Down wind with Spinnaker

Straight Line Speed with the Spinnaker

Finding your Velocity Made Good (VMG) Downwind

Presuming your objective is directly downwind of you, then you want to point the boat on the lowest angle to the wind as you can with the spinnaker still full.  Doing so will allow you to sail as directly to the mark as possible while still going as fast as you can for the conditions that day.  Sailors call this velocity made good.  It wouldn't make sense on a light air day to point directly at the leeward mark since the boat would go very slowly.  It would be better to sail an angle to the wind where your apparent wind speed increases, thereby filling the spinnaker, and allowing the boat to move quickly.  While you will sail more distance to the mark doing this (similar to tacking upwind), ultimately you will arrive at the leeward mark faster than if you went very slowly straight towards it.  On a windy day, however, you likely can point directly at the mark with the spinnaker full because there is enough true wind speed to allow it.

The trick to finding the best angle to sail downind at any given time is to sail as directly at the mark as possible with the spinnaker still full.  The crew (not the skipper) has the best feel for whether or not the spinnaker is still full because they are the one holding the spinnaker sheets and can feel when there is less pressure in the sail.  Both the skipper and the crew can see the visual cue of the sail starting to sag or hang down, but the crew will feel that happen while holding the sheet before the skipper can see it.  If it's a light air day, then the crew will want to turn off the ratchet function on the spinnaker blocks so that the block can rotate both directions.  This way the crew can get the best possible feel for the spinnaker sheets.

Skipper and crew communication is essential all the time in sailboat racing, but especially when sailing downwind.  The crew should be continually calling back to the skipper with status updates about pressure in the spinnaker.  If the crew feels good pressure while holding the sheet, then the skipper should try to sail a few degrees more downwind (pointing more directly at the leeward mark).  On the other hand, if the crew feels light or no pressure while holding the sheet, then the skipper needs to turn the boat towards the wind (away from the leeward mark) so that there is great apparent wind speed acting on the sail, thereby causing the sail to fill and the boat to speed up.

Trimming the Spinnaker

While sailing downwind there are some additional adjustments the crew needs to make to the sail to ensure the sail is set up in an optimal position for downwind sailing.  The spinnaker pole should be approximately perpendicular to the wind direction.  Once the skipper is sailing the correct course for the conditions (as low as possible with the sail still full), use an apparent wind indicator on the boat, such as a windex at the top of the mast, to determine the apparent wind direction.  Then use the windward sheet (sometimes called the guy) to either ease the pole forward or pull it aft.  If the pole needs to come quite far aft (i.e. 90 degrees of the boat's centerline), then you may need to ease the twing while haulin on the windward sheet in order to get the pole where it needs to be.

Once the pole is in place, cleat the windward sheet.  Then the crew should be continually  trying to ease the leeward spinnaker sheet to ensure it is not over-trimmed.  The crew should be watching the windward edge of the spinnaker (the edge where the pole is attached) to see that there is a 6-12" curl developing.  If there's no curl, then ease the leeward sheet.  If the crew eases so much that the curl turns into a collapsed windward edge, then trim to refill the sail, and then immediately try easing slowly again.

A slight curl on the windward edge of the spinnaker is equivalent to seeing the windward and leeward tell tales flying on your jib when sailing upwind.  Both instances show you that there is air flow on both sides of the sail.

In summary, when it comes to spinnaker sail trim, it's the skipper's responsibility to fix a hanging sail (one with no pressure) by turning the boat closer to the wind.  It's the crew's responsibility to fix a curling sail (one that is under trimmed).

Trimming the Spinnaker with Dave and Betsy Perry

Additional Video