Pick your boat from the pop-up menu and set the apparent wind speed. You can adjust

- the main traveler [main sheet]: tight is boom on the centerline, eased lets lets the boom out by 6 degrees.

- the jib sheet: tight means a flatter jib with less twist, eased a jib with a fuller foot and a more open leech.

- Heading is the course sailed: pinching corresponds to an apparent wind angle of 24 degrees, mean 26 degrees and footing 29 degrees.

The telltales on the sails reflect the airflow at various attitudes of sailtrim & heading. Green tails are on the windward (starboard) side, the red-pink tails are on the leeward (port) side. Luffing is indicated in gray

The numbers below the animation indicate the driving force [Drive], the heeling force [Heel] & the heeling moment for the mainsail & the jib, respectively. The sail areas are given in square meters, and the forces in kiloponds (1 kgf equals approximately to 2 lbs, or 10 Newtons)

Drive is the force pulling the boat forward. The more drive, the faster the boat will go. Heel is the force pushing the boat sideways, thus causing leeway. A larger heeling force means more leeway. Heeling moment is the couple created by the heeling force and its underwater counterpart at the keel. Heeling moment determines how much the boat will heel over.

You may be surprised to find out that the jib provides most of the drive, in spite of its smaller area. This varies from boat to boat, as the relative size of the jib compared to the mainsail differs. All the boats in the simulation are using jibs - with genoas the difference would be even more significant.

But don't be fooled, the jib is so efficient only because it is enjoying the benefits of the mainsail behind it:

- The mainsail is bending the air flow in front of the jib, giving it an effective "lift".

- The jib leech is in the accelerated airflow close to the the mainsail luff, preventing effectively flow separation and increasing the pressure drop (suction) on the leeward side.

Take away the mainsail, and the jib will lose its magic.

You will see how the windward tell tales start flying or the jib starts luffing as you let out the traveler. In general, the mainsail adjustments will have a greater influence on the forces than those of the smaller jib.

Since the heel angle [Heel] affects the driving force, it has been included in the simulation. As some boats heel over easier than others, the best trim for a given wind may vary depending on the boat you have chosen. The relative sizes of the mainsail and the jib also vary from boat to boat.

Lots of sideforce (Heel) will increase the leeway, damaging the boat's pointing ability. In fact, increasing leeway will decrease the apparent wind angle, measured to the boat's centerline. SailTrimSim assumes a constant leeway of 4 degrees for all the cases. To really assess the merits of various sailtrim you must resort to a full blown VPP (velocity prediction program).

In SailTrimSim the sails remain full in shape even as wind increases. In the real world, you want to flatten the sails to decrease heeling as wind builds up, for best performance.

Another catch in the simulation is that only the main traveler can be adjusted, the sheet cannot (you can think of easing the traveler as easing the mainsheet with the boom vang set really tight, too). In many cases it would be ideal to ease the sheet a bit as well while letting the traveler out.

"Telling tales" gives you an overview about using tell-tales.

"How's it done" explains what makes SailTrimSim tick and gives more thorough back-ground information.