Superyacht Adastra, 42.5m Power Trimaran.
"When you look at Adastra you look at the future of yacht design" Boat International Media. The Superyachts. 2013.
".....this project could be compared to Team GB members winning an Olympic gold in front of a home crowd., Tork Buckley, The Superyacht Report October 2012.
"One of the world's most amazing super yachts, that could spell the future for efficient long range cruising." Boat International April 2011.
The striking 42.5m Trimaran Adastra was launched on the 11th of April 2012 in China for Hong Kong clients Anto and Elaine Marden.
Adastra is the result of meticulous attention to detail and innovative design to create a yacht that meets the needs of a very experienced ocean voyaging couple and their family, and to provide the level of comfort and style expected in a yacht of this class and size. No effort has been spared in the challenge to produce a beautiful yacht that has exceptionally low fuel consumption and yet provides excellent sea keeping qualities and luxurious accommodation.
Measurements taken during the sea trials show that her fuel consumption at 10.5 knots is as low as 17 litres per hour when carrying 10% fuel and water. At cruising load (20 tonnes fuel and water) she uses just 25 litres per hour therefore on delivery trips her range is 10,000 miles starting with 30,000 litres of fuel. Her slender hulls and streamlined exterior allow her to travel faster in the open ocean and at 17 knots she has a 4,000 mile range so can comfortably cross both the Atlantic and Pacific oceans at hi-speed.
Adastra takes the power trimaran concept further than has ever been attempted before, previous vessels like Earthrace and Cable and Wireless were stripped out record breaking machines. The challenge of turning this concept into a viable luxury yacht has led us to further research and develop new thinking on stability and comfort at sea for this type of craft. Extensive tank testing and radio controlled model tests in waves have been carried out to analyse stability and performance. Outrigger height has been optimised for ease of motion at sea, and a new outrigger shape has been developed to increase stability in waves.
We have undertaken state of the art structural analysis of all the major components in the yacht in order to achieve the light weight required for very low fuel consumption. The deck and superstructure is constructed from carbon fibre with Nomex honeycomb core, the hull is glass/kevlar foam sandwich and the interior is light weight oak cabinetry using honeycomb panels. To help reduce weight further, virtually every aspect of the boat is custom built. This includes carbon fibre hatches, toilets, portlights and ladders, which are all built specifically for the vessel.
Aerodynamics and hydrodynamics above the water line were carefully considered as we developed the overall exterior shape and we kept the boat streamlined both for air and water passing over the structure. The smooth seamless, unbroken surfaces also help to reduce weight as the outer-skin forms part of the structure to absorb torsional loads.
Adastra's 16 metre beam creates a spacious saloon area on the main deck which offers superb views through a panoramic window and accommodates a lounge area, dining table and navigation station. A forward facing door through the saloon window gives easy access to a large sunbed on the foredeck. The aft deck has a sofa and bar area to port and a dining area to starboard, further aft there is space for a 4.9 metre tender and directly below is a garage which can store a 3.1 metre tender. The garage door has been designed to fold out and create a large dive platform. The main helm station, which has seating for two, is positioned in a raised pilot house situated between the aft deck and the saloon area and forms part of the cross beam structure. Extra space has been created below deck by slightly flaring the central hull just above the waterline. This area has been split into two sections with a full-width master cabin located aft with access from the deck saloon, and two further guest cabins, accommodation for the crew, and the galley located forward of the engine compartment. Adastra offers comfortable accommodation for nine guests and up to six crew members.
The anchoring system is unique with three anchors driven by carbon fibre drum winches run by hydraulics. The primary anchor is a custom 130kg Bruce style anchor that deploys out of the starboard wing. The bow anchor is 80kg and deploys out of the bow with a carbon fibre arm. The third anchor is 60kg which deploys out of the port wing and is used as a stern anchor.
Adastra in Malaysia
Adastra has now completed 4000 miles through the Phillipines, Indonesia, and up to Phuket in Thailand. The owner says she is a joy to be aboard. Seas up to 12 ft (4 m) have been encountered and the yacht has performed well.
