Magic wands - Wand control systems on Hydrofoil moths

Magic wands - Wand control systems on Hydrofoil moths
(c) Bruce McLeod 2009
The wand system and how it operates is probably the single biggest determinant of the
performance of a international moth. It plays a very significant role in how the boat will
operate whilst foiling and how fast it will go.
The idea of this document is to provide a good base of information for people who are new
to the class to understand the intricate details of the moth wand system and how it can be
changed. What this document wonʼt do is provide you with a guide on how to make your
boat faster, or give you settings to follow. It is hoped that armed with this information any
moth sailor will be able to understand their boats better and be able to tune them to suit
their own style.
The basics
The wand system, as used by most boats consists of a thin fibreglass or carbon fibre rod
or tube that is attached to a pivot point at the front of the hull. The wand is then connected
to a control rod or cable that connects to a bell-crank in the top of the centreboard. The
bell-crank also connects to the push rod, the other end of which is attached to the
centreboard flap.
The history
The first wands that were used by the majority of the fleet
were those supplied by John Ilett wither with his prowler,
or as part of his foil-kits. These consisted of an aluminium
tube, bent at 45º, into which a flexible fibreglass tip was
inserted.
These wands did their job, and the boats foiled. Sailors at
this stage were mainly focusing on how to sail the new
hydrofoil boats around the course, and little thought was
given to improving the system, let alone getting maximum
speed from the boats.
The launch of the Bladerider presented a significant step
forward with straight, stiffer wand, different gearing and reducing the amount of travel and
play in the linkages, which resulted in a smoother ride and overall a more controllable
boat.
Wand “Gearing”
Sometime in the class you hear people refer to their “gearing”. In moth parlance gearing
refers to the speed and distance that the centreboard flap moves in relation to wand
movement.
Most manufacturers ship with a default 1:1 gearing ratio. This means that the bell crank
pivot length is the same as the bell crank pivot height, as shown in the diagram below.
Bell-crank
pivot
height
Bell-crank
pivot
length
How this translates in practice is for every mm that the
wand control rod moves forwards and aft, the centreboard
flap moves up and down the same amount.
Changing the flap gearing is one of the areas where you
can tune the behaviour of your boat. However like most
things any change here a exercise in compromise.
If you shorten the bell crank height, you will increase the
amount the flap moves for a given amount of wand
movement, however you will also reduce the amount of
force that the wand exerts over the flap.
Conversely if you lengthen the bell crank height, you will
decrease the amount that the flap moves for a given
amount of wand movement, and you will increase the amount of force that the wand exerts
on the flap.
The overall size of the bell-crank also has an effect on the amount of force exerted, a
standard Prowler bell-crank has, for example has lengths of 25mm, whereas a standard
Bladerider has lengths 40mm, so a standard Bladerider bell-crank will apply a smaller
force, but move the flap through a larger range than a standard Prowler bell-crank for a
given amount of fore and aft control rod movement.
The best way to think of how this works is to compare the amount of effort that you need to
move a door pressing close to the hinge, as compared to holding onto the doors handle.
Near the hinge you need to press very hard, but you donʼt need to move the door very far.
At the handle you donʼt need to push very hard, but you need to move the handle a lot
further to close an open door.
Another effect of changing the bell-crank size is that it changes the amount of range that
the wand moves thorough, assuming that the flap movement down is limited by the control
rod fore and aft movement and flap up is limited by the size of the centreboard cut out.
An example of the kind of tweaking you can do was blogged by Rohan back in January
2008, and is also covered in the Bladerider high performance guide, available on their
website.
“Recently we have been removing the hump and cutting up the bottom/back edge of the
centreboard about 2mm so the flap can come up more (hence have more negative lift
when the boat gets too high), but the problem with that is that the wand can travel a lot
further forward now and consequently the boat bounces up and down a lot when the wand
gets pulled past vertical at full height and then gets gets pushed back 90 degrees when
the boat comes back down. [Changing] the gearing [will] stop this problem you can easily
remove the bell crank and cut about 4mm off the top (with a grinder), but you will need a
5mm tap to clean up the thread after it is done. This should increase the speed of travel of
the push rod running down the centreboard and move the flap faster, and not make the
wand flick forward so much.” - Rohan Veal 3rd January 2008
Wand range
The amount of movement in the centreboard flap is the determining factor on how big an
angle the wand can move through, unless it is otherwise limited. This can be changed by
moving the location of the wand pivot on the wand, or changing the bell crank ratios to
something other than a 1:1 ratio.
