This discussion relies heavily on the following literature

A. Stuart Walker. A manual of sail trim

B.  David Dellenbaugh. Speed and Smarts, issue #33.

Except on rare occasions where the boat feels fast, we do not seem to be doing as well as we should be. Of course this is difficult to assess when the boat is at the bottom of her PHRF class and everybody is supposed to be faster than us, however we know that we have when we are still finishing last with a good pro onboard and when the top boats from the classes below us pass us before the finish.

A. Pointing

The boat does not point as well as the boats around us. Even when boats with comparable or higher PHRF rating are around us, they are higher and we do not have enough speed on our heading to keep up with them.

B.  Dirty air

We spend a larger share of our time in dirty air being at the bottom of our PHRF fleet. Dirty air seems to slow us more than other boats in similar conditions, so we are getting a double whammy here. The boat does not feel good and we loose too many duels with others.

C.  Waves

The boat does not feel well in waves. The boat looses speed at each hit and is slow to accelerate between hits. We are doing worse in waves than in flatter seas.

Sail trim produces significant variations in boat speed and improvements in sail trim provides big gains in performance.

A. Position of maximum draft

1.      Draft forward ó full leading edge

Facilitates the initiation of attached flow, and therefore is appropriate to

a)     Waves

b)    Dirty air

c)     Gust response

2.    Draft aft ó flat leading edge

a)     Pointing in smooth water

3.    Controls of draft position:

a)     Luff tension

b)    Mast bend

c)     Jibstay sag

B.  Disturbed air

Requires

1.      Twist

Because

a)     resistance to stalling (when flow separates from the sail) is proportional to chord length.

b)    surface friction makes the apparent wind shifts aft in proportion to height.

2.    Rapid responses in trim, heading and crew weight

3.    Automatic adjustment of sails through:

a)     Flexible mast

b)    Jibstay sag

c)     Vang control of the boom

To permit the boom to rise in a gust and therefore to ease the leech

C.  Pointing

High pointing can be achieved in moderate air and smooth water. High pointing depends upon:

1.      Boat speed

2.    Jib entry fullness

a)     A jib with a flat entry can be aimed higher into the wind without luffing

b)    But this narrows the steering groove

c)     Moving the draft aft of mid-chord usually flattens the jib entry

3.    Leech position

a)     At the optimum, both the jib and main leeches should be parallel to the leeway of the boat ó 3-5 degrees above boat centerline

The fractional rig permits optimal adaptation to a particular main or jib and optimal control of main and jib shape to adapt them to a wide range of wind and wave conditions.

A. More mast bend

(If top of mast bends forward, mast bend is negative)

1.      Reduces main leech tension

2.    Increases jibstay sag

B.  More vang tension

1.      increases jibstay sag

2.    opens main leech if mid-mast is flexible

3.    closes main leech if mid-mast is stiff

4.    increases the slackening of the leeward upper shroud

C.  Low shroud tension

1.      reduces mast bend

D. Negative pre-bend

1.      reduces the slackening of the upper leeward shroud

E.  A slack upper leeward shroud

(Because the hounds will then no longer be fixed but will rotate forward, to windward, and downward)

1.      increases lower mast bend

2.    increases jibstay sag

A. Set jibstay length

1.      to provide maximum rake

B.  Set upper shrouds

1.      to maximum tension

(to keep the leeward shroud taut to maximum possible wind velocity)

2.    that does not induce compressive lateral bend in the mast

C.  Define desired (positive or negative) pre-bend

D. Set lower shrouds to permit or limit the desired lateral mast bend

E.  Set backstay to match mast bend to mainsail luff curve

A. Fore and aft mast bend

1.      is optimal when main draft is 45-50 aft at all levels

(with no cunningham and halyard just permitting horizontal wrinkles)

B.  Backstay tension

1.      is optimal when main draft is 50% in the upper third of the sail

C.  Vang tension

1.      produces flexibility in dirty air

(gusts, heavy air and waves)

a)     with a corresponding ease of the mainsheet

b)    by sagging the jibstay

c)     by permitting the end of the flexible boom to float to

(1)    open the leech

(2)   produce more twist

2.    shifts draft aft

3.    requires corresponding tension in the cunningham

4.    is increased in gusts and eased in lulls

A. Solutions

1.      Increase fullness and shift draft forward

a)     to facilitate attached flow

2.    Twist sails

a)     to ensure that some portion is always at the optimal angle of incidence

3.    Provide flexibility

a)     to let the rig self-adapt

B.  Actions

1.      Make mid-mast sag laterally to leeward

(and top of mast moving to windward)

2.    Move jib lead forward

3.    Ease outhaul

4.    Ease mainsheet and jibsheet

5.    Increase jib luff tension

6.    Use vang sheeting:

a)     To permit boom end to lift in gusts

b)    To bend the mid-mast

c)     To sag the jibstay

A. Solutions

1.      Same as for dirty air

2.    Provide pitch responsiveness

3.    Provide stability

B.  Actions

1.      Same as for dirty air

2.    Steer lower to reduce:

a)     side force

b)    leeway

3.    Keep the crew spread apart and standing

a)     to minimize rolling moment

* 5/17 race with Jeff. Tws 7-12 kn, unstable with gusts and waves. No good boat speed.