| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 864.1 | IMS POLAR PLOTS | MILVAX::HO | | Tue Jun 14 1988 10:33 | 21 |
| The United States Yacht Racing Union in Newport publishes the IMS
polar diagrams for all the boats holding IMS certificates. This
is a circular plot of boatspeed vs wind angle. Copies are obtainable
at exorbitant cost from USYRU. I no longer have their price list
but details can be had by calling them. The plots have that
scientific look about them but are projections based on black magic
as opposed to empirical data.
Looking through the annual indexes for Yachting or Sail for articles
on "Target Boatspeed" or "Polar Diagrams" should yield data for
some representative types of boats - eg plots for keelboats,
catamarans, dinghies.
With some steady wind and a few hours time, one can generate a custom
plot for one's own boat using real data.
The cheap answer is that it's never the other guy's engine, rarely
the boat, almost always the person in the boat.
- gene ho
|
| 864.2 | USYRU plots are for "ideal" boats | GRAMPS::BAILEY | May the 4 winds blow u safely home | Tue Jun 14 1988 17:23 | 33 |
| Polar plots are also available from the folks who built your boat.
At least this was the case with us and J-boats. Our skipper obtained
polar plots for a computer program he's been writing to help us
predict things like jibing angles, VMG, and favored tacks during races.
However, these ARE based on computer modeling, and are therefore
only an approximation based on assumptions relative to the "ideal"
boat (what was referred to in .1 as "black magic"). I agree that
it's better to go out and take your own measurements if you have
the time, steady wind, and really want to customize the plots to
your particular boat. Most average "club racers" however could
probably do just fine with the USYRU versions.
Some of the variables that I know of that can affect the accuracy
of these plots relative to YOUR boat are things like the shape of
your keel and the cut of your sails. For example, we got a list
of offsets for the keel from J-boats also. When we made the templates
we discovered that there was a noticeable difference between the
ideal and the actual shape. Granted we've banged into rocks a couple
of times since the boat was new, but even the upper part of the
keel was off. Also, there is a noticeable difference in performance
between the two mains we have. The older, dacron main is a lot
baggier than the newer, kevlar main. And it actually outperforms
the kevlar off the wind. But the kevlar outperforms the dacron
upwind. So this would have an effect on the polars which is not
taken into consideration on the computer models.
I'm sure there are other considerations, these are just the ones
that we've discovered while playing around with the information
we got for our particular boat.
... Bob
|
| 864.3 | A "simple" matter of tuning? | ECADSR::FINNERTY | | Tue Jun 14 1988 17:45 | 7 |
|
As long as we're on the subject, both my main and genoa have lines
which run down the leech which can be used to modify sail shape
to some degree. Any suggestions for how to adjust them properly?
Jim (Irish Sloth)
|
| 864.4 | It's called a "leech line" ... | GRAMPS::BAILEY | May the 4 winds blow u safely home | Tue Jun 14 1988 20:01 | 9 |
| Leech lines are useful in heavy winds to keep the sail leech from
flapping around. The general rule of thumb is to only tighten it
down enough to keep the sail quiet. I'm not sure exactly what the
aerodynamics of the shape are all about, but I can tell you from
experience that overtightening it will cup the leech, spoiling the
airfoil shape of the sail, and slow ya down.
... Bob
|
| 864.6 | Machine-generated procedure + grain-of-salt | ECADSR::FINNERTY | | Wed Jun 15 1988 09:18 | 90 |
|
This comes under the "don't take this too seriously" category.
I wrote a machine learning program last year which takes a set
of individual data points and generalizes them into a procedure;
though there are no guarantees that its generalizations are
always correct. It even has some suggestions for leech lines
which I had obviously forgotten...
