Attributed to Monamy: a plausible, but not necessary attribution.
"after van de velde"
The painting above, closely related to the sketch, seems to contain a suggestion of the style of Backhuysen. Left is a detail from a painting, dated 1694, in Ludolf Backhuysen, by Gerlinde de Beer, p.132, published 2002. This work addresses the relationship between Backhuysen and van de Velde, see note here. My impression is that the Younger was more influenced by Backhuysen than any English marine art historian would readily allow. Michael Robinson had a distaste for Backhuysen, and would dwell on minute inaccuracies, almost as if he feared to admit that van de Velde could have any peer. But Backhuysen was a great painter.
Appears to be a drawing either after or preparatory to one of the prints below
There is an oil painting closely following this composition: 29 x 42, signed bottom left
See also here
Canot engraving 1745/6: 10 x 15
Same plate, perhaps a different issue
Small version 1760/70: 6 x 10
206 x 290 mm, signed and dated H.Dubbels fc 1655, British Museum
The note on this pen and ink wash in Middendorf's study of Dubbels remarks that "according to M.Robinson there are a series of paintings with the same motif, which all pass for works of William van de Velde. The original of these was probably thought to be a version in the collection of the Duke of Portland, dated 1662. ... It is not easy to say why William van de Velde should have painted a picture in 1662 based on a drawing by Dubbels dated 1655.... Perhaps the young van de Velde used the drawing as a base some years later, perhaps Dubbels was involved in the creation of the painting himself."
Perhaps the van de Veldes used the drawings of other masters in exactly the same way that Monamy used the drawings of other masters. Perhaps the fact that this drawing was clearly knocking around in England centuries before ending up in the British Museum meant that it formed part of Monamy's collection which was sold at auction in 1750, eighteen months after his death. Many of his collected drawings were no doubt by van de Velde, and many were no doubt not.
Monamy's Calm looks closer to Cappelle, left, than to van de Velde