The Bruce Number vs the Sail Area/Displacement Ratio (SA/D)
Why Multihulls Need Different Performance Metrics

Key takeaways: The Bruce Number is a multihull‑specific indicator of power‑to‑weight, designed to reflect how slender hulls accelerate and carry sail. SA/D works well for monohulls but exaggerates sail power on multihulls because displacement behaves differently in narrow hulls.

The Bruce Number applies cleanly to both catamarans and trimarans, offering a more realistic picture of cruising performance, reefing behaviour and passage speeds. It is not a perfect metric, but it is far more reliable than SA/D when comparing multihulls of different sizes and design philosophies.

Why the Bruce Number exists
Why SA/D doesn’t work for multihulls
Why the Bruce Number works for multihulls
Bruce Number ranges & what they mean
Bruce Number on catamarans vs trimarans
SA/D vs Bruce Number: side‑by‑side comparison
How to use the Bruce Number when evaluating multihulls

Why the Bruce Number exists

The Bruce Number is named after Ian Bruce, a New Zealand multihull designer who recognised that monohull ratios did not accurately describe multihull behaviour. In the 1960s–70s, Bruce observed that:

multihulls do not follow monohull hull‑speed limits
displacement affects multihulls differently
sail area needs different scaling
multihulls accelerate in a different way
SA/D exaggerates sail power on multihulls

To solve this, he created a simple formula that better reflected how multihulls actually perform:

\[ BN = \frac{\sqrt{SA}}{D^{1/3}} \]

It quickly became a practical design tool and remains widely used today.

Why SA/D doesn’t work for multihulls

SA/D was developed for monohulls, where displacement is the dominant factor in performance. It assumes:

a single hull
a displacement‑limited hull speed
wave drag proportional to displacement
acceleration limited by mass

Multihulls break all of these assumptions.

Multihulls:

have slender hulls
generate low wave drag
accelerate quickly
exceed monohull hull‑speed limits
rely more on hull efficiency than raw sail power

Because of this, SA/D:

overstates sail power
misrepresents performance
cannot compare multihulls reliably

A cruising catamaran can show an SA/D similar to a racing monohull, which is clearly misleading.

Why the Bruce Number works for multihulls

The Bruce Number scales sail area and displacement in a way that matches multihull behaviour:

Square root of sail area Reflects diminishing returns of adding more sail.
Cube root of displacement Dampens the effect of weight, appropriate for slender hulls.

This makes the Bruce Number far more realistic for comparing:

cruising catamarans
performance catamarans
cruising trimarans
performance trimarans

Bruce Number ranges & what they mean

Bruce Number	Typical Multihull Type	Interpretation
≈0.9–1.1	Volume‑oriented cruising catamarans	Modest sail power, predictable handling, slower passages.
≈1.2–1.4	Balanced bluewater cruisers	Good everyday speeds with manageable loads.
≈1.5–1.7	Performance cruising multihulls	Higher speeds, better light‑air ability, more responsive.
>1.7	Racing multihulls	Very high power‑to‑weight ratios; demanding offshore.

Bruce Number on catamarans vs trimarans

The Bruce Number applies to both, but interpretation differs slightly.

Catamarans

Two hulls = more wetted surface
More drag at low speeds
Higher payload capacity
More stable platform
BN ≈ 1.2–1.4 is typical for bluewater cruising

Trimarans

One main hull + two amas = less wetted surface
Lower drag
Faster acceleration
Lower displacement
BN ≈ 1.4–1.6 is common for cruising tris

A trimaran can show a higher Bruce Number than a catamaran of similar length without being “over‑powered”.

SA/D vs Bruce Number: side‑by‑side comparison

Metric	SA/D	Bruce Number
Designed for	Monohulls	Multihulls
Hull assumptions	Single hull, displacement‑limited	Slender hulls, low drag
Displacement scaling	Linear	Cube root
Sail area scaling	Linear	Square root
Accuracy for multihulls	Poor	Good
Best use	Comparing monohulls	Comparing cats & tris

How to use the Bruce Number when evaluating multihulls

The Bruce Number is best used as a first‑pass indicator of how a multihull will behave underway. It gives a quick sense of power‑to‑weight, acceleration, and whether a design leans toward comfort or performance. However, it should always be interpreted alongside other ratios and design factors, because no single metric captures the full picture.

One of the most useful companion ratios is SA/D (Sail Area to Displacement). Although SA/D was created for monohulls, it still provides valuable context for multihulls because it measures something different from the Bruce Number. The Bruce Number tells you how a multihull behaves; SA/D tells you how much sail it carries.

The two ratios complement each other:

Bruce Number reflects slender‑hull drag, multihull acceleration, cube‑root displacement scaling and square‑root sail area scaling. It predicts how willingly the boat moves, how quickly it accelerates, and how it will feel on passage.

SA/D highlights how aggressively the boat is canvassed. It reveals tall rigs, large mainsails, big headsails, and designs that may require earlier reefing. It also helps explain why two boats with similar Bruce Numbers can feel very different underway.

SA/D is particularly helpful when comparing multihulls of similar size. It can identify outliers with unusually large sail plans or conservative rigs, and it helps explain reefing thresholds, light‑air behaviour and the “personality” of the sail plan. For trimarans, SA/D often appears higher because displacement is lower, but this does not necessarily mean the boat is over‑powered; it simply reflects efficient rigs on light platforms.

Used together, the Bruce Number and SA/D provide a more complete picture of:

acceleration
reefing behaviour
light‑air ability
passage speed potential
comfort vs performance bias

They should also be interpreted alongside hull slenderness, bridgedeck clearance (for catamarans), appendage type and payload tolerance. These factors collectively determine how a multihull behaves offshore.

Written by Dick McClary, cruising sailor and yachting journalist, publisher of Sailboat‑Cruising.com and admin of the Liveaboard Sailboats for Sale community.