Saturday, 5 February 2011

A novel oar

Ullapool Coastal Rowing are an inventive bunch, experimenting with steerboards and such. Now Topher Dawson and John Mackintyre are developing what may be a totally new type of oar.
The prototype has an adjustable blade that can be swept forward or back. The aim is to prove once and for all if it really is better to have the blade angled forward as is conventional, or if a backward-angled blade might be better.
The edge of the blade has a curious stepped shape that is intended to throw off vortices and stop the blade stalling.
Topher explains:
Hi Chris,
What is the most efficient oar? Since hydrodynamic lift is a big part of the stroke (I have lots of references) we want to delay the stall of our flat blades as long as possible. Pointed blades appear in many native paddles and some old Viking oars, and John and I think they delay stall in the same manner as delta wings and Rogallo hang gliders.
The additional points we think do the same, and are a copy of the edges of tree leaves which are basically gas exchange membranes and John tells me that a professor of his acquaintance told him that the points on leaves are vortex stimulators to encourage gas exchange. We want the vortices to energise the flow over the back of the blade and prevent or delay separation.
Anyway it'll freak out the opposition.
Cant angle is measured with positive cant having the tip further forward than the line of the shaft. My original idea was to have negative cant, i.e. make the flat blade more like a spoon by bending the tip aft. However some researchers think that positive cant is the way to go so now I have no idea and we will have to try both. The adjustable bolt thing is just to try it with different angles of cant till we think we know what we want, then they will all be made fixed at that angle.
In theory the blade is in lift mode as it travels out from the boat in the first part of the stroke. The flow is from the tip of the blade towards the root. Then the flow stalls and the blade is operating as a shovel, i.e. in drag mode. Finally as the blade comes towards the boat in the last part of the stroke, flow starts from the root to the tip and lift is re-established. Fiddling with the cant angle makes the transitions between the stages earlier or later.
Watch this space!
Cheers, Topher.
Topher wants to know more about Greenland paddles and currach oars, which are very slender. Does anybody know how these oars compare with more conventional oars such as Macons?


michael b said...

Freaks me out too.
But I'll go with it for now. These are the guys who thought they could hang a paddle off the side of the boat in place of a rudder. That looked like a great idea, but the steerboard didn't work so well.

I need to build a set of oars for my new rowing shell so I'm watching this one closely.


O Docker said...

I do know that Bonnie, over at Frogma, carries a Greenland paddle as a spare, has posted about them, and could probably tell you how they compare with conventional kayak paddles.