Archeological question

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

  1. Brian Needham
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    Brian Needham Junior Member

    Hi All
    I am writing a paper on first millennium BC Nile freighter rudders. According to the Greek writer Herodotus they sailed upstream with the prevailing wind but when coming back downstream they used a very interesting method. They were towed by a raft ahead which was pulled strongly by the current and overcame the windage of the ship and in order to slow themselves down to maintain steeridge way they towed a stone behind which dragged along on the bottom. Several have been excavated recently which have evidence of holes through the keel for a rudder shaft. This is not strange as it is known that axiel rudders were in use all through the bronze age as well as steering oars.
    One ship however has two holes just 50mm apart, with the holes themselves about 350mm diameter.
    To my mind this indicates the presence of two rudders with blades on one side only.
    In effect it would be a skeg and rudder setup with the overbalanced leading rudder needing to be lashed amidships. When going down stream the aft tiller would be lashed and when going upstream the forward tiller would be lashed amidships.
    Why would they do This? Not all of them had this system. The ship in question was about 26M but another in the same area has one rudder and is 24M.
    What we have is an equal chord flat plate skeg and rudder verses a spade rudder.
    My question then is would the skeg and rudder be significantly more efficient than the spade.
    Cheers Brian
     
  2. gonzo
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    gonzo Senior Member

    I think that there is a high probability of mistakes by interpreting an archaeological find in modern terms, or those of a different technology. Design and building techniques are not always based on efficiency or performance. Without context, interpretations are no better than wild guesses. Imagine someone 3,000 years from now trying to explain the tail fins on a 1959 Cadillac ElDorado.
     
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  3. jehardiman
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    jehardiman Senior Member

    This is right on the money; without seeing the "rudder" holes, as well as the rest of the vessel, there is no context. At ~25M for a typical open vessel using typical Egyptian (not Greek) hull construction, there are half a dozen other reasons for having two holes that size and spacing without bringing in the technical engineering issues associated with a rudder shaft.
     
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  4. Brian Needham
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    Brian Needham Junior Member

    Thank you for your interest.
    There is little doubt in the archeological community that they are indeed rudder shaft holes.
    W s aww 3 would be that a crack developed in the original hole and another was made to replace it. The problem with this is that the keel section in question seems to have been made with two holes in mind with regard to the overall shape and the placement of the tenons used to fasten it to the planks and the adjoining keel section.
    So an explanation needs to be found for ship 17.
    As you say she is built in the Egyptian fashion having very short brick like planks. She also has the "long and strong tenons" mentioned by Herodotus who also remarked that she had no ribs, also true of ship 17. So how to explain the close spacing of the two shaft holes? I am not proposing this because I want to explain it in terms of modern rudder styles, it's just that no other explanation seems more likely. With great respect if you can come up with other explanations I would really like to hear them.
    All the background papers on this are available an Academia.edu and are totally legit.
    I don't know as yet how to insert drawings etc into these pages but am happy to do so when I figure it out.
    Thanks again for your interest
    Cheers Brian
     

    Attached Files:

  5. mick_allen
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    mick_allen -

    I see that the rear most aperature is slightly smaller than the other. Although these rudders would have buoyancy to some degree, depth control as well as uplift would be of considerable importance - so a close to axial rear aperature might be of most use for a line or tackle system for both raising or deepening. Of course they could have raised/lowered by upper attachments but lower possibly would be more useful.

    I would imagine that the lines of the day might be of some diameter [hence the significant diameter], but also minor repair of the lower attachments might have been possible by reaching thru with arm or long tool.

    As it is a shortish boat, maybe fore-aft deckspace was precious for other than longitudinally spaced helmsmen and the axial rudder shafts were splayed sideways rather than on the vertical fore-aft plane.

    Another possibility would be the rear attachment for an longitudinal overhead truss line, but that keel section is quite segmented - or maybe the horizontal tenons offered rigidity. [this idea seems less likely]

    ideas anyway.
     
