Fin size vs. sail size

Fin size vs. sail size

Post by Alex » Fri, 17 Apr 1998 04:00:00


Why is recommended to put a smaller fin in the board as the size of the
sail decreases? Theoretically the push of the sail on the board is the
same with a small sail and with a big one as the wind also varies
accordingly.

        Also, what's the effect of placing the fin further appart from the tail
of the board? Is it not the same asvarying the size of the fin?

regards, Alex

 
 
 

Fin size vs. sail size

Post by sailquik (Roger Jacks » Fri, 17 Apr 1998 04:00:00



Quote:
>Why is recommended to put a smaller fin in the board as the size of the
>sail decreases?

Usually the windspeed goes up, and so does te chop. And the speed increases
dramatically, up to a point. The hydrodynamic lift of a huge fin will be
uncontrollable at some speed/water surface conditions. The only solution is
to reduce the hydrodynamic area to something you can control.

 Theoretically the push of the sail on the board is the

Quote:
>same with a small sail and with a big one as the wind also varies
>accordingly.

I do not think this is true, for a couple of reasons.
As speed increases, the apparant wind/true wind ratio becomes much greater,
allowing the pointing angle to be reduced (ie you can point much higher as
the apparant wind angle gets bigger as speed increases).

As board speed increases, you have much less of the board in contact with
the water, and ultimately can support the board almost entirely on the fin.
Big fins usually have a bit thicker foil, to work better at lower speeds.
These thick large foils tend to spin out early when driven beyond their
designed speed range. Smaller fins do not do this, even witha fairly thick
foil.

Quote:
>    Also, what's the effect of placing the fin further appart from the tail
>of the board? Is it not the same asvarying the size of the fin?

It can have some effect, usually forward makes the board more turny and
loose, back makes the board go upwind and in a straight line better.
But mast foot position may play a much bigger role in this equation.
Most modern boards do not allow fore and aft movement of the fin, so
balancing the MFP becomes the only adjustment.
hope this helps!

Quote:

>regards, Alex

sailquik (Roger Jackson) |Ph # in Md. (301) 872-9459
Lvl 1 WS Instructor      |    in N.C. (919) 995-3204
F2/Sailworks/North/True Ames/Rainbow

 
 
 

Fin size vs. sail size

Post by Bob Jacobso » Fri, 17 Apr 1998 04:00:00

In general, sail size is an indicator of the board speeds the fin will
be operating in. Large sails imply low board speed, small sails imply
high board speed. To generate a given amount of side force at a given
board speed you can either use a small fin at a high of attack, or a
large fin at a low angle angle of attack. A too small fin at low speed
will operate at such a high angle of attack that the fin will tend to
stall, causing spinout. A too large fin at high speed will operate at
such a low angle of attack that it will become unstable, causing
tailwalking. (The tailwalking is more severe with high aspect ratio
fins, since the center of effort is lower, which increase the tendency
of the board to rail up.)

So the relationship between sail size and fin size is a rule of thumb.
There is actually quite of bit of overlap, with trade-offs of speed,
pointing ability, and controllability. As discussed in a previous
thread, sometimes when the wind comes up you can regain control by going
down in fin size only, without changing the sail or board size.

Changing the fore and aft placement ot the fin is not the same a
changing size. More forward placement makes the board turn easier, rear
placement makes for better high speed tracking.
Bob Jacobson

Quote:

> Why is recommended to put a smaller fin in the board as the size of
> the
> sail decreases? Theoretically the push of the sail on the board is the

> same with a small sail and with a big one as the wind also varies
> accordingly.

>         Also, what's the effect of placing the fin further appart from
> the tail
> of the board? Is it not the same asvarying the size of the fin?

