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TerryMcTool said..
Longer lines, increase the size of the wind window, so the kite takes longer to travel through the power zone part of the wind window. So it does not produce more power on longer lines, it produces the same power for longer, which can give you a few knots, at most, better bottom end. You need to use at least 5-10M extensions on 25M lines to make it work.
Downside is the amount of room you need to launch and land safely, and in the surf they are a definitely a disadvantage, relaunching can take a lot longer if you drop the kite at the edge of the window.
Every kiter should see what happens to their kite when they put longer lines on, just so you understand the wind window better.
I have seen Jordan from Sydney at the windless nationals in Townsville a few years back stay upwind in around 7-10 knots on a 17 Edge with about 50M lines. Have also seen Lachlan Beed from Newcastle when we were in Vaunatu years ago 47M of kite (19/15/13 stacked) and about 45M of lines hold ground in about 7/8 knots. I've also land buggyed on 60M lines and once achieved 2+ times wind speed on a slightly downwind reach, people everywhere told me I was deluded, but now kites and foils are doing just that.
Long lines is an interesting thing to experience, but they are not magical and line drag eventually overcomes any gain and then you can only go downwind.
Some loose science.....
Line extensions do make a big difference in amount of time you spend in the power window.
Assuming you can fly your kite from the edge of the wind window on your left to your right (180 degrees), with 20m lines, your kite travels 62.5 metres. With 27m lines, it now travels 84m..... So you have effectively spent an extra 20 to 25% of your time in the power window.
BUT .......
The downside is your kite will turn slower!!!! (tested and verified).
IMHO:
A fat fish style surfboard (~45 litres - 23 inches wide) + a 18m Foil Kite + 27m lines = sub 10 knots in flat water going upwind
A fat fish style surfboard (~45 litres - 23 inches wide) + a 19m LEI Kite/18m Foil Kite + 27m lines = 10 knots in small surf staying upwind (don't drift the foil kite!!!!)