Competition result!
What you saw was a plastic barrel filled with water to prevent the aircraft lifting in the event of strong winds. The one you saw is on the tail (see it again in the smaller photo). There are two larger barrels attached to the wings and you can see one in the main photo, below the water.
The aeroplane is a Murphy Renegade Spirit. Bought in Belgium as a land plane and converted, with the help of some good friends of the owner, to a seaplane in
Biscarrosse in the
Landes department of France. It's classed as a ULM (microlight) in France. It is unique.
The plane is moored near to the
phare (lighthouse) in the port of Garavan in summer but in winter is moved to a jetty in the centre of Garavan, away from any bad weather. And guess what, I'm invited to fly in her one day?
Thank you so much to everyone who tried to guess the answer. On reflection I realise this was a difficult competition. Perhaps I should have given 'Jonathan Livingstone Seagull' as a clue but then it might have been too easy. Although no one got its actual use, Steve from
New Orleans Daily Photo was the first to use the word 'weight' - so a postcard of Menton goes to Steve. And another to Uselaine of
Willits Daily Photo who suggestion that it was an egg sack incubating hundreds of
these wins a postcard for originality and making me fall about with laughter.