> regards, Alex


 
 
 

Fin size vs. sail size

Post by WARDO » Fri, 17 Apr 1998 04:00:00

Roger is "spot on" in his synopsis of fin theory. The only thing I'd like to add
pertains to tri-fin sailboards. These boards have an inherently large wind range
because you can adjust fin drag. When the wind velocity (and sometimes chop) goes
up, I put in bigger side-biters, and likewise at lower wind velocities I will
reduce drag be using smaller side-biters (or no side-biters) to facilitate earlier
planing. It is good that Alex is even thinking about his fin, it seems this is a
mystery to most sailors, they just use whatever fin came with the board in whatever
position they first install it, for whatever conditions they sail in. I'm not
suggesting you have to get all ***about fins or come down with a case of
"tweakeritis" (spending inordinate amounts of time tweaking at the expense of
T.O.W.
{time on the water}) but fins are very important. The thing about fins in relation
to sail size is there are many subjective parameters, I like to be on the edge of
control on the wave face, but still be able to go to weather to catch the next
wave, I don't like too much sail(or fin) when I'm going down the line, but I like
to ride the wave to the end and be able to get back out. It's pretty personal, this
fin area to sail area ratio. I do not like the idea of someone limiting my ability
to adjust the position of the center fin (Tuttle box) because everyone's geometry
is different and a little adjustment can have a big effect (this is why the mast
track is adjustable).
The effect of  size,planform, and placement of fins is crucial to board performance
(fun factor). I know sailors that sell new boards they think they do not like
without ever trying a different fin setup and Chuck Ames (True Ames Fins) is right
here in our back yard
(talk about lazy! or maybe it's just a brain cramp). You may have to go through a
couple of iterations to get it right, pick other sailors brains (be careful who you
ask though,
I have had some pseudo-famous racer weenies blow an arse gasket just looking at
their fins, you'd think it was the secret keel on an America's Cup yacht) keep an
open mind and experiment, the cost of a fin is minimal compared to the rest of the
kit and the effect they have is substantial.

Quote:



> >Why is recommended to put a smaller fin in the board as the size of the
> >sail decreases?
> Usually the windspeed goes up, and so does te chop. And the speed increases
> dramatically, up to a point. The hydrodynamic lift of a huge fin will be
> uncontrollable at some speed/water surface conditions. The only solution is
> to reduce the hydrodynamic area to something you can control.

>  Theoretically the push of the sail on the board is the
> >same with a small sail and with a big one as the wind also varies
> >accordingly.
> I do not think this is true, for a couple of reasons.
> As speed increases, the apparant wind/true wind ratio becomes much greater,
> allowing the pointing angle to be reduced (ie you can point much higher as
> the apparant wind angle gets bigger as speed increases).

> As board speed increases, you have much less of the board in contact with
> the water, and ultimately can support the board almost entirely on the fin.
> Big fins usually have a bit thicker foil, to work better at lower speeds.
> These thick large foils tend to spin out early when driven beyond their
> designed speed range. Smaller fins do not do this, even witha fairly thick
> foil.

> >       Also, what's the effect of placing the fin further appart from the tail
> >of the board? Is it not the same asvarying the size of the fin?
> It can have some effect, usually forward makes the board more turny and
> loose, back makes the board go upwind and in a straight line better.
> But mast foot position may play a much bigger role in this equation.
> Most modern boards do not allow fore and aft movement of the fin, so
> balancing the MFP becomes the only adjustment.
> hope this helps!

> >regards, Alex

> sailquik (Roger Jackson) |Ph # in Md. (301) 872-9459
> Lvl 1 WS Instructor      |    in N.C. (919) 995-3204
> F2/Sailworks/North/True Ames/Rainbow

 
 
 

Fin size vs. sail size

Post by ajhy.. » Fri, 17 Apr 1998 04:00:00

Smaller sails usually mean higher wind.   A smaller fin should give
you the same lateral resistance in high wind, because you are moving
faster,  as a larger fin would in light wind.

Consider a long board with a centerboard.  In light wind, you might
need the centerboard to get planing.  As wind and board speed
increases, the centerboard creates so much lateral resistance that you
feel overpowered.  Raising the centerboard  (effectively reducing fin
size) corrects that situation.

Jet aircraft can have smaller wings than prop planes because they
travel at higher speeds, and thus create more lift.   Lift on an
airfoil, all other things being equal,  is related to speed.   A fin
is no different from an airfoil.    

Quote:

>Why is recommended to put a smaller fin in the board as the size of the
>sail decreases? Theoretically the push of the sail on the board is the
>same with a small sail and with a big one as the wind also varies
>accordingly.

