Dave S

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About Dave S

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  • Birthday 01/30/1971

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    http://www.i14.org
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  1. Dave S

    Aluminum Tube vs. Carbon Tube for Mast

    It will, for sure, assuming the carbon rig is lighter. The weight of your mast is responsible for quite significant additional heeling moment (because it's so far from the centre of buoyancy). Reducing that weight will make the boat heel less, and you'll respond to that by powering up the rig, which will increase all sorts of loads.
  2. Dave S

    Running Backstays - Suggestions for use

    Many moons ago I sailed a Sigma 38, which was somewhat underwinched, and in most conditions upwind you just ground on as much runner as you could get. Downwind we tied them both forward to the shrouds, and often used a jib halyard to wind the mast more upright still.
  3. Dave S

    Seamanship games, for kids

    This is an important one for them to recognise, in case they're offered a crewing job on one when they're older.
  4. Dave S

    Speed in current

    If it had been a *real* glider you could just have taken the wings off, chucked it all in the trailer, and been home in time for dinner.
  5. Dave S

    Seamanship games, for kids

    Raft building? Depending on the kids' engineering aptitude, either come up with a standard design and get them to build it in teams, or just give them the materials and let them get on with it. If the rafts are still floating after they've raced them around the course, send them back out with bailers or chopped-off plastic bottles for a water fight.
  6. Dave S

    Speed in current

    Bonus points for doing it at 130 knots before pulling up and flying a circuit...
  7. Dave S

    Speed in current

    Yes and no. You can use vector addition to model a single track, but how do you figure out which track is the optimum? If there aren't too many variables (uniform tide and wind across the course, or gliding between thermals) you can try to capture all the variables in a differential equation and then solve it to get the best course. That's really elegant, but only practical with a small number of variables (McCready being a good example). For a more complex problem you go for the sledgehammer approach, which is Monte Carlo analysis. Effectively, you do the vector addition for lots of different tracks (potentially thouands) and see which one wins. It's not as elegant, but it does work, and can handle much more complex data sets such as a detailed tidal almanac or detailed wind modelling. It's not magic - it's still only as good as the data you put into it, but it is a very effective way of combining all that data. You need a lot more computing power, but computers are always getting cheaper and more powerful. Way off topic, but for any glider pilots interested in elegant mathematical solutions to real-life problems, look up how a total energy probe works. Background for the non-pilots: glider pilots use a device called a vario to measure rate of sink/climb. This is effectively a device that measures the rate of change of air pressure; if you're climbing the air pressure drops; if you're sinking it rises. The problem is that you really want to know what the air around you is doing. If you're flying at constant speed and climbing it's because the air around you is rising, but if you're zooming along at 120 knots and then pull the stick back (which is exactly what you do when you reach a thermal) you're going to climb rapidly regardless of what the air's doing, because you're converting kinetic energy to potential energy; this makes it hard to tell what the airmass is doing because every change in speed is going to screw up your vario reading, and in a straight line you're adjusting your speed most of the time. One way to solve this problem is to measure your airspeed and then do some number-crunching to compensate for its impact on the vario readings, but gliders have been around a lot longer than computers, especially portable ones, and the basic instruments are nothing more than funky barometers. The total energy probe is a very clever mechanical device, with no moving parts, that automagically compensates for changes in airspeed and allows your vario to tell you what the airmass is doing.
  8. Dave S

    Speed in current

    They're called gliders where I come from; most of the clubs have the work "gliding" in their name, and they're affiliated to the British Gliding Association. They all work the same way, it's just that some are way more efficient than others. At one end of the spectrum you have something like an Arcus with a 50:1 glide ratio, at the other end a Space Shuttle at something like 4:1, but they all work the same way. Even a Space Shuttle would climb if you stuck it in a big enough, strong enough thermal.
  9. Dave S