Related
movement
angles
Neutral setting, wand length and ride height
The default neutral setting used by most boats is to have the flap in the middle position
when the wand is at 45º relative to the bow, and the centreboard flap not being deflected
up or down.
Flap neutral
Neutral setting is something that is statically adjustable on all boats. This is adjusted by
changing the length of the control rod that connects the wand to the top of the foil. If the
rod is lengthened then the flap will be slightly down when the wand is in itʼs 45º position,
which will give the boat more lift for a static wand length. Also, the wand may naturally sit a
bit further forwards (depending on foil loads and shock cord tension) As a result the
effective length of the wand will be longer, and as a result the boat will fly higher.
Foil Interference
One of the stranger things that occurs with a foiling moth that does not occur with such an
impact on any traditional, non foiling boat is the phenomenon of foil ventilation. This
problem occurs when the attached flow to the sides of the rudder vertical breaks free, and
in the extreme case, travels all the way down to the rudders horizontal foil, causing a
drastic loss in pitch stability and throwing the boat in to a violent capsize. An example of
this can be seen in this video: http://www.youtube.com/watch?v=r_FKCD2nNWE
The photo above shows the sort of interference that a standard straight tube or tube like
wand can place on the front hydrofoil, which adds to this interference pattern ahead of the
rudder vertical. When all three are aligned at high speed, this is a significant contributing
factor.
One of the commonly seen additions to the wand is a carbon fibre “paddle” at the end of
the wand, which increases the wand force that is applied to the centreboard.
It also causes the wand to more accurately track the surface of the water, and significantly
reduces any below the surface interference caused by a paddle-less wand. Wand paddles
are now “standard” on the Prowler Zero and Mach 2.
Adjustable ride height
Currently in the fleet there are two methods that are being used to adjust the foiling height
out of the water.
Pioneered by David Lister, a “dial” adjusts the flap neutral
position by allowing the wand control rod length to be
changed whilst sailing. The dial came in to widespread use
throughout the fleet after 2009 world champion Bora Gulari
also used one on his way to winning the world
championships.
Another method is to adjust the length of the wand on the
fly. There are a number of different approaches that have
been developed for this. Luka Damic developed a “Luka
Wand” that uses a telescopic system with springs, to allow the length of the wand itself to
be changed. A similar system was also developed in the United Kingdom that allows the
same adjustment whilst sailing.
Changing wand length adjusts ride height
Changing neutral position adjusts ride height
The flick-off
Changing from a bent to a straight wand allows an extra setup option to be used. This is a
dual-mode wand that can aggressively bring the boat back down in extreme
circumstances. The diagram below shows a moth being sailed in waves where it has been
setup to allow the wand to flick off.
When the boat starts to rise up too high, the wand will initially travel from the neutral
position (the green line) to vertical (the white line). This should be where the wand
operates normally. In extreme circumstances that would otherwise lead to a break-out
crash, the wand will loose contact with the surface and flick forwards aggressively, The
wand can almost instantaneously travel from vertical (the white line) through to the actual
position of the wand in this photo.
This will aggressively add more-flap up, and may even cause the foil to stall, crashing the
boat back down again. This is an option, not a necessarily recommendation, as there are
some circumstances where this setup may be horrible. In an aggressive, steep chop for
example the boat could porpoise up and down with every wave peak and trough as the
wand flicks-off, then the boat drops, then the wand re-engages, then the boat rises again,
then as you pass the next trough the wand will flick off again. The reason this shouldnʼt be
used with a bent wand is because a it can get caught around the bow.
The offset wand problem
When the wand is installed offset
on the starboard side of the hull,
this introduces a slight difference
in ride height on port and
starboard tack, when the boat is
healed going upwind, as shown in
the diagram below.
Starboard Tack
Port Tack
Several ideas have been used to
try and solve this problem, such
as using dual wands, or angling
the wand inwards onto the
centreline of the boat. Most
skippers however simply live with
the difference and either make a
small rudder trim or adjustment to get a little more lift, 2008 World Champion John Harris
had this to say on the UK website1 “[I adjust the rudder] a little bit tack to tack (more bow
down on s/board).”
Sources:
All images in this document are either created by the author, or images marked as royalty
free from www.bladerider.com.au.
www.mothcast.net
1
http://www.int-moth.org.uk/NewPages/FoilerFettling.htm