>    Also, what's the effect of placing the fin further appart from the tail
>of the board? Is it not the same asvarying the size of the fin?

>regards, Alex

 
 
 

Fin size vs. sail size

Post by phil » Fri, 17 Apr 1998 04:00:00

Forget all that physics mumbo jumbo. Think of it this way- when the wind
pushes on the sail you need something to push back against it (the fin ) or
you`ll go sideways downwind. Ideally you want it to push back just as much
as the wind is pushing on the sail. Water is 80 times or so denser than air
so the fin can be 80 times smaller than the sail. If the fin is too small
and can`t push back against the sail enough you get pushed downwind more
than you`d like and go forward slower than you`d like. If the fin is too big
and pushes back against the sail pressure too much the fin can start rising
out of the water and make the board no fun to handle.
Maybe not the most scientific answer but essentially the truth.

 
 
 

Fin size vs. sail size

Post by Bob Jacobso » Fri, 17 Apr 1998 04:00:00

You must do a great job with the birds and the bees.

Quote:

> Forget all that physics mumbo jumbo. Think of it this way- when the
> wind
> pushes on the sail you need something to push back against it (the fin
> ) or
> you`ll go sideways downwind. Ideally you want it to push back just as
> much
> as the wind is pushing on the sail. Water is 80 times or so denser
> than air
> so the fin can be 80 times smaller than the sail. If the fin is too
> small
> and can`t push back against the sail enough you get pushed downwind
> more
> than you`d like and go forward slower than you`d like. If the fin is
> too big
> and pushes back against the sail pressure too much the fin can start
> rising
> out of the water and make the board no fun to handle.
> Maybe not the most scientific answer but essentially the truth.

 
 
 

Fin size vs. sail size

Post by Alex » Sat, 18 Apr 1998 04:00:00

Quote:

> Changing the fore and aft placement ot the fin is not the same a
> changing size. More forward placement makes the board turn easier, rear
> placement makes for better high speed tracking.
> Bob Jacobson


> >         Also, what's the effect of placing the fin further appart from
> > the tail
> > of the board? Is it not the same asvarying the size of the fin?

> > regards, Alex

        I said so because I read in a magazine something like "that the actual
lift from the fin was determined by the railing factor which was the
distance between the mast base and the fin" -.the closer they are, the
more power of the sail that is directed naturally onto the fin and the
more lift you get from it -, so if you place your big fin in the rear,
(and even placing your mast a little forward as you separate from the
fin and also place more weight on the forward of the board keeping it
flatter and more controllable killing that extra lift you're getting
from a fin too big) the lift of the fin is decreased efectivelly and has
the same effect as placing a smaller one. The problem is that as you
say, you loose maneouvrability in turns.

regards, Alex

 
 
 

Fin size vs. sail size

Post by Bob Jacobso » Sat, 18 Apr 1998 04:00:00

Placing the mastbase further forward puts more pressure on the front of
the board and makes the board sail flatter. This counteracts the
tendency of the board  to tailwalk when over powered and also allows you
to point higher, since the additional waterline increases the lateral
resistance of the board. It also makes the board slower, since there is
more drag.

So increasing the distance between mastbase and fin can make the fin
sail smaller in terms of controllability, but it will also make the fin
seem bigger in terms of speed.

I think the best strategy is to start with finding the fin size you need
to point as high as you want with good speed. These are somewhat
subjective criteria, influenced by where you sail, and the kind of
sailing you do. (For instance, sailing the Gorge in westerleys doesn't
require a lot of pointing.) Use the mid position for the fin(s) while
you're testing for size. Once you've found the right fin size for
pointing, then adjust fin and mast foot placement to tune for the
turning characteristics you prefer.

Now, let's talk footstrap placement...........

Bob Jacobson

Quote:


> > >         Also, what's the effect of placing the fin further appart
> from
> > > the tail
> > > of the board? Is it not the same asvarying the size of the fin?

