2 Critical Behaviour in Following and Stern-Quartering Seas

  2.1.1 Trapping can occur when the craft is moving directly down-wave in waves whose length is roughly equal to the waterline length of the vessel. When cresting one wave, the craft will experience a reduction in resistance, which will cause it to accelerate into the trough ahead and immerse its fore-body in the next wave. If this does not result in a bow dive, the craft will experience a significant increase in resistance that will slow it down to the speed of the waves. It can be the precursor to a bow-dive.

  2.1.2 Warning signs:

• .1 moving at the speed of the wave, see table 1; and

• .2 one wave crest at the stern and another at the bow; and

• .3 wave height greater than 4% craft waterline length;

• .4 craft becomes trapped between two successive crests.

  2.1.3 Corrective action:

• .1 slow down and allow the waves to draw ahead.

  2.2.1 When a high-speed craft is moving in following seas which are directly astern and where the wave length is about the same as or greater than the vessel length, it may accelerate and decelerate in surge as the crests pass. Such surge velocities may differ by as much as 50% of the average speed and are caused by significant changes in resistance and propulsive efficiency as the waves pass. Without warning the craft may accelerate rapidly to the speed of the wave and surf. Surfing is best avoided if at all possible because of the almost total loss of control that occurs while it is in progress. Surfing can be the precursor to a bow-dive, or a broach.

  2.2.2 Warning signs:

• .1 large variations in craft speed at constant throttle;

• .2 craft is moving at wave speed plus or minus 10% (1/10^th), see table 1; and

• .3 the wave length is between 1 to 2.5 times craft waterline length; and

• .4 the craft has a slight bow-down pitch attitude, with a wave crest abaft amidships;

• .5 response to steering controls is poor;

• .6 breaking waves increase the tendency to surf.

  2.2.3 Corrective action:

• .1 avoid running at wave speed (see table 1) in waves of dangerous length;

• .2 if caught in a surf wait until the critical wave has passed without attempting any major helm action;

• .3 afterwards, slow down.

  2.3.1 Bow-diving occurs when a high-speed craft buries its bow into a wave in following or stern-quartering seas. This causes all way to be lost, the vessel experiences a severe bow-down pitch and the bow becomes submerged, sometimes resulting in structural damage and injury to personnel. It is particularly severe for vessels such as catamarans with a cross deck and limited residual buoyancy forward. It is different to bow immersion in head seas as the wave behind lifts the stern and worsens the situation.

 Bow-diving may have a slow onset if moving at wave speed, but may be dramatic without warning if craft is moving substantially faster than the waves.

  2.3.2 Warning signs:

• .1 If preceded by trapping (see 2.1 above):

  • .1 as for trapping; and

  • .2 wave height greater than about 75% (3/4) of bow freeboard when stopped; and

  • .3 waves from between directly astern and the quarter;

  • .4 bow almost immersed to the deck or top of cross-structure.

• .2 If craft is moving faster than the waves and:

  • .1 waves from between directly astern and the stern quarter; and

  • .2 wave height greater than 25% (1/4) of bow freeboard when stopped; and

  • .3 wave length 100% to 150% of the waterline length of the craft.

  2.3.3 Corrective action:

• .1 avoidance by attention to the warning signs;

• .2 avoiding any trim by the bow;

• .3 slow down to less than about 70% of wave speed;

• .4 alternatively, if practicable, change course, even to head seas.

  2.4.1 Broaching is a severe, and often uncontrollable, yawing movement in following seas which turns the vessel beam on to the waves resulting in a dangerously heavy roll, and a sideways sliding motion down-sea. In monohulls with insufficient stability it can result in capsize. It may be preceded by surfing.

  2.4.2 Warning signs:

• .1 desired course slightly or appreciably across the waves, up to 45° from directly down-sea;

• .2 wave length similar to craft waterline length, or slightly shorter in quartering seas; and

• .3 craft speed similar to wave speed plus or minus 15% (1/7^th), see table 1; and

• .4 wave height greater than 4% craft waterline length; and

• .5 bow-down attitude and bow burying into wave ahead;

• .6 up-sea waterjets or propellers beginning to ventilate;

• .7 severe yaw motions either side of intended course;

• .8 surfing.

  2.4.3 Corrective action:

• .1 avoid a diagonal course across the waves, i.e. up to 45° from directly down-sea;

• .2 avoid running close to wave speed (see table 1) in waves of dangerous length;

• .3 reduce speed to less than about 70% of wave speed;

• .4 after a broach, directional control is best reasserted by reducing speed.