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Adastra Exterior Details
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Adastra in Hong Kong
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After 4 years in build by McConaghy boats in Zhuhai China, the fuel efficient 42.5 m Trimaran Adastra has been lanched into the Pearl River.
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The Shuttleworth Yacht Design team for Superyacht Adastra
From left Orion, John, and Sky Shuttleworth
General arrangement drawings
Interior. Rendered images. © Jepsen Designs. Hong Kong.
Adastra Interior Images
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Adastra under construction in McConaghy yard Zhuhai China
The images and content on this site are copyright. No content or images from this site should be reprinted without the permission of John Shuttleworth Yacht Designs Ltd.
In August 2013 Mazda approached John Shuttleworth with the offer of appearing in their Challenger advertising campaign. The Mazda team felt that John's interest in fuel economy and "blank sheet of paper" approach to the design of the Adastra trimaran matched the philosophy of their new SKYACTIV TECHNOLOGY. Discussions with the Mazda production team revealed many similarities in the way both Shuttleworth Design and the Mazda engineers have challenged convention in the drive to create fuel efficient vehicles, whether on land or at sea.
John participated in creating advertisements for TV, cinema, web and print. The Shuttleworth designed Adastra trimaran uses one seventh of the fuel of a conventional superyacht of the same size, and can travel halfway around the world on one tank of fuel. The webisode shows the story of how John came to design the vessel with stunning shots of Adastra underway.
Up against a field of 19 of the best superyachts in the world today, John Shuttleworth Yacht Designs was presented with the Naval Architecture - motor yachts award, at the Showboats Awards 2013 Ceremony at Monaco opera house.
Boat International Media noted that .....
The judges gave the nod to Adastra for her "clean sheet of paper approach" to problem-solving, and noted that her tank tests, as well as the physical evidence provided by designs of similar theory indicate "this yacht will have the best performance in the most sea states of all those presented".
Orion (left) and John (right) proudly receive the three awards on behalf of Shuttleworth Design.
Images courtesy of Mark Sims, Superyacht Media.
Adastra is chosen as finalist in 4 categories in the Showboats Design awards 2013.
1. Exterior Design and Styling with Orion Shuttleworth Design.
"Adastra was very different. A remarkably low hull weight of 52 tonnes,
a usable volume of 132 GT and a top speed of 23.2 knots, combined with her dynamic exterior shape and extremely
low fuel burn of around 20 litres/hour at her economical 10.5 knot cruising speed, led the jury to award a judges’ commendation
for extremely innovative design."
Shuttleworth Design was responsible for her exterior design, naval architecture, structural design and interior layout
Adastra is chosen as finalist in the World Superyacht awards.
New video of Adastra at 22 knots.
" We have been onboard on this leg for 12 days from Cebu, Ternate, Bitung and now Raja Ampat. She's behaved well. including a bit of bad weather - 4/5 on the beam. Basically she is a Joy to be aboard."
Adastra in Raja Ampat - September 2012. Photo taken by Christian Kaufmann - Raja4Divers. www.raja4divers.com
Adastra completes maiden voyage from Hong kong to the Philippines
Anto Marden reports, "She seems to be a very good sea boat. She has what can only be described as a stately motion."
John Shuttleworth Yacht Designs Ltd, and the companies involved in the design and build of Adastra have collectively been awarded the title of "Most Innovative Company" last week at the China (Shanghai) Boat Show. The unique nature of the design has been recognised by a panel of judges drawn from local and international Media and Marine Industry Professionals.ADASTRA build and launch video
Video of Adastra being built and launched, with brief interviews with the owners, the builders, and John Shuttleworth
McConaghy boats took this short video of Adastra off the coast of China. Loaded with 17800 litres of fuel, 2750 litres of water and 17 people, she is doing 22 knots.
After 4 years in build by McConaghy boats in Zhuhai China, the fuel efficient 42.5 m Trimaran Adastra has been lanched into the Pearl River.
