Sail Trim by Gibson (a query)

[Sealong]

Gibson's Sail Trim arrived a few days ago and I started reading it. I have a problem with a diagram and text on P.9 This shows in Figure 5b, a plan view of yacht and sails with a very small entry angle (angle of incidence, attack etc.). The text says, and I quote verbatim, the parenthesis are also part of the original text:
"In Fig 5b the entry angle is too small, the leading edge of the sail is flapping (luffing) in the disturbed airflow, and the sail is stalled (not working). It needs to be sheeted in, increasing the entry angle enough to stop the luffing."

I have two problems with this and I would value the comments of others on the forum.
1) A sail or aerofoil stalls when the angle of incidence is too large not too small, so this sail is not stalled.
2) If the entry angle is too small then the sail needs to be sheeted out.

Or am I missing something ?

 

[Easticks28]

My wife has a tendency to 'overthink' things. Perhaps you do too?
If the luff (leading edge starts to collapse ( flap) I sheet in or if on the edge of 'no go' I bear away just enough to stop it.
Same goes for both main and foresail.
Simple as...

 

[Sealong]

My wife has a tendency to 'overthink' things. Perhaps you do too?
If the luff (leading edge starts to collapse ( flap) I sheet in or if on the edge of 'no go' I bear away just enough to stop it.
Same goes for both main and foresail.
Simple as...

I certainly do overthink but that is how I arrive at understanding.
What you say is correct and I know this. You are correctly describing stall, which is when the entry angle is too large, so you sheet in or bear away.
However, this is not what I am making reference to in the book.
The case there is when the entry angle is too small.

 

[Easticks28]

Without either the book to hand or an image of the diagrams and text to which you refer.......

 

[Sealong]

Yes, sorry. I should have done this in my original post. But here we go:

 

[greeny]

You're correct in my opinion. I'd ignore it and move on although it does cast doubt on the quality of the publication if simple things like this are incorrect. Probably a genuine slip up by the original writer and the proof reader (if there was one) not a sailor who is just checking for grammar and spelling.

 

[IanR]

The angle of incidence is too small for the sail to develop lift and subsequently like an aircraft wing it is stalled and creates no drive. It will come out of stall when you sheet the sail allowing flow to resume. This creates the high and low pressure zone which create drive. 5b is IMO correct

 

[Toodle-oo!]

Seems OK to me - but then in 5c it also uses the word 'stalled' for the opposite condition... In my mind, 5b is stalled...

 

[Sealong]

The angle of incidence is too small for the sail to develop lift and subsequently like an aircraft wing it is stalled and creates no drive. It will come out of stall when you sheet the sail allowing flow to resume. This creates the high and low pressure zone which create drive. 5b is IMO correct

IanR: But by sheeting in the angle of incidence is reduced.

 

[Sealong]

The angle of incidence is too small for the sail to develop lift and subsequently like an aircraft wing it is stalled and creates no drive. It will come out of stall when you sheet the sail allowing flow to resume. This creates the high and low pressure zone which create drive. 5b is IMO correct

Also, stall on a foil occurs when the angle of incidence becomes too large. This causes separation to occurs on lee / low pressure side, drag increases, lift reduces. Most fluid dynamics text books show this with a graph of lift and drag coefficient against angle of incidence.

 

[anoccasionalyachtsman]

The OP is quite right. It's not stalled, but the problem is that aerodynamics doesn't have an accepted word for the opposite of stalled. 'In a dive' is the aero equivalent of pointing too high.

 

[Sealong]

You're correct in my opinion. I'd ignore it and move on although it does cast doubt on the quality of the publication if simple things like this are incorrect. Probably a genuine slip up by the original writer and the proof reader (if there was one) not a sailor who is just checking for grammar and spelling.

Yes, I tend to agree and I will. But it is annoying.

 

[IanR]

I agree entirely but the OP point was 5b was incorrect and I do think it is correct

Ps for a proper book on sail trim, I recommend CA Marchaj

 

[Sealong]

The OP is quite right. It's not stalled, but the problem is that aerodynamics doesn't have an accepted word for the opposite of stalled. 'In a dive' is the aero equivalent of pointing too high.

Perhaps the aerodynamic term for what we call pointing too high should be "kamikazied" !

 

[anoccasionalyachtsman]

Perhaps the aerodynamic term for what we call pointing too high should be "kamikazied" !

Well, you could call it 'landing' - but I prefer yours.

 

[IanR]

IanR: But by sheeting in the angle of incidence is reduced.

The angle of incidence increases by sheeting in ie the wind comes to the sail at the same angle but the boom is in further, with the sail luffing flapping at zero degrees AoA as it would for a laser sail, sheeting it in increases its angle from zero to say 20 degrees if the boat hull remains at the same angle to the wind

 

[Sealong]

The angle of incidence increases by sheeting in ie the wind comes to the sail at the same angle but the boom is in further, with the sail luffing flapping at zero degrees AoA as it would for a laser sail, sheeting it in increases its angle from zero to say 20 degrees if the boat hull remains at the same angle to the wind

Ian you are quite right and thanks.
I can see that now (Fig 5b), by sheeting in, the cord line rotates anti-clockwise relative to the apparent wind and this increases the angle of attack.

 

[TernVI]

i'd say it's not stalled.
To me, stalled has the specific meaning of too high an angle of attack resulting in no laminar flow over the lee side of a sail or top side of a wing.

When a sail is luffing what does 'entry angle mean? all the terms borrowed from planes and wings need to be used with caution or not at all.

 

[Sealong]

i'd say it's not stalled.
To me, stalled has the specific meaning of too high an angle of attack resulting in no laminar flow over the lee side of a sail or top side of a wing.

You are correct about the high angle of attack leading to stall.
However, it is not the lack of laminar flow on the lee side that creates stall but flow separation.
There can be turbulent flow on the lee side and without separation stall will not occur.
This is actually used in the design of golf balls where the dimples induce turbulence. The presence of turbulence delays separation and so compared with a laminar flow the overall drag is less.
Turbulence and separation are two quite distinct and separate phenomena.