Cox'n steering: amendment to "boats have wings..."

Cox'n steering: amendment to "boats have wings..."

Post by Kieran Coghla » Wed, 10 May 1995 04:00:00


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
I think:

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
side.

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.

row well,

Kieran Coghlan
UCI mens Crew, '92-'95

 
 
 

Cox'n steering: amendment to "boats have wings..."

Post by Zack Grossm » Fri, 12 May 1995 04:00:00


(snip snip...)

Quote:
>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.  

(stuff deleted...)                          
                                   ...we all know that (most of the time)

Quote:
>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:

(technical stuff)

Quote:
>FEEDBACK WELCOME.   Is this totally incomprehensible?  

No. You do a fine job explaining yourself understandably.  Interesting point.
I always thought (intuitively, I haven't researched any fluid mechanics) that
roll *was* due to centripetal acceleration of the boat around the turn.  Even
though the motion is relatively slow and the turn is wide, I would think that
roll would still be noticable.  Here's why:  the force opposite the centripetal
force (the one that squishes you against the door in a turning, tightly packed
car backseat), acts on the center of mass of the boat and its contents (you),
pulling it outward.  The boat is "connected" to, pulled into the center of the
turn at the point where the boat rests in the water and the combination of
these two forces creates a torque across the radius from the bottom of the
boat to the vertical center of mass. Because of the length of this radius, only
a small torque is neccessary to get the boat slightly off center, unbalancing
it, causing outward roll.  (If this is true perhaps women's boats, with lower
center's of mass, would have less roll.)  Maybe this is exactly what you
discovered was incorrect, but I suspect it must be some combination of both.
Perhaps you could study this (using a model) by varying the height of the
center of mass and the coefficient of friction between the vessel and the fluid
it is in.
      Right, well anyway, I hope this doesn't bore you; it was interesting
hearing what you had to say.  This post can be held as evidence that some high
schoolers think about things other than the Winter Park girls, who's going to
win Stotes, schools vs. clubs, how many jr. natl. team members are in my boat,
and Philly vs. the world.

(clip clip...)

Quote:
>row well,

>Kieran Coghlan
>UCI mens Crew, '92-'95

Zack Grossman
Niskayuna High School
Go Nisky!

 
 
 

Cox'n steering: amendment to "boats have wings..."

Post by Thrasher » Fri, 12 May 1995 04:00:00

We designed a hydrofoil rudder system for 8's in 1961 at NAVY. It had some
interesting characteristics including far less drag. The prototype used a
modified Clark "Y" airfoil. The immersion process was rudimentary. Patent
held on it. Working on an embeaded version that includes drag and lift
capabilities depending upon how fast you want to turn. Call if you want to
discuss it further.

P.S. There are some versions of hydrofoil rudders employing***shrouds
that rotate about the shaft of motor vessels. Got to get faster, eh?

 
 
 

Cox'n steering: amendment to "boats have wings..."

Post by Daniel Whil » Mon, 22 May 1995 04:00:00


Quote:


>(snip snip...)
>>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.  
>(stuff deleted...)                          
>                                   ...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:

>(technical stuff)

>  Even though the motion is relatively slow and the turn is wide, I would
>think that roll would still be noticable.  Here's why:  the force opposite the
>centripetal force (the one that squishes you against the door in a turning,
>tightly packed car backseat), acts on the center of mass of the boat and its
>contents (you), pulling it outward.  The boat is "connected" to, pulled into
>the center of the turn at the point where the boat rests in the water and the
>combination of these two forces creates a torque across the radius from the
>bottom of the boat to the vertical center of mass. Because of the length of
>this radius, only a small torque is neccessary to get the boat slightly off
>center, unbalancing it, causing outward roll.

[snipped]

Quote:
>      Right, well anyway, I hope this doesn't bore you; it was interesting
>hearing what you had to say.  This post can be held as evidence that some high
>schoolers think about things other than the Winter Park girls, who's going to
>win Stotes, etc

Good point. Going back to Kieran's thoughts, it might be worth looking at
whether there was more, less, or no difference in roll when you are travelling
with the stream, as opposed to against it. If you are travelling against it,
then the torque due to the fluid should be greater, due to the higher relative
speeds involved. I'll try and have a look next time I'm out to see which of the
above is the *** factor. Here's to alt.boat.physics... :-)

Dan

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Cox'n steering: amendment to "boats have wings..."

Post by William J. Wadswort » Tue, 23 May 1995 04:00:00

Quote:

> >(snip snip...)
> Good point. Going back to Kieran's thoughts, it might be worth looking at
> whether there was more, less, or no difference in roll when you are travelling
> with the stream, as opposed to against it. If you are travelling against it,
> then the torque due to the fluid should be greater, due to the higher relative
> speeds involved. I'll try and have a look next time I'm out to see which of the
> above is the *** factor. Here's to alt.boat.physics... :-)

> Dan

Dan,
        The relative speed of boat and water are very little different
when going upstream or downstream, as the propulsion and most of the
resistance are between the boat and the water.  To observe the efffect of
different relative speeds of boat and water, you need to row into then with
a strong wind.  However the effect on technique ot the wind will probably
change things more than the relative speed factor. :-(.

W.W.