Fastest dinghy?

Originally posted by Scooby_simon Oh yeah, I wouldn't be too surprised if a 29er could drive past a Taipan downwind in 30 knots - for a start Taipans (and most other cats) stop racing at a steady 22.5. Secondly a Taipan is normally capsized or on the beach in a real (not bar talk) 30 knots - as is just about every 29er! Thirdly, yeah the cats are throttling back so heavily for survival and their dash speeds just aren't that fast AFAIK. we don't stop racing in the UK just because it's getting a little breasy Ask anyone who came to that grafham cat open (or even opens, it usualyy seams to blow old boots) Except this year !

Ahhh yes, you're much better and more macho cat sailors than we are - NOT. Can I remind you of the British v  Australian results in the world titles in most of the International cat classes (As, F 18s, Ts, Hobie 16s, etc) and the Little America's Cup?

I don't particularly agree with the way cat sailors stop racing in a breeze, myself. But if we look at the boats we sail v the boats you sail, we can see a clue. You guys often sail boats that are normally 25-50% heavier and shorter rigged. Compare a modern A with a Unicorn or Dart 15 or whatever, or a 180kg Dart 18 with a 120kg (IIRC) Taipan 5.7, or a 104kg Taipan 4.9 with a 130 kg (IIRC) Spitfire. Our boats tend to have much bigger wingmasts as well.

I'm DEFINITELY not saying we are better sailors per se. You've cleaned us up even in 18 foot skiffs, never mind the Olympics. But currently the cat sailing record is pretty one sided. Look again at the F 18 worlds for example - something like 8 Aussies or expatriate Aussie skippers in the top 10, and the best Pom in 10th.

Originally posted by Scooby_simon Are 18' skiffs approx 18 feet wide ?  and so giving slightly less that 9 feet of leverage (lee wing acts against the righting moment)

The righting moment is created by multiplying the horizontal distance between the centre of bouyancy and centre of all up sailing mass by the total mass.  Hence (assuming the skiff is upright, and the rigged boat minus crew has it's centre of mass on the centreline of the hull) the moments of the wing masses and indeed entire hull cancel out.  

This leaves the righting moment counteracting the heeling force of the sails as only the distance between the centre of the hull and the centre of the crew's mass (horizontally) multiplyed by the crew's mass.  If the wingtip beam of the skiff is 18 feet than the leaver arm for the crew will be 9 feet plus the height up the crews body to their centre of mass (about 1-2 to 2/3 height).  Hope this helps. 

P.S. heeling moment is lateral side force from sails plus side force from hull and foils all multiplyed by vertical distance between centre of effort of sails and centre of lateral resistance of underwater hull profile, thus explaing why lifting the plate when its windy makes going upwind easier (reduces force from underwater profile and also leaver arm)

If you sit down and draw a graph you'll find that the highest theoretical speeds are achieved when you are sailing at 90 degrees plus the apparent wind angle to the true wind

Which is exactly why I still contend that a Tornado is going to the the quickest over 500m 10 feet wide and easier to control than other mono's at those sorts of speeds and when flying a hull you are using all the boat except the lee hull as extra righting moment.

Are 18' skiffs approx 18 feet wide ?  and so giving slightly less that 9 feet of leverage (lee wing acts against the righting moment)

 

 

Originally posted by Chris 249 Re hotter angles at same VMG equalling a faster boat - doesn't that ignore the question of what happens when the deeper but slower boat heats up? A boat could be better of running deeper but slower to get better downwind VMG, but still capable out a better ultimate top speed.

I suspect not Chris. I'm taking the position that this discussion is about which boat can hit the highest sustained burst speed over say 500m in whatever direction suits it in whatever wind suits it. So I'd be suprised if the Twelves and Rs were in the frame because they are carrying very large kites which are optimised for VMG rather than straight speed theough the water. After all from a winning races point of view zig-zagging to and fro without actually getting very far downwind doesn't help much.

If you sit down and draw a graph you'll find that the highest theoretical speeds are achieved when you are sailing at 90 degrees plus the apparent wind angle to the true wind, and the closer apparent wind angle the faster you go. This excludes drag of course! And when you look at the graphs for the different angles the benefit from getting a bit more apparent wind angle is huge. Of course you also have to be able to hold it up...

Again, taking the position that all monohulls are pretty inefficient beasts and can't achieve much over wind speed, then for ultimate speed you have to be able to manage a lot of wind, which again points you to small sails, unlike the 12s and Rs. Even with the small rigs they have very big VMG optimised kites.

But I must stress again that the speed I'm talking about is almost completely irrelevant for race track performance. Kinda fun though!

Of course what would settle this is a whole lot of timing data. But by and large this doesn't happen. I suspect the fact that all sorts of "big fish" stories would get exploded is part of this - we've seen that on another forum recently haven't we:-)

On FDBs ratios - the numbers above are my calculations not his - but I can't see fundamentally where they could be wrong.  If the lever arm wasn't important why do the skiffs change to shorter, smaller rigs in stronger winds...

A key thing though in my mind is the use of his threshold ratio of SCP/total wt of 30% for windward planing. 

As far as I can tell this was determined back in the late 60's & early 70's based on the NS14 development and other classes at the time.  These days the rigs have improved dramatically on the development boats and the new SMODs.  Better rigs tend to have lower drag and drag causes heeling.  If you look at the ratios for a 420 and a 29er, they are designed for the same weight crew, they are the same width and virtually the same all up sailing weight.   However, the 420 has a shorter mast and therefore supposedly a higher sail carrying power - the fact that a 29er is faster inspite of this is due to the better hull design and rig design.  The better rig gives the 29er a better drive force for the same heeling force

The message from this, surely, is that the critical thing is still to generate the maximum drive force.  A modern rig, with lower drag, generates less heeling force for the same drvie force so FDB's threshold of 30% in the 70s should surely be lower now - maybe as low as 25%...

(Taipans only stop racing at 22.5 knts when they have a choice - when 30kts comes at you because of a tropical thunderstorm just upwind with no warning then you have no choice but to hang on until it blows itself out...)