Quick question just seeing how much power difference there is between a 6.6 sail and (say) a 7.4, both being Freeeride flat water sails. My biggest sail is a 6.6 and I've worked out I need a steady 12 knots to plan comfortably, so would a 7.4- or there abouts- get me up and planing in say 10 knots, or will it simply be better for keeping my planing on those marginal 10-14 knot days?

Cheers guys.

6.6 planing in 12 knots! with a 7.4 you could compete in the formula series.

does thta mean new and mast boom. if so probably not worth it.

Hi Phil,

yesterday I used a 6.6m and 7.3m sail, my new sails. Initially I used the smaller sail and my smaller board but I kept dropping off the plane. The wind was there but it wasn't enough to get me going all the time.

So I swapped to the bigger board and sail. I was planing all the time and had plenty of power, even on big downwind broad reaches of over a kilometer. Plenty of power coming back upwind too.

It was getting a bit much for me though, the wind was picking up. I'd guessimate about 25 knots. I was sailing overpowered which is fun but gets a bit full on. So went back to the 6.6m. Well powered up again however with a bit more control which was good as I was feeling a bit buggered by then.

So to answer your question, I think if the wind is there but you can't get on the plane and you want to, then a bigger sail is the go. However if you are on the plane and have plenty of power, then you don't need a bigger sail.

Honestly I'm not really sure about wind speeds. I've ordered a wind anometer from an online store. It should arrive this week. I hope the thing works as I'm keen to actually know how strong the wind is when windsurfing.

I think most wind readings are taken from the tops of poles and buildings, so there could be a big difference between the observed wind speed and the actual speed on the water.

Mobydisc said...


I think most wind readings are taken from the tops of poles and buildings, so there could be a big difference between the observed wind speed and the actual speed on the water.

Before using an anemometer it's worth reading section 3.5 of the BOM siting guidelines.

www.bom.gov.au/climate/cdo/about/observation_specification_2013.pdf

Then you'll probably decide to not even bother trying.

powersloshin said...

I think sail and wind forces work on the square power (surface), so a 7.4 would have a bit less than 30% extra power, all rest remaining equal. On the other hand between 12 and 10 knots there is over 40% power reduction, so you would not be able to plane in 2 knots less wind. This is a very rough estimate, I take no responsibility for what I am saying, i am sure that other scientifically inclined windsurfers can elaborate better. Also there is a handy sail calculator available on the web.

how do you come up with 40% less power?? the difference between 12 and 10knots is 16.666% isn't it?

There's nothing dodgier than the average windsurfer's estimation of wind speed. I always laugh when I see people posting about sailing 7.5m in 20-25 kts, or planing in 15 kts with a 5.2. Almost everyone overestimates how strong the wind is (18 kts becomes 25) or how light it is depending on the story they are telling.

Get an anemometer and have a big reality check. Then, when you've discovered that the wind you told you GF was 25 is really only 20 put it alongside one of a another brand and be amazed at the variations between the two.

DunkO said...

powersloshin said...

I think sail and wind forces work on the square power (surface), so a 7.4 would have a bit less than 30% extra power, all rest remaining equal. On the other hand between 12 and 10 knots there is over 40% power reduction, so you would not be able to plane in 2 knots less wind. This is a very rough estimate, I take no responsibility for what I am saying, i am sure that other scientifically inclined windsurfers can elaborate better. Also there is a handy sail calculator available on the web.

how do you come up with 40% less power?? the difference between 12 and 10knots is 16.666% isn't it?

The power that can be extracted from the wind goes up with the cube of wind speed. The kinetic energy in a unit, say a cubic metre, of wind goes up with the square of velocity, and then the amount of wind that goes past in a unit of time goes up with speed again.

But that's for a stationary device like a wind turbine. A moving wind surfer would be more complicated but 40% is as good a guess as any, half way between squared and cubed.

They spend a lot of time assessing the windiness of a site before installing a wind generator. If they overestimate the mean wind by 10% they're down by 30% on the expected power.

Mobydisc said...

Ian K said...

Mobydisc said...


I think most wind readings are taken from the tops of poles and buildings, so there could be a big difference between the observed wind speed and the actual speed on the water.

Before using an anemometer it's worth reading section 3.5 of the BOM siting guidelines.

www.bom.gov.au/climate/cdo/about/observation_specification_2013.pdf

Then you'll probably decide to not even bother trying.

Oh,

thats a lot to read! Its only costing me $21 including postage. I plan to post a review of it on Seabreeze after I've used it for a while. If it helps to make better decisions then it will be worth the money. If not then its not the end of the world. Plus some of the places I go sailing don't have BOM wind meters nearby, or they are don't give relevant readings, ie the Norah Head meter is pretty useless at measuring wind speed at nearby Canton Beach for westerlies.

Phil,

I'm happy with that size gap. It could be a bit of a bigger gap, however I think a 8 meter sail would have been way too big yesterday. I'm planning to buy an 8 meter when I've saved up for it as well as a new smaller 6m sail.

So my bigger sail range will be 8m, 7.3m, 6.6m and 6m.

Later on I'll be updating my smaller sails however its bigger sails first as they are what I use 99% of the time.

I've got an anometer and i find it good. I use it as a guide with my knowledge and watching what others are using ( if anyone else is around). I usually add about 5- 8kts to allow for the wind being stronger out in the middle.
I find it good if you haven't sailed for weeks or haven't had any wind for months as its better than my memory.Also if I've had an accident in an earlier sail the fear factor comes into the wind assessing.." oh my god its blowing a gale!eek.. then I'll measure it and its 15 - 20kts..

swoosh said...

