Join Date: Dec 2004
Location: Portugal, West Coast
Thanked 105 Times in 88 Posts
Rep Power: 11
Light vs Heavy Displacement monohulls
I have found this article so interesting and so according with my own thought that i will post it fully. It was written by Aurélien Poncin the founder of Aureus Yachts:
Light displacement versus Heavy displacement sailing yachts is an old and passionate debate in the monohull offshore cruising world (Luckily this debate is long gone when it comes to multihulls).
First of all, let me tell you what I mean by “Light displacement”, or “Heavy displacements” hulls, yachts or designs, and what exactly will be compared in this post:
In both cases, I assume boats are properly built, and especially the composite parts. Achieving a proper building and reaching the optimal mechanical properties to weight ratio for the composite mix you use (fiber, resin and core material) requires a lot of skills, experience, tools, researches, tests and above all care for “the state of the art spirit”.
Producing state of the art offshore yachts requires curiosity, as technologies are evolving quickly, and a passion for sailing of course. A properly built heavy displacement sailing yacht will always be a better offshore yacht than a light to medium displacement yacht built without caring for details and/or with bad processes…and vice versa.
When it comes to safety, I assume that both light displacement and heavy displacement yachts are well prepared, and carry a full safety inventory that matches the ISAF recommendations. The first safety is the sailor’s care for safety. Wearing an adequate inflatable life jacket when on the watch, a harness together with a reliable safety line, a multi purpose knife and a personal MOB device (alarm + AIS) makes any boat safer for offshore cruising. The preparation and a good knowledge of the boat are also very important.
Now, to give you a precise idea of what I call “light” and what I call “heavy”:
A typical heavy displacement 50 feet monohull offshore cruiser weights 18 to 20 tons.
A typical light displacement 50 feet monohull offshore cruiser, with the same level of equipment, weights 12 to 14 tons thanks to carbon/epoxy composite and T-shaped keels.
Today’s offshore racing sailing yachts are “ultra Light” displacement yachts. For instance, the extreme IMOCA’s 60 footers , that run the Vendee Globe, weight under 8 tons.
For more than 2 decades now, offshore sailors are debating “light vs heavy” and strangely the dominant argument of the heavy displacement enthusiasts is Safety.
I say “strangely” because the obvious and main reason to go for heavy displacement hulls today is because of the bigger inside space that can be offered and the big amount (and weight) of equipment that can be put on-board (generator, washing machine, air conditioning, water maker, etc.).
As you will discover in this article, light displacement yachts, when properly built by skilled and carrying craftsmen and properly equipped, are now as safe as heavy displacement sailing yachts, can be almost as comfortable and carry an impressive equipment inventory…and they are much faster.
As the passionate builder of the Aureus XV, you might argue that I have a strong bias in favor of light displacement hull designs, and of course you would be right. I believe light displacement sailing yachts are the best fit for demanding offshore sailors, and my point here is to illustrate why, with facts.
This being said, it’s also important to notice that whatever your choice is, none of those philosophies can, alone, make a great offshore sailing yacht. There are great boats, properly conceived and manufactured in both categories.
It is crucial to look for a builder that is coherent, masters the composite technologies and cares about all the details involved in the construction, and there are a lot. The best way for this is to visit the yard and talk to the technicians.
There is nothing worst than a light to medium displacement design that ends up weighting 10% or 15% more than the architect’s and builder’s optimal target, by lack of care or skills. Those boats are usually bad performers, uncomfortable yachts, and less safe (the center of gravity always suffers from such a non predicted weight gain).
Keep in mind that such designs won’t tolerate laxity or approximation from the builder. Being 2 tons heavier on a heavy displacement design is also bad, but the effect on the behaviour and safety of the boat won’t be as bad as on a lighter boat.
I see a boat as a long chain of choices to be made that must remain coherent with the initial choice of displacement type. Each piece of equipment is chosen not only for it’s general quality, reliability and fair price, but also for their compactness and their weight (or the weight they allow you to save elsewhere on the boat).
A “state of the art” boatbuilder, after he has chosen the design, sets a realistic target of weight and develop a coherent and balanced yacht keeping those targets at the center of any construction steps. There are now luckily many materials and technologies that offer the same quality, resistance, reliability and durability for less weight. Those materials and technologies allow builders to offer boats that are very well equipped and still light. Here are the most common and popular ones:
The infusion process, when done well, considerably reduce the amount of useless (excessive) resin in any composite part. In comparison, the way it was done 20 years ago, and is still done in some traditional shipyard is pre-historic. The pre-preg technology goes even further, pre-impregnating all the fibers with just the sufficient amount of resin.
Carbon fibers and epoxy resin allow builders to manufacture lighter, stiffer and more durable hulls, structure and deck. It’s even more true for masts and rigging. The result is a need for a less powerful, thus lighter engine, less demanding in Gasoil. It’s a virtuous circle.
