SailTrimSim is based on a series of 12 pre-calculated cases, with the aerodynamic analysis program MacSail: 3 headings [footing-mean-pinching] x 2 main trim options [eased, tight] x 2 jib options [eased, tight] = 12 different cases.

Sail areas and righting moments for the various boats were also tabulated. Using the sail force coefficients from MacSail, the actual sail forces and the heeling moment are calculated for the chosen wind speed and boat type. The heel angle is solved from the righting moment of the boat in case.

The same sailshape (of a 470s) was used for all boats. The 470 sails are fuller than those normally used for the bigger boats, which results in slightly atypical apparent wind angles for the bigger keelboats. The results suffer from this simplification very little - using true sailshapes for different type of boats would have been an overwhelming task.

The mainsail shape remains unchanged for all cases - only the sheeting angle is adjusted. In the real world, this corresponds to easeing the traveler, or if a very tight boomg vang is used, trimming the sheet. The twist is not changed - most of the time, when you ease the main sheet, both sheeting angle and twist are increased. Increasing the twist of the mainsail also flattens the head. Tight trim means a sheeting angle of 0 degrees (boom on the centerline of the boat), for the eased main trim the sheeting angle is 6 degrees.

Two different shapes are used for jib: The tight shape is flatter in the foot and less twisted in the upper leech. The eased shape is fuller down low and 5 degrees more twisted.

Since the sails are not flattened as wind increases, the boats heel over rapidly as windspeed goes up. See SailPowerCalc's "Depower" feature, for simulating the effect of flattening & feathering the sails.

The apparent wind angle is 24 degrees for the pinching, 26 degrees for the mean course and 29 degrees for the footing. The true wind angles would range approximately from 40 to 60 degrees, for the different types of boats and wind speeds. The wind speed is also apparent, not true.

SailTrimSim assumes a constant leeway of 4 degrees. In the real world, leeway depends not only on the heeling force, but also on the drive (through boatspeed). A component of the heeling force in the direction of the motion is added to the drive. The drive is increased by up to 20%, and proportionately more so for the mainsail than the jib.

The forces are given in [kgf] kilogramforce. 1 kgf = 2.2 lbs = 9.8 N.

The animation was done with 2-frame animated gifs in Gifbuilder 0.5 - crude, but serves the purpose. The frames remain "light", as Gifbuilder only saves what is changed between two frames (the tell-tails). The speed of the animation depends on the speed of your computer - on slower machines, the tell tales may appear very "lame".

The sails were designed with SailMaker 1.7 and rendered in Canvas 5. The MacSail runs and the JavaScript programming was done by Mikko Brummer, on a Macintosh Powerbook and mostly during TV-commercials/soap operas. The illustrations are by Leonard D. Winch.

With the "TrimSim engine" completed, it is fairly simple to do other simulations. More subtle trim variations a single heading, including mastbend and forestay sag ... maybe, if I find the time & energy

SailTrimSim © WB-Sails Ltd 3/97

Changes:

3/97 A leeway of 4 degrees & the drive component of the heeling force due to leeway was added, to make the forces consistent with SailPowerCalc.