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Common Hull Shapes

Displacement, Semi- Displacement, and Planning Hulls

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US Navy Littoral Combat Ship USS Fort Worth Launch Dockside

The US Navy Littoral Combat Ship USS Fort Worth Launches From the Hardtop of Marinette Marine Shipyard in Wisconsin

US Navy

Naval architects have designed some crazy looking boats and they will continue to push forward with topside design principals. The hull, on the other hand, is well refined and needs little tinkering.

Hydrodynamics research is driven by supercomputers in the modern world, but the old refined designs originally built by eye and scale models are proving to be very efficient without the help of computer chips.

These three shapes are most common.

Displacement

This is of course the classic boat hull shape. It is by far the oldest and most used hull in history. The reed barges of ancient Egypt were floating on the River Nile thousands of years ago.

The main feature of this hull is its deep and mostly symmetrical shape. The measurement of hulls is expressed as deadrise, which, in the simplest terms is the angle and distance a portion of the hull rises to meet the deck. This kind of hull usually only has one chine.

Tugboats are good examples of a displacement hull since much of the hull is submerged.

Cargo vessels also use this shape since the increased buoyancy allows them to carry more weight. The tradeoff is there is also a lot of drag because so much of the hull is below the waterline when underway.

The displacement hull is also a very stable platform because of the low center of gravity and weight of displacement vessels. A high center of gravity makes a vessel more unstable but slower to roll from side to side. Displacement hulls roll less but make the trip back and forth much more quickly.

Semi-Displacement

Semi-Displacement hulls are a hybrid between displacement hulls and planning hulls. The deadrise from the bow to mid ship would resemble a displacement hull, deep with a tall bow with a wide beam. The deadrise from mid ship back to the stern would have a shallow bottomed V shape and could be practically flat at the stern. It would also be narrower than the bow and have much less freeboard.

These hulls are common on small and medium sized vessels with a few exceptions. The U.S. Navy’s Littoral Combat Ship group is one larger example. It is a high speed shallow draft vessel that nearly planes at full speed.

The benefits here are higher speed capabilities since the forward portion of the vessel lifts from the water at high speeds. At rest or at lower speeds the vessel behaves more like a displacement hull.

Many military applications use this design for medium sized vessels since it is very versatile. The shallow deadrise of the stern gives exceptional prop clearance. In some cases the forward hull has a deeper draft than the propellers.

The draw backs are a wet ride at the rear of the boat since there is little freeboard. Plus using these boats in some conditions can lead to a very rough ride. High speed over chop is not the Semi-displacement hull’s strong point.

Some designs incorporate multiple chines to give a sort of stepped hull that has sweet spots for intermediate speeds

Planning

A planning hull has little draft. In the water the vast majority of the hull will be above water. Think of every recreational boat you have ever seen and there is your planning hull example.

The hull shape is widely used outside of the recreational boat industry by builders who want a fast and efficient hull. Fast patrol boats are common in military roles around the world and almost all designs are planning hulls.

The planning hull skips over the water and at speed it will only be in contact with the surface at the stern. At this attitude it has very little drag from the hull.

A hull of this design uses multiple chines to lift the hull clear of the water very quickly.

The deadrise at the stern is shallow except for the area near the keel. This small but relatively deep V shape give a planning hull good turning characteristics at high speeds. Drawbacks are low carrying capacity and quick and frequent rolling when at rest in even slightly rough waters.

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