Now, with all hull boxes intact, the Fuji has a speed of 7. When it
loses one set of hull boxes, the speed is 5.5 MU. Lose another row, it
drops to 4 MU. Another row gets it down to 3, and when it has only one
row left, it is dropped to 1.5 MU. The hull box section of the ship
sheet looks like this (this is vaguely ASCII art, so a monospaced font
is best for seeing it):
XXXXXX 7
XXXXXX 5.5
XXXXXX 4
XXXXX 3
XXXXX 1.5
You mark hull boxes off the top row. When the first row is lost, you
cross out the 7, indicating that the ship's speed is now 5.5. If a
second hull row is crossed out, you cross out the 5.5 and the ship's
speed is now 4. The ship's top speed will be 1.5 MU per turn when it
has one hull row left.
This is assuming that you didn't roll a Boiler Damage critical hit on
any of your threshold checks, which could drive the speed down to
zero. You simply cross off the next speed rating. When the last speed
rating is crossed off, the ship has dropped to 0 speed. Damage Control
can repair critical damage. There's another critical hit, Steam Line,
that has a similar result.
There's also a Rudder Damage critical that can have the ship turning
in circles. Oh, and I can't remember if I added it or not, but I had a
rule about a threshold check on a gun resulting in a possible magazine
explosion. That can take out a ship very quickly.