Huge ocean swells, (think 10m i.e 30 feet or more) are not necessarily dangerous to small vessels like rowing boats. What is much more of an issue is when the face of the waves steepen into a wall. These walls eventually become so unstable they collapse, forming white water and hence termed ‘breaking waves’.

NB:  The critical slope angle required for waves to break according to NOAA is anything greater than 1:7.  This is the ratio of the wave height to the wave length.  Thus a 1m high wave with a wave length (distance from peak to peak) of 7m would start to break.

50nm offshore and rowing with 25 knots of wind through the Bali Sea during the north-west monsoon – January 2017

Studies have shown that the size of a breaking wave and its ability to capsize a vessel are directly related to the length of the hull of the vessel.  If a breaking wave catches a vessel side on, then the wave only needs to be 30% the length of the hull to have the capability to capsize the vessel – some of the time. If the breaking wave height reaches 60% of the hull length and hits the vessel side on then almost all vessels will capsize.

Based on these figures, we see that Simpson’s Donkey is 6.8m long.  So a breaking wave of just over 2m in height, if caught beam on (from the side) will have the ability to capsize us. If a breaking wave of over 4m in height hits us side on, then we will almost definitely capsize.  The Tasman Sea gets some very large waves, take this example from a stormy period in September 2010 “resulting in waves over 8m over almost the full width of the Tasman Sea”.

Source: https://www.niwa.co.nz/natural-hazards/hazards/waves

Simpson’s Donkey is designed to be self righting in the event of capsize.  However I will be doing my utmost in the upcoming row across the Tasman to avoid a capsize which is an extreme event and can cause injury/damage or more.  Ways to avoid capsize include running with the weather (i.e. in the same direction as the waves in order not to become caught side on), or by deploying parachute anchors or drogues which are towed in the water behind or in front of the vessel.  These have the effect of slowing the vessel down and/or holding the bow directly into the face of the waves, instead of letting the boat turn side-on which is the safest way for a vessel to handle rough weather.  A good deal of my time currently is spent planning and putting protocols into place on how to handle these situations when they arise on the water.

An excellent book to read for more information on the subject is:  Heavy Weather Sailing, by K. Adlard Coles, Peter Bruce