The relationship between displacement, power and performance in Open 60 design

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To be clear and in simple terms without referring to GZ curves other naval architecture terms (readers can Google or read any numbers of books to understand in more depth):

Displacement is the equivalent to the mass or weight of a yacht measured in tonnes or kg. It’s a measure of it’s lightness but this is only one part of the picture. As well as knowing a yacht’s mass it’s important to know the position of that mass relative to the yacht’s waterline and centreline. Two boats with the same hull, draft, sail area and the same displacement could have completely different performance characteristics. Imagine Boat A weighs 8,000 kg and Boat B weighs 8,000kg, in a moderate breeze (ie when fully powered up) if Boat A’s keel/bulb weighs 4000 kg and Boat Bs weighs 3000kg then all things being equal everyone would understand that Boat A is going to be quicker. Note that in light airs, perhaps downwind in some conditions then Boat B might be very slightly faster for a short time on account of a larger bulb and keel fin having more wetted surface area.

Power can also be described in terms of righting moment, stiffness (ie Boat A is stiffer than Boat B) and it is a term that describes a yachts ability to carry sail. In terms of Boat A and Boat B described above, Boat A is more powerful, is stiffer, has a higher righting moment, than Boat B.  Righting moment is measured as kgm, that is; as a lever arm, kilograms multiplied by metres (displacement multiplied by the value GZ)

Performance is really a subjective term and can have any number of meanings. In this article we’ll consider it to mean straight line boat speed. There are many other factors involved in calculating the straight line speed of a racing yacht other than displacement and power. We’ll deal with some of these in future articles. For the purpose of this paper we’re going to simplify the relationship and not consider other factors such the effect on overall performance of different course types, weather conditions or the advantages/disadvantages in drag terms of different hull, keel and mast types.

The figure above is of the same Open 60 in three configurations and shows in simplified terms which ignores the vertical shift in CG (center of gravity, or center of mass) and uses only the horizontal distance between it and CB (center of buoyancy of the heeled hull) how canting keel and adding water ballast increases the power/righting moment of an Open 60.

For the sake of argument let’s say the sailing/racing displacement of the yacht is 9 tonnes un-ballasted and 13 tonnes with water ballast. The difference we can see between the first two figures is how the distance between CB and CG increases as the keel is canted. No surprise there, as the keel rotates to windward the 3 tonne bulb moves further outboard and as a result the center of gravity of the boat as a whole moves outboard and upward. The horizontal distance between the CG and CB has increased by @40% and so the righting moment has increased. When we add water ballast the difference between CG and CB increases again to a much lesser extent, but the yacht’s displacement has increased by @45% which causes another significant increase in the righting moment of the yacht.

We can’t just have any old bulb weight or add whatever water ballast we want into an Open 60 though; we’re constrained by some stability rules written into the IMOCA 60 box rule. The three figures above highlight three quite different lines of development undertaken by us over the years and work well to explain how the ten degree rule works and its affect on a yachts displacement and available power.

The IMOCA rule requires that in measurement condition with the keel fully canted and the water ballast on the ‘wrong side’ the yacht shall not heel more than ten degrees. Hexagon was a development of Kingfisher and the first of our designs to benefit from a fully PBO rigged mast. Launched within two years of Ecover 2, our widest ever open 60, these two highlight more than any other the potential to choose a ‘light or powerful’ design within the box rule. For a given keel angle the wider boat is able to carry a heavier bulb without exceeding ten degrees of static heel. Another way to look at it is that given the same weight bulb the wider beam boat is able to cant her keel further. Looking at the diagrams we can see that this is the case: Ecover 2 has a greater keel cant (and in reality also had a slightly heavier bulb) than Hexagon. Also a function of the wider waterline beam, Ecover 2 was also able to carry within the limits of the ten degree rule a greater volume of water ballast. The result was that although almost half a tonne heavier in measurement condition than Hexagon, on a Vendee Globe and every other tans-oceanic race course the heavier Ecover 2 was faster.

