The windy workshop...
Mon, 31 Oct 11 15:01
OK, it’s all coming back together. The wind has been howling here throughout our repairs. Yesterday was a real container rattler. Slowly Namibia infuses her way into the structure as the sand inevitably finds its way into various small laminates.
The kiters will be going for it further south in Luderitz today (more on that later)
The beam repair is coming along nicely. We had made up all these moulds to bring down with us but the container ended up so crammed full of boat, wings, RIB’s etc that we had to leave some things out. This means we had to make new moulds down here and that all takes extra time. Never mind, we have them now. Alex and Nick set to on the job and I lent my input after having done numerous beam repairs in the past.
YOU CAN SEE THE CRACK ALONG THE ORANGE LEADING EDGE. THE SECOND PICTURE SHOWS THE SHEAR WEB INSIDE THE BEAM. YOU CAN JUST MAKE OUT THE BUCKLE IN THE SKINS JUST TO THE LEFT OF CENTER. THERE IS ACTUALLY A LOT OF DELAMINATION ALL AROUND THIS AREA CAUSED BY THE BUCKLE. WE HAD TO REMOVE ABOUT 1.3 METERS ALL IN ALL.
Right now the beam is going together right beside me and it will be pretty much rejoined by the time this is posted. We could be ready for sailing tomorrow but seeing as no wind is forecast I have told them to spend the extra time to get it all spot on as there is no rush weather wise.
We have been over to ‘Speed-spot’ and recovered the main parts of the wing. Many bits are still over there in the ‘igloo’ shed so as to save some space back here. The wing really did come away unscathed. I have pulled the key components apart including the strut ‘Collar’ and the HARKEN car that all the breaking loads went through and they are perfect. I think that the breakage may have been initiated by a load we put on the beam early on in the sailing trials at the start of the year. We had a clearance issue between two wing sections that prevented the wing from ‘feathering’ and it put a huge load on the strut. It survived then... but failed this time under what seemed like a lot less load. Maybe cracks were initiated then that an external inspection couldn’t detect. Well, it’s definitely stronger now.
The beam is not a big issue. The cracks in the main foil are a much bigger issue. If we can’t use this new, high speed, ventilated foil then we can kiss 60 knots goodbye until we get a new one. Those little hairline cracks indicated that something was going on inside the laminate which needed to be understood. We put load on the foil at the tip... up to 360 kg and it didn’t fail.
It did deflect more than it should have and the thickness of the foil changed by 1 mm. Something was opening up inside. The only option was to cut it open in the least destructive manner and have a look.
The biggest weak area of foils of this nature is the bursting loads in the curved elbow... especially as they are loaded to open up the curve rather than fold it as on more traditional curved foil (on say an ORMA trimaran or some modern beach cats). Once I ground away the carbon tapes that hold the two foil halves together along the trailing edge of the curve, small cracks could be seen. With a tiny amount of load these cracks just opened up and spread as in glass. Not a nice noise in composites.
I ground away and chased the cracks through the thick ‘Spa-bond’ glue that was used to join the two foil halves. About 50 mm in I got to the first carbon fibre ‘shear –web’. These are used to put carbon fibres across the join line rather than just to rely on the strength of the glue. The carbon is much stronger and better at transferring the immense loads for one side to the other. It seemed the crack had also extended to this web and once again, under a little bit of load it had opened up. It was now time to chase the whole crack to its full extent. I dug out a big groove all the way around the transition foil (the curve) all the way to the shear web. I laid the foil nose down and filled the whole groove with some ultra slow epoxy resin and then pulled on both ends of the foil with a 6:1 purchase to load and unload it. This forced the crack to open and close.
I had big G-clamps at either end of the crack to stop it running further than it had to. When the foil was loaded up it ‘drank’ the resin and then spat it out as it was unloaded. I did this repeatedly until all the air had come out and no more resin was going in. Then I left it to cure. Next we jammed in rolls of fibres at +/- 45 degrees and 0/90 degrees to take the shear and bursting loads. These were vacuumed in in two stages to try and get as much air out as possible. The whole grove has now been filled in this manner rather than just having glue in there. Today I will vacuum on the carbon ‘tapes’ around the outside to seal it all up.
It is impossible now to tell exactly how well this has all bonded on the inside. I think it’s about as good a job as we could do without getting too destructive. We are also discussing the option of bolting through the foil around this bend. Of course this option has its good and bad sides. I have seen this option used in such places as Groupama 3’s beams where similar loads are exerted on the curved sections where the beams go down into the floats. We are still looking into this option as it has to offer a clear advantage as drilling holes in a highly loaded area is not something to be done lightly.
