storm sea waves

Douglas Sea State 3 – London Arbitration Award

Oct 11, 2017

A recent London Arbitration Award, reported in Lloyd’s Maritime Law Newsletter 826, has dealt with the issue of Douglas Sea State 3.

In this case, for which StormGeo provided Optimum Ship Routing, a Voyage Performance Report and follow-up support, the vessel’s description contained the following quote:


In assessing the days of good weather, the tribunal stated the following:

"On the previous day, when swell of 2 metres height was recorded, the average speed was shown as being 10 knots, and on the day before that in similar conditions, 10.4 knots. However, on 19 February, again in similar recorded conditions, the ship only maintained 9.5 knots and on 22 and 23 February, in similar conditions, she made 9.6 knots.

In the light of those facts, it seemed to the tribunal right to conclude that in conditions which satisfied the requirements of the charter for the purposes of assessing performance, the ship could not do better than 10 knots. That was 1 knot short of the effective minimum warranty and on that basis the charterers’ claim was justified…"

The clear implication is that the tribunal accepted swell heights up to 2 meters as within the good weather definition of Douglas Sea State 3. This aligns with the StormGeo interpretation as shown in the following discussion.

In some past issues of Shipping Insights we have discussed the Douglas Sea Scale and how this relates to Charter Party terms and the determination of good weather conditions. These articles are from July 2009 and February 2010.

The Douglas Sea Scale, which describes the height of the waves and also assesses the swell, is expressed in one of 10 degrees. The original Douglas Scales used descriptive terms for the sea (wind-driven waves) and swell separately, as shown in the following table:

douglas sea and swell

The main issue with the Douglas Scale is the lack of quantified values for the sea and swell. Over the years, there have been attempts to equate the descriptions used in the Douglas Scales to precise wave heights, but according to the World Meteorological Organization (WMO), there is no internationally recognized definition or official status for any of the Douglas Scales.

Despite this, it is still common to see Charter Party terms for good weather wave heights referenced to a particular Douglas Scale, most often Douglas Sea State 3, without any mention of specific values for sea or swell. This results in the interpretation of the Douglas Scales' meaning when assessing a vessel’s performance in good weather conditions — creating one of the most contentious issues that we see between owners and charterers in speed and consumption disputes.

We have often seen it asserted that Douglas Sea State 3 means a combined wave height (combination of both wind-driven waves and swell) of 1.25 meters, including by some weather-routing organizations, while others have alleged that the meaning of Douglas Sea State 3 is a combined wave height of up to 2.0 meters. But the validity of equating the Douglas Scale to a combined wave height is questionable in light of the separate terms for sea and swell in the Douglas Scale.

In the absence of specific wording in the charter party stating otherwise, it has always been StormGeo’s policy to examine sea waves (wind-generated) and swell separately in keeping with the original Douglas tables. As guidance, we refer to the following discussion taken from the WMO website:

For the length of swell waves:
Short 0 - 100 m
Average 100 - 200 m
Long over 200 m

For the height of swell waves:
Low 0 - 2 m
Moderate 2 - 4 m
Heavy over 4 m

For the height of sea waves:
Calm (glassy) 0
Calm (rippled) 0 - 0.1 m
Smooth (wavelets) 0.1 - 0.5 m
Slight 0.5 - 1.25 m
Moderate 1.25 - 2.5 m

From the above, it is seen that a low swell is described as 0–2 meters while a moderate swell is given as 2–4 meters. In the Douglas swell table, scale 2 is described as a long low swell while at scale 3 it is a short moderate swell. The transition from Douglas 2 to Douglas 3 is therefore a height of 2 meters. StormGeo considers this to be a fair and logical interpretation of the swell height for Douglas Scale 3.

With this latest arbitration award from London, there is evidence for consensus that a 2 meter swell height is indeed the preferable understanding for Douglas Sea State 3.

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