New batteries (gel or Lithium-ion technologies) and renewable energies
sources on-board allow builders to put less battery weight, while maintaining a high capacity for a good electronic installation and modern electrical comfort equipment. A Hydro-generator, under sail, and solar panels at mooring can produce a lot of energy. On a well-conceived boat, it allows to reach a high level of comfort without having to run the generator too often (reduce the amount of Gasoil to carry).
A careful selection of each piece of equipment, and accessories can lead to another saving of a few hundreds (yes hundreds!) kilos.
During the last decade, more and more light to medium displacements sailing yachts have participated to offshore rallies and they are now overtaking heavy displacement yachts in most of the events, such as the ARC. Of course, light displacement yachts trust the winnings and they have proven to be as safe as their heavier competitors in offshore conditions.
Here are 14 key facts to help you make your mind, or remake it…
Safety at sea:
Though they have usually a keel ratio around 40%, which is high, the center of gravity is usually higher on any heavy displacement yachts than on good light to medium displacement sailing yacht. This is due to the slightly deeper T-Shaped, , keels of the light to medium displacement.
When it comes to safety, know that the lower the center of gravity of the boat is, the better. The center of gravity is very important when the boat is brought to extreme angles (above 90°) as the lower the center of gravity is, the more chances you’ll have not to capsize (we’re talking about really extreme conditions here, that can be avoided with a serious weather analysis).
It means a well designed and properly built light displacement sailing yacht will perform better and be as safe in hard conditions. By the way, the center of gravity both vertical and longitudinal of your boat are precious data to collect.
Comparison heavy and light displacement yachts – center of gravity for the boat (blue points) and the keel (green points). Those are two typical designs.
The light displacement is a 13 T (Keel 4.8T – Draft 2,95m) Aureus XV Absolute, fully equipped for offshore navigation. The heavy displacement, on the left, is based on an average 50 ft heavy displacement central cockpit, weighting 19 T (Keel 8T – Draft 2,35m)
I must say that when comparing two stability curves, heavy displacement yachts will prove to have a better stability between 30° and 120°. It means their weight will reduce the amplitude of movements. It’s an advantage of heavy displacements, the disadvantage being a lower speed. But the curves also show that at threatening bank angles, the light displacement will offer a higher stability (capsizing at a higher angle and being easier to redress).
Finally, the stability in the common angles (0° to 35°) is quite close for both design. To sum it up, both designs offer a very good stability in common angles, Heavy displacements will be more stable (less amplitude and slower movements) in the uncomfortable zone between 40° and 90°, but will lose their advantage in critical angles, above 120°.
Stability curves. Comparison between the curves. In blue the heavy displacement (the one from the drawings above), in Red the Aureus XV Absolute. Both designs offer a nice stability in the common angle. The heavy displacement stability is better in the “unusual angle zone”,between 30 and 110°, due to its superior weight. But the light displacement will offer a better stability in the “critical zone” where it’s about not capsizing, or recovering from capsizing, due to its lower center of gravity.
In tough downwind conditions, when the boat is running with the waves, heavy displacement sailing yachts will be caught-up by waves more easily, due to their reduced speed potential. That will affect the steering reactivity, which is not good in hard conditions (note that the term “hard conditions” is relative.
A professional offshore sailor will be just fine in 8 to 9 Beaufort, whereas some people will define “hard conditions” as a wind above 6 Beaufort, probably 5 when sailing upwind).
Both heavy and light displacement yachts can equally benefit from fully centralized maneuvers at the helm station (automatic reefing system, electric or hydraulic furlers, electrical winches, etc…) thus allowing crew members to stay attached in the cockpit when conditions are tough.
Speed is also a safety issue. When faced with the imminent arrival of hard conditions, speed allows you to get away as fast as possible or reach a shelter.
Speed and sensations:
This is the tipping point. A light displacement sailing yacht is a much better performer than a heavy displacement one, both in terms of speed and sensations.
Light Displacement yachts are faster, the sail area/displacement ratio is more important giving the boat more power. Light displacement yachts offer less resistance to water (reduced “wet surface”) and their hull designs are less asymmetrical, meaning a light displacement design will create less rolling downwind. It’s an important point to consider, since downwind is what offshore sailors are looking for when planning trips.
When it comes to performance and balance, Light displacement yachts’ T-shaped keel are much better than the traditional long shoal fin keels usually fitted under the heavy displacement hulls. The downsize is that they have a bigger draft (the center of gravity of the keel is as much important in its efficiency than its weight). If you want to solve that issue on a light displacement yacht, go for a lifting keel.
Light displacement monohulls can sail to descent speed in light winds, where heavy displacement monohulls will have to use the engine propulsion. It means you need to embark and burn much more Gasoil and will be sailing less often. I think this is a very important point pleading for light displacements.
Generally, many things on a heavy displacement must get heavier to get the same level of resistance or reliability as the light displacement yachts. A heavier anchor, a heavier mast, heavier rudders, heavier keel to balance the heavy structure and equipment, and so on. This is a vicious circle.
