Archeological question

Discussion in 'Hydrodynamics and Aerodynamics' started by Brian Needham, Feb 10, 2020.

  1. Brian Needham
    Joined: Jan 2020
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    Location: Australia

    Brian Needham Junior Member

    Sorry perhaps the word refute was a little strong however the idea that I am extrapolating from the found archeology is rather strange when I am referencing nothing more than section K1 and the eyewitness account while you are postulating several large non-extant features and technologies to support your theory. All makes me feel a little refuted myself. If you don't mind could you please say a few more words in support of the statement that it would be less efficient and generate considerable vorticity. As far as I am aware skeg rudder combinations are at least as efficient as spades and have better stall characteristics. The only difference in this case is that the two components are the same width. Also I don't quite understand the concern with wear. After all there would only be a couple of knots of current flowing past. Just enough to give steerage way. There may also be an advantage to this system when the ship was feeling a crosswind. In that case the leading rudder might be allowed to fall off 5 or 10 degrees to windward so that it meets the oncoming current at a better angle of attack. This would make the trailing rudder more efficient. Any more than 10 degrees would generate too much torque but at small angles it might work. The test rig will determine how much torque is required to achieve a range of rudder angles and how much turning moment the two types of rudder generate at those settings.
    I am well aware of frapping it being one of my jobs to frap the lashings on the cannon on the Duyfken replica. Its a great way to fix the stretch caused by working in a seaway especially if the lashings were put on by inexperienced deckhands ! It was my privilege to work as engineer on her for the three years she was stationed in Cairns.
     
  2. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Though I feel like I'm feeding a troll, I understand the bane of ignorance is enlightenment.
    While Herodotus devotes several lines to the technical description of construction and shiphandling of a baris, the πηδάλιον appears to be mentioned in passing, so therefor may be inferred to have little interest to his contemporaries as it showed no new construction techniques. I concur that the construction of Ship 17 matches Herodotus' description of a baris type vessel, however I do not postulate "several large non-extant features and technologies". All items (steering crutch, πηδάλιον frapping, hogging cable) I mentioned are contemporary or previous art to Herodotus and in the historical record, and indeed some are required for the structural soundness of Ship 17, and therefor too ubiquitous for Herodotus to mention. What I think improbable is the fixation on a possible anachronistic axial rudder shaft. Even Belov, in the papers you cite, states, when he references Herodotus' single line about the πηδάλιον, that the features of Ship 17 Piece K1 "allows for interpreting them as shafts of an axial rudder" and refused to explicitly state that an axial rudder shaft was proved. Belov, and I, most likely will not say it is a done deal until a shaft is found in situ penetrating a keel. That is prudent circumspection.
    But what you suppose is not a skeg and a rudder, but two foils in close proximity with the leading foil poorly formed; see Hoerner, Fluid Dynamic Drag. Additionally, most skeg rudder combinations are tied at the tip forming a single apparent foil. Two πηδάλιον in tandem are not, and the bending moment will cause the loaded foil tip to fall off giving excessive tip interference between the two foils; see Hoerner, Fluid Dynamic Lift this time.
    This is one of those times I like to use a favorite statement of mine for an old textbook..."the solution is left to the student". Really, you should do the analysis of the bending moment and contact force for a spade rudder's shaft of sufficient size to control a 25 meter, 150 tonne vessel. It is not a trivial load even in "a couple of knots". Especially in a case like this where in order to prevent downflooding, there is a significant distance between the shaft support and the center of pressure.
     
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  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Herodotus was a historian, so his description of vessels, and their parts, needs to be read with suspicion. We can look at illustrations of boats that are no more than 200 years old and see gross discrepancies.
     
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  4. mitchgrunes
    Joined: Jul 2020
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    Location: Maryland

    mitchgrunes Junior Member

    I admit that I know absolutely nothing about this type of boat, of hydrodynamics, and very little of rudders. Most of what I have used are kayaks, whose use includes "sculling strokes" to move sideways.

    So - I picture two rudders, both pulling the boat sideways in the same direction, creating a balanced torque so as not to turn the boat. You don't need to change the direction the boat is pointing in to sideslip past a rock, another boat, or other obstruction. Such a design is completely intuitive and does not require sophisticated thought or technology. I can't imagine anyone who has used oars or paddles to significant extent not almost immediately thinking of this type of use.

    Or you could use both rudders, pulling the boat in opposite directions, to turn the boat very fast.

    Do these ideas make sense to you?

    However, I can think of no way to prove these were the intended uses. As people have noted, there are so many other ways to make use of rudders, and so many other uses to which holes can be put to use (e.g., to drop anchors, create fishing hole(s), live well(s), drag fishing nets between two lines, measure water depth, or to pole the boat sideways, pushing against the bottom, against a dock, or simply to hold it against a dock for quick loading and unloading), or any combination of above, that you would need a lot more data to offer definitive proof.

    One of the coolest features of archeological science, like literary interpretation or history, is that you little or no data to draw interesting conclusions, and that it is only rarely possible to refute your best imaginative theories. If you can make a good argument to support your theory, that is quite good enough.