> > > regards, Alex

>         I said so because I read in a magazine something like "that
> the actual
> lift from the fin was determined by the railing factor which was the
> distance between the mast base and the fin" -.the closer they are, the

> more power of the sail that is directed naturally onto the fin and the

> more lift you get from it -, so if you place your big fin in the rear,

> (and even placing your mast a little forward as you separate from the
> fin and also place more weight on the forward of the board keeping it
> flatter and more controllable killing that extra lift you're getting
> from a fin too big) the lift of the fin is decreased efectivelly and
> has
> the same effect as placing a smaller one. The problem is that as you
> say, you loose maneouvrability in turns.

> regards, Alex

 
 
 

Fin size vs. sail size

Post by NLW TFW » Sat, 18 Apr 1998 04:00:00

Why do some of you say we're going faster on smaller sails? There are several
counterarguments to that:

1. Even (or especially) on flat water, we strive to be powered up all the time
(i.e., going AFAP). If we're not maxed out, and own a bigger sail, we're gonna
rig bigger.

2. The speed potential is bigger on smoother water (i.e., the water we
encounter more often at 20 knots windspeed than at 35 knots). Most of us do not
have access to sandspits giving us 35 knots of steady wind with glassy water.

3. Fla***er (i.e., big-sail) boards and fins are faster on flat water than are
roughwater boards and fins.

4. Rider skill and nerve are less critical to recreational sailing speeds on
flat water.

Thus we should be going faster on flat water, which usually means bigger sails.

So I suspect increased hydrodynamic lift due to greater speed isn't the main
issue. Because the fin's job is to oppose sail DRAG (downwind force), I'm
guessing that the drag on a big sail in its wind range is greater than the drag
on a smaller sail in ITS wind range, thus the smaller sail needs a smaller fin
to counteract its lesser drag. Certainly with a 5.0 and a 6.5 sailing side by
side (i.e., in the same wind), the bigger sail will be producing more drag,
thus needs a bigger fin to oppose that drag. (Rail shapes, hull lengths, rider
size, sail design, and technique ALL affect this comparison noticeably,
though.)

Another BIG factor is that at low sub-planing speeds, neither sails nor fins
develop much lift (lift = forward thrust in a sail and upwind thrust in a fin)
in our usual beam reach direction. Both develop primarily drag (downwind force
from the sail and rider, upwind resistance from the fin) at very low board
speeds in a beam reach, so a big sail overpowers a small fin and just shoves it
sideways initially.  unless we bear way off the wind, pamper that undersize fin
to greater speed until it can develop opposing lift, then turn upwind. We need
much more fin to get planing than we need to keep planing, so much of the need
for big fins with big sails is just in the first few meters or seconds, until
we're planing. Once up to speed and thoroughly powered up, we'd rather reduce
fin size unless our objective is WAY upwind.

So why not use a tiny 11" blade on a 270 race board, for greater speed? Plenty
of people do; I've ridden combos like that in major swell and chop, and it's a
HUGE rush. You feel like it's just you, the sail, and the wind -- like there's
almost no board under you, and certainly no drag. But with an insensitive foot
like mine, it can lead to frequent spinouts that cost a racer time. The more
skillful the rider, the smaller fin s/he can use successfully -- once it's on a
plane.

Life would be a lot easier, or at least faster, if we could retract our fin
vertically as we gain speed. Expose all 45 cm until planing, then shorten it as
we speed up from there until we're showing only half that at top speed -- maybe
30 if pointing, 20 if beam reaching. A big fin is essentially like a very low
first gear in a race car. Until we get dynamically variable fin sizes, we must
either live with big fins (i.e., a low first gear) to get big sails moving, or
learn to get moving without swamping the much smaller fin that is quite
sufficient at 25-30 knots board speed.

Either that, or I'd realize that this is all wrong if I were willing to spend
more time actually THINKING about it.  

Mike \m/
Never Leave Wind To Find Wind

 
 
 

Fin size vs. sail size

Post by sailquik (Roger Jacks » Sat, 18 Apr 1998 04:00:00

Quote:
>Placing the mastbase further forward puts more pressure on the front of
>the board and makes the board sail flatter. This counteracts the
>tendency of the board  to tailwalk when over powered and also allows you
>to point higher, since the additional waterline increases the lateral
>resistance of the board. It also makes the board slower, since there is
>more drag.

