Richard Braithwaite
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Everything posted by Richard Braithwaite
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Problem with ships having a port and starboard sides is that I need to make two jigs... One benefit is that I can now work on both sides at once, which is speeding up production a bit (from my usual snails pace...)
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First check on the ergonomics with my trusty triad of marines: Still to fit their footrests...
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For some reason its taken me a long time to work out how to install and jig the upper seats... The jig ive ended up with at this stage is shown below together with a seat assembly (seat+ foot stretcher + support pillar). The jig consists of two parts. The main part is intended to hold the seat level athwartships and at the 9 degrees rake to the centerline with a removable section that can be unbolted so that the jig can be removed after fixing the seat in place. Here is the jig in place in the model: And, finally with the seat assembly in place: The seat is assembly is fixed to the seat forward of it by its foot stretcher (held in place by the steel clip while the glue dries) and to the beam underneath by its support pillar. The intention is that all the seats form part of the removable part of the model and so this seat assembly can not be glued to the sheer capping... The lathe cutting tool is functioning as a weight to hold the seat down in the jig while the glue dries...
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My triad of oarsmen completed. From right to left: Thalmian, Zygian and Thranite oarsmen holding their appropriate oars. The oars shown are based on the original oars for Olympias which were used for most of the sea trials. The blades are designed with a different shape to account for the differing vertical angle of immersion for the three different levels. A lighter version of the oar was later trialed (where all three levels used the same design).
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The middle tier of oar seats are much more straightforward to install than the lower tier as they are not raked relative to the vessels centerline. This means I can align them easily with a straight edge. Here are 3 going in in one gluing operation:
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Yes, I am finding that 1:24 is about as small a scale as I can manage. The main reason for the full scale reconstruction was to demonstrate that it was possible to arrange 170 oarsmen, each with their own oar, in a ship 37m long... So I wanted to be able to show that this could work on the model... The clearances between the oar blades, and hence the geometric tolerances for building many critical areas of the ship, are necessarily very tight to achieve this even at full scale let alone on a model...
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Starting the installation of the second tier (Zygian) of oarsmen seats. Again, my Manikin is helping to confirm that the seat is in the right place for the downward angle and sweep of the oar and that there is no interference with the lower (Thalmian) oarsmen. The oar blades ae resting on a block of wood placed to give the right downward angle for complete immersion of the blade in the middle of the power stroke (at the design displacement). One the inboard distance of the seat is established I constructed the jig for installing the oarsmen stretchers at the same distance from the tholes which can be seen in the next bay forward of the manikins position. Part of this jig holds the new stretcher in place, perpendicular to the deck beams and the other part is a simple "ruler" that fits over the thole pin marked for the correct distance inboard. I will need to remark this for the shorter oars at the ends of the vessel...
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Yes that is still the plan. However, at the moment I am concentrating on completing the model with as little concession to that as possible. I want the mechanism to be fully removable so that the model can stand with or without. That is on of the main reasons I have made it fully capable of dismantling.
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Disassembled the model today for the first time in a very long time, to check that I haven't done anything to stop me being able to get at the bolts and that the main hull outfit assembly (which includes deck beams, support pillars, inner hull longitudinals, gangway, and now thalmian seating...) is still removable. It takes me just over half an hour to assemble (there are 82 bolts to secure in total...) and I was quite pleased with how easily the main hull outfit assemble fitted into place with no flexing at all and all the bolts perfectly lined up with their holes. I'll have to do it again to fit the mast steps (I know, I could have done that before I put the deck in...) and I'm hoping it will make it easier to get at everything when I finally come to coating the wood with something.
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Confusingly, this lines plan is drawn to two different scales. The sections are at 1:10. However the waterlines and buttocks are both drawn to 1:10 in the vertical (in the case of the buttocks) and athwartships (in the case of the waterlines) directions BUT at 1:50 in longitudinal direction. I guess John Coates did this so that he could fit the drawing (with a decent size for the sections at 1:10...) on a shorter piece of paper. The drawing would be over 3.5 meters long at 1:10... Some of his other drawings give the locations of the station placement in relation to the structure of the ship (eg Plans 8,10 and 11)
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I designed a simple prototype rowing machine that produces an elliptical rowing stroke. The machinery fits underneath the gangway on a 1:24 model of Olympias (in between the vertical stanchions). there is a link to a video of the machine installed in a 1:24the section of the ship on my Trireme Olympias thread on this site. Adding a software controlled stroke, as proposed here, would be really interesting as it would give you complete control over the stroke geometry.
