Oar shaft stiffness

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Oar shaft stiffness Empty Oar shaft stiffness

Post by topherdawson on Thu Dec 01, 2016 11:45 pm

1   Quite separately from strength, an oar shaft has to have a certain stiffness. A stiff object deflects very little when stressed. Chalk is stiff but weak, nylon rope is strong but unstiff (stretchy or soft), jelly is soft and weak, diamond is stiff and strong.

2   In an oar, stiffness is measured by hanging a standard weight (normally 10kg) from an oar a certain distance down the shaft from the oarlock (normally 2.5m). In timber gig oars, a deflection of less than 80mm is considered stiff although annoyingly they use a weight of 25 pounds (11.3kg). These weights approximate to an average rower's pull.

3   Although very stiff carbon oars are said to be unpleasant to row with, in practice wooden oars tend to have the opposite problem and can bend too much. If they are too soft, the rower feels that the soft shaft is not transmitting the force generated at the handle to the water, and energy can be lost in the shaft which may not be returned when the shaft springs straight at the end of the stroke. It is a matter of taste and we need stiffness measurements of good oars from the fleet to decide what is too soft.

4   My experiments with shaft design and actual oar shafts indicate that shafts can be strong enough but not stiff enough. But if a shaft is stiff enough (less that 80mm deflection) it is more than strong enough. (experience)

5   Thus what we may end up needing to do would be to decide an adequate stiffness, design to just meet that, and minimise the outboard weight.

6   Calculating the deflection of a proposed design needs a lot of number crunching but it can be done and I have made an attempt at it.


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