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      Abbreviated rules   07/28/2017

      Underdawg did an excellent job of explaining the rules.  Here's the simplified version: Don't insinuate Pedo.  Warning and or timeout for a first offense.  PermaFlick for any subsequent offenses Don't out members.  See above for penalties.  Caveat:  if you have ever used your own real name or personal information here on the forums since, like, ever - it doesn't count and you are fair game. If you see spam posts, report it to the mods.  We do not hang out in every thread 24/7 If you see any of the above, report it to the mods by hitting the Report button in the offending post.   We do not take action for foul language, off-subject content, or abusive behavior unless it escalates to persistent stalking.  There may be times that we might warn someone or flick someone for something particularly egregious.  There is no standard, we will know it when we see it.  If you continually report things that do not fall into rules #1 or 2 above, you may very well get a timeout yourself for annoying the Mods with repeated whining.  Use your best judgement. Warnings, timeouts, suspensions and flicks are arbitrary and capricious.  Deal with it.  Welcome to anarchy.   If you are a newbie, there are unwritten rules to adhere to.  They will be explained to you soon enough.  

A R Eastwood

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About A R Eastwood

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  • Location
    UK, just moved from USA
  • Interests
    Boat design and theory of sailing centred on the canoe, IC history
  1. DC Designs

    Re last post smiley faces should have been a B and a )
  2. DC Designs

    It has always surprised me that there are not more seat breakages. It is a highly stressed part of the canoe. As correctly stated the compression loads at the contact points between the seat and the carriage are reduced by using a seat which extends across the whole beam of the canoe. These are not, however, the only things which should be taken into account. For a seat kept captive at the centreline of the boat there are four loads on the carriage which are R(1) at the windward gunwale due to the seat, = 2W(L+B/2)/B down R(2)also at the windwrd gunwale from the deck track = W(L+B)/B. Up R(3) at the centreline constraint = 2WL/B up R(4) at the lee gunwale from the deck track = WL/B down Here W is the helms mans weight, L the seat extension, and B the seat carriage width. Putting reasonable figures in for these parameters will give the loadings. Comparing them with the values of compression strength for timbers will give the necessary contact area between the seat and the carriage at these contact points. For a seat which is captive at either end of the carriage then the formulae change to-R(1)=W(L+B)/B =R(2) And. R(3)=WL/B = -R(4). Negative implies downward, positive upward forces. As has been said the load at the lee and windward ends of the seat is just about halved. Personally, I have never seen a seat fail by crushing at the contact points, perhaps other could comment on their experience. As IC Nutter says the over length seat generates no bending moments on the carriage. However, the maximum bending stresses in the seat itself will however remain the same for both setups, with the maximum stress in the top face occurring at the point where the seat is over the end of the carriage. Rough calculations suggest that for a seat with 4mm ply top facing the stresses in the top face are near or maybe above those which can be supported by the ply. This suggests to me that substantial loads are being carried by the framework inside the seat. I have done no calcs to confirm this. I have seen seat failures due to tension crack propagation at this point. The maximum bending loads in the carriage for a centre captive seat work out to be about half of those in the seat. Back to the question over long or normal seat. My answer would be to go for the normal seat. The extra weight for an added 370/400 mm of seat would probably be over a kilogram which would be difficult to remove from the carriage. I would suggest reinforcing the carriage with unidirectional carbon tape (appropriately). From my recollection a 2.5 cm wide tape will carry about 10 kilonewtons, thus supporting the entire static loading which is applied, all at a weight of a few grams. Similarly tape under the top face of the seat will allow a great reduction in the amount of wood used in the seat, again reducing weight.
  3. DC Designs

    Theory is not everybody's idea of fun, but it is for some of us. Those who just get on the water and sail may (probably) get better results thro' more practice. I, and probably most of you, have seen race where the time difference between the first and last boat has been upwards of 30%, for the one design IC. The good sailors have a combination of natural talent and practice. As an average sailor, I and fully aware that I will never be able to design a canoe that is, say, 15% faster than the average and take me from 'average' to first place. Even if I could design a canoe which was 5% faster than any other i would probably still be in the middle of the fleet and no-one would be able to detect the superior design. People will look at the best sailors and follow what they do design wise. Quite a rational thing to do, but does it guarantee that the boats are the 'best' they could be. If you can sail 15% faster than average then you don't need the best boat, you need a good reliable boat. I know I have taken an extreme case, and the average spread of times over all races is less than 30%, but the same argument still applies even if the spread is only 10%. I notice that the RS 700 and the Musto skiff both have Portsmouth yardstick below the IC, giving them an average fleet speed round the course of about 2.5% greater than the IC. These new breeds of boat have upped the anti, and can the canoe respond? Personally, My aim in thinking theory is to improve my own performance and for the fun I get out of it, I also believe that it will give me confidence in the boat which i eventually build and this will ultimately allow me to sail better.
  4. DC Designs