 OCD 60 development
                                                                            
 Launch/
re-launch
 Disp
  (tonnes)
RM
(kgm)
Beam
 (m)
 Keel
(material/type)
  Mast
  (type)
 Hexagon  2002  7.98  26,500  5.35  Fab steel  Classic
 Ecover 2  2004  8.45  29,000  5.7  Fab steel  Rotating
 Mirabaud (ex Temenos 2)  2006/10  8.7/8.45  32,000  5.55  Carbon  Classic
 Gamesa (ex Ecover 3)  2007/10  8.35/8.03  33,500  5.6  Carbon  Classic
 Acciona 100% Eco-Powered  2011  8.06  32,000  5.6  Fab steel  Classic
 OCD conventional 2012  NA  7.8  32,000  5.6  Carbon  Classic
 

For the 2008 Vendee Globe IMOCA instituted an additional rule involving a value known as AVSwc (Angle of Vanishing Stability, worst case) that effectively reinforced the connection between bulb weight, waterline beam, and the volume of water ballast one could carry. For our new boats, in consultation with our clients we had arrived at a maximum righting moment of @ 33,500 kgm and a displacement in the order of 8,300 kg as a target value. There were however several boats that pushed this powerful is faster theory further than anyone had expected. Further than many thought was reasonable for a lone sailor to handle. The extremes within the box rule for that edition of the Vendee Globe were  illustrated by:

Safran with a righting moment we believe somewhere in the region of 28,000 kgm and a displacement of less than 8,000 kg.

Pindar with a righting moment of over 40,000 kgm and a displacement of over 10,000kg and by far the widest hull and heaviest bulb of the fleet.

How can there be such a difference in concepts based on the same rule and who was right? Each yacht had its supporters; each one was high performance on paper. The reality though was that in terms of the kind of performance single-handed sailors require, high averages, Pindar was just too powerful for one person to handle. A lower righting moment, lighter boat was always going to be a better fit for single-handed racing but where to set the level?

 VG 2012 comparison Launch  Disp
(tonnes)
RM
(kgm)
Beam
(m)
Keel
(material/type)
Mast
(type)
 Macif 2011 7.7 <30,000 5.7 Fab steel Rotating
 Virbac 2010 7.8 <30,000  5.7 Fab steel Classic
 Banque Populaire  2010  7.8 <30,000  5.8  Stainless steel  Rotating
 Cheminees Poujalat  2011  8.5 32,000  5.8 Not available Classic
 Hugo Boss 2007/11 8 <30,000  5.85 Fab steel Classic
 Gamesa 2007/10 8.32/8.03  33,500 5.6 Carbon Classic 
 Synerciel 2007 8.7 29,500 5.8 Not available Classic 
 Mirabaud 2006/10 8.5 32,000 5.5 Carbon Classic 
 Acciona 2011 8.06 32,000 5.6 Fab steel Classic
 

The introduction of the AVSwc rule for the 2008 race followed in 2009 by the imposition of a maximum righting moment of 32,000 kgm and maximum mast height of 29m above waterline has constrained the designers box hugely. There will be no more boats like Ecover 3, above the 32,000 kgm limit, but not all the designers of the new boats have decided to go to that limit. While Juan K (Cheminees Poujalat) and Owen Clarke (Acciona) have boats at the limit of power, Guillaume Verdier and his skippers have chosen differently as illustrated in the table above.

While we don’t have specific numbers, specifications do leak out as we all share the same suppliers, meet crews at race starts and eventually a picture, but perhaps not the complete picture is able to be formed. In any case, the table above of the leading boats in the Vendee, plus Acciona (that made a pit stop early on) show the range of displacements and righting moments of IMOCA’s top Open 60s. The earlier Verdiers such as PRB and Safran have slightly lower righting moments in the 28,000 kgm range, while the very latest boats we believe are 29,000 kgm.  How does this come about?

