Pre­dict the sea state

Con­sider wind and waves when plan­ning and, as Nor­man Kean ex­plains, you'll be able to pre­dict and avoid rough wa­ters

Yachting Monthly - - INSIDE THIS MONTH -

Check­ing swell fore­casts and in­ter­pret­ing them will help you avoid rough wa­ter, says Nor­man Kean

T he ba­sic driver of sea state is of course the wind. The stronger the wind, the greater the dis­tance over which it blows unim­peded (termed the ‘fetch’), and the longer it blows for, the big­ger the waves – up to a limit, for the wind strength.

Waves – as our stom­achs know – are not gen­er­ally reg­u­lar. A typ­i­cal wind­driven wave pat­tern is a com­bi­na­tion of many wave trains, each with dif­fer­ent wave height (trough to crest) and pe­riod (the time in­ter­val be­tween crests). When these com­bine, the re­sult ap­pears as groups of waves. Waves pass­ing one spot will build to one or two big ones, and then di­min­ish again be­fore the cy­cle re­peats, while a short dis­tance away the same thing is hap­pen­ing, but not in step, so to speak, and the sea sur­face is a con­tin­u­ous grid of these fan-like wave groups.

The pat­tern is of­ten best ap­pre­ci­ated from the air, in breezy con­di­tions. In a small ves­sel it’s of­ten pos­si­ble to steer be­tween the groups in such a way as to dodge the big­gest waves. If you’re watch­ing waves break on a beach, it’s re­mark­able how of­ten two or three big ones ar­rive in suc­ces­sion. In a ran­dom wave pat­tern, con­sist­ing of com­bi­na­tions of many wave trains of dif­fer­ent heights and pe­ri­ods, about one wave in 25 will be twice the av­er­age height, and given sev­eral thou­sand waves – say 12 hours at sea – there is an ex­cel­lent chance of meet­ing one three or even four times the av­er­age. Ca­su­ally la­belling these as ‘rogues’ or ‘freaks’, as the me­dia of­ten do, is thus not en­tirely ap­pro­pri­ate, but it meets the need for sen­sa­tional head­lines.

Sig­nif­i­cant wave height is the av­er­age of the high­est one-third of waves. This is re­garded as the fig­ure of great­est in­ter­est to sailors, and it’s the one that’s quoted in buoy re­ports and wave height fore­casts. Long waves with long pe­ri­ods move faster, sur­vive longer and travel fur­ther, and the most ex­treme ex­am­ple of this is a tsunami.

In this pic­ture of Bernard Stamm’s Chem­inées Pou­joulat bat­tling a Force 10 in Bis­cay at the start of the VELUX 5 Oceans solo round the world race, the wave patches can be clearly seen

Wind-driven waves in­ter­act with storm-driven swell from a dif­fer­ent di­rec­tion to cre­ate patches of waves Wave di­rec­tion

Wave pat­terns are not reg­u­lar be­cause mul­ti­ple wave trains with dif­fer­ent pe­ri­ods in­ter­act re­sult­ing in a pat­tern with vari­able am­pli­tude Wave en­ve­lope

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