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Lessons Learned the Hard Way - Beware the Venturi Effect!

Norm Cooper

Sarah Jean II
Saga 43
September 30th, 2015

Norm shares another valuable lesson learned from almost sailing Sarah Jean II onto the beach at full speed while cruising in Haida Gwaii a few years ago.  The Venturi Effect is one of many land effects that can cause sudden changes in wind strength and direction.  This can lead to problems aboard your boat if not properly anticipated.  Beware the Venturi Effect!

Situation

While cruising in Haida Gwaii we met up with some friends aboard a similarly sized boat and decided to have an informal race to the next anchorage.  The wind was moderate at about 12 knots.  Our course would take us dead down wind.  We decided to forego the mainsail and deployed only our furling asymmetrical spinnaker that was already rigged and ready to go.  With a quick pull of the spinnaker sheet and easing of the furling line we were off.  Our opponents were using a standard genoa and we quickly left them behind.  At the time I’m sure we were quite pleased with ourselves.

The anchorage we were sailing to was located at the end of a long inlet with beautiful high bluffs on either side.  Part way down there was a sandy build up of land that crossed the entire inlet creating a dead end.   We would anchor just off the beach.  Beyond the sandy build up the inlet continued, allowing the wind to blow along its length unimpeded, depending on the direction of the wind.

On that particular day, the wind was directly behind us and blowing directly into the inlet.  We thought this was very fortunate and decided to make a dramatic entrance into the anchorage with our colourful spinnaker flying.  That was a very dumb thing to do!  As we entered the inlet the wind began to accelerate, quickly rising from 12 knots to over 20 knots!  Suddenly we were flying headlong into a dead end bay at well over 8 knots, barely in control of the boat!  We had nowhere to run and very little room to maneuver in the narrow channel!  Sailing right onto the rapidly approaching beach seemed imminent!

When wind blows between islands with high terrain or through narrow passes it is squeezed and will often accelerate.

When wind blows between islands with high terrain or through narrow passes it is squeezed and will often accelerate.

Recovery

Fortunately the furling spinnaker system allows fairly quick take down of the sail.  We released the spinnaker sheet to depower it.  Beth took the helm and brought the boat abeam to the wind.  The spinnaker was flogging madly to leeward.  We were no longer heading toward the beach but were closing on the shore at the side of the inlet.  Beth fired up the engine and started heading us up slightly into the wind.  We feared wrapping the spinnaker in our rig creating further problems.  Pulling with all my might I slowly furled in the sail, one wrap at a time.  After what seemed like an eternity the spinnaker was fully wrapped and control was resumed to the boat.  We looked at each other wide eyed.  It was clear we would never be making that mistake again!

Mistake Made

When wind is forced into a narrow space it accelerates.  This is known as the Venturi Effect.  It occurs between islands and along inlets where the wind is squeezed.  We did not anticipate this effect.  We should have taken down our spinnaker well before entering the inlet.

Lessons Learned

Understand and Anticipate Land Effects: Local land effects can create significant threats.  Be aware of how land can change the speed and direction of the wind.  Wind will accelerate between islands, in narrow passes, around capes and at either end of islands.  Be prepared.

Sailing from seaward into San Francisco Bay requires extreme caution as the wind can suddenly accelerate. Be cautious and reduce sail before going under the bridge.

Sailing from seaward into San Francisco Bay requires extreme caution as the wind can suddenly accelerate. Be cautious and reduce sail before going under the bridge.

Reduce Sail Early:  Know about these land effects.  Be cautious and reduce sail early when going through a pass or between islands.  Certainly take down your spinnaker.  Consider partially furling your genoa and putting a reef in the main.  Standby on the mainsheet so you are prepared to control the main should the wind move across to stern and cause the main to gibe.

In the Hawaiian Islands, the Venturi Effect often causes the wind to howl at over 35 knots in the Maui Channel between Maui and the Big Island.

In the Hawaiian Islands, the Venturi Effect often causes the wind to howl at over 35 knots in the Maui Channel between Maui and the Big Island.

Do Not Sail Into Dead End Anchorages:  Sailing into an anchorage and anchoring under sail is good seamanship and should be practiced regularly.  But do this in open anchorages, ideally with wind blowing parallel to the shore or blowing off the shore.  Do not sail into a dead end bay, especially with wind blowing towards the shore, unless it is an emergency, such as an engine failure.  Make sure that your windlass is on and check that it is working.

When Flying a Spinnaker Put Your Mainsail Up:  Sailing with only your spinnaker limits your options to depower it for the take down.  It is best to put you mainsail up too.  Keep the mainsail centred or partially reefed if it is blocking wind to your spinnaker.  When the time comes to take down your spinnaker, ease the main sheet so the mainsail blankets and depowers the spinnaker.  This will make the spinnaker much easier to take down, especially if the wind speed has increased.

Know the Maximum Wind Speed for Your Spinnaker:  Determine a safe and comfortable TRUE wind speed for flying your spinnaker through practice.  This wind speed should allow an easy and comfortable take down.  On our boat this is 12 knots TRUE.  After this wind speed we can sail quite fast downwind using our white sails and whisker pole, but with no risk in gusts or building wind.  It is important to set your take down wind speed in TRUE because the downwind APPARENT wind will always be lower and can easily lull you into stalling the take down until it becomes too late.

Repairs

There were no repairs required this time.  It could have been much worse!

Have a Story?

We have all made mistakes and learned valuable lessons the hard way. Do you have a story you want to share with BCA members, so we can learn from your experience? If so, please submit it for publication in this column.  Thanks!

Send your story to currents@bluewatercruising.org with ‘OOPS!’ in the subject line.

 

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