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A Thalmian oarsman's view (looking aft) of the 21 seats Ive installed so far on the starboard side of the ship (149 to go...)
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Yes, I've tried some modelling of humans using a similar program that is freely downloadable (DAZ3D). I guess you could save in a file format readable by a 3D printer?
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Great to see someone producing a model based on another of John Coates reconstructions! Building frames first and then planking them seems to work fine. I produced my trireme hull the other way round... Planking first onto a jig of temporary frames at the hull stations. Here is the hull planked up to the level of the floors: And then fitting the frames from the inside Working up level by level until all the planks and frames were in place and then removing from the frame jig:
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Upping the tempo on the Thalmian beams, stretchers and seats. Here a Jig for installing the stretchers between the thalmian beams: The little wire clip is useful in holding the stretcher at the right height while the epoxy cures. Unfortunately this jig will only work on the starboard side. I think Ill have to completely rebuild for the port side. Would have preferred to come up with something a bit more ambidextrous (oh well...) For the seats themselves, I'm building them in strip fashion so they can be cut off one by one and finished before installation...
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Just looked through your Dromon Thread. Great reconstruction. I see you carved individual oarsmen! I don't think I have the endurance to make 170 for my model! I am thinking of three of my manikins to demonstrate a single triad or oars. I suppose one could make a CAD model and 3D print them? Has anyone tried that?
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I do take your point. Not the most efficient stroke... Apparently the diagram was taken from a video record so should be reasonably accurate representation of what this particular oarsman achieved at this time. However, "the video record did reveal the variable quality of the bladework". Looking at the report which shows similar traces of what some of the other rowers were doing it appears that the diagram is one of the better ones (by a long way!). Bear in mind that this was 170 people rowing together for the first time in a vessel that they were unfamiliar with often from constrained seating positions with very little visibility. So technique was not optimal at all... The report does comment on the large difference observed between the effective stroke length and the total length of the stroke saying that "the reasons why they mostly took their blades out of the water long before they had finished moving them sternwards require investigation.." One reason suggested was that "the high moment of inertia of the oars which meant that they could not be manipulated quickly: if they were slowed prematurely because of this they would have to be taken out of the water early to avoid backwatering" Even in a modern high performance racing 8 the blade is moving before it enters the water in order that it is at least travelling at the same speed as the water that is passing the boat (otherwise there will be a degree of backwatering at the catch and a negative force on the boat). For similar reasons the oar will be moving at some speed at the finish. The distance taken to accelerate the oar to this speed before the catch ("catch slip") or decelerate it after the finish (the "release slip") will largely be a function of the inertia of the oar (as well and the strength/skill of the oarsman) as suggested in the report, and the oars fitted to Olympias were much heavier and had higher inertia than those of a modern racing shell. They did make some effort to address this with lighter oars in later trials on Olympias. This did enable higher speeds to be achieved, but I haven't seen any traces of oar path to see if the catch and release slip had been significantly reduced. There are some good diagrams at the following link which shows this effect and oar traces for modern racing shells. http://biorow.com/index.php?route=information/news/news&news_id=30 even these guys seem to waste some energy moving the oar up and down in the water during the power stroke
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This document contains a bit more detail on the trails and calculations described above and application to a working model of Olympias using the elliptical machine shown in the video on a previous page of this thread. Rowing Machine Calculation.pdf One interesting finding is that with The top tier only (i.e. 62 oarsmen) the average speed is predicted at 6.53 knots, tis increases to 7.72 knots with all 170 oarsmen. So a significant increase in speed, but not as much as one might expect for all these additional oars. The main benefit would have been acceleration and maneuverability (very important in combat) which, I guess is why it was so important to pack as many oarsmen as they could into the boat.