    So if you like your theory better - remember that academia has nothing to do with being right, and everything to do with coming up with new and novel publishable theories. You've got one. Why not stay with it? :)

    You can bolster your theory sufficiently to meet all reasonable academic standards by building a craft with such holes, and demonstrating it can be done.
     
  5. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    Might I suggest that, if these two holes were used for rudders, on a river where the vessel had to sail upstream, then warp/drag or drift downstream, where the size and shape of the blades of a steering rudder would be either a compromise to both applications or designed to be more efficient to one or the other, but not both, and in light of the depictions shown of vessels with such an elevated poop, there may have actually been two different rudders, each hoisted and secured out of the way while the other was lowered and engaged.
    For instance, when sailing upstream against the current, way through the water would be hard and fast, a thinner, lighter blade would be responsive and easier to handle. Conversely, while dragging a kellet or being towed with the direction of current, way through the water would be less dramatic and a broad heavy blade would be desirable for maneuvers at slower speeds and to act as a break.

    The stowed blade would be turned laterally, so as not to interfere with the shaft of the engaged rudder and the helmsman would simply control the forward rudder from the front while he would move aft to control the rear rudder.

    Once the barges were loaded, fighting with an oversized rudder or trying to break and dock at slower speeds with an undersized rudder would be undesirable.

    Herodotus aside, there are other examples of ancient historians who were believed to speak in analogy or generalities, when later evidence bore out what they said as much more precise than the experts originally thought. Homer describing the battle between Arcadia and Troy, and Pythias who discovered Thule, are two examples that come to mind. It is important to remember that the idioms and common phrases and metaphors that every ancient would understand, may not have the same meaning or connotations today. Reading Herodotus as if he were writing today would be a mistake. It is also a good first step to assume he knew what he was writing about. A historian, such as he, would have traveled by boat. Many educated ancient Greeks practiced the science of navigation as part of their celestial studies. That is why Euclid couldn't rectify his infamous Parallel Postulate. It didn't work when navigating across the Med. Now that's precision for you.

    When it comes to woodworking, the ancient Egyptians were highly sophisticated. Furniture in their tombs contain the earliest example of the dovetail joint. Even today, the dovetail exemplifies the highest mastery in woodworking. They had the patience to cut stone in intricate shapes, boring a couple of holes through a heavy wood beam would be nothing.

    I think if the forward hole was for a lashing skeg, it would have been mortised square, not drilled round, but there are some deck posts that are depicted which seem to be pretty clearly round. I don't know if they had hinges.

    As for the chisel marks, sometimes new boats sank too.

    -Will (Dragonfly)
     
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  6. alan craig
    Joined: Jul 2012
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    alan craig Senior Member

    Sailing upstream with the prevailing wind sounds straightforward but could drifting downwind, slower than the current because of a headwind, have been with the aid of a sea anchor type device? A sea anchor could be carried on board but how would the rafts mentioned be moved back upstream? Also the boat would be more controllable if it was facing upstream when drifting downstream as the rudder(s) would be working in their normal sense. Also, and apologies as I didn't read the paper only looked at the pictures, are the round holes situated above the waterline?
     

  7. mitchgrunes
    Joined: Jul 2020
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    Location: Maryland

    mitchgrunes Junior Member

    Ah, I hadn't read the article yet either. The holes are possibly too close together, and are not not symmetrically enough placed along the boat, to make particularly likely my idea they were designed for two parallel rudders, intended to be able to create sideslip. They are certainly too close together to argue that it is a redundant design, in case one rudder is damaged.

    Some minor thoughts - though I'm not expert on ship design:

    The high degree of longitudinal hull curvature (rocker) in the craft you show makes sense if you assume that a small turning radius steering was essential. So the hypothesis that the holes were associated with a rudder system is obviously plausible.

    An alternate reason for such curvature, is to allow the boat to land on a shore for easy loading and unloading, or for keeping the boat out of the water for repairs, to prevent rot, or to drydock it during storms - but a rudder would be vulnerable, unless lifted away.

    In figure 10, you provide no way to lift the rudder completely away, because the hole is too small. And there appears to be no obvious way in figure 10 to move the rudder between the two holes. BTW, how do boats with holes ensure waves and splash don't flood the boat through those holes? I hate the whole idea of holes in a hull, even above the waterline, unless you can count on eternally calm conditions. I've seen boats with rudder shafts mounted through the bottom, but the shafts were sealed against water, but which sometimes required repairs to avoid leakage.

    You might want to adjust trim, and the "center of lateral resistance" in terms of being turned by winds and waves, if they had such a concept, so less steering effort and its associated drag is needed. Perhaps, taking into account the drag any rudder creates even when centered, moving the rudder between two holes - assuming that was possible - was a way to adjust to minimize steering effort? Would that be easier than changing the weight distribution? Would two holes be enough?

    One should argue multiple solutions, and argue that this justifies additional funding to look for more such boats. :)
     
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