JulianB

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About JulianB

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    Sailing, eating, designing

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  1. Angus is now 6' 3" and 90kgs, he ws the best part of 20kgs in that shot! Really interesting sails, with black trim, you completely lost the boat at 200m, quite amazing. (hence now the orange piping) But the sails where really good, and the cloth amazing. Pity North did not continue with it. You could puncture it with a knife, and put a rope through the gash, and it would move 200-250mm and then stop. Very simple cross-cut joints at each batten. It was probably too cheap to make.
  2. What, no life jacket??? Its under the polo shirt!!!
  3. Dicko was asking about a rig for his kid and mate that are 100kg combined. For 1-2 years they shoudl be sailing with a XS rig. They could probably survive a std 29er rig but it would not be much fun. Got to let them have fun. Sorry if I was a bit obscure, at 100kgs, the kids will have a ball with a XS rig. As they get up into the 115-120kgs combined range, then swicth it out for a std rig. The plan in Italy was the club would lease these rigs out for a year or so, then lease them to the next hopefuls. Nice idea. 1997 sailing with my then 4 year old son Angus on the Mk2 29er, probably 115kgs combined.
  4. Hi, so a XS main is 4.289m² and the jib 2.133 m². Approx 6.45m² in total. (it obviously reefs under 5m²) The Spinnaker I can't find a numbers. Std 29er main is 8.64m² and the jib is 3.76m². Bit over 12m² in total. (Std spin is 18m² from memory, but don't bank on it) XS sails are much higher ratio, which is deliberate, area is down, so it's really easy to handle, but lots of luff length so as you start to get going, you can induce plenty of power. Bottom line, YES, they could manage it, like anything, don't send them out first time in 20 knts, maybe 10, get them understanding the power and stering for balance and then witin 3-4 sails they will be good for just about anything. It was design for kids out of Opies, and those kids are late 40's to early 50's so 100kgs combine is perfect. I would forget about the reef unless you live in Wellington (NZ). jB
  5. JulianB

    49erFX Tuning Guide and guidance

    So in answer to your cant question. I once again went to 9eronline.com and downloaded the rough 3d model of a 49er (done by Martin Billoch in Argentina approx. 2009) And I then did 2 things, I worked out where the 33%, so the nominal CoE line of pressure (differential) was and projected that line onto the mainsail (yellow line) So below you have that line projected head-on and side-on to the mainsail. Martin did a really nice job, the mainsail is twisted (washed out) at the tip, as it should be approx. 22°, so this would be a up-range wind scenario, with minimum vortex and therefore minimum drag. On the next page I have rotated the whole boat, rig, sails, foils around to 22°. We know from the vector diagram (attachment #1) that the AWA [Apparent Wind Angle] is going to be somewhere from 21-24° in the normal course (excuse the pun) of sailing. So with any sort of windward heel that 99% of the good 49er sailors do, the cant, so the angle between the red vertical line that I am dropping from the mast head and the cyan line that I have also started at the mast head, but is running through the heel of the mast, will be about 5°. Dead flat, as this is showing its 4°. I went a bit further with the yellow line, and it’s a lot more vertical, (2.4° positive cant) but it is also has a nice concave shape, which, along with wash-out/twist, reduced cambers and reduced cords high in the rig reduce, probably eliminate most of the time any tip vortex and therefore the resulting drag. So this is how the wind see the rig. WRT the FX and the tip flick, just visualise where the tip would be if you have 20mm less cap shroud tension. Angle has ½ ved, plus the sail will have flattened off substantially because sideways bend adds to fore n aft bend, so the pressure will spill off the top of the main, you will get SWD [Span Wise Drift] upwards in discernible amounts from the top spreader up and that in turn will reduce leach load, it will reduce camber and all of those things will drop the CoE of the sails which will reduce the ARM and increase SCP. Again, you need to see this from the winds POV, the sails are an obstruction, it wants to get around them in the lowest energy scenario that it possibly can. The yellow line the line of pressure, is angled aft, so the air will naturally want to rise, but with firm Cap Shrouds, the tip comes out to windward, it's like a albatross with dropped wing tips, (and they have been doing this stuff for millions of year before us). Don’t know how much or what but at some point the curved and canted rig to windward will cancel out the aft racked pressure line. Pretty obviously the loose FX cap-shrouds dose it, so we must be very close to a tipping point!~ Its actually quite clever. Now I have done all of this on a 49er, I have not done it on a FX. I make no assertions and I can't back any of it up. Very happy to hear other POV! ~~~~~~~~~~~~~~~~~ I promissed you some CoD and CoL graphs, the 1st graphs are for 0012 section, very similar to a Laser foil The commonality between the 2 graphs is the CoL, so Section Lift Coefficient which is the vertical scale on the LH graph, and the horizontal scale on the RH graph. Same book (it was publish before I was born) 6 pages earlier, a section not to dissimilar to a 49er/29er section. (Tasar section is thinner) Notice that the CoD comes down to 0.004 with the 0010-34 (49er) where as with the Laser section 0012 is 50% more at 0.006. The 0012 section is far more tolerant, virtually has no bucket, there is not much penalty (WRT foil drag) whether you Yawl at 2-3-4°, and the big reason for that is it’s a turbulent section. The 0010-34 has a very defined bucket, and that is where it has laminar flow, bit more precious, you have to be a little more “just so”, you had better be under 2°, better if your at 1.7-1.8° Yawl, but the reduction in drag is 50% (all other things being equal which as I said before rarely are.) I am going to leave you for 2 days. (I need to do some real work) jB
  6. JulianB

