Subj : Re: Row360 - Issue 2 - Carl Douglas article
To   : rec.sport.rowing
From : Matthew Farrow
Date : Fri Mar 26 2021 03:35 am

On Sunday, 12 April 2015 at 18:29:13 UTC+1, Carl wrote:
> On 12/04/2015 09:00, robin_d...@hotmail.com wrote: 
> >> 
> >> And how's about a golf-ball-textured film. 
> >> 
> >> Enough for a Friday! 
> > 
> > Another question which has come to mind after watching the Boat Races - in
proper headwind conditions, would the reduced diameter of the scull-like C2
skinny sweep shafts compared to more conventional diameter of either normal C2
or Croker sweep blades have a noticeable effect on a crew? 
> >
> In short, yes. 
> 
> For a 5cm diameter cylinder transverse to a 10m/sec air flow, the 
> Reynolds number is about 450. At that point the drag coefficient for a 
> smooth cylinder will have fallen almost to a plateau of around 
> 1.2<Cd<1.4 & may rise only slightly as diameter, & hence Re, is reduced). 
> 
> Since reduced diameter reduces frontal area, & since drag is 
> proportional to the product of frontal area times velocity squared, 
> slimmer oars should probably reduce wind drag, but not quite in direct 
> proportion to the diameter reduction. 
> 
> That said, it is within the range 50<Re<800 that you might expect the 
> greatest benefit from fitting boundary layer trips as a means to 
> re-energise the surface flows & thus to reduce Cd by reattaching an 
> otherwise separating boundary layer. 
> 
> For the slimmer oar you must be sure that its bending and torsional 
> stiffness remain within acceptable limits, but if the reality is that 
> you are outside the zone within which these become critical to your use 
> of the oar, then smaller diameter should be better.
> Cheers - 
> Carl 
> -- 
> Carl Douglas Racing Shells - 
> Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories 
> Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK 
> Find: tinyurl.com/2tqujf 
> Email: ca...@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682 
> URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells
Hi Carl,
I appreciate I am very late to the thread but I am currently doing some
research on the topic and have a couple of enquiries.
If this finds you, please can you explain how you got these values for Reynolds
 numbers? 
Transition from laminar to turbulent for a smooth cylinder happens at Reynolds
numbers in the region of 200,000 (see Fundamentals of Fluid Mechanics, p501,
http://civilcafe.weebly.com/uploads/2/8/9/8/28985467/fluid_mechanics.pdf).
Personally, I calculated the Reynolds number of the cylinder in conditions you
described to be around 32,000, much higher than your value of 450. Some
discussion on this would be great :)
Matt
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