Naval Architecture, Structural Design and Exterior Design - John Shuttleworth Yacht Designs Ltd
John Shuttleworth Yacht Designs Ltd. is based in West Sussex, England and has gained a reputation for designing fast, strong, and safe boats. In the 1980s Shuttleworth's design team developed the concept of "Integrated Structure," resulting in boats that were lighter, stiffer, and stronger than any previous multihull designs. Their designs became consistent race winners, and several have broken and still hold some of the toughest long-distance ocean sailing records. Many of Shuttleworth's racing and cruising multihulls have completed round the world voyages. They continue to develop and use this technology to design both sailing and power multihulls, where significant savings in fuel consumption and increased range can be achieved. Always working closely with their clients to achieve their wishes, Shuttleworth Designs have kept to their philosophy of "form follows function" which has resulted in a unique style that can be recognised by anyone who is familiar with their work.
Exterior Design and Design Project Management - Orion Shuttleworth Design Ltd
Orion Shuttleworth Design Ltd specialises in yacht design and visualisation for a variety of clients including yacht owners, naval architects, engineers, boat builders and manufacturers. The company offers a wide range of services including yacht exterior and interior design and styling, custom component design, rendering, animation, 3D modelling and design project management. With over 10 years' experience in yacht design, Orion has worked on a diverse range of projects from production motor yachts to high performance sailing multihulls. In 2009 he joined the ground breaking and multi award winning 'Adastra' project, working closely with the client until the launch in 2012. The company is now focused on the design of multihull and monohull, motor and sailing yachts, as well as visualisations for a variety of industries. Go to www.orionshuttleworth.com
Structural Design – Asta Ltd
Asta is a specialist composite engineering company providing working solutions for designers and fabricators in all applications of structural FRP. Their clients are: yacht designers, architects, civil engineers, boat builders, and designers of lightweight structures. They provide: stress analysis, FEA, production engineering, composite solutions, construction drawings and reports. In many cases the structural engineering has been unrestricted by codes or regulation and as a result genuinely innovative solutions have been developed.
Builder - About McConaghy Boats
For more than 40 years McConaghy has been at the cutting edge of race yacht construction; leading the way in research and application
of new materials and technologies to build some of the strongest, lightest Ð and most successful Ð racing yachts ever launched.
But that is only part the McConaghy success story. With manufacturing bases in both Australia and China, McConaghy now deploys its wealth
of talent and its mastery of laminated construction techniques across a number of different divisions, extending the strong commercial
platform that has allowed it to defy industry trends and continue to thrive.
Interior Design - Jepsen Designs
Jepsen Designs is a Hong Kong-based architectural and interior design Firm. Although not exclusively working on yacht interiors, Jepsen Designs has worked with clients, naval architects and boat yards around the world for the past 20 years. Projects have varied from the interior design of modern sailing and motor yachts to the restoration of classic motor launches to the conversion of Chinese wooden junks into high-class pleasure vessels and house boats. Whilst drawing from Scandinavian modern design principles fused with Eastern accents, the Jepsen Designs team has developed versatility in creating designs and styles to match unique client expectations. Years of sailing and racing experience within the team further bolsters Jepsen Designs' understanding of the technical and functional challenges a naval project offers. Jepsen Designs is today widely acknowledged as a well-established interior design firm in Hong Kong.
The Search for a Seakindly Fuel Efficient Vessel
By John ShuttleworthClick here to Download a printable PDF of the article.
Part 1 - Fuel Efficiency
In recent years there have been a few attempts to find a new solution to achieving low fuel consumption in large ocean going yachts. In this article we will look at the design considerations and compare performances of some of the types of vessel in commission today. We will review aspects of the design of various vessels, not with a view to criticising them, but to show how our thinking has been guided by what has gone before, and then to give our ideas and design concepts on how we have taken up the challenge to reduce fuel consumption and still retain sea kindliness. Finally we will endeavour to demonstrate how successful our solution has been and to explain why the concept has worked so well.
Length to Beam Ratio,
Most vessels in the superyacht category cross oceans at about 13 knots. At these relatively low speeds it has long been known that a thinner hull will be more efficient. This is because frictional drag dominates the resistance of the hull at low speed. In fact research conducted by the US navy many years ago indicated that efficiency would continue to improve past length to beam ratios of 13.5.