    49erFX Tuning Guide and guidance

    Thanks for this first instalment Julian. My pleasure, I recognize some of it as being an elaboration of what was in one of your father's/ your books (they are lent out at the moment so I can't check) and answers some of the questions I had after reading that; especially the calculation of (I think it was) Sail Carrying Power (SCP) the formula of which wasn't entirely clear from the book - although the factors that went into it could be inferred. Just to confirm my interpretations (and maybe help others wrestling with the formulas and concepts), my understanding is BS is Boat speed YES AWS is Apparent wind speed YES While I can now see how ARM is measured, do the initials stand for anything or is the word 'arm' itself the concept (a bit like 'righting arm') Not that I am aware of, ARM = distance between CLR and CoE "Yawl" is what I might have thought of as side slip or leeway (expressed as a degree off course)? Correct, it could be expressed as simply as HDG [Heading] vs CoG [Course over Ground] which these days seems to be set by GPS. It’s not a lot, if your Centreboard is doing a good job, most skiffs it will be 1.5-1.8° and WRT a skiff which is on-top of the water, as compared to a displacement boat which is in the water then Yawl drag is smallish, hang-on. So we generated this graph, which is a 49er, sailing 14kgs light, so around 265kgs all-up weigh in 4th mode, so nose down 50 - 100mm. The green line is generated by pulling it straight, the magenta line is at 2° off axis. This was done in 2008, with Simon Watin (nb1) So Yawl drag of a 49er at hull-speed (5.6knts) will be the difference between 7.26kgs and 8.6kgs (I went back into the data to dig that out.) That is a 18% drag increase to put it in perspective. Couple of points, skiffs, even Tasars the yawl drag is pretty low because they sit on top of the water, not in it. The 2 bell-weather classes are the I14 and the 5o5 who spend a lot of resources getting gybing boards to work and they do it because it is important, and it works. So they can opt for “in the water” with board gybed and minimising Yawl drag or they can jump up ontop of the water (planning to windward) at which point they rake their board aft a tad and the locks off the gybing function. Pretty obviously a boat “in the water” will have a dramatically higher Yawl drag signature than a boat that is ontop of the water. I have never measured the drag of a “in the water boat” or attempted to verify what the Yawl component is, so read into that what ever you wish! (nb2) Very few classes can Plane Up-wind (on top of the water) and even fewer body swung classes can, the Tasar being one that can. And by plane up-wind, sure you can get your Laser and sail at a few degrees above abeam and plane, but in a meaningful manner, the word is up-wind, so to be able to Plane Up-wind you need to be able to generate a better VMG to the windward mark, by opting to allow the boat to crack off a few degrees and plane. Yes a moth can (before they foiled), but it’s using wings, as dose a Contender and Musto skiff (using trapez) etc. And a lot of boats with trapeze can only effectively Plane to windward (and achieve increased (faster) VMG’s) when conditions are just so. Just like a Tasar, plenty of wind is the key factor. ISAF, now WS used to coin a term, they can plane to windward [with increased VMG] most of the time. So that would assume 50% or more and the 49er/FX are the only Olympic classes that can do that, 29er is the only Youth Class that can do that. This was then the definition of High Performance! That went out of favour pretty fast, so that has disappear from the WS tecahing, and possibly that is a good thing! Tasar probably dose not meet that criteria as it needs conditions just so, lots of wind, to make it work, so it’s probably 15% of the time at best. But it's probably is the only body swung class that can lay claim to being able to do it at all! So ASP, I will come back to Tasar’s and it’s rig in a day or 2, and it’s the rig, it’s lightness and it shape (of the hull) that allows it to do this, but let me come back on track to Rambler. I'm not quite understanding the "half of that doing anything" concept in relation to the centreboard, combined with the suggestion that only the 300 to 400 mm under the keel is the bit doing anything. This is a new concept to me; the suggestion that the bottom section of the board is ineffective? OK, 49er Centreboard is 1.2m LOA, 350mm is in the case, so it’s 850mm into the water, CLR is say 45% of the way down it, 850 x 0.45 = 382mm. Now pull that centreboard 200mm up, its now 650 into the water, 45% of 650 = 292mm. I rounded it out to 100 for simplicity. And the reality is there is less area low down, and there is less camber towards the tip (as a %) so the true effect will be closer to 100 than 89 (382-292 = 89-90mm) I presume the 5 degree canter to windward at 22 degree apparent wind is the rake of the mast? Yep, hard concept to grasp, let me re-do that dwg. It’s easy for me in 3d, give me a few hours. COD on the centreboard is Centre of Drag? [This