Myboatplans 518 Boat Plans

s there really any need to calculate the strength of a boat? For centuries boats have been built from scantling rules that are based on experience, rules of thumb, guesswork and luck, with no actual strength or load calculations being made. The forces of the wind and sea are the same today as then, so reasons for the need to calculate boat strength must be sought elsewhere.

To begin with, modern boats of the 1990s have more highly loaded rigs compared with boats just 50 years ago. Aluminium spars, stainless stays and shrouds and sails from synthetic fibres deliver more power and need not be reefed as early as before, which lead to high loads from the rigging which must be absorbed by the hull.

Another factor working in this direction is today's more aggressive way of doing things, comparing ourselves and competing with our neighbour and consequently driving our boats harder.

With scries production of boats the cost of production has become more important. Since the cost relates directly to the weight of the boat, the importance of not building too heavy plays an increasingly important role. Performance, on the other hand (almost always sought), is inversely proportional to the displacement, but still this has pushed the development towards lighter and lighter boats.

So, higher loads from the rig and an aggressive and competitive owner must be taken care of by an increasingly lighter hull structure. All this means that the margin of error gets smaller and that the need for accurate calculations of strength becomes more important.

Other factors that put higher demands on the structure are the development of increasingly shorter fin keels that increase the stress on the keel/hull joint, and separate rudders that are supported only by their own rudder shafts, or by a skeg so small that it hardly contributes to the strength.

Before calculating strength requirements one must know the loads, and this is perhaps the most uncertain part of it all. The loadings can be divided into two parts: global and local. Global loads affect the vessel as a whole, ic loadings from the rig when underway try to bend the hull girder, and the stresses and deflections can be calculated by means of simple beam theory (to be discussed later). Local loads can be divided into hydrostatic/dynamic loads imposed on the vessel by the sea and waves, and loads brought into the hull from chain plates, keel, rudder, winches, sheet blocks and tracks, stanchions etc.

In this chapter we will discuss the influence of different loads, global and local, and what deflections they induce. Then we will show how the keel and rudder affect the hull structure, and finally survey different kinds of materials and their use, including exotic materials and sandwich construction. Details of the actual dimensioning of the YD-40 will be given in the next chapter.

Concepts in structural Although this is not meant to be a chapter 011 general structural mechanics mechanics, we will describe some basic concepts in structural design that are used in this chapter.

When we are talking of a material's stress, we mean the amount of force acting over the cross sectional area of the item in question, expressed in Newtons per square millimetre (N/mm2). The ultimate breaking stress represents the breaking stress, and the yield stress for metals means the maximum useful static stress.

Strain is the extension of the material per unit length when loaded, and it is expressed as a percentage. So if we have a piece of wood, steel, glassfibre etc, initially 100 mm long that when loaded becomes 103 mm long, the strain is said to be 3%. Obviously the lower the strain value the more brittle the material is.

The stiffness of a material is the ratio of stress to strain. If we compare two equal wires, one of nylon and the other one of stainless steel, both carrying the same load and stress, the stainless one will stretch just a little while the nylon will stretch quite a bit more, reflecting the different levels of strain. Dividing the stress by the strain you get a measure of the stiffness known as the modulus of elasticity: E = stress/strain [N/mm2]. This relationship is only true when the material is within its 'elastic region", which means that when the load is released the piece in question retains its original size. For metals this region is quite small. Typical permissible levels of strain are 0.2-0.3%. The level of stress at this point is called yield strength, as opposed to ultimate strength which describes when the material actually breaks.

When bending a beam or a panel one side will be subjected to compressional forces and the other side to tensional forces, both of them normal to the surfaces. Somewhere in between there will be a layer with no stress, called the neutral axis. In a homogeneous material this will pass through the geometrical centre of gravity for the cross-section. If the cross-section consists of parts of different moduli of elasticity, the cross-section is modified in the same proportion as the moduli of elasticity. If, for instance, one part has a 40% higher modulus of elasticity, this part is widened the same amount before calculating the centre of gravity for the cross-section.

The combined moment of inertia (I) for a composite section is the sum of each part's own moment of inertia plus each part's distance from the total neutral axis squared, times its area. When calculating the resistance to bending for a beam or a panel, we need to know the section modulus (SM), which, put simply, is the moment of inertia divided by the longest distance from the neutral axis to one of the surfaces.

As stated earlier the bending force induces compressional and tensional forces 011 the surfaces, and also a transverse, or shear force

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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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