    Last edited: Feb 15, 2020
  6. Brian Needham
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    Brian Needham Junior Member

    Thanks Mick.
    Yes the rudders would have had substantial buoyancy and would have needed lifting from time to time but although blocks had been in use for centuries multi part tackles were not known until the time of Archimedes.
    Late period rudders are shown as more rectangular than in earlier times but no remains have been found so far. The holes do not display any lateral wear marks which would suggest a splayed setup. They seem to be quite cylindrical and unworn.
    To me it seems unlikely that the extra hole was used as access to the other shaft. Ship 43 did not show this feature and Herodotus did not mention it but it remains a possibility.
    I have been in contact with Alexander Belov, who wrote the attached paper and he thinks the twin rudder scheme I described is at least worthy of proper investigation. This is simply because the alternatives do not seem to be any more likely. I am building a test rig to compare the two rudder types but in the mean time would really like some expert opinion on the theoretical difference in efficiency between an equal chord skeg rudder and a symmetrical fully balanced spade rudder. There must be plenty of people on these fora who know everything about this stuff. Thanks again for your interest.
    Cheers Brian
     
  7. jehardiman
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    jehardiman Senior Member

    Sorry I'm so late getting back to this, had to go to Maui.
    Anyway, while many funerary models from the middle kingdom forward (understanding that the middle kingdom is as separate in time from Ship 17 as we are from it) show a single axial rudder, that rudder is almost universally mounted at the extreme transom, often in a cleft between the gunwales/garboards. This would allow for the rudder to be shipped to prevent damage while landing. A rudder shaft penetrating through the keel (Herodotus not withstanding as he only states rudder, not rudder shaft) would be very susceptible to grounding damage and additionally require extensive quay faculties to be developed because the rudder cannot be shipped. Additionally, the lack of wear noted on the holes, in my opinion, serve to prove these features are not dynamic; i.e. the wear of the shaft on the hole due to the drag of the blade would have left some mark, most likely an elongation in the aft direction; as well as the wear from any feature required to support the rudder in the axial direction to keep it submerged/lifted.
    Let me offer another more practical alternative in agreement with Herodotus' observations, with evolution of the aft rudder cleft, and the surviving structure of Ship 17. Piece K1 tapers aft because it tenons into the crutch piece (now missing) that the rudder shaft rests upon. Aft of this crutch, the planking extends to the extreme transom, where the plank ends are possibly joined with a horn timber (now missing). This produces a slot in the hull that allows the rudder to be shipped as well as explaining the in situ remaining structure of Ship 17. Structurally, everything aft of the rudder crutch is mostly non-structural. These 8 or 9 starboard strakes (as indicated by the surviving port side structure) would easily be the first material lost due to their raised and exposed position (it appears that the vessel sank and rested on its port side) as well as only being held in place by tenons. This may also point to the reason that the vessel was 'scrapped'. Structurally, the vessel is very poorly supported longitudinally; all of its longitudinal strength coming from shear of the tenons, which is weak. This makes any cantilevered stern structure aft of the keel (which effectively ends at the rudder crutch) susceptible to failure. Possibly the starboard stern upper strakes were damaged/lost prior to the vessel being discarded, leading to the total stripping of the ship, including the rudder and rudder crutch. It would be interesting to know if the "poles" depicted in Fig 2 are contemporary with the wreck as the placement of the after ones would be down through the rudder slot, possibly aiding in the vessels dismantling.
    So what are the two holes? Earlier and contemporary Egyptian and Greek vessels used a longitudinal hogging truss cable (hypozomata). Many depictions of this cable show that the stem is girdled but the aft end of the cable just disappears into the hull, most likely seized back on itself after passing through the keel for ease of installation and maintenance . Because this is not a working cable (i.e. there would be no wear on the holes in K1) would most likely be taken up tight by internal and external wedges to prevent wear.
    But my personal opinion is that these two holes represent the gammoning for the rudder shaft. This also allows for the rudder shaft head to rest on a vertical stanchion/support frame as depicted in many carvings and funerary models. In operation, the buoyant rudder and shaft is held submerged and in place against the rudder crutch and support by the gammoning. This provides full support for the rudder. By having a rudder shaft that is connected to the hull by a flexible connection, the rudder could quickly be tilted up to reduce draft just by moving the rudder shaft head off the supporting stanchion. It also makes the rudder easy to remove for maintenance. This arrangement requires no technological advancement from the Middle Kingdom and better fits the needs of a cargo vessel which may have to beach anywhere along the river.