So once we have joined it all together and cured it under high temperature... we have to go and do a full load test. For me this will mean taking it up to a load equivalent to 65 knots with a safety margin. We will do this here in Walvis Bay. We also did it before in the UK and it passed. We did hear cracking during this procedure but it still took and held the 1 ton load at its centre of pressure. The foil was not faired and painted then and I wonder if the cracks we were seeing were not just because the fairing/paint had not been subject to this load until our last high speed run. It’s entirely possible that the foil had developed cracks during the initial load test but still managed to take the load without failing. The best way to see this is to measure the deflection during the load test. The cracks may also have propagated whilst being worked. We noted a much larger than expected deflection during the last 360 kg load test which is why we chose to operate on the foil.
Hey... we just joined the beam back together with the new section. A tricky job that took a lot of jigs, clamps, lasers, spirit levels... and spabond. It looks good.
THE CONTAINER FLOOR IS THE FLATTEST SURFACE WE HAVE. RIGHT NEXT TO WHERE MY ERM... OFFICE IS. THIS IS WHERE THE MAGIC HAPPENS;)
KITERS GOING FOR IT...
It’s getting pretty windy here now. Hitting 30 knots. It’s hitting over 40 knots down in Luderitz and the kiters are tearing it up in the new channel. Apparently Seb Cattelan is already at 48 knots. Today should be a big day for them and a great trial for the new channel. I reckon it’s pretty damned dangerous to be doing what they are doing down a ditch with dry land either side... but then there are lots of things we humans do which is far more dangerous i.e. I never really understood how dangerous it is to race bicycles down tarmac alpine roads until I did it this summer. It’s bloody dodgy and people get seriously messed up. Still, it’s your choice whether you do it or not and I’m all for freedom of choice. Still, I recommend a helmet at least.
As I have said before, I fully expect the kite surfers to go a fair bit faster than they already have. There are many things that can give them gains. I’m not sure how much they rely on their small fins for speed as these would be the only areas limited by either ventilation or cavitation. The rest of the aero/hydro package is only limited by efficiency. For VESTAS Sailrocket 2 to beat them we have to iron out all the bugs relating to strength and overall balance and then we too can just focus on efficiency. If this foil passes on the upcoming test bench... then we will give it as hard a push as is necessary in the upcoming record attempt. I will be wearing a nice thick, expensive helmet.
Just heard it’s gusting 50 now in Luderitz and the new channel is working fine. Sebastian Cattelan has hit just over 50 knots already despite having a crash on an earlier run. The container is starting to rattle again. It's all happening in Namibia. WOO!
Cheers, Paul
z-pinning technologie
Submitted by Florian Prevel on Tue, 11/01/2011 - 15:40.Hi Paul,
Happy to see that the work goes well.
Your foils problems remember me a presentation of a composite technologie which could be use for your next foil : z-pinning
http://www.cranfield.ac.uk/sas/clientservices/facilities/page5257.html
Maybe you could contact this university to do some tests ?
best regards
foil repairs
Submitted by AdrianThompson on Mon, 10/31/2011 - 19:06.Great progress guys - I'm green with envy with that 50+ ride!
I hate to see your foil looking like the one we had in gamma - same problem on the curve as the catinary loading on the inside laminates separated the structure as they tried to straighten out. We ended up threading some carbon tows through the stack although drilling the 3mm holes felt quite wrong. It did work though - we spaced them about 50mm apart along the length of the bend. The holes were chordwise, so we just had a little fence a couple of mm high. I guess you would have to fare that in with your super critical foil.
Armchair contributions should all end up in the waste bin I know. Just couldn't stop myself!
Cheers AT
repairs
Submitted by Mike Ewart on Mon, 10/31/2011 - 18:04.Hi Paul great to hear it is all going back together well, it must be difficult to keep the conditions and cleanliness right down there, I was in Walvis Bay many years ago on a ship and I can still remember the constant haze of dust/sand in the air.
Very interested in the repair of the foil, I think that could have caused a disaster if you hadnt caught it in time.
Good luck
Beam repairs & Luderitz
Submitted by Tim on Mon, 10/31/2011 - 16:22.Paul,
Great read on the beam repairs efforts, and very glad to read about your progress towards new speed runs. The carbon-fibre materials you work with are materials with which I have no direct experience except flying high performance gliders, so I find your descriptions of the repairs process interesting.
Of course, I wish the kite boarders all sucess and safe runs, and new records...that's the game, yeh? But really, forgetting, for a second, the skill involved and the risks incurred in such endeavor, breaking world speed records in winds blowing almost as fast as the boarder seems rather in-elegant....like falling off a cliff to say you are the fastest sprinter in the world. More power to them, but my money lies with the VSR2 approach to exceed the wind speeds by at leasst a 2X factor.
BTW, I note with interest the Rick Cavallaro engineering marvel "Blackbird" which apparently can "sail" (move) directly downwind at approximately 2.8X the speed of the wind!! I understand the energy of the sytem, but I haven't fully gotten my head around the physics. It's a bit of a mind bender...
Best to Team VSR2!
Tim, Vermont