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MartinRF

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Everything posted by MartinRF

  1. Yeah, I thought about that block too. A ball bearing block for a static load does not make much sense to me. This study may interest you: http://hem.bredband.net/b262106/Boat/Blockfriction.pdf /Martin
  2. Let me start with a disclaimer: I have no first-hand experience of synthetic standing rigging. My all-steel implementation is very similar but with three extra toggles to minimize fatigue inducing bending of the wires. To me the implementation shown in your photos looks really good. I would replace what is worn without changing anything. Others may have wiser advice... /Martin
  3. Nice! Your sailing grounds are very different from mine. I'll post a video showing mine sometime. Over canvassed is very relative. Single handed sailing I am fully powered up at 10+ knots -- to windward that is. Fully crewed I have a lot more righting moment. /Martin
  4. Or: I don't kno the guy who posted this but I know the one who made the video. /Martin
  5. Just to show that there is more than one active Tennant cat in the northern hemisphere. After work sailing this Tuesday testing my new jib and a Sony action cam. Sunset at 22.09... /Martin
  6. Been there, done that -- because we ignored the manual and used self-tailing winches for the sheets. Long time ago, 40 ft French former racing cat. Another very successful French cat, a racers-cruiser was named "inoui" beccause it reads the same upside down. I don't think she has ever capsized. Owned by a family who had built her. Teachers from Bordeaux and very enthusiastic sailors. Sailing to Norway on summer vacation? Sure, why not? Crossing the Atlantic? Absolutely! Met them in Gonthenburg in the early 1990s. /Martin
  7. My two pence on balance: If rudders are not contributing to lift they are *only* drag. Drag vs Lift follows a quadratic curve so adding a little lift (from zero) on the rudders increase their drag slightly while the daggers are operating further up their drag-vs-lift curves and will loose more drag as the rudders start to share the load. This reasoning assumes rudders are reasonably sized and deep. If the centre of effort of the rudders are on or very close their axle through their pintles there will be very little tiller load and if you are used to a heavy-on-the-tiller mono you m
  8. He already got the cruising cat, sailed it home from South Africa. /Martin
  9. I think you should try to find out what mast bend and mast characteristics this sail's luff curve is designed for. If it is significantly different from what you can achieve you should factor in a luff curve re-cut. /Martin
  10. Interesting. I have a similar set-up these days but I have to help the car or it will stop too close to the centre-line of the boat. I usually only have to pull it down after a tack and then the car stays in the right place. I higher winds, however, I have to apply the outhaul all the time as I want to open up the slot and still sheet the jib flat and that does not happen of itself with a straight track -- not on my boat at least. Call it semi-self-tacking... /Martin
  11. The red ball is on the unloaded side of the dagger board case -- where the board isn't even touching the case. /Martin
  12. Photos or it didn't happen Here is one I created just before bicycling home today: This is for the floor-less case with the board pushing on the left side of the hull opening. Red arrows show material in tension and blue arrows to the left of the hole shows the material is in compression. The red 'ball' is just an indicator showing the centre of rotation when I was manipulating the view. /Martin
  13. Here is the 1986 calculations explained in English. This is kind of brief but I hope it makes sense. DaggerBoardLoads.pdf I tried to attach the LibreOffice spreadsheet as well but that was not permitted. It is my current understanding that real composite experts do not design reiforcements looking like this but it may still be the best option for paxfish. (The presence of the floor calls for a modification in the laminate scheme though.) One thing not addressed here is how to avoid locally crunching the hull laminate. /Martin
  14. It is easy to impress by using computer generated graphics I think it does a good job of illustrating where potential problem areas are. Unfortunately it does not offer a way forward to determining proper laminates. The reason is I don't have tools for modelling composites - I have used homogenous, isotropic materials and composites are layered and orthotropic. What I miss is something like this: http://www.ansys.com/Products/Simulation+Technology/Structural+Analysis/Structural+Technology+Leadership/Technology+Tips/Efficient+Workflow+for+the+Design+of+Thin+Composites+Structures It
  15. First results. /Martin Stresses_on_Hull_from_Daggerboard__version_1.pdf
  16. No FEA results yet but I have created an illustration to show the importance of having a healthy separation between upper and lower (boat bottom) supports for the dagger board: Figures are from my boat and the support separation in the drawings we bought is roughly 20 cm! The Wildfire design looks better in this respect but I think I would prefer even more separation in the interest of reducing material stress. /Martin PS Only using free and open software this time: LibreOffice on a Linux system.
  17. Using open source programs you mean? I think that could be done using Freecad and maybe Z88 Aurora or Elmer or the tools collected in CEALinux. I will try that some day but for now I am lazy and use tools I am used to from work. /Martin
  18. So I couldn't help myself and started modelling the board-hull interaction: Very much work in progress and I can't guarantee the usefullness of the results but the idea is to load it up with some of those finite elephants and hopefully understand better where stresses go. /Martin
  19. No sole in my boat. None specified and my boat is small with some 1.3 m head room so adding a sole would not really add to comfort. I see Tennant stuck to the same design we rejected back in 1985. There were a number of things we change in the structural engineering. The only things that have broken are things we did not change such as the cross beams. Yes, the sole helps distribute the load but only from one side unless you wrap fibres around the dagger board case and spread them onto the sole. Are you in a hurry? If not, why not spend some time on a bit of proper engineering befo
  20. Four layers of glass? What kind of glass and why? Here is what we did 30 years ago instead of following Tennant's instructions. This has worked but I (now) understand there are even better ways of reinforcing this area of the hulls. Some years ago there was a very long thread on the renovation of a tri called Timberwolf. Unfortunately all photos are missing now. Having longer boards with the upper support further up in the dagger board case also helps -- less crow-bar like. My dagger boards reach deck even when fully down. /Martin
  21. Underengineered and underbuilt foils are far too common. There is some information on this here. I have enclosed a section that works real well. You can scale thickness a bit to fit your dagger board case. The first coordinate is cord and starts at the trailing edge (1) with zero at the leading edge. Recommended for rudders too. /Martin prulsY.txt
  22. This four year old video demonstrates what impact on sail shape down-haul-induced mast bend can have. /Martin (envious, I don't have this level of shape control)
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