Currently it appears that the limiting length to beam ratio of a monohull in the 40 m range is about 7. Increasing the L/B ratio above 7 starts to become problematic for two reasons. Firstly the boat will have an increasing tendency to roll uncomfortably at sea and at anchor, and secondly in order to meet current safety standards the Vertical Centre of Gravity (VCG) will have to be kept low in order to increase the stationary stability to required levels. Keeping the VCG low increases the tendency to roll and limits the accommodation space. Most monohulls have to have some form of added stabiliser, usually using hydraulic fins, or gyroscopes, or both. Palmer Johnson have recently introduced a new type of stabilisation for a monohull with a length to beam ratio of 7. They have added small outer hulls aft to increase the righting moment of the hull and further reduce rolling. The first vessel is due to launch in a year or so.
Catamarans in this size and accommodation range, on the other hand, have length to beam ratios of around 10 which is an improvement on 7 as seen on monohulls, however having two hulls in the water increases wetted surface for the same weight carrying ability. Thus a catamaran and a monohull of similar length with length to beam ratios of 10 and 7 respectively have similar fuel efficiency. The catamaran gains by having much more accommodation and is inherently very stable.
In the case of a trimaran the centre hull has no inherent stability of its own and all the stability is created by the outriggers. These vessels can achieve length to beam ratios in excess of 17 which has been shown to significantly increase fuel efficiency and has been proven by boats like Earthrace and Cable & Wireless which were stripped out record breaking machines, and now by the sea trial results of Adastra, which is a fully fitted out superyacht, with space for 6 crew and 9 guests. The comparisons in the table 1 and Fig. 1 show the differences in the length to beam ratios of a number of vessels in the 40m range.
Table 1. Length to beam ratio for various 40 m vessels.
Fig. 1. Length to beam ratio for five 40 m vessels
The other key factor in achieving fuel efficiency is weight. The lighter the boat, the easier it will be to propel through the water. Composite materials and modern analysis methods allow us to design much lighter structures. The easily driven hull of the trimaran which needs much smaller engine/s can be significantly lighter than other types of vessel. This is shown in Fig. 2.
Fig 2. Displacement in tonnes of four 40m vessels.
Displacement to Length Ratio
Naval architects use a formula (see appendix) to calculate the displacement to length ratio of a vessel. The lower the displacement to length ratio, the more efficient the vessel.
Table 2. Displacement to length ratio for four 40m vessels.
In a catamaran the displacement to length ratio of each hull will be less than a monohull, but the fact that there are two hulls in the water means that the catamaran performs like a monohull with displacement to length ratios of approximately 50 % higher than the displacement to length ratio of each hull. Hence the 40 m Catamaran will have similar performance to the Outrigger stabilised LDL 42m monohull and the 41,2 m LDL monohull. All of these vessels will use about half the fuel of the 40 m semi-displacement monohull.
Fig 3. Displacement to length ratio of four 40 m vessels.
Earthrace trimming bow up at speed.
The picture of Earthrace shows how some vessels trim bow up at speed. As the length to beam and the displacement to length ratios are critical in creating low drag, it is essential that the vessel remains trimmed flat throughout the speed range. The above image shows that the waterline of Earthrace has reduced to about 80% of the stationary waterline. By tank testing we have been able to develop a hull shape for Adastra that has near zero change in trim up to 30 knots, thereby using the full waterline length for maximum efficiency through the whole speed range.
Adastra at 23 knots trims level. Using the full waterline length.
Speed and Powering
Adastra could have a top speed of over 32 knots, but on balance we calculated that by keeping the top speed at a maximum of 23.2 knots, we could keep the engine weight on Adastra to a very reasonable 1.2 tonnes compared to the two engines on the Outrigger stabilised LDL monohull weighing 15.6 tonnes. This approach increases the efficiency considerably throughout the speed range because the boat is not carrying the extra weight of large engines. 23 knots is still a very respectable speed for a 40 m superyacht as shown in Table 3 and Fig. 3.
It is clear that the trimaran Adastra will be orders of magnitude more efficient than other solutions, on the basis of the light weight, the displacement to length ratio and the length to beam ratio.