is definitely a question; not really a concept I've struggled with before - if that's what it is] CoD = Coefficient of Drag, it is rarely linear there are buckets and assorts of other stuff but from -2.5 -> 3° it could be assumed to be pretty flat line, the bucket starts at about +/- 4°. Let me dig out a graph and explain that when I come back with the 5° cant! But there's some really interesting stuff in what you've written that goes well beyond what I've managed to absorb from previous writings (mainly your family's) I'm really looking forward to the next instalment. Much as I asked your father to do all those years ago, If I was to take the sum of your writings and try and break it down and express it in a way that could be presented as a lecture to reasonably intelligent people, might you be willing to review it? I will look fwd to writing it. jB ~~~~~~~~~~~~~~~~~~~~~~~~~~ Nb1, occasionally I get a person “debentured to me”. I think it's happened 4 times. Simon was from Revolution Island in the middle of the Indian Ocean, French. He was one such person, by the time I meet him he was 23-24, had a few degrees under his belt, I remember being with Dina Kowalyshyn (really smart Naval Architect, Amercan, presently chairs Equipment Committee at WS) at Orly Airport in Paris and this long hair crazy frog descended on us and Dina being really impressed. Cut a long story short, Simon arrived in Sydney few months later, and spend 3-4 months with us! I was his “professor” (what a joke, this guy was so smart) but he and dad hit it off beyond belief. It was 2008, so dad was 88. I remember one night where dad was doing maths in radians, and long hand, realms and realms of paper, and Simon was using MatLab. Simon when one way and dad went the other, few glasses of Red later, lots of French swear words, and they ended-up within a few fractions of a 1% of each other. Error was so small. That empirically proved what we were doing had an extremely high level of credibility. That whole report is on 9eronline.com. Last I heard he was working for VPLP in Breast. Nb2, just re-reading this and it struck me that we all use centreboard that are way bigger than then need to be. Especially on slower in the water boats. Then I had a bit of an epiphany. If you are in a “in the water boat” then the cost consequence of allowing the Yawl angle to increase very much is so devastating that carrying 20% more Centreboard area than what is needed is a small price to pay. The extra drag from another 20% at 5-6-7 knots is trivial compare to a) the increase in Yawl drag from say 1.8 – 2.5° and b) the increase in CoD of the foil, when from 1.8 – 2.5°. (I will post that bucket drag curve) Again, I have never attempted to measure the Yawl drag of a “in the water boat” and when we did the extensive tow tests of 470’s 3-4 years ago we did not think of doing it then. Pretty interesting paper out of the Wuhan Sports University re 470 hulls and the effect rudders have on them. I got it off Scuttlebutt, those of you far more intelligent than I should have a read. I probably need to re-read it! Skiff's the sum goes the other way, at 10-11-12knts, the ability for the Centreboard to resist lee-way (Yawl) is a X² law, so a Laser probably has far more surface area than a 49er yet has a RM that is less than ¼ . Skiffs get very judicious about how far we pull the centreboard up, and it has a major bearing on overall speed. Off the wind, with a skiff, it’s already up, so getting it further up, is not so important, where as a in the water boat, getting the board up is paramount! Interesting, need to ponder that! Jb
  7. Lifting it up another 200-250 would probably add 1-2 knts to max speed, especially given its a 0009 section. But point taken! jB
  8. For the foil to hmmmmmmmm, it has to use energy and that comes from you, or more to the point, it's slowing you down. I remember wires "starting to harmonic" on a 18teen in 1993 and it took atleast a knot of the speed going up wind. Easyiest way to stop it, champher your TE at 30°, especially towards the tip, it will trip a tiny vortex that will remain stable on one side and no more hmmmming!
  9. Hi Steam Flyer, re reefing, we have become pretty proficent at getting the possitioning of things like shrouds/vs/forestay and from memory the boat has D1's so as you go up the wind range, more and more down haul (not neccassarily vang) along with correct tension on the rig and the jib sheet allows the boat to go right up range and still remain in control. What JimC is refereing to is very very real, you reef the main and you end up with a mast that inverts, when you want it to bend. And that's not good at all. Makes it far worse! "Weird unbalancing lee helm issues" are the least of your problem's. The other issue is when is it too much wind, particularly in this legalistic society we are heading towards, and especially with kids. Kid's are evolving, and unless we want to cap that, what we offer them to keep them engaged has to also evolve. Whether that is a change of boat, and change of venue or some new gear, really matters not.
  10. JulianB