I agree here, in concept, but there is one big problem you haven't
mentioned which can get so big as to make moving the MFP a huge problem.
As the MFP comes forward, it "drives" (and I do mean DRIVES) the rocker of
the board, forward of the planing flat aft of the front footstraps, into
the chop, and this sets up a viscious "washboard effect" which can result
in less control than simply being over finned.
Compensating for too much fin, other than an immediate trip back to the
beach for a smaller or curvier planform, to solve the "too much fin"
problem, makes absolutely no sense to me.
Why would anyone "want" to sail a board/rig/fin combination that's
not working/unbalanced.
If you are a beginner, and have never experienced anything but the one fin
that came with your board, it's OK, as you do not know any better, YET!

But if you are an experienced sailor, and have a few fins, and know how to
change boards/rigs/fins to give you improved control/speed/comfort as
conditions change, I really don't know what to say, without getting myself
in troublr.

Quote:
>So increasing the distance between mastbase and fin can make the fin
>sail smaller in terms of controllability, but it will also make the fin
>seem bigger in terms of speed.

Not sure how this computes, but if you are saying that moving the MFP
forward adds some control, I agree. But it does so at the expense of speed,
maneuverability, and overall ride smoothness, which is far too much to give
up, considering how long it takes to change fins.

Quote:
>I think the best strategy is to start with finding the fin size you need
>to point as high as you want with good speed. These are somewhat
>subjective criteria, influenced by where you sail, and the kind of
>sailing you do. (For instance, sailing the Gorge in westerleys doesn't
>require a lot of pointing.) Use the mid position for the fin(s) while
>you're testing for size. Once you've found the right fin size for
>pointing, then adjust fin and mast foot placement to tune for the
>turning characteristics you prefer.

Unless we are talking wave or B&J boards, moving the MFP forward is only
something I consider in 3 situations on a shortboard.
#1 If I can't get on a plane, sometimes moving the rig forward will help
get the weight forward and the board up out of the water. But this is in
the most marginal conditions, and I can do better, in most situations by
pumping the board up over it's ow bow wave and leaving the MFP back where
it's efficient and the designed planinf "flat" works on the water at the
most effiient angle.
#2 If I really, really, need to go upwind, and want to present as much rail
(waterline length) as possible, but I don't care about the speed.
#3 If the wind has increased to  point where the board is tailwalking and
I'm spinning out and out of control, moving the MFP forward will give me
enough control to get back to the beach to rig gear more sutable for the
conditions. Moving the MFP forward kills the speed, gives a terrible ride,
but can allow me to stay more in control on my way in.

Quote:
>Now, let's talk footstrap placement...........

Except for the training position, all the way back, with a comfortable
spread between the front and back straps. Back footstrap, rear***even
with or a bit behind the leading edge of the fin.
If I can't get planing, stay planing with the straps in this position, I
need a bigger board!
later
sailquik (Roger Jackson) |Ph # in Md. (301) 872-9459
Lvl 1 WS Instructor      |    in N.C. (919) 995-3204
F2/Sailworks/North/True Ames/Rainbow
 
 
 

Fin size vs. sail size

Post by Bob Jacobso » Sat, 18 Apr 1998 04:00:00

Mike,

If Sail lift is in the forward direction of the board, the drag has to
be in the opposite direction, toward the rear. so the lift of the fin
(upwind, or perpendicular to board direction) can't be counteracting the
sail's drag.

But, enough nit picking. The lift of the fin opposes the side force of
the sail. A bigger sail will produce a greater side force at a given
wind speed and board speed (say 20 knots wind speed and 20 knots board
speed) than a smaller sail. Therefore, either a larger fin or a greater
angle of attack on smaller fin will be required. If the fin is too
small, the angle of attack gets so great that the fin stalls. Even if it
doesn't stall, at high angles of attack the induced drag of the fin is
very high, so a larger fin operating at a lower angle of attack will be
faster.

Adjustable fins have been tried, but there was a problem with sticktion:
the side force on the fin is so great that the fin is wedged in the fin
box. To pull the fin up board speed had to be reduced drastically. Not
the way to win races. We're still waiting for someone to come up with a
way around that problem.

