over powered (but comfy)

grumplestiltskin said...

I would have thought that

was a contradiction of terms. How can you be overpowered but comfy?

It also basically answers your own question. If you are still comfy then the sail is not too big.

Bender said...



NOTE- the boards used were a Iso86/34GW and a small 48cm wide F2 missile/26F1
Speed experts your input here would be very much appreciated.

keef said...

i'm not an expert bender and i'm not familiar with your boards or your fins,if the 34wg&26F1 are the fin's, that's a big difference,i'm reading between the lines here, so you used the 7.2m with the iso86/34 and the 6.2m with the F2 missile/26F1 and you was doing the same speed on both boards

Bender said...

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 .

nebbian said...

Are you sure that there wasn't any weed on your fin for both runs?

Ian K said...

Interesting numbers Yoyo. They show that when speed is held constant lift varies linearly with angle of attack. And Slowboat's graph showed that when lift is held constant, the angle of attack is inversely proportional to velocity squared. So each fin must have a K value such that

Lift = K * AoA * Velocity squared.

Which is consistent with the basic understanding of how lift is generated ie. F = d/dt(mv)

But getting the K for a particular fin requires of course a more complicated program like Yoyo's used.

I also checked out how the induced drag varied as a percentage of fin lift as angle of attack is varied. See Yoyo's table for the other settings.

AoA................. Lift................... Induced drag.......................... % induced drag of lift

.5 deg...............19kg.................0.048kg ( 2% of total drag)....................... 0.25%
.75 deg ............28.7kg ..............0.1145kg (4.5% of total drag).................... 0.40%
1deg.................38.7kg...............0.1827kg (7% of total drag.)......................0.47%

(Sorry about the dots, the editor didn't allow spaces to get things in columns )

You can see the lift is precisely linear with AoA at constant velocity, the % induced drag sort of linear. But yes well below the lift*sin AoA ( almost but not quite an order of magnitude)

But it seems induced drag is negligible when you're only trying to generate 19 kg of lift (if that ) at 47 knots.

I'm still curious to fully understand what is meant by induced drag, friction is easy. How do these programs calculate it??

Do they work out the pressure distribution over the whole fin. Then do they integrate the pressure times normal-to-the surface vector over the whole fin. The component of the total vector in the direction transverse to motion to be called lift, and the component along the direction of travel called induced drag?

Bender said...



My question is at what time/rate of Knots does carrying a bigger sail make you go slower

sailquik said...



I think it depends a LOT on the design of the sail, fin, board and angle of sailing.
But: My simple 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.
I think this is because:
-the larger sail has too much wind drag for the amount of lift it can now develop in it's crippled shape.
-The larger sail is heavier and does not allow the board to release from the water as easily meaning more board drag.
-The smaller sail has a lower C of E meaning I can direct the power in the desired direction better.
-I can set the smaller sail fuller to develop more power deep off the win from a smaller drag area.

Anyhow, it works for me. 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.

Haggar said...

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

firiebob said...



Vando switched down from his 6.8 & 34cm C3 combo to I think his 5.6 & 30cm on that day, it would be good if he chimed in and let us know what was faster. I had the same model 6.8 and a 30cm C3 and at no stage felt I needed a smaller sail but Vando was about 3 knots faster (but it's not just the gear ).

Bender said...

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.

decrepit said...

.....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.