Solid trapezoidal outboard shaft of a light timber

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Solid trapezoidal outboard shaft of a light timber

Post by topherdawson on Sun Dec 11, 2016 2:10 pm

One design route which would be easy to build would be a solid outboard shaft out of Western Red Cedar or spruce. The light timber allows a more bulky shaft which puts the extreme fibres further from the neutral axis and thus makes more effective use of them.

John Murray of Gaco has pointed out that since timber is half or less as strong in compression as tension, it makes sense to design an oar section with the aft face half of the forward one. If made symmetrical top to bottom, this section is an isosceles trapezoid. Easy to make with flat faces, and can be cut tapered with a table saw with its blade canted over.

In the spreadsheet which calculates stress and stiffness, the width of the section at each station can be optimised. Interestingly, adding ash strips to the fore and aft faces, or just the forward face, does not reduce the UHF for the same deflection, and it does not reduce the deflection for the same UHF.

The current design for a 4.5m oar has a solid WRC outboard, a solid rectangular section oak inboard, and a light plywood spoon blade of Macon outline, 200mm at the widest and 600mm long.

It weighs 5.6kg, has a UHF of 2.5kg at a true gearing of 3, and a deflection of 80mm. Width at the oarlock is 75mm and depth is 58mm. Three gears spaced 40mm apart would be 3, 2.85 and 2.7.

If made with the more expensive spruce, which is slightly more dense and a lot stronger, it comes out with a UHF of 2.7. The spruce's superior strength is not being used, and it's only marginally stiffer.

Either way this is an oar of similar total weight to existing oars but importantly much better balanced.

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