Basic downwind question

[HAL]

The boat: fractional, 110 jib, swept back spreaders, boom out to maximum of 120 degrees.
The statement has been made.
”With sweptback spreaders I never sail below a broad reach of 140 degrees. It is faster, even with gybing, because the sail still has aerodynamic lift as opposed to more DDW, were the wind is just pushing the boat.”
My question is:
If the sail is at 120 degrees (this is right into the spreaders) then a wind direction of 60 degrees would make it luff. With a maximum angle of attack of 15 degrees before stall, then the lowest AWA that would provide lift before stall would be 75 degrees. If the boat speed is 5 kts and the TWS is 15kts and the TWA is 120 then the AWA is 101. So how can you possible keep the airflow attached with a TWA in the 110 120,130,140, direction? These angles would appear to create a total stall. It seems the lowest TWA would be 105-110 to bring the AWA forward enough to maintain lift.

 

[Don12364]

HAL,
Not quite sure what you mean by "Sail at 120°" but I imagine you mean "Boom let out to 50°". i.e. boom against shroud on a Hunter with the spreaders swept back 30°.
In the case outlined by you with a true wind of 15 kt at 140° and boat speed of 5 kt then the apparent wind is at 124° and the "Angle of Attack" is 64°.
Prof Marchaj's

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book Sailing Theory & Practice shows a cambered sail (fully battened) as having a total force of approx 70% of max drive at this angle and it certainly is not stalled.
Have done a sketch of this as below.

 

[Franklin]

If I let my boom out more then 35 0r 40 degrees from center, my main is all over the spreaders which isn't good on long runs (ok for a couple hours).

 

[HAL]

Thanks

Thanks for comments and the diagram. Yes I mean the boom is let out as you describe and this would be the theoretical maximum as was mentioned in regards to chafe. The boat speed and TWA and AWA agree with the calculator that I used. It graphically shows what I was trying to ask. The boat in question is a Hunter by the way. The sail is creating drive at these angles but it would appear that all aerodynamic lift is lost at such a steep angle of attack. I learned that when the angle of attack exceeds approximately 15 degrees the sail begins to stall and its aerodynamic lift qualities are lost and it is no longer pulling but instead it is being pushed by the wind.