    49erFX Tuning Guide and guidance

    Friday, 3 July 2020 Sydney Australia Theory of skiff rigs. Hi Rambler, you asked for it! Sailing is all about drag minimisation, You have a fixed amount of RM (Righting Moment) and you are exploiting that RM to generate lift to leeward (and sightly forward) in the sails, and lift to windward (and slightly forward) via yawl on the foils and the resulting combination of the 2 fwd. vectors minus drag = boatspeed. My father used to say, a foil in the air, a foil in the water and a bit of low drag floatation! Foil in the air/water, interestingly you can do the maths, it’s a x² law stuff, water is 812 times as dense as air, but your rig is operating in AWS where as your foil is operating at BS, so a 49er “say” has 22m² in the air, at 18 knts it has an AWS 29.5 knts and a BS of 14 knts. (see attachment #1, I’m re-cycling dwg’s) So 29.5(knts)² x 22(m²) = 19,145 air widgets. 19,145/812 = 23.6 water widgets 14(knts)² x X (m²) = 23.6 And that = 23.6/14² = X So X = 23.6/14²= 0.12m² & at 14knts a 49er has lifted it centerboard 200mm but regardless it’s probably only operating on the 300-400mm from the keel-line downwards, boat has a cord of 325mm, so the .12/.325 = 360mm of the top of the board is actually doing any work. (the board is 1.2m LOA, 350mm is in the case, 200 is lifted so there is only 1200-(350+200) = 650mm in the water and ½ of that doing anything is probably about right. The reality is you could probably pull the centreboard up another 300mm and still maintain a yawl of sat 1.5° with a reasonable CoD.) Every Naval Architect/Engineer is going to say CoL, AoA and CoD considerations and yes yes yes, but as a rule of thumb, it works and it’s fascinating! So, get back on track, you want to do everything you can to maximise RM and minimise Drag. Your question re tip flick is all to do with RM & Drag. Re RM and the corresponding SCP (see attachment #2 from the 9eronline.com blog) The important thing to remember about RM is once you leave the beach, the max RM you can generate is given, you simply can’t generate more, especially these days with people dress in rubber and not greasy wool jumper, (when you went under a wave, with a greasy wool jumper, you put on a “stone” (14lbs about 6 kgs)). So to increase (or not decrease) BS you need to reduce drag or increase SCP. SCP = Sail Carry Power, and it’s simply the ARM between the CLR [Centre of Lateral Resistance or the CoE [Centre of Effort]] of the centreboard and the CoE of the sails. If you go back to 9eronlince.com and click on Library and then go to 49er sail working dynamics you will find a diagram spells that out, in the case of a 49er, at the design wind, the ARM is 4380mm. Go one step further the RM of a 49er is crew weight = 165kgs (approx.) x distance of CoG [Centre of Gravity] of the crew from CoB [Centre of Buoyancy] of the hull. 2.9m/2 (width of the boat) + approx. 1m (distance from your toes, to the CoG of your body) so it then = 165 x (2.9/2 +1) = 404 kgs/m if you now divide that by the arm which is 4.380m you get 404/4.380 = 92.3kgs/m² SCP Interestingly SCP also =’s the sideload on the Centreboard and the sideload in the Rig (every reaction has an equal and opposite reaction) You can increase SCP by lifting the Centreboard, or flattening out the tip of the main, easing the sheet, sheeting along the foot of the jib, easing the jib sheet, and feathering the rig (luffing briefly) etc etc etc. You can also do it by allowing the tip of the mast to flick to leeward and re-distribute the pressure differential across the mainsail. I will do the easy one first, lift the centreboard. Pull the centreboard up 200mm and what happens, well if all other things are equal, (which they rarely are) then say ½ of that 200mm will decrease the ARM by 100mm so SCP is now 404/4.28 = 94.4kgs/m² so a 2.5% increase in effective power. One of the big things about lifting the Centreboard is that