The very nature of displacement boats is that they displace water, which must then go somewhere.

There is a formula for the top speed on flat water of such a boat:

4/3 the square root of the length at the waterline – the speed is in knots.

For a boat to climb its bow wave and ‘get out of’ its trap, it must use design features – thus the dinghy can rise above it and plane on top of the water, many modern yachts can too.

Question for you is – what is the displacement boat maximum speed if she is 40 feet, 10 inches LWL? Before design features enter into it of course.

8.52 knots, assuming I didn’t fail to carry the 1 somewhere.

That’s where I came too, roughly – to 8.5 or thereabouts for my purposes. My hull will plane if wave action lifts her out downwind, but not upwind.

Both wrong. I have it on good authority it is 42. Read it in a book somewhere.