    I have found a point on which ICNutter, BalticBandit and I agree on. Discussing theory can help generate ideas about design, but does require the effort to learn and understand the concepts, which is not everybody's idea of fun, and let's face it, it's fun we are all after. One point, the derivative of PV = nRT is VdP/dt + PdV/dt = 0, not the expression given by BalticBandit.
  5. DC Designs

    For those who would like to see the end of this rather theoretical discussion, I intend this to be my last post on the matter (unless of course something significant comes up) Phil S - don't let the thread mess with your head, 'cos there's not much theory here. C Maas - I think there is one point on which we all agree, even if we don't see eye to eye on such things as energy and pressure recovery. You go fast enough already. I C Nutter - although I am unconvinced by the method you have used, trying to calculate the acceleration of the water being pushed aside by the bow and hence getting a handle on the forces is a good one. You don't give sufficient information to follow your development of the idea. It is essentially the same as the way the planing theory is developed through the added mass concept. They use experimental findings as their starting point. If you haven't already looked at it, then I suggest you might find it useful. I think that behind the stern of the boat, after flow separation has occurred when there is an indentation in the water surface ( for want of a better word, a void) then to find how gravity makes the water flow in is a good idea. My guess that it may respond as a soliton, a solitary wave, or a standing wave on water, but without your maths development cannot really comment on whether i think your method has validity. For the section of the hull between the point where the cross section ( underwater) is a max and the stern I don't see how you calculate the work done in creating an infinitesimally thin void ( it must be infinitesimally thin otherwise we would see it) into which the water flows. If you have achieved a method I would be pleased to see it, and it may indeed lead to a method of calculating drag. BalticBandit - Have you even vacuum bagged a hull? Is so then you might have "pulled" the air out with the vac pump. If not have you ever used a drinking straw? " pulling" the drink into your mouth by sucking. If not have you even used a syringe to measure out epoxy? Perhaps you "pulled" the epoxy, or hardener, into the syringe by pulling on the plunger. If you have done any of these things you may have noticed that you have to apply a force, so they consumed energy, as the point of application of the force moves. In any of These examples it wouldn't matter what the vac pump, mouth parts, or syringe was made of, the effects would be the same, it would be material independent.. In the last example you may have noticed that if you pull too hard then you can create a vacuum in the syringe. To do this you have to reduce the pressure in the syringe from atmospheric to (essentially) zero, and so the force with which you pull on the plunger can easily be calculated as the x-sectional area of the syringe times the atmospheric pressure. As you increase the length of the void in the syringe (by pulling the plunger) you do work equal to force times distance moved. I didn't say that the means of creating the hole was important, just that to create a hole took energy, and that energy must come from the boat. Thus even if you use the thought experiment you suggested and gravity fills the void then you have to create it in the first place, and that takes energy, and that translates into drag. If you consider the water to be pushed into the space left by the hull as it moves forward we should be able to calculate the work done by this pushing. If we look at the boat as it moves ( at a constant speed) then one instant will look identical to the next. The water surface level remains constant! So the potential energy of the water remains constant, also the kinetic energy remains constant, as the flow field around the hull does not change, except by translation. As these are the only forms of energy available to the water then by the conservation of energy, it cannot be doing work. It is thus apparent that the work is being done by the hull and so it is more rational to think of it a pulling by the hull. No doubt you will argue that looking at the boat instant by instant shows no change and so the same conclusions could be drawn. I.e. The boat cannot be doing work. This would be true except for the fact that the boat has wind force acting on it and this force is doing work on the boat. As the boat is traveling at a constant speed it is not gaining energy, so it must be losing energy at the same rate as is being supplied by the wind ( the continuity equation ensures this). Thus the boat does work on something and that something can only be the water. The wind force does work on the boat which does an equal amount of work on the water. The 'theory' which you suggest sounds, to me, very similar to the Aristotelian theory of motion, which was superseded by the work of Galileo, Newton and others. If you don't know this theory look on Wikipedia. This is NOT sloppy thinking.
  6. DC Designs

