4.6m solid all Douglas oar

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4.6m solid all Douglas oar

Post by topherdawson on Sat Mar 04, 2017 2:05 pm

This oar is being made by Finlay Robertson in North Berwick.

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Re: 4.6m solid all Douglas oar

Post by Finlay Robertson on Sun Mar 26, 2017 9:13 pm

Design Intent
This oar was designed to be as simple and cheap to build as possible, to see what kind of performance could be achieved with basic build processes.

Description
The oar is of solid spruce construction with a flat plywood blade. It may be built from a single 100x50 section or two 50x50 sections glued together. A flat, unprofiled edge forms a datum to simplify marking-out.

Build
Total build time was around 3 hours including marking out.

Cost
The prototype was made from dirt-cheap timber and requires glue only for attaching the blade; total cost was only around £30. Robins prices for fir are much more than what I paid (higher-quality Canadian Douglas, perhaps?), but timber and ply for a full set would still only come to around £60 an oar.

Weight and Balance
The spruce was a lot heavier than the expected 510kg/m3 - it was actually around 610kg/m3. I suspect the wood was not particularly dry. At a 2.8 gearing, the UHF was 2.82kg. Wooden counterweights (2.3kg in total) were affixed to the inboard end during testing bringing the UHF down to 2.31.

Stiffness
The design is the stiffest of those prototyped, with a measured deflection of 48mm.

Ullapool Testing
Despite being a bit heavier than the other oars tested, feedback was reasonably favourable with the main criticism being the length - 4.634m is really a bit on the long side.

Post-Test Modifications
I've produced some updated calculations to reflect the actual density of the wood and include corrections to account for varnish, glue, etc. Shortening the overall length to 2.3m means an UHF of 2.3kg can be achieved without counterweights (or 1.8kg with).

I was surprised at how much heavier the Douglas fir was than I expected. If this was, indeed, due solely to excessive moisture content, then properly seasoned wood would slightly improve the balance (UHF of 2.1kg without counterweights, or 1.6 with).

Finlay Robertson

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Re: 4.6m solid all Douglas oar

Post by topherdawson on Sun Mar 26, 2017 9:35 pm

Design 14 4.635m Solid all Douglas Fir oar
This oar was made from one piece of Scots grown Douglas Fir.

Plan here: https://www.dropbox.com/sh/yes11ade5keib4d/AADovXOwevFmZDNAjW6JhMZYa?dl=0

The Unbalanced Handle Force (UHF) @ gearing of 3.0 = 3.40kg. @ 2.8 = 2.82kg @ 2.6 = 2.33kg.(fairly heavy)

Total weight = 7.00kg and deflection at 10kg at 2.5m =48mm which is very stiff.

With wooden counterweights added, the total weight rose to 8.29kg but the UHF @ 2.8 fell to 2.31kg.

Build time 3 hours. Cost of timber about £35.

This oar is stiff and fairly light to row with, although very heavy to move around the boat. It is easy and cheap to make. Rowers like rowing with it although it felt massive to move fore and aft.

If made at the shorter length of 4.3m this design should have a UHF of 2.3kg @2.8 gear.

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