A stiff ship is one with a very large GM caused by KG being too small. This occurs if too much weight is placed low down within the ship. The ship will be excessively stable, righting moments will be so large as to cause the ship to return to the upright very quickly when heeled. Roll period will be short.
In other words we can explain stiff vessel as:
- Greater GM due to high density cargo on bottom
- Ship rolls very fast
- Very uncomfortable