Stern or fin base rudder

[James HS]

We have a shell (Empacher) with a stern rudder (i.e. it is remote from the fin) and athletes report that it is really hard to steer as a 4- but ok'ish as a 4x

There has been discussion/debate about moving the rudder to be in the 'traditional' location behind the fin and I wondered what other's views are?

Are there plusses and minuses to the two locations?

The athletes steering are 'reasonably' experienced and we use the shell mainly on the tideway and for head races, and occasionally on multi-lanes and the likes of Henley Masters.

Would welcome the view as to whether swapping from the stern to behind the fin would improve manoeuvrability for our use cases.

Thanks


James

[A3aan]

Hi,

Does "hard to steer" mean that the boat doesn't turn enough, or that the boat is uncontrollable (reacts to quickly and/or unpredictably)?

I would expect that in the former case, moving the rudder closer to the boat's center of rotation would worsen the situation.
If the steering is hard to control it might help, but a boat builder would know better.
I think that having the rudder and fin next to each other is mainly a matter of reducing drag.

My 2 cts.

Adriaan.

Op dinsdag 25 januari 2022 om 16:38:26 UTC+1 schreef James HS:

[Andy McKenzie]

How 'smooth' is the steering foot plate.? I row in a variety of oldish shells, and one thing I have found is that a 'good' foot plate, with bowden cables, smooth movement and with the shoe heel not snagging on the keel, makes steering much easier. It might be worth doing all you can in that area first - I assume moving the rudder would involve some structural work to insert a rudder tube.

Another low cost experiment might be changing the area of the fixed fin? Reducing it would probably make the boat a little harder to sit, less directionally stable and more reactive to the rudder.

Having said that you might get 'better' steering with a fin/rudder combination. With a stern mounted rudder you are fighting the fin, which will be resisting the turning force, but with an integrated fin/rudder you are getting the fin to do the turning work.

[stan livy]

WIth the rudder at the stern it is a drag rudder- not very efficient except as a brake

With the rudder immediately behind the fin it works as a crude aerofoil and is vastly more efficient

With the newer style integrated rudders such as the aerofin, they work as even better foils

[carl]

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
Sydney 2000.

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.

Cheers -
Carl

--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
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