How do you quantify a yacht's comfort at sea?

An essential part of any comfortable blue water cruiser’s armoury is an easy motion at sea. While sailors might tolerate a bouncy or skittish motion on a trip round the bay, they will grow to hate it on a long cruise. 

 

In one of his last interviews, the great designer Olin Stephens said that sea-kindliness is important, adding “I have done less cruising than I might have wished, but I have felt that the rough and uncomfortable ride characteristic of sailing in a modern racing boat a few hours a day is more than enough.”

 

But working out how comfort and sea-kindliness can be achieved and recognising the factors that contribute to this holy grail of blue water cruising is not straightforward.

Charles Doane (wavetrain.net) put it well, saying: “smart cruising sailors are very interested in a boat’s motion and how it affects comfort aboard. Like all sailors, they are happy when sailing fast but are happiest when they are physically comfortable. This one factor has the biggest influence on a cruising sailor’s sense of psychological well-being.”

" For the technically minded, Brewer’s Comfort Ratio is: Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x B1.333). "

He goes on to mention designer Ted Brewer’s comfort ratio, adding “I urge you to use it when analysing cruising boats, particularly blue water cruising boats. It provides a much-needed third dimension to complement the simplistic two-dimensional picture painted by the D/L and SA/D ratios.”

Kraken designer Kevin Dibley (selected ‘2017 Yacht Designer of the Year’ at the Asian Marine and Boating Awards) puts great store in this Comfort Ratio (CR) for cruising yachts invented by Ted Brewer who wrote: “It is based on the fact that the faster the motion the more upsetting it is to the average person. Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.” 

 

He says that wide beam “generates a faster reaction” adding “the formula takes into account the displacement, the WL area, and adds a beam factor. The intention is to provide a means to compare the motion comfort of vessels of similar type and size.”

For the technically minded, Brewer’s Comfort Ratio is: Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x B1.333). 

Kevin Dibley puts this into context by explaining how Brewer’s CR figure equates to modern yachts, saying: “20 to 30 indicates a coastal cruiser; 30 to 40 a moderate blue water cruising boat; 50 to 60 a heavy blue water boat and over 60 an exceptionally heavy blue-water boat. If evaluating a larger boat, say 45 feet or longer, expect your results to be skewed a bit higher. If the boat is quite small, say 25 feet or less, they will be skewed slightly downwards.”

 

Reassuringly, the Kraken 50 has a CR of 34.24 (in ‘light ship’ mode - stores and fuel etc. will add weight and increase the CR), putting it firmly in the 30 to 40 CR ‘moderate’ blue water cruiser class. The larger Kraken 58 and 66 have ‘light ship’ CRs of 39.2 and 40.1 respectively. 

 

In other words the semi-custom Kraken 50 ft yacht, 58 and 66 are heavy enough for blue water comfort... yet not so heavy that performance is impaired. For comparison, modern volume production yachts such as the Bavaria C57, Hanse 575 and Oceanis 60 have CRs ranging from 25.34 to 31.9. 

 

Of course there are other factors that contribute to a sea-kindly motion. For example, motion is more comfortable in a centre cockpit than in an aft cockpit above a wide stern. But Brewer’s CR is very relevant and often overlooked. It’s an important indicator of how a blue water cruiser will behave at sea. 

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