Note how in the diagram the (blue) True Wind vector is always the same length as it represents equal strength on all points of sail. The Apparent Wind (red) however, changes dramatically in strength and direction, especially on an upwind course. This example illustrates why it is often more advantageous to tack downwind rather than sail on a dead run.
On a straight downwind course, the Apparent Wind will diminish as the (yellow) Boat Wind will cancel out part of the True Wind. On a reach and beat the Apparent and Boat Wind become greater than the True Wind.
Asymmetric Spinnaker Controls
You will get to the yacht club bar earlier than if you would have taken the "Rhumb" (as in direct, not Rum as in drink) Line.
The farther downwind your destination is the more it pays to perform a series of gybes. This is especially true in winds under 15 knots true. VMG or speed made good towards your goal is an important concept to understand. It is inherently associated with Apparent Wind and your best boat speed as compared to sailing a certain course. The skipper's decision whether or not to diverge from the Rhumb Line will depend on the wind speed, the sea state and the multihull's polar diagram. Generally, in light air, multihulls will go appreciably faster if the Apparent Wind is brought forward. This means if you are sailing on a broad reach, it will pay to steer upwind and sheet in, especially if the wind is fluky and inconsistent. Heading changes do not have to be excessive. A course correction of only 10 degrees could make a substantial difference, resulting in an increase in Apparent Wind pressure and 2 knots more speed. Needless to say other parameters, such as un-navigable areas, vessel traffic and sea state, will also have to be accounted for.
Just as paying attention to the jib telltales when sailing upwind or on a reach, downwind sailing will have you scanning your boat's instruments. Your true/Apparent
Wind speed and direction indicator and your boat speed gauge will become your best allies. It will be key to keep the Apparent Wind angle constant, and by monitoring the wind gauge or masthead Windex, one simply steers, or lets the autopilot steer, a course relative to a constant wind angle. In fresher conditions, your multihull could be accelerating down a wave and start to surf momentarily, bringing the wind forward. One would bear away to keep the sails at the angle to which they have been originally trimmed. This will assure that all sails are drawing, even if the boat will slalom slightly. It sounds more complicated than it is, but in essence this is sailing a downwind acceleration curve and maximum speed will be attained.
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