This is "A Dutch Yacht saluting" painted in 1661 by Willem van de Velde. The painting depicts a marine occasion fairly common in seventeenth-century Holland, but it's executed with such accuracy and fine detail that it becomes spectacular and exciting.
The central vessel is a Statenjacht, or state yacht, flying the Dutch colours. You'll notice on the stern are the arms of the Province of Holland and above them, those of Amsterdam. The puff of smoke rising from the cannons is a salute to the officials approaching in the barge alongside, which also bears the coat of arms of Amsterdam.
On the right side of the painting, there's a small pleasure yacht with a pennant featuring a gold wreath with the initials 'W/VV' in the centre. This might suggest that van de Velde has portrayed his family's yacht, but it's uncertain whether the people on board are indeed the van de Veldes.
The composition is beautifully balanced, with the masts upright and keeping the overall layout even keel. The calm sea shimmers like glass between the boats moored on either side of the waterway.
A Dutch Yacht saluting hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.