Forces acting on a Vessel

Throughout any given moment there are a variety of forces and stresses acting on a boat or yacht. Hardly ever is a boat in a completely static environment, whether it is in the water or on the hard. The basic elements such as water, wind and weight/gravity are working 24/7. Stress is a measure of the load put on a vessel whilst strain describes what happens if the stress becomes too big and the structure deforms.

Types of stresses:
1.Tension – is a stretching force – referred to as Tensile Strength.
2.Compression – is a squeezing force – referred to as Compressive Strength.
3.Torsion – is a twisting force – referred to as Torque.
4.Sheer – is a chopping force – referred to as Modulus of Elasticity.
5.Flexural strength – materials ability to yield and return for X cycles.

When bending a piece of material, e.g. fiberglass, it is subjected to all the aforementioned forces at the same time. This is very evident when a boat encounters a head sea or swell.

Stress concentrations tend to occur in a structure at abrupt changes in shape such as around holes. Ragged edges are a source of stress and may result in crazing and cracking.

The stresses that act on a vessel can be divided into two classes, Static and Dynamic.

Static Stresses:
These occur when a vessel is floating at rest in still water, with water pressure acting at right angles to the hull. Bulkheads, frames and floors resist these stresses. A vessels hull thickness is increased in thickness towards the bottom of the vessel where water pressure is greatest. Uneven distribution of weights throughout the length of a boat is opposed by buoyancy, and this counteraction results in shearing stresses within a hull. The weight of the engines is a good example of this. Large shearing stress is caused particularly when the boat is hauled out of the water.

Dynamic Stresses:
A vessel when working in a seaway causes these stresses. These stresses can also be divided into two types, namely Rotational and Lateral.

d)Twisting or Torsional

b)Surging (often incorrectly referred to as surfing)

Hogging and Sagging stresses occur when the vessel is subjected to bending longitudinally whilst working in a seaway. Longitudinal stiffening in the form of longitudinal stringers, intercostals, keels and garboard strakes provides resistance. Bending stress is greatest at the mid-ship sections.

Photo courtesy of A condition in which a hull is sagging with age in also referred to as hogging. It is an accurate indicator that this wooden hull has reached severe degradation and cannot be considered safe.

Shearing stresses are concentrated at the boat’s neutral axis when hogging and sagging. They will also occur in a vertical sense as the distribution of buoyancy alters in a seaway. Large weights, such as those of engines, should be distributed over a broad area by means of engine beds.

Racking stresses are caused by the rolling motion of a vessel, and may cause transverse distortion of the hull. Web frames and transverse bulkheads provide resistance to these stresses.

Pounding in a head sea due to the pitching motion of a vessel causes local shearing stresses as the fore part of the boat slams down on a wave. This is most pronounced in light displacement crafts with full bows.

Panting is also associated with pitching and causes shearing stresses due to the increase and decrease in water pressure as the bow rises and falls through the water as she moves. It is most pronounced in fine bows with a lot of flare.

Vibrations from the engines and propellers are resisted by special strengthening in the immediate area. Local stresses also occur in the vicinity of rigging and deep keels on sailboats. This is minimized by fairing off the structure with fashion plates to distribute stresses more evenly.

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Types of Corrosion
Affecting Boats & Yachts

The following types of corrosions are found commonly on boats and yachts.

Galvanic Corrosion - It is corrosion resulting from the electric current flow between dissimilar metals or dissimilar surfaces on one metal in contact with the same electrolyte (i.e. water) This same natural phenomenon we see every day used in flashlight batteries, automotive batteries and electro plating.

Stray Current Corrosion (a.k.a. Electrolytic Corrosion) – It is similar to galvanic corrosion in that the more positive areas loose material to the less positive areas of metals in an electrolyte, but is caused by an outside source rather than spontaneously. The boats AC or DC system, the dock’s AC system or other boats on the dock could be the source of the stray currents (this is not electrolysis).

Electrolysis – Chemical and/or electrochemical changes in a solution due to passage of electric current. Some boaters use this term incorrectly to mean galvanic corrosion or stray current corrosion.

Cavitation Erosion – Extreme turbulence, such as a propeller in action, forms and collapses bubbles when pressures due to the mechanical action fall below the vapor pressure of the seawater. Crystallization and fatigue of the metal results. These areas are anodic (more positive) than the other areas of the same surface and thus corrosion results. This is a common occurrence on Outdrives with dual props. A zinc or an impressed current system would help even out the voltage and reduce the effects.

Crevice Corrosion – Some metals develop a film that protects against corrosion. In cracks and crevices this film is often broken because of a lack of oxygen, thus resulting in corrosion. Stainless steel is very susceptible to this form of corrosion. Often a yard is blamed for using one mild steel bolt or screw in a place such as along a window and the rest was stainless. It could be that this specific bolt that has rusted away so quickly was mild steel or only a low grade of stainless steel or there could indeed be a larger problem.

Stress Corrosion – Heat transfer, “working” of the metal and chemical attack all combine to cause brittleness and early failure.

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