Fastest dinghy?

The numbers for the cherub and 49er are true but they miss one thing, and that is that a cherub is four and a half foot shorter so yes, downwind in a blow the cherub will go faster in a straight line (length not as much of an issue provided you can keep the bow up).  Round the cans the 49er will win becasue the extra hull length makes speed upwind far superior.  Remember also, the most important factor in all of this are the two monkeys trying to hold the thing down in 30-whatever knots of breeze. 

Wow, lots of positive vibes out there for my favourite toy  !

I think that the numbers might look even better if you put more realistic crew & helm weights in for the Cherub: most of us sail with an all up crew weight of under 135Kg (not 150Kg as shown above), but my maths might be a bit wonky.

If anyone it interested in seeing how fast they REALLY feel, there will be loads of Cherubs at Carsington Water Sailing Club on Saturday November 13th. I can't promise 30 knots of breeze, but anything above 11 knots is usually enough to put a grin on your face for the rest of the day

Lucy (Cherub sailor, so very biased)

 

For a start, how realistic are FDB's ratios? Consensus among skiffies seems to be that they are OK apart from the fact that they include the "lever arm" which is generally thought to be irrelevant or misleading. After all, the Cb is easy to pull up and tall rigs tend to be more sophisticated so they can be twisted off and depowered more easily - this makes up for the supposedly higher C of E. Just compare a Laser rig to a taller sailboard rig, the C of E on the sophisticated board rig is way lower in reality but higher under Frank's measurements.

And surely neither Cherub or 29er are fastest under any calcs. The R Class has no minimum weight limit and has hull significantly lighter than a Cherub (32-35 kg). It also allows wings (but doesn't use them) and it almost certainly has heavier sailors than the Cherub so therefore more power.

In practise, of course, in all conditions the 12' skiff beats the Rs despite the fact that many Rs are built on 12' hulls. Even when the 12 is down to its number 3 rig, the extra weight of the 12 hull (45kg??) is more than compensated for by the fact that it has a very flat, efficient rig designed just for heavy air.

So the 12 is lighter (IIRC) than the Cherub, carries a heavier crew (more power) because it has a bigger light-air rig, and it has a much more effective heavy-air rig.

Finally, it would be a hell of a brave person who'd say a Cherub would blast past an 18. Or even the Skate, an Australian 14 footer with twin sliding seats (the forward hand's on an 11 footer which makes an IC plank look small) and a skinny flat lightweight hull.

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.

For example, before they went to assys I think Skates sailed fairly low downwind  so assy boats would go higher and faster - but when it actually came to a top-speed contest the Skates could wipe just about everything, reaching with the power of a 7' (?) and 11' sliding seat.

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...)

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:-)