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Preventers: A Comparison of Approaches

Preventers for off wind sailing has always been a popular discussion topic.

A recent tragic event off the New Zealand coast, has again brought the topic to the forefront. A valuable engineering analysis performed by Maritime New Zealand shed some light on the loads imposed on preventers, booms and the support hardware.

The Gulf of Maine Post recently held a lively discussion lead by Leslie Schneider. Following their lead, the Buzzard's Bay Post held a Safety Moment discussion on use of preventers.

This summarizes the discussion and alternatives.

Preventers are essential gear for off wind sailing  but are not a substitute for proper reefing for anticipated conditions and sailing within abilities of crew and boat

The issue is uncontrolled, unplanned gybing

  • Injury and death, man o/b
  • Gear breaking.
  • Rig failure

Ways to avoid:

1. A midships preventer, Dr. Garry Fischer describes a permanently installed midships preventer system that was installed on a Morris 46 on CCA website:


  • pre rigged, restricts gybing in “moderate” conditions.
  • Can be handled entirely from cockpit, including tacking/ gybing
  • Nipping the gybe early greatly reduces load buildup


  • preventer attachment mid point or forward on boom.
  • Smaller load angle, thus greater load on boom and gear.
  • Also Vertical load angle increases load on Boom and gear
  • Risk of boom breaking, bending, gear failure.
  • CCA Member, John Harries writes, “amidships preventers-a-bad-idea-that-can- kill”  “Attainable Adventure Crusiing
  • He addresses the loads based on the engineering analysis models of the New Zealand tragedy and estimates the loads on midships preventers are 12-24 times the wind force versus 2 times the wind force for bow preventers the load on a boom end preventer


2. Bow preventer (or End boom preventer), a  preventer from bow to end of boom

  • Optimum approach for heavy weather sailing
  • Greater load angle, thus great gybe resistance for given amount of force.
  • Lines along boom can be pre-rigged
  • Single or double line from bow block to aft winches
  • Does Require going to mast to rig preventer.

See diagrams below for comparison of loads between the two arrangements.


3.  Other approaches:

  • Vang to toe rail, better than nothing. Poor load angle
  • Boom brakes

There is no one best approach one needs to consider boat, crew, weather and available alternatives.


Comparison of loads on a midships versus bow preventer

modification of Gary Fischer's sketch by Paul Bushueff