Why rake sail back?

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maker
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Why rake sail back?

Post by maker » Sat Jun 27, 2020 7:48 am

I am 85kg and sail 78l with 3.7m up to 125l detuned slalom with 7.5m twin-cam. I heard it is best to rake sail back as that closes the gap so you get a cleaner flow of air exiting the sail over leech, and move weight forward to keep board flat. On my 125l/7,5m I notice an improvement especially planing in light winds. Yesterday I did same blasting in strong winds on a wave board and 4.7m and it felt better, though unsure why. So what is going on when you lean sail back?

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BTB
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Re: Why rake sail back?

Post by BTB » Wed Jul 01, 2020 9:06 am

maker wrote:
Sat Jun 27, 2020 7:48 am
I am 85kg and sail 78l with 3.7m up to 125l detuned slalom with 7.5m twin-cam. I heard it is best to rake sail back as that closes the gap so you get a cleaner flow of air exiting the sail over leech, and move weight forward to keep board flat. On my 125l/7,5m I notice an improvement especially planing in light winds. Yesterday I did same blasting in strong winds on a wave board and 4.7m and it felt better, though unsure why. So what is going on when you lean sail back?
The reason for racking the sail back is misunderstood. It has a lot to do with the board. At low speed with all the board in the water the resistance from the board (the COE) is close to the midpoint, when you are at decent speed the board has lifted out of the water and most of the resistance coming from the fin(s) so the COE is somewhere under your back foot. You have to move back to keep the forces aligned with the COE and as you move back you rake the sail back.

With older gear the boards were longer and everything was further forward from the fin, moving back was a process of gaining speed and moving back, modern kit has everything further back and the COE moves much less as the boards are shorter, you can start of with your feet in the straps on very small boards. The rig does not naturally rake back as much on modern kit but from a control perspective I think you naturally move everything back when overpowerd / sailing fast v's forward and upright when underpowered / getting going.

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WetSoot
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Re: Why rake sail back?

Post by WetSoot » Wed Jul 01, 2020 10:01 am

Just to add to the above (and gently correct the terminology - sorry BTB! :D ) the actual point of the board that BTB refers to (that is in reality the pivot point), is called the Centre of Lateral Resistance (CLR), and the central point of power production (usually in the middle of the sail) is the Centre of Effort (COE).

As BTB says, when the board is powered up things move back and actually its the the CLR that moves back.

Turning moment, ie how much the board wants to head downwind is defined by the distance fore and aft between the CLR and COE, and as you get on the plane, this widens..... so you rake the rig back to narrow the distance again.

The lower the turning moment, the less energy wasted (by the sailor in practice, but theoretically in drag) in keeping things pointed the right way!

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