• A wave is a disturbance that propagates
through space and time, usually with
transference of energy.
• A mechanical wave (eg. a water wave) exists
in a medium.
• Waves travel and transfer energy from one
point to another, often with little or no
permanent displacement of the particles of
the medium (eg. the water does not move,
rather the shape of the water changes).
How do waves form?
• Waves result from distant winds.
• Three factors inﬂuence the formation of
• wind speed, distance of open water that
the wind has blown over (fetch), length of
time the wind has blown over a given
• The greater each of the variables, the larger
• A non-breaking wave, no matter how high,
will not capsize a conventional boat with
• Breaking waves are the dangerous
• Can easily capsize a boat if the height of the
wave is equal to or greater than 60% of the
length of the boat when hit end on.
• When hit beam on, the breaking wave has
only to be higher than the width of the
beam of the boat to capsize the craft.
• A breaking wave is one whose base can no
longer support its top, causing it to
• A wave breaks when it runs into shallow
water, or when two wave systems oppose
and combine forces.
• When a wave is too steep, breaking is
Options in Big Seas
• Make for a windward shore.
• Reduce sail/run with bare poles.
• Surf with the wind.
• Use a drogue.
Options in Big Seas
• Heave to.
• Use a sea anchor.
• Hoist a small sail at
• Lie a-hull.
Planning for Waves
• Wind/wave analysis charts are available
from many providers.
• Show wave heights and direction.
Reading Wave Products
• Wave height over a period of time is usually
expressed as signiﬁcant wave height.
• This ﬁgure represents the average height of
the highest one-third of the waves in a
given time period.
• The largest individual waves are likely to be
about twice the reported signiﬁcant wave
height for a particular day or storm.
Information on Waves and
other Marine Topics at
A presentation to the Bluewater Cruising
Association Weather Group
by Darusha Wehm
March 12, 2008
C This work is licensed under the Creative Commons
Attribution-Noncommercial 2.5 Canada License.