    CMaas - I think I can see, ARE, how on your slow moving boat the energy it takes to push the water aside would be (completely?) "recovered" by the water flowing back together over the after part of the hull. So at low speeds form drag is near nil. But we're going faster than that one would hope which makes me think that IC Nutter is right - that there is a speed, I'm going to guess about 5kts, at which you transition from energy recovery to "suck". I'm going to say it again: On ICs you need to design to minimize suck in the aft section and the drag from wind waves. Ignore wave making. I think I must have worded me post very badly, as you took exactly the opposite meaning to that intended. In the example I was trying to avoid the complications of the wave pattern set up by the boat, and so avoid adding gravity as a possible source of energy. The boat pulls the water in around the stern, and this takes energy. So that energy is lost thro pushing the water apart at the bow and pulling it in at the stern. No recovery.
  7. DC Designs

    Baltic Bandit -Consider a cylinder with a disconnectable cover on the bottom. We insert this cylinder into the water vertically. Now - because this is a thought experiment we can do this - we instantaneously remove the cylinder, leaving the bottom in place so that no water can come up from below. What FORCE causes the water surrounding the column of air to move laterally? For this thought experiment you are perfectly correct. It is the water pressure which generates the water movement and this is due to gravity. Hence you could say that gravity is the driving force. However this is very unlike a boat moving thro' the water. In your experiment you insert the cylinder vertically, where does the energy for this process come from? When a boat moves thro the water we never see a 'hole' adjacent to the boat. Consider a thought experiment which, I think, is identical to yours. Consider two hull shaped surfaces, initially in the same place and let one of them move forward, leaving the other in it's original place, so there is a gap between them.( this is essentially the same as your cylinder immersed in the water, except for the shape). Next we remove the surface that didn't move. The water flows in to fill the gap under the action of gravity. This is exactly right in the thought experiment, and would be exactly right if we could do the experiment in the real world. I have absolutely no problem with it. Again in this thought experiment we have neglected the actions needed to produce the void below the water surface. To move the surface which we have associated with the boat and create a void we would need to apply a very large force to the surface. this is equivalent to saying that a lot of energy is needed to move the hull forward. To create such a void is the same as cavitation and is very energy expensive.Even if you stipulate that your cylinder is neutrally buoyant you stil have to do work ( supply energy) for the experiment. The cylinder will be neutrally buoyant at only one depth. To move it to this depth we have to apply a force and move it though a distance.
  8. DC Designs

    BalticBandit - "In this case the acceleration vector in the aft section is provided by gravity accelerating the water laterally. We cannot fully extrapolate the "slug in a pipe" because the slug is fully immersed whereas the hull is operating in boundary conditions where absent gravity water would not need to flow to level in the hull's wake, IE you could very well leave a trough behind the hull. you also have the bernoulli effect of the hull shape as well providing acceleration towards the center. But remember that F is a SQUARE function in time of distance so the shorter the distance (ie the longer the chord from max thickness to TE) accelerated the shallower the slope of that power function and thus the net force from parasitic drag is reduced." I'm still not convinced that energy can be 'recovered' from the water flowing around the hull. I do agree that the slug analogy doesn't fully model the boat sailing at a water/air interface. I didn't follow your argument, but didn't think that gravity has much to do with it except astern of the boat. Gravity produces forces vertically downwards only. If you mean that it produces a higher pressure away from the hull, then this is possible. Perhaps we agree that the pressure at the surface of the hull is less than that away from the hull. Even in your scenario this is the only way to get horizontal water movement. However it is the boats movement reducing the pressure adjacent to the hull which produces the pressure difference and so the flow of water. Perhaps we could do a thought experiment, and move the boat very slowly such that the water surface remained essentially flat. Remember that i can specify this speed as near zero as i like and however close i make the speed to zero the water would still have to flow in behind the boat, but there would be no way gravity could act to produce a force. It is the fact that a void cannot occur between the hull and the water that ensures that the water flows in behind the boat. This is ensured by the forces between the molecules of the hull and those of the water, the water molecules pull the hull back and out and the hull pulls the water in and forward. Your point about what would happen in the absence of gravity is well taken, and I entirely agree that it is gravity which is affecting the water astern of the boat and filling any wake effects.
  9. DC Designs

    I don't think (but i don't know) that sailing boats suffer from cavitation around the hull, but has been discussed in papers on planing theory applied to power boats. I was giving the extreme case to make a point as to how different the bow and stern are. More likely would be the effect of vortex formation at the stern if the hull has a lot of curvature. These would also extract energy from the boat and require power for their maintenance.
  10. DC Designs

