engineering

Hogging and sagging

Hogging and sagging is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Hogging and sagging rather than just read about it. In short: In solid mechanics, structural engineering, and shipbuilding, hogging and sagging describe the shape that a beam or similar long object will deform into when loading is applied. Hogging describes a beam that curves upwards in the middle, and sagging describes a beam that curves downwards.

Key takeaways

Hogging and sagging belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Hogging and sagging to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Hogging and sagging from memory before moving on to harder problems.

Reference excerpt

In solid mechanics, structural engineering, and shipbuilding, hogging and sagging describe the shape that a beam or similar long object will deform into when loading is applied. Hogging describes a beam that curves upwards in the middle, and sagging describes a beam that curves downwards.

Ships

Dynamic stress Hogging is the stress a ship's hull or keel experiences that causes the center or the keel to bend upward. Sagging is the stress a ship's hull or keel is placed under when the length of a wave is the same length as the ship and the ship is in the trough of two waves. This causes the middle of the ship to bend down slightly, and depending on the level of bend, may cause the hull to snap or crack. Sagging or dynamic hogging may have been what sank the Prestige off Spain on 19 November 2002. The 2013 loss of container ship MOL Comfort off the coast of Yemen was attributed to hogging. Subsequent lawsuits blamed the shipbuilder for design flaws.

Time-induced stress Hogging, or "hog", also refers to the semi-permanent bend in the keel, especially in wooden-hulled ships, caused over time by the ship's center's being more buoyant than the bow or stern. At the beginning of her 1992 refit, USS Constitution had over 13 inches (33 cm) of hog. The keel blocks in the drydock were set up especially to support this curve. During her three years in drydock, the center keel blocks were gradually shortened, allowing the hog to settle out. Additionally, the diagonal riders specified in her original design to resist hogging, which had been removed in an earlier refit, were restored. The similar-sized USS Constellation had 36 inches (91 cm) of hog before refitting in the mid-1990s.

Cargo loading During loading and discharging cargo, ships bend (hog or sag) due to the distribution of the weights in the various holds and tanks on board. The maximum amount of cargo that a vessel can load often depends on whether her Plimsoll mark is submerged or not. Therefore, sagging can reduce her effective cargo capacity – especially if her loadline has already been reached prematurely due to the sag. This is taken into account when calculating cargo, by applying what is called a "3/4 mean draft". This method is also called the "two-thirds mean correction", directly derived from Simpson's first rule.

Structural beams In building construction, the sagging of beams is called "deflection". The amount of deflection varies in accordance with the beam's stiffness, the span between supports, and the load it carries. Sagging is a common problem in older houses.

See also Glossary of nautical terms (disambiguation) Hog chains Strength of ships

References

This article incorporates text from the public domain Dictionary of American Naval Fighting Ships.

External links Constellation's 1994 restoration article has several photos showing the hogging. Diagram showing hogging stress and the diagonal risers.

Worked examples

Example 1 — a first encounter with Hogging and sagging

Start with the simplest possible case. Write down what Hogging and sagging claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hogging and sagging before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hogging and sagging ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hogging and sagging

In research: Hogging and sagging appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hogging and sagging in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hogging and sagging is common in secondary-school and first-year university syllabi. It links to neighbouring topics Nautical terminology, Structural analysis, so understanding it makes those chapters shorter.
In everyday life: Look for Hogging and sagging outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Hogging and sagging” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Hogging and sagging in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Hogging and sagging means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Hogging and sagging out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Hogging and sagging in simple terms?

Why does Hogging and sagging matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hogging and sagging?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hogging and sagging.