    <edit of to off>
     
  8. Richard Woods
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    Richard Woods Woods Designs

    Cannot usefully comment except to say that when I used a similar steering oar/paddle the tiller was at 90deg to the rudder. So it was a push/pull operation. I don't see how the helmsman in fig10 can make big changes of course??? clearly the relief drawings are 2d not 3d and also probably drawn by an artist not a sailor. Although even so he cannot have confused the twin rudders. Was it possibly a structural solution not hydrodynamic??

    And, again in my limited practical experience of 18C ships boats the hull drawings/models are well documented but the rigs and sails are often depicted by a non sailing artist. Or the sails were made by the model builders wife....

    Having said all that there is lots of data on line regarding relative efficiencies of skeg V semibalanced rudders etc. and some on twin rudders fitted side by side as on catamarans for example. But I don't remember seeing anything about twin rudders one in front of the other. The Collins tandem keel is the nearest I can think of

    RW
     
  9. mick_allen
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    mick_allen -

    I love the gammoning possibility if not for the sound of the word itself! . . . but the logic from the lack of wear is provocative. [although one hole could be a semi omni bearing support for a smaller rudder shaft] It is also interesting the implication that the rudder shaft would likely then be a lot larger. Of related interest would be trying to discern the upright differing angles of the two proposed gammon holes with respect to the keel [and therefor the boat itself].

    With respect to a overhead truss line [hypothelion] possibility, rather than wedging for fixing it could be indicative of a girdling loop with the two separated strands that were then inserted up thru the keel and then used for both gammon [if forward] and rudder support as well as separation for whatever twisting approach was utilized for tensioning. In that case, the angles of the two apertures would be angled almost identically as well as slightly forward.

    It would be interesting to see that block in more detail.
     
  10. Brian Needham
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    Brian Needham Junior Member

    Thanks Ladies and Gents for your continuing interest in this post.
    I don't think that Herodotus can be disposed of that easily. I don't read ancient Greek but it seems that the translation of his statement regarding the rudder leaves little doubt in the minds of most researchers in the field. Boreau 1925:248 Edgerton 1923, Landstrom 1970:26, S vinson 1998, Basch 1999, Belov 2014.
    Before the excavation of ship 17 and ship 43 at Thomas Heracleion there was no preserved archeological record of it but both these ships attest firmly to its literal meaning.
    "there is one rudder which passes through a hole in the keel"
    As for the unworn state of the holes, they could easily be "trued up" from time to time. The larger diameter of one may be as a result of this process. After all it would take nothing more than the skillful application of a chisel.
    I am not aware of any other reference to "gammoning" in the literature.
    Also re using the holes to affix the hogging truss, I don't know about you but I would not be inclined to make large and greatly weakening holes in a keel structure which is inherently weak and then fasten a truss which is guaranteed to impose huge loads. Better as you say to use a girdle around the hull.
    Lastly using a rudder hung thru a cleft in the stern would make it rather difficult to tow the stone from the same place unless there was some kind of towing bridal, also undocumented and unobserved in the iconography.
     
  11. jehardiman
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    jehardiman Senior Member