Table 3. Six current versions of Power Yachts in the 40m range, showing published figures for top speed and HP. Arranged in order of top speed.
Shuttleworth Designs "Adastra" 42.5 m - top speed 23.2 knots - 1150 HP
Fig. 4. Top speed (red) in knots vs maximum HP/200 (blue) for six vessels. shows how efficient Adastra is compared to other 40 m vessels.
Comparing Fuel Consumption for the Same Weight
Accurate figures for fuel consumption for most yachts are very difficult to obtain, however using our own data we find that Adastra uses one third of the fuel of a semi displacement monohull of the same weight over most of the speed range. Table 4 shows how Adastra compares speed and fuel consumption for an equal weight semi-displacement monohull.
Table 4. Litres per hour for Adastra vs. same weight monohull
Fig. 5. Speed in knots vs Litres per hour for trimaran Adastra and a semi displacement monohull of the same weight.
Comparing Fuel Consumption for the Same Length
A semi displacement 40m monohull power yacht will use approximately 250 to 300 litres per hour at 12 to 14 knots. Published figures do not state whether they are for full fuel or empty lightship.
A 40m LDL monohull or as predicted the outrigger stabilised monohull, and a catamaran, will use half that at 120 to 150 litres per hour. The outrigger stabilised monohull is predicted to use 112 litres per hour at 13.5 knots. We assume that this is at light load.
At 12 knots Adastra uses a measured 38 litres per hour with 19 tonnes of fuel, and 29 litres per hour at light load (10% fuel)
At 13.5 knots Adastra uses a measured 65 litres per hour with 19 tonnes of fuel and 43 litres per hour at light load (10% fuel)
Comparing fuel consumption on a length for length basis, Adastra uses less than a seventh of the fuel at 12 knots of a similar length semi displacement monohull, and a third of an LDL monohull.
For maximum range Adastra has extremely low fuel burn at 10.5 knots. 23 litres per hour with 19 tonnes fuel and 17 litres per hour at 10% fuel load. So if time is not an issue the range could be 10,000 miles starting with 30,000 litres of fuel.
It is clear from these figures and the actual measured fuel consumption of Adastra, that if all the factors that improve fuel consumption are achieved in one vessel the gains that can be made are huge.
Fig 6. Fuel consumption at 12 knots with minimum and maximum fuel for four 40m vessels.
At 40m LOA it is clear that the trimaran does not have as much accommodation as a 40m Semi displacement or planing monohull, however compared to the Hang Tuah or other similar LDL vessels the accommodation space is similar to Adastra. The outrigger stabilised monohull is an improvement because they have been able to widen the vessel at deck level, but they still do not achieve the same accommodation as the heavier wider designs.
If fuel economy is the aim, we suggest that LOA has to be much higher for the same interior volume. Due to the fact length and weight reduction are the key factors in achieving displacement to length ratios in the region of 20 and below, and length to beam ratios of 17 and above. In a superyacht like Adastra increasing the length of the main hull does not significantly increase the cost of the vessel, compared to the other costs of systems and accommodation, as long as the added length is in the bow, which is very low volume and low surface area compared to a conventional yacht.
In developing the Adastra concept we have found that when the LOA increases to 65 m and above, the same concept as Adastra can be retained, but with full standing headroom inside the wings enabling us to significantly increase the accommodation space in relation to the LOA.
Further Increasing the LOA to 75 metres and keeping a length to beam ratio of 17 it is possible to fit two double cabins side by side with a corridor between on the lower deck, and very large cabins on the mid deck extending into the wings. The additional length also enables us to maintain the displacement to length ratio required for maximum fuel efficiency.
Formulae referred to in the text.
The Displacement/Length ratio is determined by the following formula:
Displacement to Length ratio = Displ. / (0.01 x WL)3
Where: Displ. is the displacement in long tons (2240 lbs)
WL is the waterline length in feet.
The Length/beam ratio is determined by the following formula:
L/B ratio = WL / B
WL is the waterline length. B is maximum beam at the waterline.
The Search for a Seakindly Fuel Efficient Vessel
By John ShuttleworthClick here to Download a printable PDF of the article.
