I have been pondering this for awhile now. I seem to be able to sail faster on bigger sized sails than on smaller ones.
Yesterday was a prime example where i was fully powered (but comfy) on 7.2m RSR and in the nasy chop at hardies i was getting 33's and 34's. keen to go faster i switched to a 6.2m RS6 (i could have easily sailed on my 5.4m RSSMKII). The wind was in the 20-30knt range as per BOM readings. On the 6.2m i could only match the speeds got on my 7.2m.
NOTE- the boards used were a Iso86/34GW and a small 48cm wide F2 missile/26F1
I have always believed that going fast really broad is all about the biggest sail possible for the conditions on the smallest board.
My question is at what time/rate of Knots does carrying a bigger sail make you go slower
Speed experts your input here would be very much appreciated.
I would have thought that
From the "SmartFins?" topic.
It might be all about the fins
How would you have gone with changing/alternating fins as well as change of sails???
answer that works for me with my gear in my spot at the optimum angle (hows that for a qualified answer 
) is as soon as I can be fully powered on the smaller sail I go faster on the smaller one. This assumes that I could crank the downhaul/outhaul on the larger sail so I can keep control on the run. If I have to do that, I can go faster on the smaller sail. although as the KA sails I use are developing, I find that crossover point to change down from the Koncept 5m to the 4.4m it at a slightly higher wind point.
My tuppence. I think that the water state was your limiting factor. The bigger fin sail combo probably kept the board riding over the chop well. Smaller board fin sail would have gone faster on flatter water. Looking at my track the rolling chop meant that I could run down the trough get high 32's but then started to go over the back and lost speed. Could not get a full broad run. Best speed was in the flatter water near the point but it still was not flat. Unless you are able to do a Claude / Jesper / Slowboat and have a big enough fin and or sail (and cahones) to skip across the top. The water state will limit your speed. At Pt W Yoyo is able to use a smaller sail and fin than most and kick butt. I don't think the same set up would work in estuary chop.
answer that works for me with my gear in my spot at the optimum angle (hows that for a qualified answer ) is as soon as I can be fully powered on the smaller sail I go faster on the smaller one. This assumes that I could crank the downhaul/outhaul on the larger sail so I can keep control on the run. If I have to do that, I can go faster on the smaller sail. although as the KA sails I use are developing, I find that crossover point to change down from the Koncept 5m to the 4.4m it at a slightly higher wind point.
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Bender, I have been poundering this all year ......... It was a good incite at the KA speed contest at Burrum earlier this year. Pretty much all the fastest speeds where recorded with smaller sails on speed boards. I was using a 6.7 RSR on an i86, with a 34 cm C3 and broke my downhaul rope on the 1st run. Since the wind was picking up to 20 - 25 k, I changed down to a 5.8 RSR with a 30 cm C3. Back out the setup felt really fast, but alas my speed was quite a bit off my PB with my 6.7 with the same board(another day elswhere) But the wind did drop off a bit and the angle was not really broad enough for best speeds. The water state was super flat. My feeling is that on a slalom board go big with what feels comfy, especially when sailing at an optimal broad angle, and the water is very flat. But others argue for efficiency. More experimenting required 
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I would stick to the idea of slalom boards go for a bigger sail size, and speed boards go for a slightly smaller sail size. However things may even out alittle if the course is very broad say 130+. As Vando is faster than all of us, just copying him is the safest bet!
when the sail is so powered up that it gets to the point its shape is starting to deform under load i.e the draft starts to wander aft..this is the point when you should be faster on a smaller sail.
If snides is no expert, that makes me a complete amateur.
But I was speaking to Bugs recently along similar lines. From what I've gathered from him, although modern sails have a huge range, they will still have a sweet spot, where they are the most efficient, the optimum downhaul/outhaul, to get the sail in the right shape, for maximum lift to drag ratio. The wind speed for this is going to vary for different riders physique, (weight and leverage).
In up and down conditions, over a longer time period average speed will be better with a bigger sail because it doesn't slow down that much when overpowered, but goes faster in the lulls.
For 2sec/10sec bursts, a sail that's in the sweet spot for the gusts, will be faster.
I agree Decrepit,
In general the rule to follow is hold onto the largest sail you can handle going off the wind for any given conditions. Likewise use the smallest fin for the given sail combo that allows you to sail in control.
With my Koncept 6.6 or 5.0, Ka assym' 23 or 21 respectively, both have done around 45knots, Slowie & Tonster I think have had similar high speeds with larger sails too.
Sometimes I think where you are located in a gust plays a part in how the wind is circulating/flowing around the sail that makes it feel somewhat underpowered, yet you record a great speed without much of a fight/effort. Aka "The magic carpet ride".
A few have had such rides, mine I ended early at 45knots whilst still steadily accellerating, I had know idea how fast I was going and thought it was a slow run, so I stopped early
Would be interesting to see on water test data from small sensors on a sail that indicate airflow direction/pressure at various points across both sides of the sail. Not talking bits of stickytape and wool either.