    I have no doubt that the authors listed translated Herodotus correctly, what is in question is whether Herodotus meant what he said the way modern authors interpret it. The whole statement (Book II, Chapter 96) is "There is one rudder, passing through a hole in the boat's keel." The word used is "πηδάλιον" translated as "steering paddle" or in modern terms (15th cent) "rudder", though rudder itself is from Old English meaning "paddle". From a Hellenic shipping perspective, where dual side hung rudders was almost ubiquitous, Herodotus, writing for his contemporaries, whished to show the difference between the Egyptian vessel and contemporary Greek vessels. His statement cannot be extrapolated to mean the explicit technical statement that "a single round rudder stock passed through a perfectly round hole made in the center keel timber." That is an assumed anachronism. As I pointed out, I have yet to see a funerary model that actually has the rudder shaft passing through a hole in the keel timber. Given the detail of these models, it is unlikely that Herodotus knew more about the vessel than the modelers and therefore we shouldn't "cherry pick" how we modern readers interpret our historical sources.
    That theory fails Occam's Razor and you cannot just toss off how "easily" it could be done. My main problem with the assumption of a rudder with a shaft passing through the keel is the huge amount of effort needed to install and remove the thing. Even up until the 20th century, ship rudders (and 25m is a ship) were not fitted this way because of the effort needed to make it work. We are talking a round timber a foot in diameter, 25 feet long weighing up to 900 lbs, and that is just the shaft. In order to remove such an item we would need 15 to 20 feet of water depth quayside besides the necessary shearlegs and tackles. Gudgeons and pintles were developed to specifically avoid this type of problem. The amount of effort to constantly un-ship the rudder, true up the hole and replace the rudder shaft for fit would be immense. A better theory is that the hole was lined with lead for lubrication and wear and that the lead was recovered when the vessel was scrapped
    I wouldn't doubt it. Naval Architecture and seamanship is a very technical field with its own vocabulary. I could say "sail gasket" to an automotive engineer and he would have no idea that it was something an illiterate 18th century topman would instantly know what I meant. There is probably a very specific archaic Greek nautical term, unknown to me and possibly now lost to time, for the lashing that held the πηδάλιον in place against its bosses. A term so known to contemporary Athenian citizens that manned the triremes that none thought it worthy to record. Since gammoning is rather specific these days, perhaps you prefer the more general term frapping? Frap /frap/: verb, Nautical, gerund or present participle: frapping; to bind (something) tightly.
    As a Naval Architect I know that a hogging truss improves the weak keel structure because it does impose "huge" loads...in compression! I am unsure of your technical background, but to put it simply the hull of ship 17 as built would poorly support tension in either the sheer strakes or keel. However, the thickness and construction of the shell is ideally suited to support compression. Like modern pre-stressed concrete construction, the hogging truss imposes a constant compression to hold the hull tight. Additionally, acacia is slightly stronger in tension than compression (which is fairly high for most woods) due to its interlocked grain structure. Based upon the strength of rope that could be produced (papyrus is similar to palm fiber; Perry: Engineering the Pyramids), and the minimum area between the holes, I would be inclined not to worry about the strength of Piece K1.
    Again, I see this is a simple problem of those unfamiliar with the actual nautical application. Turning again to Herodotus (Book II, Chapter 96); "and the stone is connected by a rope to the after part of the boat." The word used is "ὄπισθεν": "at the back". He does not use the word "πρύμνα": stern of a ship he uses elsewhere in his histories. Herodotus, from Halicarnassus, a port city, would definitely know the difference. The towing of warps or the dragging of kellets is often used to slow a vessel down and to provide control is very old. Generally these are lowered from the aft quarter specifically to keep them out of the rudder. The even with the use of the raft with the kellet to keep the vessel aligned with the current, you still need to steer. Sailing ships, dropping down river from Foochow when the wind was contrary, would trail a kellet from the bow, slowing the ship and giving the rudder bite as they backed down the channel. Additionally, you can also steer somewhat by shifting the kellet line side to side.​
     
  12. Rumars
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    Rumars Senior Member

    The reconstruction in the article can not work and if the archeological community believes that it can they should explain following points:

    1. Any rotating or sliding part inside the holes would have left marks on the wood. A river full of sand and a rotating rudder shaft would have abraded the hole and shaft in no time. Even with a bushing of wood or bronze the effect would have been the same and the bushing and rudder would have had a very short life. The theory of a shaft in a pipe (bushing) would have also left evidence of securing said pipe in the hole.
    2. The distance between the holes is to small. There are ample forces on the rudder, the bridge between the two holes would break in service. A heavy bronze bushing cast so it is one piece for two holes would solve that, but you return to point 1.

    The only way to explain the presence of chisel marks in the holes and the small bridge is if they were plugged with something that remained static and completely and solidly filled the hole. What that structure was and its purpose remains unclear. It could be the upright rudder support poles from some depictions, or a centerline cable anchorpoint or a platform support. Ruddershafts passing trough simply does not work.
    This is in addition to all said by jehardiman to wich I agree completley.
     