Obviously there will be some differences in the engineering hardware and general layout of the boats that will contribute to different displacements. However these aren’t going to greatly affect the righting moment of the boat and so I believe that it’s more than likely that the Verdier designs are using less water ballast, not maximising the size of the tanks within the rule, in turn, because of the way that the AVSwc rule works this then permits a lighter bulb. In this way the reduction in displacement also correlates with a reduction in righting moment and slightly more reliance on the yacht’s form stability to increase stiffness.

So which option is correct? Well, as we developed our knowledge of how the Open 60 rule worked and the ways to best optimise within it, we became convinced about two points:

  • ·          On an average or Vendee Globe course all things being equal a higher righting moment boat was always quicker than a lower righting moment one in any apples for apples comparison.
  • ·          That it was more efficient and less drag to get that righting moment by having the lowest drag and so narrowest waterline beam hull form possible in the un-ballasted condition and then top up the power when required by adding water.

That didn’t stop Mike Golding however asking in 2003 for a lighter wider boat when shown this information for the first time. The reality is that the difference isn’t that great and on some points of sail and wind speed the lighter boat is quicker. The tank and CFD doesn’t tell us the whole story either. It’s probably true that a slightly lighter boat is gets on the surf a little earlier and perhaps takes that surf a little longer, accelerates a little quicker in fluctuating breeze etc. It’s also natural for a sailor to want to gravitate to the lighter option with the less complex ballast system which is what Mike saw with Ecover 2. The reality is that over a Vendee Globe course the advantage in the context of that race is small, a little over a day between comparable designs.

In 2007 it was decided that we would make a moderate increase in the righting moment for Mike’s newest Open 60 and we would achieve the maximum righting moment using water ballast. Ecover 3 would be 100kg lighter than her forerunner but with a bulb that was some 300kg heavier, a clear demonstration that it’s pointless talking about weight unless there is some reference to power. Although only 100kg lighter when built the performance of Ecover 3 over Ecover 2 is an order of magnitude greater.

For the design of Acciona (her water ballast tanks are illustrated above) we took the concept a stage further. With the new rule  limit of 32,000 kgm  placed on righting moment, after a further in-depth study of the Vendee course using Router we decided that Acciona would use her full allocation of righting moment for reaching and running conditions by only having tanks aft of the mast.

I’ve used the term “ all things being equal” a number of times in this article. Obviously there are many complex factors affecting the performance of a racing sailboat. In future articles dedicated to Open 60 design we’ve a range of topics we can explore including the merits of rotating verses classic spars, curved and straight daggerboards and different keel types. Interesting to note the different keel types in our first table which exemplify again that when talking about weight we need to understand about VCG and power.  On equivalent boats the difference in the combined fin and bulb weight of a fabricated steel fin compared to a carbon fin would be more than 200kg, but the difference in performance measured in a few hours. Those few hours or perhaps just the thought of the extra weight would be important to one skipper while another will make a different choice based on a range of inputs; overall displacement, cost etc.

Finally, going back to the original remit: to look at the relationship between displacement, power and performance in Open 60 design. I hope that this article without getting high bound into VPP, numbers etc. has been helpful and hopefully clear enough so that when someone in the bar or in a magazine article waxes lyrical about the advantages of power and/or a light boat (depending on their view point) you can be the devils advocate and make them aware of an alternate reality. After all, although there are clearly a lot of factors at play in the current edition of the Vendee Globe, the top four boats are all very close together after a range of conditions and seven thousand miles sailed. We know Bernard’s boat weighs 8.5 t as she was weighed at the same time as Acciona, nevertheless she at present is leading a group of Verdier Open 60s that displace in the range of 7.7 to 7.8 tonnes. Either way there’s not a great deal in it providing you’ve done a good job of designing all the other aspects of the boat that make it go fast !
 

For more information on the design process go to: Naval Architecture


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