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Modifications to the Ballast on the Phantom Yacht

Rudders Chain Plates Mast fitting for shrouds forestay Mainsheet fairlead Water proofing the hatch Improving Ballast Replacing Shrouds 'B' rig sails Radio Setup Clew/Tack Adjustment Improved sheeting system Gooseneck repair


Improving Ballast

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.

Lead Ballast
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 essential modification.

page last edited on 24/04/2012