not only do you increase SCP but you more than likely will reduce CoD of the Centerbaord (and wetted area) and if you have enough speed you won’t increase yawl, in-fact the increased speed, you may reduce it with the corresponding reductions in hull drag. Far harder concept to get your head around is allowing the tip/mast head to flick to leeward. First thing is a FX or 49er (hopefully a 29er) are being sailed up right, and if you now look at that boat from 22° to windward, if the boat is flat, then the rig is in fact canted about 5° to windward. (there is a diagram of this somewhere in SA, I will go find it, or maybe someone else can) Plus if the sailmaker has done a good job, and the crew has the right sort of controls set and right rig tensions, then the square head will be bladed-out nicely and making almost a 0° AoA to the AWA (at approx. 22°) so there will be very very little SWD [Span Wise Drift] so movement of the air upwards and over the mast head (which generates a tip vortex), (just have to include the photo out of Cape Town of the V60’s and their mast heads generating these vortexes) and that is pure drag, the tones and tones of air being spun into that vortex comes from your RM and SCP and that’s slowing you down. Now with a FX, with it’s really long head length, yes it’s prone to generating a vortex, but empirically it would appear that in-terms of a bucket of drags, it’s less of a problem to allow the mast head to move to leeward, and by doing so, allowing the air pressure on the windward side of the main, from ½ way down the sail (that’s a guess) to move upwards getting greater and greater as you move towards the tip, and that in turn, sure it will generate a vortex, how big and how long living I don’t know, but given that they do it, it would suggest not for long, but the reduction is presure high in the rig, that in turn drops the CoE and that reduces the ARM and that then increases the SCP. And that’s why they probably do it. And very possibly without knowing why! With a pin head rig, you never do it, the drag consequence is too great and the benefit to small. Best pin-head rig is a over-rotating mast, (like a Tasar) and again a Tasar is sailed flat, so you more than likely have air moving down the rig in a breeze so allowing the upper leach to hang right off is low drag, increases SCP so it’s a bit like lifting a centreboard, as in everything is working for you, for your advantage. With the 49er rig, with its far shorter mast head length, they tend not to do it, (let the head flick to leeward) they ease sheet tension just a little and again allow the upper leach to feather. (need to stress lots of downhaul (approx. ½ tonne at the head board) and plenty of rig tension. As you get longer and longer with the mast head length (look at some of the windsurfers) the top of the sail inverts. (A windsurfer is a little different, in that it has a big head so if has lots of grunt downwind, where as a 49er/FX sets a spinnaker) Let me get into that in a few days, probably enough to absorb right now. What is the ideal mast head length is a really interesting debate. The sailors tend to say a 49er has far more range than a FX and that may be the case now, but where will it be in 10 years’ time? The FX coaches may have pulled another rabbit out of the hat by then, it’s simply too early to say. And is more range a good thing or not. On the 18teens where you have a selection of rigs, probably not, but then again, David MacDiarmid (NZL) has just won his 3rd JJ Giltinan with far shorter mast head lengths than his Australian counterparts. No doubt there are other factors at play! Also remember that it’s only 12 years since I watched my father light up (2008) watching Simon (Watin) and Harry (my son) sail the first 49er/skiff square-head and to his credit completely flick his 50 years of perfecting pin-heads and go hard on a new direction - squareheads.
  11. JulianB

