"Entry is open to boats of between 5 and 9 metres LOA........ subject to the Organising Authority reserving the right to refuse admission to any boat without giving reason. The Organising Authority may also admit boats of LOA slightly less than 5m at its discretion."
They are mostly worried about small boats being swamped on 20 mile long Loch Ness, where the waves were over a metre high, short and very nasty in 2011. With a big widely dispersed fleet, it would be difficult for the safety boats, even though there are a lot of them, satisfactorily to cover smaller and more vulnerable competitors.
The fleet splits on the first day on Loch Ness - one lot doing the gruelling Loch Ness Challenge, which is one and a half times the length of the loch, and the others racing on the southern part of the loch. The Challenge is sometimes described as by invitation only. Even if you want to do it, the organisers have to be convinced that both the boat and its crew are capable of completing it without assistance. Only two boats (both BR20's) took part in 2012 but this year it was seven.
