Hull shape for ocean-going small autonomous boat

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

  1. andy47
    Joined: Oct 2016
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    andy47 Junior Member

    What about a shielded rudder like this?


    It's a cross section. The shield would cover the full range +/- 30 degrees for all rudder positions. And if I want to get crazy about it, I can mount a brush on that sticking part of the rudder, so nothing could enter the gap.
  2. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    You really should read more books instead of trying to assume an re-invent everything.
    You assume that apparent wind directly from beam is (universally) the best. This is not correct.

    On fast boats real wind direction from side can be fast point of sail but as the boat accelerates the apparent wind spins around towards the front. At the same time apparent wind speed becomes much higher than true wind.

    The lift vector will not be towards the front of the boat but a fair bit to the side. You assumed that for best performance the direction of the lift being towards the direction of travel is the key. It is not. The keel (esp. tru on a fin keel) will leverage and direct this sideways force forward. Getting much larger diagonal force is more optimal than smaller force directly towards the direction of travel.

    I am not the best at explaining it. So go read a book.
    Imagine someone from your field came and asked for advice on his project while making very basic faulty assumptions. You are being that guy.

    Couple of nights with a few good books will go a long way.

  3. andy47
    Joined: Oct 2016
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    andy47 Junior Member

    I didn't make these assumptions, I have guessed that the preferable wind direction would be approximately sideways just to stress that this boat behaves differently than a typical sailboat. Also I wanted to say that going downwind is not preferable as the lift would be exactly zero. On a typical sailboat, going downwind is fine. What's also interesting (and unbelievable for many experienced sailors) that the boat can completely forget the wind direction. In case the wind sensor fails, it would navigate only by steering the rudder while ignoring where the wind is coming from, because the sail is self-trimming. It wouldn't be optimal, but it works. A free-rotating self-trimming sail may behave differently in some aspects and I can only get it by trial. Now the design is important, but the sailing algorithm can be tuned any time later. And even if I don't have much experience with sailing, well, there is AI and machine learning...
  4. tspeer
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    tspeer Senior Member

    I've built and sailed a landyacht with a freely rotating wingsail. I can assure you tacking is definitely required. No sailcraft (save those with a rotating turbine) can sail straight into the wind without tacking. The fundamental sailing performance relationships are:
    Vb = Vt * sin(gamma - beta)/sin(beta)
    beta = arctan(D_hydro/L_hydro) + arctan(D_aero/L_hydro)

    Vb = boat speed through the water
    Vt = true wind speed over the water
    gamma = course relative to the true wind (gamma = 0 means heading into the wind)
    beta = apparent wind angle, measured between course through the water and the apparent wind at the reference height
    D_hydro = hydrodynamic drag (force component tangent to the course)
    L_hydro = hydrodynamic lift (force component at right angles to the course)
    D_aero =aerodynamic drag (force component tangent to the apparent wind)
    L_aero = aerodynamic lift (force component at right angles to the apparent wind)​

    These relationships are exact and apply to all sailing craft. Tacking is simply the result of maximizing the velocity made good to windward (Vmg). There is an optimum gamma at which Vmg is a maximum, and this occurs near 45 deg to the true wind. It is possible to sail higher than the optimum gamma, but the loss of speed will exceed the decrease in distance sailed. And symmetry requires that eventually the angle of attack of the wing will need to change sign, as will the apparent wind angle. Hence, tacking is unavoidable.

  5. andy47
    Joined: Oct 2016
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    andy47 Junior Member

    It will tack, because it's optimal in terms of speed as you described. I want to say that it's not critical for the long-term mission. I have seen models that are sailing directly into the wind. The trick is that the wind is rarely blowing in the same direction, the free-rotating sail responses instantly to the wind direction and even small fluctuations make the boat move slowly forward.
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