Sir, how can the V A in the open water and back of the ship is same?
You have taken it to be same.
Then we know the V A in open water, but we do not know in that what is V A.
You are just calculated know; you are just calculated what is the V A by measuring wake. We have done that just now. The open water test must be done at that speed not at shift speed. That is how the V A will be same. Do not confuse with the open water test being done at ship speed, no. The open water speed test is done at that speed V A which you have calculated.