4.7m hollow cedar/ply outboard oar

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4.7m hollow cedar/ply outboard oar

Post by topherdawson on Sat Mar 04, 2017 2:06 pm

This oar is being made by Robert Graham in Dundrum.

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Re: 4.7m hollow cedar/ply outboard oar

Post by topherdawson on Sun Mar 26, 2017 9:30 pm

Design 13 4.755m hollow outboard oak/cedar/ply oar

The Unbalanced Handle Force (UHF) @ gearing 2.8 = 2kg

Total weight = 5.5kg and deflection at 10kg at 2.5m =89mm which is soft.

Build time 50 skilled hours. Cost of timber about £50.

This oar is light to row with but soft. It is the longest prototype made, and took a long time to make. It has extensive internal zigzag bulkheads. It has a flat blade. Its length makes it feel fairly massive to change direction with, but it rows well.

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Re: 4.7m hollow cedar/ply outboard oar

Post by Robert Graham on Wed Mar 29, 2017 8:00 pm

Prototype Oar

Aims
To make a 4700mm long oar with a balance UHF of 2kg at 2.8 gearing and a deflection of 80mm.
To trial Topher's slotted oar concept.
Overall oar weight was not a primary consideration.

Construction
Inboard - Oak (845kg/m3) 82 x 63mm section.
Outboard - Flanges WRC 8mm thick along shaft to blade.
              Webs 4mm plywood
Blade -      4mm plywood 800 x 140mm
Internal -   Lattice girder stiffening, 4mm plywood (not recommended)
Taper -     Straight taper from oarlock to blade (not recommended)
Build Time - Approx 50 hrs which includes "thinking time" on a first
               prototype and making the numerous components of the                          
               internal lattice which would not be repeated.

Final Outcome - 4750mm long
                     2kg UHF
                     89mm deflection
                     5.5kg overall weight

Trials - Oar is light to row with but the higher than target deflection
         of 89mm produces a disconcerting flick at the end of the stroke
         which is difficult to manage effectively.

Conclusions -
It is feasible to make a long balanced all wood oar.
The slotted oar sits centrally over the pin which eliminates twisting loads at the rowlock and provides very positive location of the oar in the fore/aft plane. There has been no issue with oar strength.
Stiffness is the second most important design parameter after UHF and I would revise my ideal target from 80mm to 60-70mm. To achieve that I would follow Topher's lead and use a 90 x 45mm section outboard of the rowlock with a flange thicker than 8mm. I would also revise the target UHF to 2.2-2.5kg.
It is very difficult to predict the end result due to variation of wood density, working tolerances of wood and effects of epoxy coating and varnish. It has been suggested elsewhere that supplying an oar kit along with the skiff kit may be an option.


Prototype Rowlock Pin

Aims
To avoid having to drill a large vertical hole in a solid gunwale which could affect the structural integrity particularly at 1 and 4 where it is quite angled.
To have a robust vertical pin mounted in a wooden block bolted to the solid gunwale.
The oar supported so that constant pitch of the blade and gearing is maintained throughout the full stroke movement without any input from the rower.


Construction
To ensure accurate presentation of the oar blade to the water the rowlock pin is vertical to provide a reference datum to work from.
The top face of the wooden block(routh) at the rowlock is level to provide a datum to maintain constant pitch and gearing throughout the stroke.
As acetal does not glue well with epoxy I trialled a 16mm stainless pin
welded to a base which was then encapsulated into the oak block.

Conclusions
The concept of a ss pin with a flange base encapsulated in a wooden block provides a simple and strong pin.
As a stainless steel pin will not be universally accepted a machined flange acetal pin would be an alternative.

Prototype Rowlock Rocker

The rocker sits over the pin and a transverse raised bead support maintains an accurate presentation of the blade throughout the stroke. The system also enables the gearing to be adjusted in small increments within the selected range.
Spreading the induced loads over a wider area reduces wear and maintenance of the components.
This prototype is a variation of our standard design and has 3 components so is not practical for a production model.
The next option would have been to revert to a one piece development of our standard design.

I have photos of the oar and rowlock and I can whatsapp a video of the oar in action but unfortunately I'm not tech savvy enough to get it on this post after a number of attempts.

Robert Graham

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Re: 4.7m hollow cedar/ply outboard oar

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