  13. Brian Needham
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    Brian Needham Junior Member

    Am attaching a paper which details Herodotus narrative line by line. It details the similarities and the differences between between it and ship 17. I am of the school of thought that accepts Herodotus literally on this question.
    It is from this line of thought that I am proceeding.
    Thank you Jehardiman for your most exhaustive and thoughtful reply. I am flattered that you went to such pains to point out the errors in my thinking :) I am not an academic or a Navel Architect, just an old sailor with a lifetimes experience in everything from sailing dinghies to square rig. I hold an unrestricted coxswains ticket for the Australian coast and have used it mainly for long distance yacht deliveries. Amongst other things I designed and built an overbalanced auxiliary rudder and trim tab self steering gear with an inclined axis vane. It was particularly effective in automatic yaw correction. It was in advance of other similar overbalanced setups because the whole thing could swing aft from a pivot point rendering it unbalanced and therefore usable under power. When under power we would unship the windvane and while sitting on the pushpit seat steer the yacht with two fingers by slight movements of the vane counterweight. Now retired I like to spend time trying to find answers to archaeological puzzles.
    Sorry but I don't follow your objection to the literal meaning of Herodotus statement. You admit that Herodotus meant to say rudder although if it is translated as steering paddle it makes no difference to the proposition. You also agree that he meant to describe a single rudder. So it comes down to weather it was mounted through the keel as Herodotus plainly stated or in a cleft which he clearly did not. So I ask you exactly where the extrapolation is ? Seems to me that you are making the extrapolation rather than the experts in the field. Herodotus did not see the models (which had been made centuries earlier) he saw the actual ship. The rudder either came through a hole in the keel or it didn't.
    We have an expression in Australia which goes like this. "If it walks like a duck and quacks like a duck its a duck.
    There is no reason to confidently assume that the rudder was 25 foot long and weighed 900 lb. The profile is unknown but may have been wider and shorter. This may seem a radical idea but after a couple of thousand years they may have worked it out. As for the docking facilities, these people built and loaded the heavy lift barges used to transport the 375 ton obelisks in pairs to the temple of Queen Hatshepsut. These vessels were estimated to be 81mt in length. Docking facilities would pose little problem to them. It is worth noting that the rise in water level at Aswan, for instance, is approximately 8mt in the high season rendering draft under the keel and rudders irrelevant.
    Re the possible use of a raft on the towline aft. It is tempting to postulate such a device but it is unsupported by Herodotus or the iconography. Also I doubt if it would be a practical or desirable proposition to manhandle the loaded towline across the stern, particularly with the encumbrance of a cleft hung rudder to deal with.
    I could carry on all night doing my best to refute your objections although I doubt if I could ever change your minds but please consider this. The vessel which perhaps corresponds best to Herodotus is ship 43 although she has not to my knowledge been fully excavated. She has only one hole through the keel. My object here is to understand ship 17 which has two. Now if you will please indulge me for a moment and briefly assume that I am right and the two holes were used in the way described in my original post unlikely as that may seem to you. I am constructing a test rig to supply real world results for the two types but would be very grateful for some theoretical input.
    We would have (in effect) a skeg rudder combination inclined at 30 degrees acute to the oncoming current. The skeg is the same width as the rudder. How would that compare in terms of efficiency to a fully balanced spade rudder of the same profile and inclination. Please answer that and lets put this one to bed.
    With great respect
    Brian
     

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    Last edited: Feb 24, 2020 at 4:12 PM
  14. Rumars
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    Rumars Senior Member

    If you want to asume the holes are for rudders then I would say the first hole is for a rudder support post to wich the actual rudder was lashed. Your arrangement of two one sided blades negates the biggest benefit of balanced rudders, the reduction in the needed operating force. Not to mention that having a fixed aft part and a moveable front makes no sense when steering.
     

  15. jehardiman
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    jehardiman Senior Member

    I do not understand why you need to "refute" me.
    It would be less efficient and generate considerable vorticity which will cause the shaft to oscillate transversely more when compared to a steering paddle. Again, I point to the lack of wear in the holes as them not being part of a dynamic system.
     
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