Part 2 - Motion at Sea
General principles of motion
Fig. 1. Six degrees of movement
Fig. 1 shows the 6 ways in which a vessel can move in a seaway. Comfort at sea will depend on how large these movements are, the frequency of movement and the accelerations. Slow movements can be less disturbing to crew than fast movements and high acceleration will cause discomfort and sometimes injury.
Safety is connected to sea kindliness, since a vessel that is easy on the crew will result in less fatigue and hence leave the crew more alert and energetic should a difficult situation arise.
The primary considerations are - how does the vessel get induced to move and how is that movement damped or controlled.
The main design aims for easy motion at sea are:
1. Reduce accelerations.
The study of these effects in the following discussion shows that with careful attention to the relationship between weight distribution and hull form, we can achieve a vessel that is seakindly and fuel efficient. The trimaran configuration has exceptionally good damping of movement thereby creating a vessel that is very seakindly, both at rest and powering through waves.
Roll accelerations are well documented as being the primary culprit that induces seasickness. All vessels are partly or only stabilised by the shape of the hull. In order to achieve fuel efficiency the trend is towards longer narrower hulls. In a monohull as the hull is narrowed the vessel becomes more prone to rolling and in all cases some means of roll damping is added to reduce this effect. A mathematical analysis of the hull forms in Fig. 2 shows that the LDL hull shape will have the least resistance to roll of the five hull forms. The catamaran and the trimaran have the highest, and although the trimaran has less of the outer hulls in the water than the catamaran, the wide beam gives the same or higher righting moment and hence the same or higher resistance to roll as the catamaran.
Fig 2. Underwater hull forms for five different vessels.
Conventional monohull yacht design theory states that ...."in general we observe that while greater beam will provide less roll angle, greater beam will also provide much more harsh, rapid, aggressive roll accelerations. Other factors being equal, stiffness (initial stability) varies as the cube of the beam. In other words, small changes in beam have a dramatic effect. We conclude from this that widening the water plane (increasing beam) will increase stiffness, but will at the same time reduce comfort and degrade seakindliness." Michael Kasten 2012.
In a monohull this is true, but in a trimaran provided the outriggers are immersed to the correct depth, the wider beam does decrease roll angle but does not create worse roll accelerations. In fact the opposite is true. The very high initial righting moment in a catamaran and trimaran of the Adastra type has a huge effect in damping roll. This has been proven by many sea miles covered in large catamarans, and in the case of Adastra in radio controlled model testing in waves and now in the full sized vessel. The owner of Adastra describes the motion as stately, and "like an old transcontinental steam train". Basically a small slow easy movement from side to side.
This is caused by the hulls of a catamaran and a trimaran reacting to waves in a completely different way to a monohull.
Monohulls in Roll
Wind warrior at sea. LDL monohull in waves.
The reason that a vessel will roll is that a varying force is being applied to the hull side by the passing waves. Fig. 3 shows three different positions of a LDL monohull in relation to the wave fronts. If the pressure from the waves was fixed then the boat would roll to a fixed angle of heel and stop there. However the diagram shows that the pressure changes constantly. Therefore the boat will roll back and forth in an attempt to stabilise itself in relation to the changing force on the side of the hull. This causes the vessel to roll.
Fig. 3. Waves passing along hull of LDL monohull. Long thin monohull length/beam = 7. LDL type. Wavelength 12.5 m. wave height 1.5 m.
LDL monohull and PJ 42m Supersport, bow on.
The photographs above show a typical low displacement to length ratio (LDL) monohull shape and the Palmer Johnson outrigger stabilised monohull (PJ 42m Supersport) side by side. Both vessels have a main hull length to beam ratio of 7, and it is clear that the outriggers on the PJ Supersport will reduce rolling, as the LDL hull on its own has very low resistance to rolling. In the standard LDL underwater hydraulic wing stabilisers have to be added, or a Gyroscopic system is required. These boats will still roll to some extent depending on the added damping system.