Bob Jacobson

Quote:

> Why do some of you say we're going faster on smaller sails? There are
> several
> counterarguments to that:

> 1. Even (or especially) on flat water, we strive to be powered up all
> the time
> (i.e., going AFAP). If we're not maxed out, and own a bigger sail,
> we're gonna
> rig bigger.

> 2. The speed potential is bigger on smoother water (i.e., the water we

> encounter more often at 20 knots windspeed than at 35 knots). Most of
> us do not
> have access to sandspits giving us 35 knots of steady wind with glassy
> water.

> 3. Fla***er (i.e., big-sail) boards and fins are faster on flat water
> than are
> roughwater boards and fins.

> 4. Rider skill and nerve are less critical to recreational sailing
> speeds on
> flat water.

> Thus we should be going faster on flat water, which usually means
> bigger sails.

> So I suspect increased hydrodynamic lift due to greater speed isn't
> the main
> issue. Because the fin's job is to oppose sail DRAG (downwind force),
> I'm
> guessing that the drag on a big sail in its wind range is greater than
> the drag
> on a smaller sail in ITS wind range, thus the smaller sail needs a
> smaller fin
> to counteract its lesser drag. Certainly with a 5.0 and a 6.5 sailing
> side by
> side (i.e., in the same wind), the bigger sail will be producing more
> drag,
> thus needs a bigger fin to oppose that drag. (Rail shapes, hull
> lengths, rider
> size, sail design, and technique ALL affect this comparison
> noticeably,
> though.)

> Another BIG factor is that at low sub-planing speeds, neither sails
> nor fins
> develop much lift (lift = forward thrust in a sail and upwind thrust
> in a fin)
> in our usual beam reach direction. Both develop primarily drag
> (downwind force
> from the sail and rider, upwind resistance from the fin) at very low
> board
> speeds in a beam reach, so a big sail overpowers a small fin and just
> shoves it
> sideways initially.  unless we bear way off the wind, pamper that
> undersize fin
> to greater speed until it can develop opposing lift, then turn upwind.
> We need
> much more fin to get planing than we need to keep planing, so much of
> the need
> for big fins with big sails is just in the first few meters or
> seconds, until
> we're planing. Once up to speed and thoroughly powered up, we'd rather
> reduce
> fin size unless our objective is WAY upwind.

> So why not use a tiny 11" blade on a 270 race board, for greater
> speed? Plenty
> of people do; I've ridden combos like that in major swell and chop,
> and it's a
> HUGE rush. You feel like it's just you, the sail, and the wind -- like
> there's
> almost no board under you, and certainly no drag. But with an
> insensitive foot
> like mine, it can lead to frequent spinouts that cost a racer time.
> The more
> skillful the rider, the smaller fin s/he can use successfully -- once
> it's on a
> plane.

> Life would be a lot easier, or at least faster, if we could retract
> our fin
> vertically as we gain speed. Expose all 45 cm until planing, then
> shorten it as
> we speed up from there until we're showing only half that at top speed
> -- maybe
> 30 if pointing, 20 if beam reaching. A big fin is essentially like a
> very low
> first gear in a race car. Until we get dynamically variable fin sizes,
> we must
> either live with big fins (i.e., a low first gear) to get big sails
> moving, or
> learn to get moving without swamping the much smaller fin that is
> quite
> sufficient at 25-30 knots board speed.

> Either that, or I'd realize that this is all wrong if I were willing
> to spend
> more time actually THINKING about it.

> Mike \m/
> Never Leave Wind To Find Wind

 
 
 

Fin size vs. sail size

Post by Bob Jacobso » Sat, 18 Apr 1998 04:00:00

Quote:

> >So increasing the distance between mastbase and fin can make the fin
> >sail smaller in terms of controllability, but it will also make the
> fin
> >seem bigger in terms of speed.
> Not sure how this computes, but if you are saying that moving the MFP
> forward adds some control, I agree. But it does so at the expense of
> speed,
> maneuverability, and overall ride smoothness, which is far too much to
> give
> up, considering how long it takes to change fins.

