On our sail north from the Azores, we saw whales most days.  On one occasion 3 huge fin whales followed us for 50 minutes.  Difficult to estimate their size but probably around 20+ metres; certainly made our boat feel very small.

 

The video clip is a bit shakey - but we were more than a little apprehensive as they came so close to the boat and stayed with us for a long time.  The turquoise colour is the under-belly of one whale that went under the hull.

 

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