The drawings [his watercolors] usually succeed in one day or at most two days or they develop difficult and usually don't finish well, then…. [I hope] the end will be as good as the beginning. (translation from original Dutch: Fons Heijnsbroek)
Ships, houses, mills… in one word everything that is made by people must stand upright and be painted with care. This is actually a good presentation compared to other, less symmetrical things, like the trees, skies, etc. It doesn't create the painting, but it certainly strengthen the illusion. It's just like somebody who is neatly dressed, but whose tie is coming off. The windows of a house must be straight, a mill in a pure construction, the blades well-positioned in perspective.
.. when making a painting after a study, it costs me a lot of effort to follow this study very well. One is very much inclined to make something different, so-called better, and that's why people usually get confused. A good outdoor-study has a breath of nature in it which must not be neglected or destroyed. You have to get everything out of that study and not just a third or half. If you can really improve one or the other: a la bonheur, but otherwise it is advisable to follow the study obediently as a guide.