Light Displacement — For Those With a Taste For Speed?

To stand up in favour of light displacement cruising yachts is to risk facing the force, and sometimes fury, of established and cherished conventions. For ‘serious’ offshore sailing it is decreed that that light displacement is to be eschewed, and that the characteristic that really counts is weight – within reason, the more of it the better. And, over the years, enough people have intoned those sentiments with enough vehemence to give it the ring of absolute truth, accepting no dissent.

And you can see why this notion is so ingrained. That the sea can be violent is not in doubt. When faced with any threat, our ancestors first hid in caves and later built castles. And when they became navigators they took these instincts with them. Even today the word fo’c’sle (forecastle – a fighting tower in the bow) survives commonly, if less aptly, to show how the first ships were regarded by our forebears: literally as floating fortifications with high platforms from which to fire down upon their foe. There were exceptions, of course: the Viking longships and the galleys of the Mediterranean were enthusiasts of light displacement. But these were mainly propelled by oars – and here it’s interesting to reflect that only when man had to bend his own back, did he fully seem to appreciate the benefits of light weight and a slippery hull form.

Other maritime cultures – most notably in the Pacific – were never intrigued by heavy displacement. Light displacement is all they have known. Whether engaged in war or migration, they put their faith in speed and agility. In unballasted boats, tied together with plaited ropes, small groups of sailors colonised the choicest of the 25,000 or so islands that pepper over half-a-million square miles of ocean. Each voyage was an immense feat of seamanship conducted in craft that most of us would deem unsuitable. And, for those that haven’t been there and might think it eternally benign, the Pacific doesn’t always live up to its name. Storms rip across its vastness as they do through every tropic region. No, this is very, very serious cruising indeed.

Of course, I’m not suggesting that everyone should do exactly likewise but it does demonstrate that, when it comes down to technology, there’s more than one way of doing things.

Inevitable trends

But before I move on to the merits and demerits of each type, it’s worth mentioning that there’s an inevitability about the trend towards lighter displacement. Take a given amount of material and spread it more thinly and you get a bigger boat for the money – an appealing concept for probably the majority of potential boat owners, whose wishes the manufacturers will respond to with alacrity. Of course, there’s a point of diminishing return. To achieve extreme light weight can be very expensive, calling on exotic materials and the sophisticated techniques used to apply them. If you want an Open 60 racer, for example, it’s going to cost you – not because there’s much in them but it wasn’t cheap putting it all there.

Yet to present the purely commercial view seems rather dusty. There’s also the matter of preference. My first tentative design was a ferro-cement sloop and after we had built the hull I was so appalled at the result that my next was a cold-moulded trimaran, a bit heavy by today’s standards but fast for the times. A couple of years later I entered another tri, Three Fingered Jack, in the Round Britain Race. At a little over 26ft she was one of the smallest – and by far the lightest – boat in the race, yet we still managed to finish in the front half of the fleet, ahead of several much larger yachts. And that year none of us had an easy time. 1970 saw a tough race, with the weather ranging from challenging to awful. If ever I needed proof that a lightweight craft could do better than just survive in those conditions, this was it. I’ve never since had any reason to think otherwise.

Design basics

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But of course performance isn’t everything. The point is often made that lighter boats have a twitchier motion, and that’s basically true. Being inherently more buoyant they’re more prone to being tossed about on what is, after all, an extremely mobile and irregular surface. However, that’s not all bad news because it’s that inclination to stay on the surface that also makes them less likely to dive into the sides of waves, thus inviting all that green stuff to sweep back along the decks to drench the crew. So, in some respects it’s a matter of how you define comfort.

