Ways to make the outboard lighter

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Ways to make the outboard lighter

Post by topherdawson on Sun Dec 25, 2016 10:41 pm

I'm not sure what other makers of hollow oars have done but our oars at Ullapool had walls of a uniform thickness because it was easier to make them in the thicknesser. Don Currie on the other hand tapered his, starting from 750mm out from the oarlock, from 16mm to 9mm at the tip. This turns out to have a very beneficial effect.

If the section tapers evenly from the gunwale, the parts nearer the oarlock are too soft and when they bend, they cause a lot of deflection at 2.5m out because any bend near the oarlock is magnified as it goes out. Don's solution was to keep the inner sections stiff and save the weight in the outer sections where it really adds to the UHF.

The result is nearly the lightest and stiffest oar design in the class as an inspection of the oar comparison spreadsheet shows.

He also uses a much thinner web (top and bottom) (6mm) than the flanges (front and back) (16 to 9mm) because the material in the flanges is working harder. Our webs and flanges were all 9mm.

He used Radiata Pine, which has a density of about 500kg/m^3. If we use Western Red Cedar with a density of 370kg/m^3 we can reduce the weight and UHF of the outboard although it increases the cost a bit and the wood is less resistant to knocks. This is despite the fact that WRC is weaker and less stiff than Radiata.

Another potential saving is to make the webs of 4mm okoume plywood, the same species the hull of the skiff is made of. Ply is arguably a better material for the webs as it is less likely to crack longitudinally in thin strips.

If the box section is made wider, 75mm instead of 70mm, the flanges can be thinner and some weight saved. There is a limit to how big the box can be made because the strips are getting thinner and wider, which may make them or the joints fragile.

Finally if we use blocks on the inside of the gunwales to mount pins 2 and 3, we save 70mm or so on the inboards and can save 200mm on the overall length of the oar. Looking at the numbers for a 4300mm oar with WRC and ply for the outboard we can get a total weight of 3.4kg, a UHF of 1.8kg and a deflection of 80mm. This is shown as Design 4 on the comparison spreadsheet.

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