VPP iteration notes

Laser VPP briefly outlines the cycle of calculations from an initial guess of Boatspeed.

Here are the numbers for the first cycle of calculations
TWA = 45° and TWS =8kn
BS =5kn for the starting value, which triggers calculations for
AWA = 28.1° and AWS = 12.0kn

Individual drags are calculated:
Hull Friction = 3.2kg
Hull Wave = 3.4
Board Friction = 0.8
Sail Induced = 2.1
Windage = 0.9
Sum of Drag  = 8.3
Thrust  =5.6 

The net force on the boat is Force = Thrust - Drag = 2.7kg
Acceleration = Force/Mass  - Newton's 2nd law
New BS = Old BS + Acceleration x timestep = 4.97

RM = 72kgm and HM = 49kgm are compared,
the crew is moved inboard -0.15m to reduce RM

2nd cycle
AWA = 28.2° and AWS = 11.9kn

Individual drags are calculated:
Hull Friction = 3.2kg
Hull Wave = 3.3
Board Friction = 0.8
Sail Induced = 2.1
Windage = 0.9
Sum of Drag  = 8.1
Thrust  =6.1 

Net force = 2kg
New BS = 4.95

RM = 57kgm and HM = 54kgm are compared,
the crew is moved inboard another 1.5cm to reduce RM

And so on...

After 25 iterations RM = HM =51.8 but the net force is 0.3kg.  BS = 4.6kn
After 80 iterations RM = HM =51.2 and the net force is 0.01.  BS = 4.56
After 300 iterations RM = HM =51.2 and the net force is 0.  BS = 4.55

In the acceleration and heeling calc, "Timestep" governs how much the boatspeed or heel angle changes. Larger timesteps increase errors in the same way that sudden gusts do!

If you're trying to find the speed at the top end of hiking and twist then reduce Timestep to say 0.1 seconds. Keelboats seem OK with 0.5 - 1 sec - possibly because their RM increases smoothly with heel.

Timestep is a Named Range and can be edited by pressing Ctrl-F3.


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