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.
A very large GM should be avoided for the following reasons:
- The ship will return to the upright very quickly whereby the motion will be jerky causing excessive strain on cargo lashings and possible cargo shift.
- Loose gear will be thrown about.
- It is uncomfortable for crew and injury may result from the ship’s quick motion.
- Structural damage to the ship may occur due to racking.
A tender ship is one with a very small GM caused by KG being too large. This occurs if too much weight is placed high up within the ship. The ship will have insufficient stability, righting moments will be very small when heeled causing the ship to be sluggish and slow to return to the upright. Roll period will be long. (A tender ship is still a stable ship i.e. M is above G.)
A very small GM should be avoided for the following reasons:
- Because of the small righting moments the ship will only offer limited resistance to being rolled, causing the ship to be rolled to larger angles of heel. This will increase the risk of water being shipped on deck.
- The ship will be slow to return to the upright and will tend to remain at the extent of the roll for a comparatively long time. This will create greater and more prolonged strain on cargo lashings and increase the risk of cargo shift.
- Rolling to excessive angles of heel is also uncomfortable for the crew and injury may result.
As a guide, a GM of between 4-8% of the ships breadth is desirable. Container ships that have containers stowed on deck may probably be more suited to a GM value on the tender side of these limits to minimize the stresses on deck container lashings.
Typical curves of statical stability for both a stiff and tender ship are shown.