I've just read through it again. Very interesting.
Genesis 7.10.
And it came to pass after seven days that the waters of the flood were on the earth
11.
In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened
12.
And the rain was upon the earth forty days and forty nights
...
17.
And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lift above the earth.
18.
And the waters prevailed and were increased greatly on the earth.....
...
.24
And the waters prevailed upon the earth an hundred and fifty days.
...
Genesis 8.4.
And the ark rested in the seventh month, on the seventeenth day of the month, upon the mountains of Ararat.
So the ark was only afloat between days 40 and 150.