This weekend several UBO's were sighted on the beach of the German isle Spiekeroog. Close to each UBO's were 4 or 5 lonely kayaks. No paddlers to be seen.
After extensive research these strange objects on Spiekeroog turned out to be "Bothy Bags" or "Kisu's": portable shelters made of waterproof nylon just big enough to accommodate two persons sitting together protected from the harsh environment. Larger ones accommodate 4,6, 8, 12 or more persons. In the United Kingdom "Bothy Bags" are a quit familiar survival equipment, but on the continent they are rather unknown. Outdoor shops in the Netherlands and Germany don't sell them. Unfortunately, as they are a brilliant piece of kit!
While the other groups that stranded on the sand tried to create protection against the chilly wind by putting kayaks on the side, by digging holes in the sand and were improvising with bivy or emergency bags, the groups with a bothy bag simply pulled the bothy bag over their heads and were protected against the cold wind: fast, simple and very effective!
The body heat of 4 or 5 people soon creates a warm local environment and provides protection from the threat of hypothermia and wind chill. Very cosy inside. And you also feel very isolated from the environment/outside world: it's not a "room with a view..."
A bothy bag is a very simple concept. It is nothing more really than a very big waterproof nylon bag. The front usually has one or more windows and the back usually has one or more vents.
Using one is very simple; you take it out of it's pack sack and put it over your head(s). Typically the base of the bag has a drawstring closure that will help stop it flying off. Larger ones may have interior handles.
A bag will typically be made out of 100% waterproof non-breathable nylon and have taped seams. The better ones are effectively frameless tents.
Using a bothy bag will provide you with some warmth and shelter. Condensation may well be a problem but it is assumed that you will carry on wearing all your normal outdoor gear and use them to stay dry.
A one-person bag weighs around 300g and a 4-person bag weighs around 400g. 300g is lighter than most plastic bivy bags and 400g is lighter than almost all single-man bivy bags. For the low-weight and weather protection provided they are an essential part of any autumn/winter pack.