NotWal said...

Is there some simple way to pin-point the centre of lift of a sail?

Obviously it lies on the line from the mast foot through the centre of the harness lines if the harness lines are positioned correctly, but how do you find the height?

It wouldn't be easy but you could.

You'd have to lash a 2nd mast from the mast foot through the harness lines point and up into the leech area. You could then climb a step ladder and hold the 2nd mast at the balance point which would be both the back-front point and the top-bottom point.

It would be different on the water so you'd have to find a boat with a step ladder...

That doesn't take weight distribution into account. When the sail flies horizontally the mast booms and extension are all supported by the sail. The apparent centre of lift would appear biased towards the bottom of the sail where the weight bias is. It would be nice to be able to isolate the aerodynamic centre of lift from the weight of the gubbins.

I just had a thought. You could add weight to the mast tip to compensate for bottom weight bias. You could, just for the sake of repeatability and standardisation, add enough weight to put the centre of gravity half way up the luff. Maybe a third of the way up would be more practical from the point of view of flying the rig.

You wouldn't need to fly it from a bridle either. You could just stand in the wind and hold onto the mast at the balance point. You have the line through the mast foot and harness lines as one datum. You only need one other datum line and that would be perpendicular from the mast at the balance point presuming the sail was held perp to the wind.

So you would need some scales and weights and a felt tip pen and a string line and a calculator.

Notwal this is so easy.

Get planing and perfectly balanced. The COE lines up with the centre of lateral resistance and then is just up a bit.

Hmm OK a slightly easier method:

Make a two way bridle from the clew to a clamp that can be clamped at different positions up the mast.

Move the clamp up the mast until the windward deflection of your boge* is zero. At this point look along your single support line.


* When sailing along, my boge joint has a fair bit of visible shear deflection to windward. You might find an easier way to figure out the windward force on this point.

Scott COLR has nothing to do with the sail. It is the lateral resistance of the board. ......the combined effect of fin and rail length stopping the board going sideways.

The sail has a Centre Of Effort (where the power comes from) dependent on draft position and sail shape.

The board has a Centre of Lateral Resistance which is the combined effect of fin and waterline.

On old longboards it was shown in diagrams when learnign as the board turned when the COE and COLR did not line up.... move the sail forward and the nose was pushed downwind (bear away) and if the sail moved back the tail was pushed downwind (rounded upwind)

You are talking about the sail's resistance to the wind - which is not the COE. You are talking about the sail as a 2D shape being exposed to the wind - like a piece of paper has it centre in the middle obviously. That si irrelevant as the centre of EFFORT (power) is affected by draft. It is not in the middle it is forward of the middle

I'm going to get myself a clamp and try this test. If it works I'll check all my sails and the effect of downhaul on the height of the coe.

It would be nice to know this dimension for other sails. The height of the coe is a significant performance indicator. This is one of the key differences between the S1 and the Blade. It is an indicator of the suitability of a sail for high jumping. It is important for speed and racing to keep the coe as low as possible. I have long been curious about Boxers. They reputedly have a high coe but who really knows?

Surf mag would probably do measurements like that. They are anal thorough and precise Germans. I wonder if they have done it in the past.

sboardcrazy said...

Not meaning to sound like I'm having a go at anyone but why do you need to know? What use is it?

I was thinking the same.

I just rig an appropriate sail size for the conditions and get out there an sail it!

Then adjust lines to suit.

The centre of lift in a flexible sail structure must surely wander a little (or a lot) in different wind forces and directions of air flow.

Even on finding its average position, what would it tell us that sailing won't?

So you want to spend a loooong time ascertaining that your S1 will jump 1.8% higher then your Blade?

I'd rather know I had a stable COE (no draft shift) than know where it is.

Mark _australia said...

So you want to spend a loooong time ascertaining that your S1 will jump 1.8% higher then your Blade?

I'd rather know I had a stable COE (no draft shift) than know where it is.

Not really. I want to look at a sail test and know relatively where the heights of the coes are. I want to know the same of my own sails for reasons of comparison.

I said "simple" test. I hope for nothing more complicated than an IMCS test for a mast and this proposal seems to promise that.

Didn't JP jump out of helicopter with his board? Could be fun.

NotWal said...

Mark _australia said...

So you want to spend a loooong time ascertaining that your S1 will jump 1.8% higher then your Blade?

I'd rather know I had a stable COE (no draft shift) than know where it is.

Didn't JP jump out of helicopter with his board? Could be fun.

Nah look closely - it is faked

I'd be interested to see your results Notwal

Might even be interested enough to do some experiments myself... Mmmmmm science

Gestalt said...

it's impossible to measure as every time you rig the sail you can vary the position of the COE

the COE is where you rig it to be.

if you want a high coe then use less downhaul,

if you want a low coe then use more downhaul,

want it forward use less outhaul, want it back further use more outhaul.

and so on.

True. I might have to start wearing a lime on my head too.......

Oooh idea for a new thread!