    Speed in current

    A couple more thoughts: I think the McCready approach would work if you know (from experience) that you're always going to hit the eddy and tack up it. In a glider you'd have some combination of electronic vario and glide computer to help you, and you could write something similar to do it on a boat; XCSoar, which a free open source glide computer, might be a good starting point. If you think you're going to make it without tacking and just want to know when to foot and how much, that's more analogous to being on final glide, where you still dolphin through lift, but have a fixed arrival height. Again, XCSoar should contain the algorithms you need, and you're actually in a better position than you would be in a glider, because you should know where you're going to hit sink, and how much. Getting it wrong and having to tack up the beach is also rather less embarassing than sticking it in a farmer's field 2 miles from the finish. In practice though, I'd go with the Monte Carlo approach; in a glider you have very data to work with (instantaneous lift/sink, expected lift in next thermal, wind velocity and glider polars) so a simplified approach works. There's a lot more data available in sailing (including a full tide atlas), and Monte Carlo seems like the easiest way to crunch that data.
  10. Dave S

    Speed in current

    I think this could work for the simplified problem described, but only if you're not going to make it to the mark without getting into the eddy. The way to visualise it is to replace your vario with a device showing your upwind VMG relative to the ground. Your upwind VMG when you get out of the tide (upwind polar VMG + 0.5 knots) becomes your McCready setting. Set the ring on your "vario" appropriately, and head up or bear away in the same way you'd put the nose up or down when you're flying. I suspect it wouldn't work in real life though. McCready theory relies on the fact that you "know" your rate of climb in the next thermal, and also relies on your not being able to get where you're going without using that thermal to climb. The equivalent is knowing how fast the eddy is flowing (which you do) and needing the eddy to get there (which, in 20 knots of breeze, you probably don't). I think the real World situation is more analogous to ridge flying in the mountains, where some bits of ridge are liftier than others, and with some patches of sink. The best approach there seems to be leave the electronic vario in thermalling mode, and judge how much to speed and slow down by feel. In a boat you'd instinctively foot off a bit while you're out in the tide, which feels sort-of equivalent.
  11. Dave S

    where is it?

    Yep, HISC Laser 2. Just dig it out and sail it, I'm sure the blocks will all be fine.......
  12. It's pretty much a full Moon at the moment, so Spring tides, and anyone who's raced at Cowes will know the tide's pretty strong there; my guess is they hit it sideways.
  13. Dave S

    Confused

    Yeah, we did quite a bit of two-boat testing (though not so much in the heeled-to-windward soaking regime that displacement boats occupy in lighter conditions). TBH, it depends how the kite was cut in the first place. As someone else suggested, if your class rules allow unmeasured kites, as ours did, then your sailmaker is likely to have designed the sail with the longest luff possible for whatever sort of sailing it's designed for. So we would never have wanted to make the luff even longer (which is what easing the tackline effectively does) but we did have a case for making it shorter on triangular courses (hence the cunningham). Conversely, if your kites are measured or your boat is a one-design, it's possible or even likely that your luff isn't as long as you might want it in some conditions; in that case, easing the tackline in some conditions would make sense. If you're sailing a one-design, what are the leaders doing? If you're sailing a one-off, ask your sailmaker. If you're sailing a big boat with multiple kites, they may not all want to be trimmed the same way.
  14. Dave S

    Confused

    At one point on the I14 we were using kites with a cunningham. Leave it off for normal sailing, crank it on to flatten the luff when struggling on a tight reach. Worked really well with that particular cut of sail, we even managed to tack the thing in very light winds...
  15. Dave S

    Port Tacking the Feet.

    I remember an Int 14 Supercup regatta once, where we started most races on port, ducking the fleet. Nice lift off the back of all the other boats, first onto the layline, and when they started tacking over, none was far enough ahead to cross us, so they had to duck. We led round the windward mark in most races, until people started cottoning on and copying us. Caveat: Only worked because it was a small fleet, everyone was on their game so we didn't have to duck any stragglers, and the course was short enough for shifts not to be a big factor. A bit like trying to cross the fleet on port, it worked best when we were the only boat trying to do it! To answer the OP's other point: calling out fake rules gets no respect from me. Win the race by sailing better than than the opposition.