In favour of light displacement

The notion that heavy displacement craft are safer and generally more seamanlike offshore has persisted for at least a century. However the trend towards lighter displacement suggests that today's sailors disagree.

To stand up in favour of light displacement cruising yachts was once to face the force, and sometimes fury, of established and cherished conventions. For ‘serious’ offshore sailing, many pundits will tell you that the characteristic that really counts is weight - within reason, the more of it the better. And, over the years, enough people had intoned those sentiments to give it the ring of absolute truth, brooking no dissent.

And you can see why this notion was 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.

There were exceptions, of course: the Viking longships and the triremes of the Mediterranean were wonderfully spare. But these were principally 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. 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. To sail there offshore 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.

But before we 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 many, probably the majority, of manufacturers and customers alike. Of course there comes a point where extreme light weight becomes expensive. An Open 60 racer has barely a smear of materials over most of its surface, but the exoticism of those materials and the technology necessary to apply them vastly outweighs any paltry savings in quantity.

But the purely commercial view seems a rather arid argument. My first serious design was a ferro-concrete sloop and I was so appalled at what I’d done 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 (8m) she was one of the smallest and certainly 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 1970 is remembered as a tough year, with the weather ranging from challenging to downright horrendous. 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.

But why?
Essentially, a sailing boat is a machine which must overcome the forces that hold it back (inertia and the various forms of drag) by harnessing the available sources of power (the wind and, in its absence, an engine). The extension of this principle is that the more eager a boat is to move, the less you need to push it. In more direct terms, it means that with lighter and more efficient hull shapes you can get away with smaller sail plans. This relationship between power and weight is best expressed by the Sail Area/Displacement Ratio (SA/D ratio).  Clearly, the more sail area (power) you have compared to the displacement (weight) the more sprightly the boat will be. Most boats fall into the 14 to 20 range, with the larger figures towards the most powerful rigs. Our current boat, Shindig, has a SA/D ratio of 18.7 -- quite respectable enough for a couple of threshold ancients who prefer to sail two-up.

So, the inescapable truth is that if you want heavy displacement with a respectable SA/D ratio you either must choose a smallish boat or struggle with a monstrous sail area. The alternative is a larger, under-powered boat which, I hope, comes with a patience transplant for the crew because they may be spending a long time at sea.

“Ah,” some might say, “We’re not racing. We’re cruising so we don’t care how long it takes to get there.” To associate cruising with sailing slowly is a curious connection. Racing skippers need sail only better than their own and others’ handicaps to win the race. Cruisers sail in real time where they can find themselves unable to round a headland because the tide has turned or arriving at a port ten minutes after the pubs have shut.

So, summing up:

  • Lighter boats usually cost less to build 
  • Light weight means lighter gear and sails
  • Lighter boats usually have less wetted surface area. Better in light winds.
  • Average wind strengths favour lighter boats
  • Faster passages makes it easier to meet tidal gates

    Remember:You can sail a fast boat slowly but you can't sail a slow boat fast
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