Ian K said...
The power that can be extracted from the wind goes up with the cube square of wind speed. The kinetic energy in a unit, say a cubic metre, of wind goes up with the square of velocity, and then the amount of wind that goes past in a unit of time goes up with speed again.

I think you made a typo here, don't wanna confuse the masses!

No this time it's not a typo. It is the cube of windspeed for power. The force on a wind turbine blade goes up with the square of velocity, but then the blade can also spin faster. Power = force times velocity, so there you get the cube. Not quite that way in a turbine as many are synchronised to the mains voltage and have variable pitched blades, but the result is the same.

en.wikipedia.org/wiki/Wind_energy


Betz' law looks at this from an interesting perspective.

en.wikipedia.org/wiki/Betz'_law

You can argue whether force or power is more important to get planing. Most likely it is power as the power required to push a non-planing hull thru the water also goes up with the cube of hull speed. But then the force to push that hull only goes up with the square - so it's a bit of a circular argument.

It's all a bit academic for something as complicated as a windsurfer. All sorts of issues with apparent wind, what angle do you bear away at, and then there's the pumping power of weet-bix. Doing the calculations for powering a 40 watt light bulb with a stationary turbine is relatively easy.

powersloshin said...

I think sail and wind forces work on the square power (surface), so a 7.4 would have a bit less than 30% extra power, all rest remaining equal. On the other hand between 12 and 10 knots there is over 40% power reduction, so you would not be able to plane in 2 knots less wind. This is a very rough estimate, I take no responsibility for what I am saying, i am sure that other scientifically inclined windsurfers can elaborate better. Also there is a handy sail calculator available on the web.

SAIL SIZE

7.4-6.6=0.8
0.8/6.6=0.12
0.12 * 100%= 12% at the best
(BTW surface is already square )

WIND

P=1/2*ApV^3 (V x V x V)
12-10=2
2x2x2=8
8/10=0.8
0.8 * 100% =80%

Wind power is proportional to the third power of the wind speed -
to illustrate the double the wind - eight time more power

Ian K said...

en.wikipedia.org/wiki/Betz'_law

nobody is stupid enough to question Betz law !
but I have my doubts that is wrong
suppose that wind turbine blades instead of just stopping molecule to zero velocity
bounce it back like tennis racket or eat it (absorb) ?

powersloshin said...

Macroscien said...

I stated in the beginning that I am not a scientist, but according to your calculation if I can plane in 12 knots with a 6.6 I will need an 80% bigger sail to plane in 10 knots, which makes almost 12 metres....this is beginnning to be fun...

well, I just did try to correct your % calculations
sail 7.4 is 12% bigger then 6.6
Wind 12 knots is 80% stronger then 10 knots
and if we simplify all over variables that looks most likely

Macroscien said...

Ian K said...

en.wikipedia.org/wiki/Betz'_law

nobody is stupid enough to question Betz law !
but I have my doubts that is wrong
suppose that wind turbine blades instead of just stopping molecule to zero velocity
bounce it back like tennis racket or eat it (absorb) ?

Unlike a tennis ball, there's no room to bounce it back. More air keeps coming in.

Hence the sweet spot for how much to slow the air down to maximise power extraction. Slow it down too much and you hinder the air coming in, it'll go around rather than through. Slow it down not enough and you haven't extracted enough power.

Absorb it? That would allow you to extract every last bit of energy. You're onto something there.

OMG! cant stop laughing!
Poor Phil! He just wanted to know if a bigger sail was worth spending some hard earned on, instead he got a nuclearwindphysicsmathamaticalsciencecomprehensiveanaylisisonrelativewindspeedv'ssqauremeteargev'sterminalvelocity report!

Dear Phil,
Yes the bigger sail will help in lighterwinds, if you are going in light winds already on a 6.6 you will really notice the difference in sail size buy going up to a mid 7m, you will then probbaly use your 6.6 only on stronger days!

Happy uncomplicated sailing!

JonesySail said...

OMG! cant stop laughing!
Poor Phil! He just wanted to know if a bigger sail was worth spending some hard earned on, instead he got a nuclearwindphysicsmathamaticalsciencecomprehensiveanaylisisonrelativewindspeedv'ssqauremeteargev'sterminalvelocity report!

Dear Phil,
Yes the bigger sail will help in lighterwinds, if you are going in light winds already on a 6.6 you will really notice the difference in sail size buy going up to a mid 7m, you will then probbaly use your 6.6 only on stronger days!

Happy uncomplicated sailing!

All good, I switched my brain off about 8 post back... Bigger sail, go faster in light wind- got it

petermac33 said...

On Sunday was struggling to get planning on my 60cm wide sonic and 6.3m in 14-15 knots. Swapped boards to my 69cm wide Fanatic Falcon with 44cm fin,also reduced a little the downhaul on the 6.3m. Was not only planning but pretty powered up.....amazing.

Swapping over from a 6.6m to 7.4 will make a reasonable difference provided you have enough board size to support the larger sail. A 2cm or so larger fin probably needed to,otherwise your board may feel like it's sliding rather than lifting.

I'll often go up a board size before I try a larger sail.. Bigger board gets going earlier and planes through the lulls better. Not much help if you only have one board..sorry..