Catamarans in Roll
Catamarans have very high roll moment of inertia because most of the weight is concentrated in the hull. The theory behind why this reduces roll will be discussed later in this chapter. The high volume of the hulls and the high inertia prevent any significant roll. However, the varying loads on the two hulls does create sway. There can also be a sense of dipping the bow as the overall buoyancy of the platform changes. So the boat does not go through the water as if there are no waves, but the effect is different and rolling is virtually eliminated.
Moving diagonally to the direction of the waves, the catamaranÕs two hulls are crossing the wave face at different times. This means that the motion of the catamaran can be more complex due to the period of the lifting force varying in an irregular manner.
Fig. 4. Waves passing along the hulls of a catamaranEach hull length to beam ratio 10. Wavelength 12.5 m. Wave height 1.5 m. The sideloads on both hulls are large, varying, and are different on each hull.
Catamaran in big waves.
Sunreef 40 m Catamaran.
Trimarans in Roll In earlier versions of the power trimaran concept like Earthrace and Cable & Wireless, the outriggers were designed to skim the water. This reduced drag by about 8% compared to the position of the outriggers on Adastra. However the problem with having very low initial stability is that when a wave lifts the windward outrigger the downwind one has to sink to the depth required to counteract the lift on the opposite side of the vessel. If the outrigger is just touching the water it will sink very rapidly and then slow very suddenly as it picks up buoyancy. The image below shows how the outriggers of Cable & Wireless skim the surface, and hence how easily the boat will roll with high accelerations.
Cable & Wireless trimaran.
Adastra.Adastra on the other hand has both outriggers firmly in the water. As a wave lifts the windward hull, the opposite outrigger picks up buoyancy immediately and damps the roll. This is the same roll damping effect that is used in the PJ Supersport, except in Adastra the outriggers are 14 meters apart as opposed to 7.5 m, and are higher volume and more deeply immersed.
Fig. 5. Adastra (blue) with Cable & Wireless type outrigger in red.
Another difference between Cable & Wireless and Adastra is that Adastra's outriggers are longer and have more overall volume lower to the water which has a further damping effect on rolling.
PJ 42 m Supersport and Adastra - bow on view.
Comparing the PJ Supersport to Adastra in the above picture, with both boats at similar scale, it is obvious that the outriggers placed at 7 metres from the centre will have a massive effect in roll damping. Note that the main hull of Adastra with a length/beam ratio of 17 has no inherent stability. Not having to design the main hull with the constraints of creating stability from the hull form, allows us to design a hull that is optimised to reduce pitching, heave and yaw.
Fig. 6. Waves passing along the hull of Trimaran Adastra.Main hull length to beam 17. Wavelength 12.5 m. wave height 1.5 m.
Fig. 6 shows how the wave pressures affect the main hull and the outriggers. Although there is a varying force on the three hulls, the forces on the outriggers are much lower than the main hull. The larger lateral pressure on the main hull is not enough to cause the boat to roll provided the outriggers are immersed to the correct depth and have the correct volume. It can also be seen from the diagram that the effect of varying force on the hulls will be virtually eliminated compared to a catamaran. With the long thin hull, directional stability is excellent and the boat will not have the same uneven motion that can occur in a catamaran.
Effect of Inertia on Roll
The next important element is inertia. This is the vessel's inherent ability to resist movement and, rather like a flywheel, it is dependent on the weight relative to the centre of rotation. If the weight is concentrated at the perimeter it will be hard to get moving and then equally can be hard to stop once it is going. The rate at which a vessel begins to roll and then stops rolling is therefore highly dependent on the vessel's weight distribution and beam. While the hull form dominates the reaction of the vessel to the waves, the distribution of the vessel's mass has a very significant effect on the momentum and accelerations as the boat responds to the effect of the interaction between the hull/s and the waves. The roll moment of inertia is a measure of the vessel's resistance to rolling. The higher the roll moment of inertia, the more the vessel will resist rolling and the more slowly it will return from movement to a neutral state. Moment of inertia is calculated using the square of the distance between the resisting mass and the centre of rotation (C of R.). The effect of the square rule is that weights far from the centre of rotation have significantly more effect in increasing the moment of inertia than those close to the C of R.