Comfort on board:
Heavy displacement hulls generally offer more living space as comparable length. To get the same level of equipment and space on a modern light displacement hull, the solution is to go for compact and clever accessories. Their draft is also generally lower (60 cm in our example), meaning you can go closer to the beach.
Heavy displacement sailing yachts are generally slower and less comfortable downwind. But in return, they can provide an improved comfort at helm station. The reduction of the boat’s windage is not a real concern on heavy displacement yachts, and a lot of them offer central cockpits with hard tops structures. The aesthetic is not really modern, but the result is a dryer helmsman in agitated condition.
Basically, if getting a little wet in tough conditions is a central concern to you, a heavy displacement is what you need. I really consider this as a comfort issue, not a safety issue.
A light displacement sailing yacht’s cockpit is as safe as the cockpit of a central cockpit, but definitively exposes the skipper more to water projections and wind. I find those conditions thrilling and rely on a modern, very comfortable and light full weather Gore Tex® gears, but others are OK to trade the thrill for a dry place under heavy conditions.
Light displacement sailing yachts tend to roll less downwind.
Light displacement sailing yachts above 50 feet can now benefit from a lot of equipment without having to embark tons of fuel and dozens of heavy batteries. Modern batteries offers 2 to 5 times more disposable energy than traditional lead batteries and their lifecycle is 2 to 3 times longer. And wait for it: They are lighter, at comparable capacity.
With integrated hydro generators, light yachts are now rewarded with an easy and durable energy source while sailing and the fact that they sail faster allow them to reach an incredible autonomy at sea. A single very low drag hydro generator, such as a Watt and sea hydro generator can produce up to 500W at 8 knots, and you won’t see the difference in your speed. Have a try of the Aureus XV absolute and you will be amazed by how effective a modern sailing yacht can be in terms of energy consumption and production.
Price and value:
There was a time, in the boat industry, where people said: you can estimate the price of a yacht looking at its weight, and this was quite coherent. This was correct when all builders and brands were stuck with the same materials (Basic Fiberglass and poor quality Polyester). Since everybody was using the same materials, the heavier the boat was, the more raw material was included, the higher the cost. So naturally, heavy displacement yachts were higher priced and considered as the “premium or luxury solution”.
But now, carbon fibers, titan and new technologies are available on the market and allow major gain in weight…at a relatively heavy cost. So this old wisdom is not right anymore. Now you can reach the same mechanical properties at half the weight.
A carbon hull costs more to (properly) produce than a heavy displacement hull, even twice as heavy. A multiplexed DC cabling is half the weight of a traditional DC cabling but also costs three times more. Gel batteries cost twice as much as standard lead batteries and Ion-Lithium batteries 10 times more.
Today, to offer the same level of comfort and equipment as a heavy displacement sailing yacht, a light displacement sailing yacht must be built in carbon/epoxy using vacuum infusion (or pre-preg) process. This is the only possibility to counter the weight of equipment such as Generators, descent fridge and freezer, dishwasher, water maker, hydraulic and electric manoeuvres, etc… So a state of the art offshore cruising light displacement sailing yacht will be more expensive to build than a heavy displacement yacht.
On the other hand, carbon/epoxy hulls are much more durable than Polyester or Vinylester hulls. A remarkable light displacement yacht using carbon/epoxy offers, contrarily to what is generally said, a much better durability than a traditional E-glass (fiberglass) using polyester or vinylester. This value in time is to be considered. To learn more about resins or fibers, check out our post: Polyester vs Epoxy and Fiberglass vs Carbon.
Light displacement offshore sailing yachts have considerably reduce their dependence to combustion engine or generator. It reduces the need for Gasoil consumption, isn’t that good news for environmental concerned sailors ?
Most of the heavy displacement sailing yachts brands keep using the same materials as 20 years ago (Fiberglass, Polyester resin and plywood).
In the meantime, materials like carbon and epoxy have made their proof on hundreds of sailing yachts’ laminate, they are now totally reliable and allow innovative builders to produce much faster, safer and durable yachts. I really think that a “state of the art” modern light displacement sailing yacht is the best fit for offshore sailing, even though the appearance of a good old heavy displacement keeps on reassuring some sailors.
I think a lot of brands are very conservative both in design and construction because they are used to it, and because a lot of customers are also used to those good old materials and design. They assume their qualities are better, since they are used for years on good quality yachts, and they assume they will naturally last longer. Well, that’s kind of wrong ! A properly built epoxy hull will automatically last longer and with better performances than a Polyester hull or deck, plus you don’t have to fear any osmosis any more.
I hope this plea for lightness will resonate and encourage skeptical sailors to give a try to well-built light displacement offshore sailing yachts, for more sensations.
Please feel free to disagree or comment, for the sake of the debate.
Thanks for reading,
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.