Hi guys,
Please excuse the
unnatural writing style but this is my first experiment with ‘speech
to text’ and it seems to be disrupting my line of thought.
It's nice to see
this thread with a bit of cutting and trying in it.
There are a number
of interesting points that I believe need closer examination and I
believe they come under a number of headings.
First the problem
with camber and the various panels.
Secondly a question
of twist in the sail.
Thirdly the shape of
the head of the sail being square or whatever.
And the laser dinghy
called Lores.
Colin’s Collins
observation that the position of the cambered panels affects the
performance of the sale is particularly interesting. I believe we
need more information before making a judgement or decision on these
facts.
I noticed that Colin
only sheets the lowest batten or boom and that he uses a kicking
strap to control the twist in the sail. I also notice or fail to
notice any sign of tell tails on the various panels on the sale so
that is difficult to determine whether the various panels are stalled
or not. Until it is clear that all the panels are in an un-stalled
mode it is difficult to assess the efficiency of the setup.
In some of his
writings Jan’s fellow countryman Tony Marchaj gave an indication of
how the wind speed decreased at lower levels due to the effect of
wind-shear , and I remember some years ago calculating that for the
Westerly Longbow Poppy where the bottom of the sail was some 10 feet
above the water and the head of the sail some 30 feet higher that the
twist in the relative wind when sailing close hauled was approximately
five degrees over the height of the sail. For a Laser dinghy where
the bottom of the rig is some four feet from the water and not such a
tall mast the relative wind change over the height if the rig would
possibly be something similar to the five degrees (this is only be a
rough estimate). However, this can have a big effect on the entry
angle of the relative wind to the sharp edge off the sail, and to the
performance of the flat cut panels in particular.
There is also the
question of what is a flat panel and what is a cambered panel. If we
think that 8% camber is fairly normal but remember that that is the
camber in the mid height of a panel and that the camber is zero along
the battens then the average camber over the height of the rig must
be something less like say 5% overall. Most people believe that you
need flat sales in stronger winds are therefore flatten the top
panels as they are considered to be the storm canvas.** Colin found
that with flat panels at the top and cambered panels at the bottom of
the rig when sheeted for best efficiency the performance was not as
good as with the cambered panels at the top of the rig and the flat
panels at the bottom.
I believe this can
be explained by considering the entry of the wind to the leading edge
of each panel by the twist of the relative wind. In the untwisted
sail the twisted wind when producing drive in the lower cambered
panels will be striking the upper flat panels are too steep an angle
and there will be knife edge separation and these panels will be
stalled, therefore these panels will produce negligible lift but
significant drag and degrade the lift/drag performance and windward
ability of the total rig.
With the cambered
panels at the top of the rig and the sheeting set for best
performance the lower flat panels will be receiving the twisted wind
at a lesser angle and will be less inclined to be stalled and
probably producing some lift and lesser drag and therefore a better
total lift/drag ratio of the rig and overall better performance than
with the panels arranged in the earlier order. To verify this it
would be necessary to attach tail tails to each panel and see what is
going on.
I believe best
performance would be achieved by having camber in all the panels and
with tell tales to assist trimming to make sure that all the panels
have attached flow and are not stalled.
I’ll post this and
continue later. (I’m not sure I like the ‘speech to text’ as
there’s still a lot of editing to do)
Cheers, Slieve.
** I have had a particular experience that makes me believe that storm canvas should not be flat, and I will try to remember to explain at a later date.