sail shape and sideways force
 

Is sideways force always exactly perpendicular to the boat's centerline, as
shown by the vectors in Sleight's Manual (link below)? Can the camber of the
sail be forced to shift that force slightly forward?
http://scout235.tripod.com/Sleight_p_31.jpg
Scout

"Scout" wrote in message
...
Is sideways force always exactly perpendicular to the boat's centerline,
as
shown by the vectors in Sleight's Manual (link below)? Can the camber of
the
sail be forced to shift that force slightly forward?
http://scout235.tripod.com/Sleight_p_31.jpg

I think that whoever did those diagrams didn't understand very much about
physics.

What does "sideways force" actually mean? If you think about it, sideways
force must always be perpindicular to the centreline.

In reality, a force is exerted upon a boat in line with the wind. The sails
and keel combine to produce a forward motion that may not be directly
downwind.


Regards


Donal
--

I thought something seemed off with the diagram too, and also believed your
last paragraph to be true. I can see the perpendicular force in the first
figure, but when close hauled, as in fig 2, especially considering the wind
on the structure, I'm not sure why that vector isn't running "slightly" more
toward the stern?
Scout

"Donal" wrote in message
...

"Scout" wrote in message
...
Is sideways force always exactly perpendicular to the boat's centerline,
as
shown by the vectors in Sleight's Manual (link below)? Can the camber of
the
sail be forced to shift that force slightly forward?
http://scout235.tripod.com/Sleight_p_31.jpg

I think that whoever did those diagrams didn't understand very much about
physics.

What does "sideways force" actually mean? If you think about it, sideways
force must always be perpindicular to the centreline.

In reality, a force is exerted upon a boat in line with the wind. The
sails
and keel combine to produce a forward motion that may not be directly
downwind.


Regards


Donal
--

"Scout" wrote in message
...
I thought something seemed off with the diagram too, and also believed
your
last paragraph to be true. I can see the perpendicular force in the first
figure, but when close hauled, as in fig 2, especially considering the
wind
on the structure, I'm not sure why that vector isn't running "slightly"
more
toward the stern?
Scout

I think that the author of the diagram simply failed to express himself
clearly. If we substitute "leeway" for "sideways force" then his diagrams
make a bit more sense. Leeway is not a force, but it is the result of
combined forces.

In fact, leeway is *always* directly downwind. I learned this on my recent
dinghy sailing holiday. There was an area known as "the Graveyard". It
was a lee shore that had very gentle winds. Once you entered the Graveyard,
you were bound to end up on the rocks. The reason was that after a tack,
you would make leeway before you got forward motion. Furthermore, the wind
was so light that it was impossible to get forward movement without bearing
away --- which meant going even closer to the shore..... The first time
that it happened to me was very disspiriting. I thought that I had
forgotten everything that I knew about dinghy sailing.


I'll do a drawing if I get time.

Regards


Donal
--

Donal wrote:


In fact, leeway is *always* directly downwind. I learned this on my recent
dinghy sailing holiday. There was an area known as "the Graveyard". It
was a lee shore that had very gentle winds. Once you entered the Graveyard,
you were bound to end up on the rocks. The reason was that after a tack,
you would make leeway before you got forward motion. Furthermore, the wind
was so light that it was impossible to get forward movement without bearing
away --- which meant going even closer to the shore..... The first time
that it happened to me was very disspiriting. I thought that I had
forgotten everything that I knew about dinghy sailing.


Was there a current to drive you onto the rock or are you saying you
can't sail to windward in light winds???

Cheers

"Donal" wrote

In fact, leeway is *always* directly downwind. I learned this on
my recent
dinghy sailing holiday. There was an area known as "the
Graveyard". It
was a lee shore that had very gentle winds. Once you entered the
Graveyard,
you were bound to end up on the rocks. The reason was that after a
tack,
you would make leeway before you got forward motion. Furthermore,
the wind
was so light that it was impossible to get forward movement without
bearing
away --- which meant going even closer to the shore..... The first
time
that it happened to me was very disspiriting. I thought that I had
forgotten everything that I knew about dinghy sailing.

So, did you end up on the rocks?

Scotty

Scout wrote:
Is sideways force always exactly perpendicular to the boat's centerline, as
shown by the vectors in Sleight's Manual (link below)?
http://scout235.tripod.com/Sleight_p_31.jpg

The short answer is yes. Because sideways force is *defined* to be
perpindicular to the boat's centerline.

Think about it this way: there is a net force from the wind. It's a
vector (i.e. it has a magnitude and a direction.) You can always
express a vector in terms of it's components relative to a coordinate
system. In this case, the've chosen a coordinate system with axes
paralell to and perpindicular to the boat's centerline. So of course
sideways force is perpendicular to the boat's centerline - in the same
way that the x-component of any vector is parallel to the x-axis.

