Modifications to the
Ballast on the
The ballast situation is one that needs attention. The
instruction say to fill the keel bulb with lead shot. This
leads to a maximum amount of lead that can be cajoled into the
space as about 1500g-1600g With this amount the yacht is
very tender in winds over 10-12 knots. A much better value
is to aim for 1700g-1800g. Can this be achieved? The
answer is not if lead balls of fixed diameter are used as
mathematics can be used to show that only 65% or thereabouts of
available space can be filled if the spherical balls are packed
as closely as possible. As they are in fact placed in a
random fashion it is more likely that only 60% of available
space is used. You can check this by filling the space
between the lead with water until absolutely full then pour the
water into a measuring device and you will find about 60-70mls
of water is present which means 60-70mls of space was there.
If the space is left as air then this volume of air will produce
a buoyancy of 60-70g thus reducing the effective ballast by
60-70g - you thought you had 1500g of ballast - now in fact you
only have 1430g or thereabouts no wonder the Phantoms lie down
in anything more than a zephyr.
Here are two solutions
1. Replace the plastic filled shape with a moulded lead shape -
now there is no empty space no air buoyancy all lead is ballast
make it to 1800g
2. Fill the air space in the keel bulb with something heavy.
The best would be powdered lead mixed with resin. If the
resin and powder is warmed and the existing lead in the bulb
warmed then a 50-50 mix of resin and powdered lead should flow
enough to run into all spaces. It may be necessary to add
a few more holes to achieve this but as the whole space is
now filled with resin mix, this is no longer a problem as a
little filler can fair off any slight depressions once the resin
has set. My calculations estimate an additional 400-600g
of ballast could be added. I have been unable (unwilling?)
to source powdered lead but I have sourced powdered brass (SG of
8 compared to lead SG of 11.6). Despite the lower specific
gravity of brass it should be possible to get and extra 250-400g
of ballast into this space. The brass is about $40/kg so
is quite reasonable when shared 5 ways. Considering the
poisonous nature of lead particularly when powdered perhaps
brass doesn't sound so bad after all and still get the ballast
up to the 1800g.
Either of these two processes allow for another problem with the
Phantom which is that if the lead is spread evenly through the
bulb, the centre of gravity of the bulb ends too far forward.
This becomes more obvious as more weight is added and the yacht
is pushed harder in the stronger winds. Using a solid lead
bulb allows it to be positioned on the fin a little further back
(~10-15mm). When adding the powdered metal/resin mix it
can be added to the aft section first, continuing until enough
weight has been added. This will effectively shift the C
of G aft as required.
It has been suggested to Ben that the powder could possibly
added dry and with judicious shaking it should settle in the
spaces between the lead spheres. This might allow the mass
to be distributed better in the aft section of the bulb.
Maybe pure resin still needs to be poured in through the front
part of the bulb to ensure the brass stays put (maybe need a new
hole near the front to do this).
The effect of this added ballast will be to slightly decrease
the downwind speed in light airs but significantly improve its
upwind performance particularly as the wind picks up and as this
is where most gains are to be made in a yacht race for me there
is no doubt - 1800g is essential.
A current project is to make a lead ballast keel of about
1900g to see the effect on performance. The hull does not
appear to sink to any significant degree when extra weight is
added so making sure the ballast is aft rather than forward will
also alleviate the nose digging in to some extent. The
plan is to make a wooden plug, construct a split mould and pour
the lead in one go. Images and outcome over the next few
weeks. A mould has been produced and about 10 or so bulbs
produced. These are clearly effective in allowing the
yacht to sail in the higher wind range while not reducing the
speed in the lighter winds significantly. In accordance
with our specifications a maximum
ballast weight of 1900g has been established.
1800g of ballast is the minimum to allow the yacht to drive
effectively in a breeze 1900g seems about optimum as the hull
starts nosediving too readily at weights much above this.
Remember tho that the weight needs to be moved back from the
original bulb so that its C of G is about 15mm further aft.
This has transformed the potential of the yachts and is an
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