    49erFX Tuning Guide and guidance

    Sorry guys, just came back from Melbourne, avoided Covid-19, and walked into a huge caldron of oppotunity. Need another few hours, and it will come in 2-3 x 3 page instalments over a few days. Got a bit carried away! jB
  12. Where and when is "Skiff - Chicks" ??
  13. Before I get to Rambler, Hi Stanno. The XS rig came about because of the drive of the FFV (French) and FIV (Italian) federations, and was driven particular hard by Paolo from Nautivela. He obviously speaks Italian and French (and German, and Spainish and English (Aussie) so your right, they where into him for a rig for the 6-8 year old hulls. 2 things have happened, a) the 29er hulls and lasting longer and longer, so the 6-8 year old hulls are no longer lying about & b) the FFV & FIV had a change of position. The wheel turns, but it's interesting it's now doing another revoloution, and the rig is coming back into favour. But the XS rig is perfectly suited for ex-opie sailors, and as you mentioned they (the rig and the sailors) are the right size, and when you put them in a XS they are also fearless and can manage what ever is thrown at them. It's very inspiring! I will leave the rest to you, just one intersting point, you will notice there is a reef in the main, this is to bring the area in under 7m² (could be 5, can't remember) the point being, its a EU regulation but it adds a singnificant cost to the main sail, In Australia, probably everywhere, even inside the EU, it will never get used so next iteration, we probably need to get without the zippers and save the money. Enjoy, and I am glad to see it being enjoyed! jB
  14. JulianB

    49erFX Tuning Guide and guidance

    Hi Mate, so how many pages do you want?????? I'm in Melbourne right now (how stupid am I), drive home tomorrow. Let me come back to you on Friday but in short, while your seeking power and efficency mast bend to leeward is not a good thing. The almost perfect boat/rig is the Tasar mast, when you over rotate it, the mast come out to windard, a near perfcet elipse, my father was not dumb, infact most "over" rotating masts do this, to some extent. Once you get to the point where your over powered, in a skiff, then it's all about de-powering in the first instance, and minimising drag in the second instance. With a FX, with it's longish head length, then the sum "or drags" falls heavily on allowing the tip to move to leeward to spill excess pressure. With the 49er with it's smaller head board lenght, then it still pays to use a lot of downhaul, keep the mast head up (to windward) and feather it off. Talk Friday, jB
  15. JulianB

    Masthead float on a sleeved mainsail?

    Hi guys, there are 3 things to this. (plus 2 other subs) #1, the amount of air you can trap (by plugging the tip)and the resulting amount of bouancy it trivial, take a C5 rig, think about the topmast only as that's the bit that might stop inversion, you have a ID at the tip of about 19mm and a ID at the joint of about 50mm, 2.6m long. Keep the sum super simple, = 2.5kgs floation (I did it in a 3d program). Then remember that in a best case senerio, the tip weights 1.4kgs, plus the sail so the reality is it wont even be able to hold up it's own weigh, plus to get even that amount of support, the entrie length of the top mast need to be emersed, and on a boat like a last that floats high, by the time you get any meaningfull "push-back" the boat is well on it's way to a turtle position. #2 is that a FRP tube you need to keep under it's TGi temp, and the best way to do that is to allow air to flow up the inside. Plug the ends and you get a oven, and the temp can quite easily get to 150c plus (wont happen in Scandinavia, but Singapore) and that's well above most FRP laminate TGi's & #3 most of the boats I design have mast head spinnakers, and sure you can run (the halyards) them outside, but that has a lot of additional complications, bow-stringing, frictons, etc. Then i) - is a conversation I had with the late Ian Bruce, which was early in the laser days in Florida, where a boat capsized and the tip did not sink fast enough and it continued to flip down wind at a rate faster than the sailor could swim, and from memory, it did not end well. ii) - is if there is a cavity, water will get into it, and you need to get it out. So plugging the tip is not a meaningful soloution on a wide range of fronts. We have developed a sock, quite similar to what ldeikis has suggested, 5mm per side, that can be added very quickly to mast head, it conturs the sail, it comes down to the 2nd batten and it generats about 4.4kgs of bouancy right at the tip, which it is where it's most effective. Roughly = to 2 milk bottles. It weight approx 1/2kg, so people can opt for it to be on or off. Kid's think it's a lot more cool, it's more effective, can go on and off in seconds and in shallow areas, it's pretty effective. The big thing is you get the result right at the tip, and it starts working as soon as it hits the water. It's really up to the individual. jB