Can the camber of the
sail be forced to shift that force slightly forward?

You can definitely do things to make the net force more forward and
less sideways. i.e. make the forward component larger and the
sideways component smaller.


--
// Walt
//
// There is no Volkl Conspiracy

I had a vaguely similar experience only with anchored fishing boats instead
of rocks - damned embarrassing, and most power boaters seem to think only in
terms of "just give 'er more throttle!".
Scout

"Donal" wrote
[snip]
... after a tack, you would make leeway before you got forward motion.
Furthermore, the wind was so light that it was impossible to get forward
movement without bearing away --- which meant going even closer to the
shore.....

Thanks for the input Walt,
At first glance, I thought the author was ignoring the force of the wind on
the boat itself for simplicity's sake, and making a point about forces
developed by the sail. But then I realized that while the wind is
approaching the sail at the same angle in both diagrams, the sideways force
stayed relative to the boat's centerline and not the vertical plane of the
sail.
But from a practical standpoint, does total force = net force? And if so,
shouldn't the net force vector show the direction of the actual path, or
movement of the boat?
Scout

"Walt" wrote in message
...
Scout wrote:
Is sideways force always exactly perpendicular to the boat's centerline,
as
shown by the vectors in Sleight's Manual (link below)?
http://scout235.tripod.com/Sleight_p_31.jpg

The short answer is yes. Because sideways force is *defined* to be
perpindicular to the boat's centerline.

Think about it this way: there is a net force from the wind. It's a
vector (i.e. it has a magnitude and a direction.) You can always
express a vector in terms of it's components relative to a coordinate
system. In this case, the've chosen a coordinate system with axes
paralell to and perpindicular to the boat's centerline. So of course
sideways force is perpendicular to the boat's centerline - in the same
way that the x-component of any vector is parallel to the x-axis.

Can the camber of the
sail be forced to shift that force slightly forward?

You can definitely do things to make the net force more forward and
less sideways. i.e. make the forward component larger and the
sideways component smaller.


--
// Walt
//
// There is no Volkl Conspiracy

Scout wrote:

Thanks for the input Walt,
At first glance, I thought the author was ignoring the force of the wind on
the boat itself for simplicity's sake, and making a point about forces
developed by the sail. But then I realized that while the wind is
approaching the sail at the same angle in both diagrams, the sideways force
stayed relative to the boat's centerline and not the vertical plane of the
sail.
But from a practical standpoint, does total force = net force?

Um, yeah. The terms are more or less interchangeable.

And if so, shouldn't the net force vector show the direction of the
actual path, or movement of the boat?

Yes, if you include all the forces. But this diagram leaves out an
important force - the resistance force of the keel.

Simple physics analysis usually starts out with a free body diagram
where you draw all the forces on the object. Let's take a really simple
example: an apple sitting on a table. There's the gravitational force
pointing straight downward. So why doesn't the apple fall downward?
Because the gravitational force is not the full story; there's an equal
but opposite resistance force from the table pointing up that exactly
cancels the gravitational force. The net force is zero, and the apple
doesn't move.

In the sailboat example, there's a lateral resistance force that opposes
the sideways force and nearly cancels it out. They don't show it in the
diagram, but it's there. It comes from the centerboard or keel, due to
the fact that it's hard to push the boat sideways through the water.
Add it in and the net force vector will be aligned with the direction of
the actual path of the boat.*

If your boat has good lateral resistance (a big keel or centerboard)
this force will be nearly equal to the sideways force. If your boat has
little lateral resistance (i.e. you've pulled up the centerboard or
you've let the boat heel) this force will be less than the sideways
force and you'll crab sideways slipping to leeward.

--
//-Walt
//
// http://cagle.slate.msn.com/working/040514/matson.gif


* or more properly, the instantaneous direction of acceleration. If the
boat's on a steady course, this is the same direction as the boat's
path. Let's not split hairs where they don't need splitin.

Scout wrote:
Thanks for the input Walt,
At first glance, I thought the author was ignoring the force of the wind on
the boat itself for simplicity's sake, and making a point about forces
developed by the sail. But then I realized that while the wind is
approaching the sail at the same angle in both diagrams, the sideways force
stayed relative to the boat's centerline and not the vertical plane of the
sail.
But from a practical standpoint, does total force = net force?


Walt wrote:
Um, yeah. The terms are more or less interchangeable.

Unless you're trying to work out structural details ;)


And if so, shouldn't the net force vector show the direction of the
actual path, or movement of the boat?


Yes, if you include all the forces. But this diagram leaves out an
important force - the resistance force of the keel.

Agreed- the diagram ignored the 2nd set of forces on the foils & hull,
and tried to oversimplify to the point of being misleading IMHO.


