MATERIALS FOR OARS

The model. 

Mechanically speaking, an oar is a beam, loaded in bending. It must be strong enough to carry, without breaking, the bending moment exerted by the oarsman, it must have a stiffness to match the rower’s own characteristics and give the right ‘‘feel’’, and—very important—it must be as light as possible. Meeting the strength constraint is easy. Oars are designed on stiffness, that is, to give a specified elastic deflection under a given load.

Figure 1 An oar. Oars are designed on stiffness, measured in the way shown in the lower figure, and they must be light.

The upper part of Figure 1 shows an oar: a blade or ‘‘spoon’’ is bonded to a shaft or ‘‘loom’’ that carries a sleeve and collar to give positive location in the rowlock. The lower part of the figure shows how the oar stiffness is measured:
a 10-kg weight is hung on the oar 2.05m from the collar and the deflection δ at this point is measured. A soft oar will deflect nearly 50mm; a hard one only 30. A rower, ordering an oar, will specify how hard it should be. The oar must also be light; extra weight increases the wetted area of the hull and the drag that goes with it. So there we have it: an oar is a beam of specified stiffness and minimum weight. It is that for a light, stiff beam :

where E is Young’s modulus and ρ is the density. There are other obvious constraints. Oars are dropped, and blades sometimes clash. The material must be tough enough to survive this, so brittle materials (those with a toughness G1C less than 1 kJ/m2) are unacceptable.

The selection. 

Figure 2 shows the appropriate chart: that in which Young’s modulus, E, is plotted against density, ρ. The selection line for the index M has a slope of 2, as explained in Section 5.4; it is positioned so that a small group of
materials is left above it. They are the materials with the largest values of M, and it is these that are the best choice, provided they satisfy the other constraint.

 Figure 2 Materials for oars. CFRP is better than wood because the structure can be controlled.

Table 1 Material for oars

Material	Index M (GPa)1/2/(Mg/m3)	Comment
Woods	3.4 –6.3	Cheap, traditional, but with natural variability
CFRP	5.3 –7.9	As good as wood, more control of properties
Ceramics	4–8.9	Good M but toughness low and cost high