MOTHER Nature looms as the greatest obstacle for crews taking on the George Bass Surfboat Marathon.Four teams from Kiama, Coledale, Bulli and Wollongong City will be among 25 crews lining up for tomorrow's opening leg from Batemans Bay to Moruya.The 190km biennial event runs over seven days and finishes at Eden.Kiama sweep John Ford is competing in his fifth George Bass Marathon and believes the constantly changing swell is the real test for crews."The biggest thing is the changing conditions of the ocean and how they can adjust their rowing to suit the conditions," Ford said."It changes every day and it's quite demanding, but they enjoy that side of it. That's certainly one part of the attraction of the event."If you get a running nor-easter you can be running a 10 or 15-foot swell. I do all the work then and they [rowers] just bludge."Six members of Kiama's crew have never undertaken a George Bass Marathon."I've told them to expect a world of pain," Ford said."'We don't concentrate on marathons much but we have this year for the first time for a while. You have to be realistic but we're certainly here to win." Coledale have teamed with Woonona rowers to form an all-women crew.Only two members have had a previous taste of the George Bass Marathon."We're all kind of feeling a little bit nervous, but in saying that we're ready to take it on," Coledale rower Vida Oakes said."There's a lot to think about - the distance we have to row and doing it seven days in a row, the water changes and all the organising that comes with it, moving and setting up camp every day."We've been training for about 12 months and organising things. It's going to be a hard seven days and we're really thankful to have a good support crew of 25."Bulli have sent an experienced crew, while City will be represented by a young crew with no George Bass Marathon experience."Everyone's fine and ready to go," Bulli rower Craig Lunney said."Most of the guys have put in a fair few miles on the ocean. We've had a little wind-up through Christmas and now we're all looking forward to getting into it on New Year's Day."