I'll try the method from 2001 USN maneuvering board manual chapter 6, example 31, "COURSE AND SPEED TO PASS ANOTHER SHIP AT A SPECIFIED DISTANCE".
If we don't take the submerged part into account, we basically just need to pass ahead of the fleet with a CPA of 4km. This matches the situation in the manual very much.
1) So let's plot ownship at the center of the Mobo, and M1 representing the ship 7km ahead of us.
2) Draw the fleet vector r (the red one) with course 120deg at 8kts.
3) From M1 draw a line tangent to the 4 km red circle at M2. This satisfies the requirement of passing with a CPA of 4km from ownship.
4) From r draw a line parallel to M1-M2, intersecting the 15-knot speed circle at the blue arrow.
5) The blue vector is the course we want to follow.
Proved with MoBo V1.3 using the advance function.