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You can get strain guages for full size oars and I have thought about making a full size mockup of a single rowing position to measure forces to validate my computer simulation. However, it is probably impractical to measure the strain in a model oar shaft as the loads on the oars are really tiny. For example the maximum thrust I measured in a zero speed trial of my Mk 1 galley (750mm long, 500 grams in weight, 12 oars, 150mm outboard length at 45 strokes per minute) was 0.0123N (measured using a fine thread attached to a post on the stern running to a pully with a hanging weight). So that's only 0.001N/oar! The average speed achieved by this galley at this stroke rate was 0.09 m/s. If we say that this galley was at a scale of 1:24 then this speed equates to around 2.16m/s or around 4 knots (if we scale speed by length). This isn't particularly fast, but then a constant speed circular oar motion is not particularly efficient! The motor in my Mk 1 galley was way over the power required, but that's not really a problem (I wasn't interested in efficiency here) but it does ensure that any friction in the mechanism (much more significant than the propulsive forces) is easily overcome. (Now if you fitted your rowing machine to a full size galley, or a really large scale model, the loads would be larger and easier to measure!!) It might be better to measure the boat speed somehow and program your oar motion accordingly rather than try to measure these tiny forces? Alternatively you could increase the speed of a small scale model, but that would require ridiculously high stroke rates! Ive Just found a simple spreadsheet model that I used to predict the oar forces in the first trial (zero speed 45 spm) I referred to above: RowingMk2Trial1.xls And here is the spreadsheet set up for the second trial (steady state speed at 45 spm) RowingMk2Trial2.xls Since this rowing machine moves the oars at a constant (circular) speed no matter what the boat is doing its relatively easy to simulate with a spreadsheet. Still I was quite pleased at how close my spreadsheet came to the measured performance of my little model. Adding a human being into the simulation does, however, makes it a lot more complicated!!
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I did some trials with an early prototype rowing machine. If you go for a "realistic" stoke rate of say 50 spm the speed of the boat is very low (it can never be faster than the oars move through the water!) and the force required (and hence torque on your servos) is trivially small. A more significant factor determining how strong your servos need to be will be the friction in your rowing machine mechanism. A couple of pictures of my Mk 1 Galley and its rowing machinery (simple, circular path, constant speed...). I found, that as the scale speed is so low you need a really calm day if you do the trials outside! My calculations suggest that this will still be the case with a 1:24 scale model of Olympias even with its 170 oars...
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A couple of figures from the Olympias sea trials that may be of interest (Ref The Trireme Trials 1988 Report on the Anglo-Helenic Sea Trials of Olympias, J F Coates, S K Platis, J.T. Shaw, Oxbow Books) The first shows a trace of the oar handle and blade of one of the Thranite oarsmen (top tier, who tended to provide most power as they had the least constrained positions): The next plot is of speed from a standing start (Thranites only). Acceleration and maneuverability are particularly important for this type of vessel so that they could break from a slow moving formation faster than the opposing vessels and so achieve a tactical advantage and get into a position to ram. You can see that the first few strokes are at a lower rate, accelerating as the boat speed increases and the pressure on the oar blades reduces for a given stroke rate (at higher speeds the inertia of the oars will become more dominant in determining stroke rate). My current simulation code allows you to set a maximum load that the oarsman can apply to the oar handle and then works out the dynamics of the oar handle (and the resulting forces on the boat) accounting for water pressure on the blade and oar inertia. When I use the data for Olympias with Thranite oarsmen only I get the following match for the trials data: For a working model (it you wanted to make starts more convincing), you already have the ability to vary the rate of the power and recovery parts of the stroke. You might be able to add a sensor for boat speed (gps or spinner?) and adjust stroke rate accordingly? I just noticed that this thranite oarsman achieved a 700mm stoke (the design hoped for 800mm ...), which is similar to the stroke length that my manikin figure is managing on my model (on the more constrained, lower level). I've just posted a GIF animation of him in action on my blog!
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Lovely model great workmanship. I have 170 oars to make for my current model and the jig you used for your oars looks interesting...
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