A while back, I posted a letter about my ideas on the fluid mechanics
involved with steering a racing shell. Well, I have been thinking about
some of the theories I had, and in trying to observe my theories in
action while on the water, I noticed that I was wrong! So I tried to
figure out why, and find an explanation for what I observed. Here's what
In my previous post, among the many points I made was one in which I said
that the force acting on the rudder to turn the boat also acted to roll
the boat, and that this was the reason why the boat became unset during
turns. Well, it is true that the force on the rudder acts to roll the
boat, but this is not the reason for the boat tipping to one side during
a turn. If you think about it, if what I said were true, the boat would
roll to the INSIDE of turns. This means that in a turn toward starbord,
the boat would roll to starbord. But we all know that (most of the time)
the boat rolls to the OUTSIDE of a turn. A lot of people think that this
effect has to do with the centripetal acceleration around the turn, like
in a car when you feel you body "pulled" to the outside of the turn.
This, however is not the case, because the velocity of the boat is so
small, and the radius of the turn is so large, that the centripetal
effect is almost unnoticeable. Here's why it happens: when the cox turns
the rudder, the STERN swings around and the BOW stays put. This is because
the boat is steered from the stern (unlike a car, where the front swings
around in a turn and the rear stays (or follows, depending on speed.) SO
WHAT! you say. Well, next time you are in the boat, and the cox (or you
if you are a cox) makes a fairly big turn at a fairly good speed, look at
the water on either side of the boat. during the turn there is a lot of
wake comming off the side of the hull that is inside the turn, and almost
no wake comming off the hull on the outside-side. This is due to the
fact that the stern has swung (word?) around, while the bow stayed put.
So now, the boat is facing a new direction, but still traveling in the
old direction (or close to it -- gradually the path straightens out.)
The boat is now doing what piolts (and aero-engineers) call side-slip.
For people without any background in vector based math, side slip is a
little hard to explain without using my hands. (well here it would be
impossible without my hands - I couldn't type!!)
If you cant see what I mean by what I have said above, let me try this:
imagine the boat is stationary, and the water is moving past it, and you
can't change the direction the water flows, only the direction the boat
points. At first the boat is aligned with the dirction that the water is
flowing. Now you turn the boat a little, so it is at an angle to the
dirction of the water flow. Now the water does not flow "nicely" around
either side of the hull: it is kind of hitting one side. In
vector-speak, a component of the water velocity is normal (perpendicular)
to the boat.
It is this part of the flow that is the culprit. As the boat is turning,
some of the water is flowing ACROSS the hull (i.e. from one side to the
other) as opposed to ALONG the hull (i.e. from one end to the other.)
Well, there is friction between the water and the hull, and the water
flowing ACROSS the hull also pushes on the hull. This is what causes the
boat to roll to the outside of the turn. As the water "pushes" on the
outside of the bottom of the boat during a turn, the boat topples to one
However, this effect is primarily noticeable on wide, sweeping turns.
During a race, when only small adjustments are (should be) made by the
cox, the roll due to the rudder is more noticeable.
TRY IT AT HOME! Try to notice these things when you are on water. Watch
the wake when you make wide turns, and notice the set (or unset) of the
boat. Also notice how the set is affected when you are at race pace and
the cox makes a small turn or, "directional adjustment" (double speak).
FEEDBACK WELCOME. Is this totally incomprehensible? Are you sitting at
your monitor tinking, "man, this guy has got his head up his but! I don't
know what the heck he is saying!" Well, some of my team mates may agree
with you, but for different reasons :P Anyway, let me know. I am
thinking about writing my research paper for a technical writing course on
the physics of rowing.
UCI mens Crew, '92-'95