The term stiff and tender ship is a key part of the vast topic i.e ships stability. A ship or boat floats in water when its downward acting weight is balanced by the upward buoyancy.
In most cases this axis of flotation passes through the center of gravity and center of buoyancy cutting through a straight line. Under this condition the downward gravity and upward buoyant force are in equilibrium.
In the event when the center of gravity and center of buoyancy does not lie in the same line. The ship is acted upon by a righting moment that tends to stabilize the ship. This can be seen in the form of metacenter the point at which the upward buoyancy force axis cross that of the center of gravity.
It is on these stability forces and the nature of stabilization; a ship is said to be a stable, unstable, tender ship, or stiff ship.
A ship is said to be stable if the C.G lies below the center of buoyancy. In such a case the ship will auto stabilize rolling back to its initial condition when acted upon by the external forces.
What Is Stiff And Tender Ship?
For a floating vessel, the distance between its center of gravity G and its metacenter M is known as the metacentric height. This metacentric height is what defines whether its a stiff or a tender ship.
Then there are other factors such as the distance of C.G from its keel, inclination, weight distribution, righting moment, and angle of heel that decides if the ship is tender or stiff.
From the point of designing the ship deciding on a suitable metacentric height of a vessel is an important factor in setting the comfort and stability level of a vessel. Tender ship and stiff ship help us know of the two extremes of ship stability; while the actual ship lies somewhere in between.
1 ) Stiff Ship
A ship or vessel is said to be stiff when its metacentric height is much larger thus making the ship highly stable. With large righting moment for a much smaller angle of heel, these ships return to a stable position within a fraction of seconds when forcibly heeled to one side.
This will occur when you place a large portion of the ships weight at the lower part of the ship closer to the keel. These ships are most stable but are quite jerky and uncomfortable for the crew.
Due to the short roll period of these ships went through large structural stress and one prone to damage. Further due to jerky behavior it is difficult to maintain proper cargo lashing.
In this ship, due to a large righting lever, it rolls violently with a small-time period. The chance of structural damage is high, is more suitable for the bulk cargo, and put more stress on the hull.
A ship designed for more stability like the bulk carrier and other heavy cargo vessels are designed with a metacentric height much close to the stiff ship.
2 ) Tender Ship
A ship or vessel is said to be tender ship when its metacentric height is very small thus making the ship less stable. With small righting moment for a larger angle of heel, these ships return to a stable position much smoothly and are sluggish to return when forcibly heeled to one side.
This will occur when you place a large portion of the ship’s weight at the upper part of the ship far away from the keel. These ships are insufficiently stable but are quite smooth to ride and comfortable for the crew.
Due to the long roll period, these ships went through lesser structural stress but create more strain on cargo lashing. As these ships are slow to return to its upright position it’s easy for water to get on the deck.
In this ship, due to a lesser righting lever, it rolls excessively to sides and tends to remain at the same position longer. These ships are also uncomfortable for the crew but are much better than the stiff ship.
The ocean liners, large ferries, container ships, and cruise vessels designed for the comfort of the passengers tend to be more towards tender ship.
Difference Between Stiff Ship And Tender Ship?
1. Response to wave profile rapidly with a large righting moment.
1. Sluggish and slow to respond to waves i.e lesser righting moment.
2. Most stable
2. Just enough stable
3. Due to a large righting lever, it rolls violently and quick.
3. Due to a lesser righting lever, it rolls excessively to sides.
4. Uncomfortable to ride.
4. Comfortable to ride.
5. Due to the short roll period, these ships went through higher structural stress.
5. Due to the long roll period, these ships went through lesser structural stress.
6. Bad for crew, cargo in containers wile better for carrying cargo in bulk.
6. Bad for carrying heavy cargo load, carrying cargo in bulk but better for crew, passengers.
7. As these ships are quick to return to its upright position it’s hard for water to get on the deck.
7. As these ships are slow to return to its upright position it’s easy for water to get on the deck.
8. These ship have small portion of the ship’s weight at the upper part.
8. These ship have large portion of the ship’s weight at the upper part.
9. Due to large structural stress they are prone to damage.
9. Less structural stress it last long but create strain on cargo lashing.
10. metacentric height is large
10. metacentric height is very small
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