Sorry, I wasn't too clear here. What I meant by seeming bigger in terms
of speed was slower, due to the higher drag.

Quote:

> >I think the best strategy is to start with finding the fin size you
> need
> >to point as high as you want with good speed. These are somewhat
> >subjective criteria, influenced by where you sail, and the kind of
> >sailing you do. (For instance, sailing the Gorge in westerleys
> doesn't
> >require a lot of pointing.) Use the mid position for the fin(s) while

> >you're testing for size. Once you've found the right fin size for
> >pointing, then adjust fin and mast foot placement to tune for the
> >turning characteristics you prefer.
> Unless we are talking wave or B&J boards, moving the MFP forward is
> only
> something I consider in 3 situations on a shortboard.
> #1 If I can't get on a plane, sometimes moving the rig forward will
> help
> get the weight forward and the board up out of the water. But this is
> in
> the most marginal conditions, and I can do better, in most situations
> by
> pumping the board up over it's ow bow wave and leaving the MFP back
> where
> it's efficient and the designed planinf "flat" works on the water at
> the
> most effiient angle.
> #2 If I really, really, need to go upwind, and want to present as much
> rail
> (waterline length) as possible, but I don't care about the speed.
> #3 If the wind has increased to  point where the board is tailwalking
> and
> I'm spinning out and out of control, moving the MFP forward will give
> me
> enough control to get back to the beach to rig gear more sutable for
> the
> conditions. Moving the MFP forward kills the speed, gives a terrible
> ride,
> but can allow me to stay more in control on my way in.

What I mean here was that assuming the distance from mastfoot to fin is
correct, when the fin is moved forward the mastfoot should be moved
forward the same amount to maintain the relationship. I agree with your
three reasons above. You are right, changing to the right fin is the
best course of action.

Quote:

> >Now, let's talk footstrap placement...........
> Except for the training position, all the way back, with a comfortable

> spread between the front and back straps. Back footstrap, rear***
> even
> with or a bit behind the leading edge of the fin.
> If I can't get planing, stay planing with the straps in this position,
> I
> need a bigger board!

Mostly true for slalom sailing. However, if you do a lot of upwind
sailing, then a more forward position would be more comfortable. And I'm
not so sure about wave sailing. More forward straps may be better there
for some conditions.

Bob Jacobson

- Show quoted text -

Quote:
> later
> sailquik (Roger Jackson) |Ph # in Md. (301) 872-9459
> Lvl 1 WS Instructor      |    in N.C. (919) 995-3204
> F2/Sailworks/North/True Ames/Rainbow

 
 
 

Fin size vs. sail size

Post by NLW TFW » Sun, 19 Apr 1998 04:00:00

RE:"If Sail lift is in the forward direction of the board, the drag has to
be in the opposite direction, toward the rear."

Aerodynamic (sail, body) or hydrodynamic (fin and hull) drag, by definition, is
downstream, to my understanding. So sail drag pushes us straight downwind at
very slow (initial) board speeds, and fin drag resists that. As board speed
increases,  apparent wind and true wind directions diverge, confusing the
issue, and there are even different types of drag (e.g., parasitic, induced,
and probably more). In general,  hydrodynamic fin thrust (lift) points upwind
to oppose sail drag, while sail thrust points "forward", ideally (it's
inefficient if it points other than in the direction we're sailing). Also, sail
thrust and fin thrust direction depend on several factors, which fill whole
textbooks I've hardly skimmed. Because I just sail the stuff, not design it, I
haven't taken the time to study it. It just makes interesting bookstore
browsing.

As board speed increases, the apparent wind tends to head us, and net sail drag
and fin drag swing more towards our wake, opposing the sail's forward thrust to
ultimately limit our board speed.

Mike \m/
Never Leave Wind To Find Wind

 
 
 

Fin size vs. sail size

Post by TS » Sun, 19 Apr 1998 04:00:00

Have to correct you here. Density of air is 1,293 kg/m3, for water it is 997
kg/m3. The ratio between air and water is approximately 770:1. Your
assesment is not valid, but the conclusion close.

TS


Water is 80 times or so denser than air

Quote:
>so the fin can be 80 times smaller than the sail.