The shear force between water & a hull's surface falls from bow to
stern, so a shell's drag centre lies ahead of its centre of mass. With
cross-sections approximating to flattened semi-circles, shells have
side-slip easily &, having less resistance to side-slip at stern than at
bow, are easily deflected from their intended course when a boat's axis
departs from its direction of motion. As stern swings out of line, the
the opposed force vectors of forward momentum of boat & sternwards drag
on hull go increasingly out of line, amplifying the initial yaw rate.
Optimum performance requires a system giving proportionate, controllable
side-force, first to control the stern's tendency to swing, but also
able to be steered.
In a single-scull a simple fin, towards (but not at) the stern, resists
any initial swing and the sculler varies blade loads to over-ride &
maybe stall the fin. But safety & speed require a crewed boat to be
steered, by 1 person rather than a committee, by some form of rudder.
How best to steer & control a crew shell? With a fin-sized 1-piece
rudder; with a fin towards the stern (as in a 1x) and rudder right at
stern; will flat plates so; or can you have an efficiently-combined
fin & rudder?
A fin which stalls readily at a small angle of attack (a flat plate) can
prevent that initial yaw, allowing a separate rudder to over-ride it.
But when a rudder forces a direction change that stalls the fin inducing
lots of extra drag. To steer well in this situation a rudder needs a
recognisable & relevant aerodynamic section as it must operate over a
wide range of loads & angles of attack in which conditions a flat plate
will just stall.
The close-coupled, but separate, plate fin & rudder is currently the
most common system - a large plate fin with a much smaller, less deep
rudder right behind it, or even cut into it. Here the plate fin resists
turning while the little rudder must be heavily rotated to get the fin
to stall, wholly or in part. The 2 elements do not cooperate. So cox
rides on a knife-edge, between no useful steering & over-steer as they
increase rudder rotation. And, of course, the crew always blames the
cox who can't fully control their nearly 1 tonne of boat-plus-crew with
that £20-worth of tatty tin plate!
You can do far better. Aircraft wings are shaped & aligned to generate
lift with minimum drag. Their hinged trailing edges (flaps & ailerons)
continue the smooth shape & can be drooped to increase the wing's
effective camber (curvature) & thereby modify the flow over its top
surface, increasing the lift with almost no drag increase.
The value & function of "foils" (as wings of all kinds are properly
termed) being poorly understood in rowing, we blindly waste part of our
effort on the "parasitic" drag from inefficient steering systems &
resulting poor directional control whose side-slip adds further drag.
A well-designed, articulated, aerofoil-section steering foil gives cox
perfect boat control - able to make fine course adjustments or major
changes with minimal parasitic drag. The response is precise & almost
immediate. And the boat is more stable - an added bonus.
Our "AeRowFin" is exactly that - a retro-fittable foil-based steering
system which has been improving boat speed & course control ever since
As an add-on, we offer a forward-mounted "CanardFin" - a much smaller
foil, mounted around bow's frontstops position on an eight. It keeps
the bow on track so that, when cox steers/changes course, the bow now
goes immediately where you point it rather than side-slipping &
generating leeway drag which slows the boat & delays the turn. We call
this combined system "HyperSteer", for obvious reasons.
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
& now on Facebook @ CarlDouglasRacingShells