Based on Episode 5 of Catamaran Conversations with Phil Berman (Balanced Catamarans), Patrick Shaughnessy (Farr Yacht Design), and Herb McCormick
If you’ve ever gone down the rabbit hole of comparing cruising catamarans, you’ve probably encountered Bruce numbers thrown around on forums and social media as some kind of definitive performance metric. The reality, as the designers themselves will tell you, is a lot more nuanced than that.
In this installment of Catamaran Conversations, Phil Berman and Patrick Shaughnessy unpack the tools that actually drive catamaran design decisions — and why the shortcuts consumers rely on can be misleading.
The Bruce Number: Useful Shorthand or Oversimplification?
The Bruce number (attributed to Edmund Bruce, who also invented the Bruce anchor) is essentially a power-to-weight ratio. It takes your sail area — using a 100% foretriangle plus mainsail — calculates the square root, and divides it by the cube root of your displacement. The resulting figure gives you a rough sense of how much sail you’re carrying relative to your boat’s weight.
The appeal is obvious: it’s a single number that lets you line up boats side by side. The problem is what it leaves out. As Phil put it bluntly, the Bruce number is the same whether the catamaran is going forwards, backwards, or sideways. It doesn’t account for hull shape, appendage drag, the beam of the boat, or — critically for catamarans — how far apart the hulls are spaced. That spacing has an enormous impact on sail-carrying ability and stability, and the Bruce number ignores it entirely.
There’s also a slightly more sophisticated version called the Real Performance Index (RPI), which factors in the height of the sail plan’s center of effort. That’s a step in the right direction since it starts to capture how easily a boat can be tipped, but it still misses hull form stability and hull separation. Better than a Bruce number, but still a blunt instrument.
VPPs: The Designer's Real Tool
Where the actual design work happens is inside a Velocity Prediction Program. A VPP takes a detailed description of a boat — hull shapes, sail plan, appendages, weight distribution — and simulates performance across a range of conditions. Designers iterate through changes (add sail area here, adjust hull shape there), run the VPP after each modification, and compare results against a baseline until they converge on the performance profile they’re after.
Patrick noted that only a handful of commercially available VPPs are widely trusted in the marine industry, and that top-tier design offices often run proprietary versions refined through campaigns like the America’s Cup and Volvo Ocean Race. The key point is that a designer needs to deeply understand the tool’s assumptions and limitations in order to trust its output.
And those assumptions matter. A VPP assumes perfect sail trim, a clean bottom, an ideal skipper, and flat-water conditions unless told otherwise. Every one of those assumptions creates a gap between predicted and real-world performance.
The Displacement Problem
One of the most important takeaways from this conversation is how much the displacement figure you use changes everything — in Bruce numbers, in VPPs, and in the polar diagrams that come out of them.
European manufacturers commonly publish figures at “lightship” displacement, which is basically the bare boat with nothing on it. As Phil pointed out, nobody has ever actually launched a lightship boat. Every customer orders what Balanced Catamarans calls the Essential Voyager package: multiple sails, air conditioning, refrigeration, watermakers, life rafts, and all the gear that makes offshore cruising possible.
Balanced Catamarans has moved to publishing performance data at what they call “average launch weight” — the equipped weight of the boat as it actually leaves the factory. That represents something closer to a coastal cruising configuration, which is far more relevant than a theoretical lightship number. If an owner wanted polars for a full-load departure from Cape Town with five crew, full fuel and water, and provisions aboard, those would look meaningfully different again.
The implication for consumers is clear: when you’re comparing Bruce numbers or polars between manufacturers, make sure you understand what displacement each builder is using. Without that apples-to-apples foundation, the numbers are essentially meaningless.
Polars: Reading What Your Boat Wants to Do
Polar diagrams are the graphical output of a VPP — a wind rose showing predicted boat speed at various wind angles and wind speeds. They’re powerful tools, but they come with the same caveat as the VPP itself: the output is only as good as the input and assumptions behind it.
Patrick made an interesting distinction between two uses for polars. The first is the shopping phase, where you’re trying to cross-compare boats. That’s inherently difficult because builders use different displacement baselines, different VPPs, and different assumptions. The second use — and arguably the more valuable one — comes after you own the boat. Polars can teach you what your boat wants to do: the optimal angles, the conditions where it excels, and where it starts to struggle.
The practical reality is that most cruising sailors operate at somewhere around 65–80% of their polar speeds. In benign conditions — flat water, 12 knots of breeze — you’ll get closer to the predicted numbers. As conditions deteriorate, the gap widens. That percentage matters for passage planning: if you’re estimating an arrival time based on raw polar speeds, you could be significantly off.
Phil also shared an illuminating anecdote about a customer considering a carbon mast. The weight savings were modest and the cost was significant. The customer concluded he’d save more weight by simply running half tanks in the Caribbean — a free optimization that would outperform an expensive upgrade.
The Narrow Hull Trade-Off
The conversation closed with an important observation about hull design philosophy. Narrower, flatter-sided hulls with high fineness ratios tend to have superior raw speed numbers — but their performance degrades more sharply as displacement increases. Phil illustrated this with a story about racing a Switch 51 against a Fountaine Pajot Sanya in the Caribbean. The lighter, more voluminous Switch was faster in real-world cruising trim. But stripped down to bare minimum weight, the long, narrow hull had the edge.
The takeaway for prospective buyers: a boat that looks fast on paper at lightship displacement may lose that advantage quickly once loaded for cruising. Hull form, volume distribution, and structural weight all interact in ways that a single performance number can never capture.
The Bottom Line
If you’re shopping for a cruising catamaran, by all means glance at Bruce numbers — but don’t build your decision around them. Ask builders what displacement they’re using for their published figures. Look for polar data at realistic sailing weights. And understand that the real story of a boat’s performance lives inside its VPP, a tool that even the designers who built it will tell you is an estimate built on imperfect assumptions.
The best performance tool, as always, is experience on the water.
This is the fifth in an eight-part series. Catch up on previous episodes covering catamaran design from the inside out, hull shapes, dagger boards, and more. Next up: sail inventories and sail plans.