Simple physics analysis usually starts out with a free body diagram
where you draw all the forces on the object.

AAAARRRRGH no! Now you've summoned the dreaded Navspritigator, who will
soundly denounce anyone who dares utter the dread incantation "free body
diagram"...


In the sailboat example, there's a lateral resistance force that opposes
the sideways force and nearly cancels it out.

Right. And the net forward force accelerates the boat until it is
cancelled out by the drag (which increases as speed builds up).

... They don't show it in the
diagram, but it's there. It comes from the centerboard or keel, due to
the fact that it's hard to push the boat sideways through the water.
Add it in and the net force vector will be aligned with the direction of
the actual path of the boat.*

It's also important to keep in mind the circumstances under which the
underwater foils generate this leeway-countering force. The boat must
have some forward speed. It is quite possible for the boat to move sideways.

Donal's observation that you have to bear away and give up some distance
to leeward to gain speed for the daggerboard (or other underwater foil)
to get a grip is quite accurate. There are a few boats that will
accelerate from a standstill close-hauled, but not many... and the ones
that will, they'll make notably more leeway at first, too.

Fresh Breezes- Doug King

"DSK" wrote
AAAARRRRGH no! Now you've summoned the dreaded Navspritigator, who will
soundly denounce anyone who dares utter the dread incantation "free body
diagram"...

I caught that but was pretending it didn't happen.


It's also important to keep in mind the circumstances under which the
underwater foils generate this leeway-countering force. The boat must
have some forward speed. It is quite possible for the boat to move
sideways.

I didn't realize that the forward speed increased lateral resistance.
Er..wait a sec, could this also be called "lift"?

Scout

"Nav" wrote in message
...


Donal wrote:


In fact, leeway is *always* directly downwind. I learned this on my
recent
dinghy sailing holiday. There was an area known as "the Graveyard".
It
was a lee shore that had very gentle winds. Once you entered the
Graveyard,
you were bound to end up on the rocks. The reason was that after a
tack,
you would make leeway before you got forward motion. Furthermore, the
wind
was so light that it was impossible to get forward movement without
bearing
away --- which meant going even closer to the shore..... The first
time
that it happened to me was very disspiriting. I thought that I had
forgotten everything that I knew about dinghy sailing.


Was there a current to drive you onto the rock or are you saying you
can't sail to windward in light winds???

Current??? Now why didn't I think of that?

Sadly, there wasn't any current.



Regards


Donal
--

DSK wrote:

AAAARRRRGH no! Now you've summoned the dreaded Navspritigator,

Is that like a crocagator? Not to fear, it's too cold up here for
crocagators.

who will
soundly denounce anyone who dares utter the dread incantation "free body
diagram"...


Did I say free body diagram? I meant to say Free Beer.

(There, that should put them off the scent...)


--
//-Walt
//
// http://cagle.slate.msn.com/working/040514/matson.gif

"Scott Vernon" wrote in message
...
"Donal" wrote

In fact, leeway is *always* directly downwind. I learned this on
my recent
dinghy sailing holiday. There was an area known as "the
Graveyard". It
was a lee shore that had very gentle winds. Once you entered the
Graveyard,
you were bound to end up on the rocks. The reason was that after a
tack,
you would make leeway before you got forward motion. Furthermore,
the wind
was so light that it was impossible to get forward movement without
bearing
away --- which meant going even closer to the shore..... The first
time
that it happened to me was very disspiriting. I thought that I had
forgotten everything that I knew about dinghy sailing.

So, did you end up on the rocks?

Twice!!!! ...And to make matters worse - the second time I had my wife in
the boat. I had assured her that the wind was gentle, and that there was
*no* chance of a capsize at all. Fortunately, she seems to have forgiven
me. AFAICR, she didn't get her hair wet in the capsize.


Regards


Donal
--

"Walt" wrote in message
...
If your boat has good lateral resistance (a big keel or centerboard)
this force will be nearly equal to the sideways force.

I think that things are a bit more complicated than this.

The boats that have the best performance to windward usually have fin keels.
Fin keels have a smaller area than long keels, don't they?

I think that this conundrum demands an explanation from an extremely
intelligent engineer. Jax ... here's your opportunity!


Regards


Donal
--

Scout wrote:
I didn't realize that the forward speed increased lateral resistance.
Er..wait a sec, could this also be called "lift"?

Sure. As long as you don't mind confusing the unwary... who might not
grasp the concept of "sideways lift."

The funny thing is that you can't have lift without an angle of attack.
If you have an angle of attack, you have leeway. So you must have at
least a little bit of the thing you're trying to avoid, so that you can
have less of it!


PS- this doesn't apply to boats with toed-in bilgeboards, twin
asymmetric daggerboards, or gybing centerboards.

Fresh Breezes- Doug King