    Amati. - "But water will fill the hole created by the the hull. Conversely, the hull will dig a hole in the water. And if the hull is going faster than the speed water can move away from it, or come back to it, what happens?" The two situations, at the bow and at the stern, are subtly different. At the bow the boat is pushing the water away, the water is under positive pressure. At the stern the boat is pulling the water into the hole, the water is under negative pressure. For our purposes the water can be pushed with any acceleration limited only by the force applied, there is no 'faster than speed water can move away'. At the stern things are different, if you try and 'pull' the water into the hole too quickly you will reduce the pressure to the point when the water will cavitate. This is equivalent to reducing the boiling temperature of the water to the ambient temperature, so there does become a 'faster than the speed water can come back in'. Indeed i understand that this is one of the reasons that for planing powerboats the immersed volume of the hull doesn't decrease towards the stern.
  11. DC Designs

    BalticBandit- "Think of energy recovery this way... Imagine if you were towing a slug through a pipe. ahead of the slug is just air. as the slug passes any point in the pipe, a microvalve opens up and the pipe fills up laterally with water. So in essence the water has horizontal accel towards the center of the pipe. When that water hits the back half of the slug, what is the net force effect on that slug? " Again not sure that the situation is the same. Why does the pipe fill with water? There seem to be two possibilities1). The water is pumped in, in which case energy is supplied from outside the system, and this is not the same as water flowing in behind a moving boat.Or2) the slug pulls the water in by suction, in which case the slug will have a force in the opposite direction to it's motion, trying to slow it down. This is more like the case of a boat moving through a fluid.
  12. DC Designs

    So many definitions of planing. All different and all trying to capture some aspect of what we see the boat doing. If we go back to basics what we are trying to do is understand why the speed of some designs doesn't just keep increasing with increases in wind speed ( power), or increases more slowly than other. One point about some of the discussions so far. Some have mentioned a peak in the resistance- speed curve. This is incorrect there is no such peak. The peak is in the slope of the curve. I guess most know this but it may be misleading to others. I find the talk of energy recovery and pressure recovery confuses the issue. A streamlined stern reduces the expenditure of energy, i don't think it recovers energy from the flow (convince me if I'm wrong). It is much easier if we talk about forces. For any hull which has a waterline which narrows toward the stern, the hull effectively accelerates the water inwards and forward in the direction of motion. This creates a reaction force on the hull which is a part of the resistance.
  13. DC Designs

    No. All it indicated is a moment. To decide if it is lift at the front or suction at the stern we need to know whether the C of G moved up or down.
  14. DC Designs

    Power to weight canoes vs sailboards. I meant to say in my last post that power to weight ratio will not determine the speed which will be determined by the power to resistance ratio. Power to weight will give acceleration.
  15. DC Designs

    As you probably know the National Physics Laboratory in the uk did test a full sized canoe, 'Wake'. As you, Phil, say it could be a costly process. However, I am with Jim and agree that it would give valuable information. It would help to separate good boats from good sailors. What we need is a tame Marine engineering Ph.D student looking for a project! Canoes and sailboards will behave in a markedly different way, due to the way the lift from the sail is used. The sailboard will have it's sail canted to windward so producing a vertically upward force as well as a horizontal force, which will reduce the wetted area. Canoes cannot be sailed in this attitude and often heel to leeward, so the sails don't help to lift the boat in the same way. I don't think that a straight sail area weight comparison is really appropriate( post #2159). For the sailboard, much of the weight will be supported by the aerodynamic lift of the sail rather than the hydrodynamic lift of the hull. I have never sailed a board so don't know how much weight can be supported by the sail, but I think it could be quite a large proportion of the 90 kg, so could alter the ratios quoted quite markedly. With the sail canted at 30 degrees the vertical force could approach 70% of the horizontal driving force but this depends on the exact flow across the sail. I would guess that in the pic of chris' canoe ( post #2165) the reduction in wetted area is still less than 20%, and probably still has a wetted surface of 21-24 sq ft, whereas a sailboard will probably halve this (just a guess). The resistance curves (post #2169) look pretty much as I would expect, 2 degrees stern down trim will effectively eliminate the rocker (this, of course depends on the amount of rocker designed into the canoe) so should provide a good planing area with little or no curvature and get rid of most of the suction. (post #2174) remember that the bow can be raised either by an upward force forward of the C of G or a downward force behind the C of G. With such a small amount of the boat out of the water, the reduction of the wetted area will be almost negligible, and if the trim is being altered by a suction at the stern, the wetted area could be increasing. With such a large amount of rocker I would expect both, and most probably more suction at the stern than lift at the bow. You will probably only be changing the trim by about half a degree to achieve effect that you are reporting if your design is for the bow to be touching the water and not submerged when the boat is in it's designed trim.