Gene Nygaard wrote:
"What is the relevant factor here -- that it is pressing down with a
force due to gravity of 9000 pounds due to gravity of 9000 pounds force?
Or that it has a mass of 9000 pounds?

The tonnage of a ship is the weight of the water it displaces.

The force pressing down (normal force) in mechanical problems is
significant when friction is involved.

Force equals mass time acceleration. So, the mass opposes and increases
the force required to get an object moving, or slowed, for that matter.
That includes a ship. It has inertia and requires force to change its
velocity. Drag is imposed on the submerged portion of the hull,
especially when coated with barnacles.

I shipped out of Long Beach in WW-2 on the LSM 472. I returned to San
Francisco on the LSM 94. I was transferred to the LST 604 to take it up
river to Stockton to be decomissioned and scrapped. While at the ship
yard there I witnessed a curious sight. A large merchant vessel was
moved from one berth to another using a small boat with an outboard
motor as the tow boat. River current in the basin was almost nil, yet it
took several hours to move that large ship with the power of only an
outboard motor. It worked! There must have been nothing more powerful
available and there must have been no rush to get the berth swap made.

Point is that it is likely that neither mass nor weight is as important
as current in many situations. How soon you can get up to speed depends
a lot on mass as Newton predicts. That motorboat would have done its
thing much more quickly with a waterskier in tow than it did with a big
merchant ship in tow.

Best regards, Richard Harrison, KB5WZI