case (windspeed)
{
[light]:
{
case (tack)
{
[close-hauled]:
(cunningham loose)
(outhaul loose)
(main-sheet-tension enough-to-remove-twist)
(main-traveller centerline-or-above)
(boom-vang none)
[reach]:
(cunningham moderate)
(outhaul moderate)
(main-sheet-tension verge-of-a-luff)
(main-traveller at-least-two-thirds-leeward)
(boom-vang enough-to-remove-twist)
[run]:
(cunningham moderate)
(outhaul moderate)
(main-sheet-tension verge-of-a-luff)
(main-traveller at-least-two-thirds-leeward)
(boom-vang attach-to-rail)
}
(main-halyard loose)
(leech-line form-fair-leech-curve)
(jiffy-reef none)
}
[moderate]:
{
case (tack)
{
[close-hauled]:
(main-sheet-tension enough-to-remove-twist)
(boom-vang none)
[reach]:
(main-sheet-tension verge-of-a-luff)
(boom-vang enough-to-remove-twist)
[run]:
(main-sheet-tension verge-of-a-luff)
(boom-vang attach-to-rail)
}
(main-halyard tight)
(cunningham form-fair-leech-curve)
(leech-line very-loose)
(outhaul form-fair-leech-curve)
(main-traveller modest-weather-helm)
(jiffy-reef none)
}
[heavy]:
{
case (tack)
{
[close-hauled]:
(main-sheet-tension tight)
(boom-vang none)
[reach]:
(main-sheet-tension verge-of-a-luff)
(boom-vang enough-to-remove-twist)
[run]:
(main-sheet-tension verge-of-a-luff)
(boom-vang attach-to-rail)
}
(main-halyard tight)
(cunningham form-fair-leech-curve)
(leech-line very-loose)
(outhaul form-fair-leech-curve)
(main-traveller modest-weather-helm)
if (backwinding-of-main == to-battens)
(jiffy-reef two-reef-points)
else
(jiffy-reef none)
}
}
|
| 864.7 | More on leech lines ... | GRAMPS::BAILEY | May the 4 winds blow u safely home | Wed Jun 15 1988 09:19 | 42 |
| Re .5
Well, if the leech line is inaccessible when reefed, the only thing
I can tell ya is maybe you should have it rigged differently (not
a terribly expensive job for a sailmaker to do). On both our mains
the leech line is accessible from three points; right above the
clew, and right above each of the reefing points. What they do
is bring it outside the leech "pocket" and put a jam cleat there
so you can grab it and tug, then cleat it down when you have the
desired tension.
As to the leech line in the jib, yes it's a pain to set. Usually
because you have your leeward rail in or close to the water when
you need to set it, and you have to go down there to get at it.
What we did is to make the job easier we had the sailmaker put the
end of the leech line on a 10" strip of velcro, with velcro on either
side of the pocket that it sets in. So when you go down there
all you have to do is grab it, tug, and close the pocket around
the velcro strip (no messing with cleats which don't seem to hold
so well when the wind comes up anyway).
As to capturing more of the wind by cupping the sail, this DOESN'T
work. What you want to do is only use enough tension to keep the
leech quiet (thing sounds like a motor when you don't have enough
tension on it). Cupping the sail spoils the aerodynamics, and it
WILL slow you down.
Remember that a sail works like an airplane wing. You want the
air on the outside of the sail to travel farther than the air on
the inside. If you cup the sail, you're forcing the air on the
inside of the sail to travel farther than it has to, and decreasing
the efficiency of your airfoil. This isn't desireable if you want
to go fast. But I'm not really good at explaining these things,
so perhaps someone who's more learned in this area can explain it
better, or maybe you should read up on sail dynamics. It'll give
you a better picture of what I'm trying to say.
Bottom line ... don't just haul this thing in and crank it down
if you're interested in trimming for maximum speed.
... Bob
|
| 864.9 | Is adding an inboard track a good idea? | ECADSR::FINNERTY | | Wed Jun 15 1988 16:26 | 14 |
|
The only track for the jibsheet block/car on my boat is the toerail;
there's no inboard track as on many other designs. In light air
I'd guess that I'm spilling too much wind by having the block so
far from the main.
Can anyone make an informed speculation about whether adding a new
inboard track would allow me to point "significantly" closer to
the wind?
- Jim
|
| 864.10 | Temporary Block Extension | CSMADM::SCHLEGEL | | Fri Jul 01 1988 13:20 | 10 |
|
To determine whether a further inboard track would improve your
jib shape or performance, temporarily tie your block to a suitably
strong line between the track you have and the block. This is
essentially an extension between the block and rail. If this
improves performance, then you might consider a permanent extension
or re-design
|
| 864.11 | How high does your boat point? | ECADSR::FINNERTY | | Tue Jul 19 1988 23:58 | 39 |
|
re .1
USYRU plots currently cost $75, and include a notebook full
of measured and projected data. Not that I would have spent
that much for it anyway, but they haven't measured the Cat 25
yet.
re .0
this is probably obvious, but the ability to point into the
wind is about as important as waterline length at that point
of sail; the table below indicates the relative VMG to
windward for a range of pointing ability (including leeway):
Tacking Angle: 120 115 110 105 100 95 90
Relative Perf: .82 .87 .93 1.00 1.05 1.11 1.16
where relative performance is relative to a boat that tacks
through 105 degrees, including leeway; i.e. tacking through
90 degrees would improve performance by 16%.
Looking at waterline length vs performance:
LWL: 23 25 27 29 31 33
Relative Perf: 1.00 1.05 1.09 1.13 1.17 1.20
in other words, in the range of LWL and tacking angle that
you'll commonly see on any given weekend, pointing ability has
about as much to do with performance as boat length.
like i said, that's probably obvious, but then why is it so
hard (or so expensive) to get the data on which boat points
how high?
- Jim (Irish Mist)
|