FerroCementTho
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    FerroCementTho Junior Member

    Thank you! More of an illustrative example than a literal one :) Meant to say please excuse the crudeness of the sketch.
     
  9. andy47
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    andy47 Junior Member

    I understand from this definition that my boat has a neutral lead - neither positive, nor negative. As I explained above, the keel is not vertical and the lead weight at the end of the keel is exactly below the mast. Maybe it looks differently from the angle the screenshots are taken. Here is the picture how the boat looks from starboard:

    Clipboard01.jpg

    When the wind is blowing to port or starboard, the sailwing is at about 75 degrees (unlike standard sails which are at 45 degrees or so), because the sailwing works much like an airplane wing. The center of the sailwing (which is where the wind force acts) lies on the same vertical line as the center of gravity of the keel weight.

    Do you suggest I can (or should?) move the keel a bit further to the stern to get a positive lead? Like this?

    Clipboard01_mod.jpg

    I am not familiar with boat terminology, so I am using different terms. I am a physicist and I understand the forces. From my point of view, if the center of gravity is behind the center where the wind force acts (which is close to the mast in my case), the wind blowing to port or starboard would rotate the boat and change its course.
     
  10. kerosene
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    kerosene Senior Member

    It is not the weight of the bulb that gents above are worried about but the sideways pushing effort. Your boat will have serious "weather helm" ie. It wants to turn the front towards the wind (which I have learned to be dedirable in modest amounts).
    Really read a bit more. Principles of yacht design is going to take some evenings but you will be far better off.
     
  11. andy47
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    andy47 Junior Member

    If the center of gravity is exactly below the center of the sailwing (the center of acting wind force), and you have a super-strong wind to port or starboard, the boat would lean, but it will not change the heading. If there is something else written in a yacht book, I would be against the first principles.
     
  12. kerosene
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    kerosene Senior Member

    Think. Google center of lateral resistance.
     
  13. kerosene
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    kerosene Senior Member

    And priciples of yacht design is name of a book. Often considered _the_ book for learning the fundamentals.

    Feel free to ignore it but don't expect the physical world or pros here to care.
     
  14. andy47
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    andy47 Junior Member

    If you mean that the keel area is too much in front of the boat, this is why I suggested the fin between the rudder and the keel. I am asking what would you suggest:

    1) A fin:

    1.jpg

    2) Moving the keel a bit more to the stern:

    3.jpg

    I don't like the second idea for the reasons I provided, but if you say this is better, I am open to investigate it further. I need to deeply understand it before I do it...
     

  15. FerroCementTho
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    FerroCementTho Junior Member

    The center of gravity has nothing to do with it. It is all about the center of effort, i.e. the center of lateral resistance of your keel vs. the center of lateral resistance of your rig, and the difference in the longitudinal positions. If your center of lateral resistance (not center of gravity!) of your keel is forward of the center of lateral resistance of your rig, then when a wind force is applied to the rig, an equal and opposite reaction is applied at the center of lateral resistance of your keel (ignoring the canoebody's CLR), and the boat will swing upwind. What Kerosene says is true, that a relatively modest amount of "weather helm," as this is known, is desirable. This is usually achieved with a very small amount of negative lead, no more than 5% of LWL if I'm not mistaken. On a regular sailing yacht, this is normally counteracted under controlled sailing by pulling the rudder farther. However, if you lose control of the rudder, the boat will naturally (and slowly) yaw upwind, taking the power out of the sails, and slowing you down. With a large amount of negative lead, when a direct wind is applied to one side of the boat, the lever arm the wind has is much greater, thus it will be almost impossible to counteract with your rudder, and you will have a very frustrating and short journey.

    Looking at your arrangement, I see now that your keel is swept back at a pretty good angle, the other pictures were deceiving. However, just by eye-balling it, I can see that your keel center of effort is too far forward of your sail center of effort.
     
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