4.3m 75x50 solid cedar outboard oar

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4.3m 75x50 solid cedar outboard oar

Post by topherdawson on Sat Mar 04, 2017 2:02 pm

This oar was made by Topher Dawson in Ullapool and has been rowed with for about 3 weeks.

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Re: 4.3m 75x50 solid cedar outboard oar

Post by topherdawson on Sun Mar 05, 2017 12:57 pm

4.3m oars work at the bow position and also at 2 and 3 if the pin is mounted in a block fastened to the inside of the gunwale, effectively reducing the beam of the boat at that point.

This oar is solid Western Red Cedar (WRC) in the outboard and solid ash in the inboard. The basic idea is to have a heavy hardwood inboard and a light softwood outboard, joined by an 8:1 scarph joint just outboard of the pin. Slight variations of the design exist to suit spruce and Douglas fir outboards.

The inboard is rectangular section 75x50 and the outboard is trapezoidal, i.e. the aft, tension, face is half as wide as the forward, compression face. This is because WRC is fairly weak in compression and doubling the compression side reduces the stress. It may not be necessary to do this and a rectangular section would be easier to make, besides 4% stiffer.

The outboard reduces in section to 30x20 at the tip.

The total weight after varnishing and adding the pin slot is 6.16kg. UHF at gears of 3.0, 2.8 and 2.6 is 3.16kg, 2.52kg and 2.14kg. Deflection is 72mm.

Cost: Assuming planks of cedar in the rough sawn 22x150 size, the 50 thickness can be made with three layers of 17 thick, and two tapered pieces can be cut from one plank. This means that for a set of 4 oars, 6 planks would be needed. At Robbins prices that would be £275 including VAT. The ash needs to be 50x175x2.6m = £93 and a quarter sheet of 4mm ply for the blades costs £16. Add £20 for glue and varnish to make £404 including VAT but without carriage, or say £100 per oar.

This oar uses more timber than the hollow ones but is simpler and quicker to build. I would estimate 30 hours for a set of 4.

Rowers like it because it is lighter in UHF than our existing oars and the spoon blade grips the water well. It gets 9's and even a 10 from some. Bigger rowers with heavy hands still like it when they find the hollow one with UHF of 1.5 too light in a wind.

There is a slight vertical vibration in a headwind during the recovery stroke, I think caused by the trapezoidal shaft. This shaft shape is pretty good for low drag, especially since the corners are rounded for robustness, but it may be acting like an aerofoil section and shedding vortices. I think making it rectangular would cure this.

The solid oar is smaller in cross section than the hollow oars and so has lower air drag. I will put photos in the Dropbox folder.

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Re: 4.3m 75x50 solid cedar outboard oar

Post by topherdawson on Sun Mar 26, 2017 9:40 pm

Design 2 4.3m 75x50 solid ash/Western Red Cedar oar
It is 4.3m, and has a solid ash inboard 90mm wide and 45mm deep. It has a trapezoidal section, but in fact a rectangular section would be easier to make and 4% stiffer. It tapers as it gets further out. The outboard is scarphed on to the inboard with a 6:1 scarph joint, and the oar has a 4mm plywood spoon blade.

The Unbalanced Handle Force (UHF) @ gearing of 3.0 = 2.98kg. @ 2.8 = 2.52kg @ 2.6 = 2.11kg.

Total weight = 6.23kg and deflection at 10kg at 2.5m = 61mm which is good and stiff.

Build time 10 hours. Cost of timber about £100.

This oar, the first to be made, is popular among rowers and adequately stiff. If the inboard was made 90x45 the oar would be better balanced.

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Re: 4.3m 75x50 solid cedar outboard oar

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