When beating upwind, lighter boats are said to be disadvantaged because they’re more easily baulked by wave action. Again, there’s some truth in that but we have to be more specific. Racing boats with their fine bows and efficient rigs are demons on the wind, despite being very light. Unfortunately, many modern cruising designs have been compromised to allow them to flourish in a market where the principal selling point has become the accommodation. Designers, understandably eager to please their customers, have been coerced to maximise internal volume – particularly forward where the clammy burrow-like forepeaks so hated on older boats can then be developed into sumptuous forecabins worthy of the name. Unfortunately, this greed for space comes with a hidden price. Water disapproves of being shouldered aside suddenly, preferring to be eased apart with decorum. Whatever a boat’s displacement, bluff bows do nothing for windward performance, and although lighter boats with their lesser momentum will be the first to suffer, this is a fault of design, not of principle. You can have windward performance or lots of space forward. But not both.

Can she carry the load?

You will often hear it said that, length for length, light displacement boats are a poor choice for long distance cruising because they can’t carry as much gear and provisions as their beefier brethren. The customary argument goes something like this. Add a tonne of extra weight to a 10 tonne boat and you will increase the displacement by 10%. Do the same to a 5 tonner and you burden it by an intolerable 20%. Although superficially true, this is misleading as can easily be explained.

Contrary to popular opinion, the fact is that ‘sinkage’ is not dependent on displacement but on the ‘waterplane’ area – that’s to say the horizontal ‘footprint’ of a boat at its waterline. The amount by which a boat sinks as you load it is aptly called ‘sinkage rate’ and is commonly measured in kilograms for every centimetre of vertical effect on the waterline (kg/cm) or pounds for every inch (lb/in). For example, if loading an extra 100kg would make a boat float 1cm lower in the water, its sinkage would be described as – wait for it! – 100kg.

Now let’s take the two hulls represented right. They are of the same length. Boat A is a typical modern light displacement cruiser while boat B is an older design of heavier but by no means extreme displacement.

Because of her traditionally narrower beam and longer overhangs, B has a smaller waterplane than A, as we can seen if we superimpose the two (C). Indeed B’s waterplane area is only about 85% of A’s. If we keep the sums simple by assuming that A’s waterplane area is 100m² this makes B’s quite obviously 85% of that – 85m² of course. To continue this fiction, we’ll now lower a 1 tonne weight onto A’s deck. It will come as no surprise to learn that she will sink exactly 1cm. How about B? Well, the same weight would sink B further – by 1.176cm to be precise – demonstrating that the lighter boat is less influenced than the heavier one.

Oh, come on, you might think. You can prove anything by juggling with figures. This is all so counter-intuitive. So, let’s put these principles into a real framework. My own 12m light displacement boat Shindig displaces just over 7 tonnes in her regular, fully equipped cruising mode. She has a sinkage rate of 252kg, meaning that a 2 tonne overload – more than enough extra payload for, say, an Atlantic crossing ­– would raise the waterline by about 8cm (3in). That’s not exactly high drama, would you not agree?

The object of this article isn’t to condemn heavy boats, of which there are many fine examples, but rather to try and dispel some of the misconceptions that might otherwise influence our choices. I’ve heard the argument that sailing brings patience, but it hasn’t yet worked for me. To complete a passage both quickly and as safely as possible still gives me tremendous satisfaction and, in those dreamier moments when the adrenaline has abated, I can always remind myself that you can sail a fast boat slowly but not a slow boat fast.

The choice is yours, but here are my reasons for preferring light displacement:

• For any given size, lighter displacement bring economies in building costs and equipment.
  
  
• The smaller rigs are easier to handle, particularly for shorthanded crews.
  
  
• Lighter boats usually have less wetted surface area which means better light weather performance. In many popular sailing areas, average wind strengths favour lighter boats. Heavier ones can be dogs in light conditions.
  Very light boats (with D/L ratios below about 150) can exceed hull speed off the wind, turning in spectacular passage times. However, when talking of ‘performance’ it’s important to think of average speeds not momentary bursts. But even an extra half knot can make the difference between rounding a headland before the tide turns foul or having to wait till it turns again.
  
  
• Within reason, light displacement boats are better weight carriers than is commonly imagined. Anyway, as performance is the first to suffer, they encourage you not to hoard.
  
  
• Oh, by the way – they’re also more fun.
  

Also of interest ...?

For really light displacement, how about a multihull?  To read more on their vices and virtues, go to:   Monohull or Multihull