sail shapes/styles

what are the key differences in sail characteristics with regards to luff legth- boom length.i like the look of the square head type sails AUS 222 AUS 52 AUS230[pics in register]with that style sail what would longer luff-shorter boom compared to shorter luff-longer boom be.is there a formula ie. a 4 sq m sail with 4mtr luff[square head style]should have a boom length x?-y?

for your little beasty , I would consider a foot length of 1.2m to be the max , after that the low down grunt of the sail will pull you sideways, if you have a flexible mast EG one of those early blue windsurfer masts, it will be nice and flexible and you will be able to have a nice tall sail up around 3.5-3.8m

Newtons 3rd law of motion....To every action there is always opposed an equal reaction
With sailboards leaning the rig into the wind , lift is created and will give the board less drag in the water.
If a rig leans away from the wind like in sailboats and to some degree land yachts this does give some downward force. And downward force creates drag in the hull in water or wheels on land.
Land yachts over time have come to use a more flexible and chassis than was used 20+ years ago. The Lefroy mini uses a torsional frame and flexible rig to take some of the wind force (Newtons law)
In a basic principle..... a taller rig will make the CE centre of effort higher and tip the yacht easier, but with a flexible rig this bends, twisting the sail and depowering it, this lowers the CE...making the yacht easier to keep upright.
Like most things it becomes a balance between yacht, mast / boom and sail....
I hope this answers your question

Ok ... I will try and answer your questions.
1.
Sail twist gives a more "forgiving" sail and works over a wider range of wind angles, and suits new sailors and shifting and or gusty wind conditions.
In an hypothetical ideal world sail twist would be a negative but as we sail in the real world a slight twist in the sail seems to be good....
Spend time and watch a soaring bird in the sky and have a look at the wing tips and how they move them....fascinating
Dont get to hung up with CE and CR except in the initial building of the yacht as these change slightly when you are sailing.
This is a good read ....
http://www.neilpryde.com/insiders-guide/insiders-guide/windsurfing-sails-design-function.html

2.
Very little has been done with IMCS numbers and testing as most of us are just happy to get a mast rather than pick and choose.
for more info in IMCS here is a link
www.neilpryde.com/insiders-guide/insiders-guide/understanding-mcs-and-imcs.html

3.
Yes... aerodynamic drag is possibly the most limiting factor of landyachts, I often say put your arm out of a car window at about 60kmh / 40 mph and feel how much wind drag is on your arm... this is a similar speed to what land yachts travel at....and on a landyacht your complete body is exposed to the wind.

hope that has sort of answered some of your questions....

best comparison weve had was on lake gillies last year . 2xmelbourne BK club blowies( arjay and ben?) with 2m sails , carbon and pods vs me and faze 5 in minis. glass seats , windsurfer masts and a 3.5 and a 4m sail. the blowies would have had less drag and probably more control, but speed and acceleration was about the same.
arjays rig would basically be the one he had for his yepoon 106kph

I am sure that aero drag is indeed the limiting factor, but not only the rig. Colin Palmer's class 2 was originally designed so that a cross section of the body at 15 degree angle to the centre line was an elipse, as wind tunnels show elipse to be the SYMMETRICAL shape with least resistance. Symmetrical cos we sail on both tacks. 15 degrees cos that is a reasonable angle of attack for apparent wind.
Take this to the smaller classes, and a wee experiment first. Go on the beach (lake etc) on a reasonably windy day, and feel the differing windspeed close to the ground and move up. If you have a windmeter even better. Boundary layer is what this is called.
It's also the reason why many designers try to keep the chassis close to the ground for dry surfaces.
Water has immense drag by comparison, so the high chassis yachts diminish this, at the cost of wind drag. The dreaded C word (Compromise)
I often wondered how much drag the old A frame mast supports had.
Lots of this is theory,but in Europe where races are around 50 minutes long, and often won by 4 or 5 metres, every little bit was thought worth chasing! 1/2% is well worth having. Several 1/2%s are a winning formula.

best comparison weve had was on lake gillies last year . 2xmelbourne BK club blowies( arjay and ben?) with 2m sails , carbon and pods vs me and faze 5 in minis. glass seats , windsurfer masts and a 3.5 and a 4m sail. the blowies would have had less drag and probably more control, but speed and acceleration was about the same.


Hi Landytch,

Not Arjay, It was Ben Graham & Crash

B,

Regarding aero drag, I had a chat with Clem today, and we discussed drag from the frame.
As a clarification to my rant above.
If we are going say 50km, then the frame is actually seeing more than this, cos the wind is probably around 15km/hr from roughly 15 degrees off centreline ahead. (Apparent wind) So the frame feels 65 (50 forward speed + 15 windspeed), IF EXPOSED TO THE FULL WIND FORCE. If the frame is close to the ground, it will still feel the 50 but less of the 15. It's called wind gradient. The further from the ground, the stronger the wind until all "boundary layer" is negated.
This is why I believe that on a hard surface, with no water, a low frame will be aerodynamically better. This takes absolutely no account of the mechanics of frame twist!

Getting way too serious, but there is truck loads of research to back this up. However it all states that the smoothness of the surface has an effect, smoother = less gradient.
Perhaps the Salt surfaces don't give the same effect? It seems trhat there is more gradient as the surface over which the wind is blowing has more drag in itself. ie ice has very lttle.
Likewise, the "benefits"? of large wheels don't take such a part too?
Just posing silly questions now!

Ohope hard!, more like "hardly" .
We did this on tarmac airfields in the UK and found very little difference on them. On the south wales beaches which make Ohope look like a sponge there was a difference, the old story? Horses for courses?

sometimes you can stare at the data for hours , but just cant fault it,so you might as well accept it. Ive personally seen 110kph from a mini on lefroy, so i'm confident it can be done, and maybe even repeated.

30mph=48kph If your using a car or speedo to get your speed you can probably add on up to 15% as your max speed giving you 56kph, with the size sail your using your doing extremely well The high speeds are in strong winds small sail, good surface and max speeds are only hit for seconds-ie that's why the speed record for blo karts have a record in the high 80's but a number of people have gone quicker

hi kiwi303,

I think you are right about the height verses boom +surface.
What I have found is that my tallshort boom sail is at its best on hard surfaces such as Lefroy.

At Lake Walyungup the club88 with shorter /longer boom sails are able to handle the rougher surface and keep moving of the line better and also get through the areas that I have problems with in lower wind conditions at there lake.

I sail with an r@j sail I think you would be familar with it as I have seen a few on the NZ web site. I also use a sail from NZ that Bill Naine recomended (windward) and I find that is the best sail for lighter winds.
Cheers
AUS230

I found this diagram that might explain a little about sail shape /aspect ratio and how it relates power / speed.



Often the sailing surface conditions and - or class limitations like mast height often dictate rig setups.