One way of increasing the inertia in a monohull is to raise the Vertical Centre of Gravity (VCG), but as previously explained this can only be done to a limited degree in an LDL because the vessel will not pass the required stability requirements of the classification societies if the VCG is too high. On a trimaran we prefer to keep the VCG low as it means a smaller and lower volume outrigger is required to achieve stability in waves. The total volume of the outer hull is determined by the lift required when the hull is following the face of a steep breaking wave. It can be reduced as the VCG is lowered and the beam is increased. The smaller the outriggers the less they will drive the pitching or yawing motion of the vessel which is discussed later in this article.
In a trimaran like Adastra we placed generators and engines for back up propulsion, and for maneuvering in the outer hulls. These weights are 7 m from the centerline and therefore create very high roll moments of inertia. A further benefit is that the generators are kept separate from the main hull and living areas. A weight positioned 7 metres from the centre of rotation has a roll moment of inertia of 49/14 = 3.5 times the roll moment of inertia of the same mass at 3.75 metres from the centre of rotation, as in the PJ 42m Supersport LDL monohull. It is not surprising then that we find that Adastra extremely stable at anchor.
In a catamaran most of the weight is in the two hulls, and so this type of vessel has a very high roll moment of inertia and is very resistant to roll accelerations. The effect of this is that the catamaran is very stable in long wavelengths where the hull will follow the wave face in a gentle manner while the monohull will always roll to some degree. Adastra will also follow the wave face in a similar manner to the catamaran without the complex swaying movement as described in the section above on catamarans in roll.
The next most important movement in causing uncomfortable motion is pitching. This is where the vessel is rocking in the fore and aft direction. High accelerations and large movements in this direction can be very disturbing and in extreme cases can cause injury due to the high G forces that can be generated.
In a similar way to the factors that cause and control roll, in pitching the hull form reacting to the wave movement along the hull or hulls is the primary factor inducing the motion. The displacement of the vessel and the distance of the centre of gravity from the centre of rotation (C of R) will be the main factor in creating damping. The volume of the forward part of the vessel and its relationship to the volume aft will also pay a part in damping pitching.
Effect of Inertia on Pitching
The pitch moment of inertia is the sum of the moments of inertia of all the weights in the vessel x the square of the distance of each weight from the C of R.
It is a common misconception that a vessel will pitch either around the centre of Gravity (C of G) or the centre of the waterplane area. A fairly simple calculation of the relationship of forward momentum and rotational momentum as a vessel accelerates shows that the centre of rotation moves aft from the centre of the waterplane area, and eventually ends up about a quarter of the waterline length forward of the stern.
It is clear then from the diagram in Fig.7 that concentrating some weight in the vessel further forward will increase the lever arm of the pitch moment of inertia, and since that effect is proportional to the square of the distance between the C of R and each weight, the effect of pitch damping can be quite dramatic. This approach has been proven on the sea trials of Adastra at full size. It is possible sometimes to create a resonant pitching frequency in the vessel that can match the frequency of the wave fronts, and then the boat will pitch more. This can be controlled by moving weight if necessary. In the case of Adastra we have fuel tanks distributed along the bottom of the hull with a capacity of about twice the normal full fuel load. This means that weight (fuel) can be moved along the hull fore and aft to create optimum trim and/or pitching control.
Fig. 7. Adastra - Distance of Centre of Gravity (C of G) from centre of rotation (C of R) at speed.
The Effect of Hull Shape and Position on Pitching
In the case of a trimaran it might seem sensible to put the outriggers right at the aft end of the vessel so that the effect of the two outer hulls pitching forces are completely damped by the inertia of the main hull. The picture below shows the approach taken in the Earthrace design of pulling all the weight aft, including the engines. We believe that this will cause the boat to pitch more than it would if the outriggers, accommodation, wing beams and engines were further forward.
We have found that the correct size outriggers do not have the power to make the vessel pitch even if placed further forward. This means that the outriggers can be placed to create maximum stability, and further away from the C of R thereby increasing the pitch moment of inertia of the vessel.
The image below shows the bow of Adastra"s outrigger and it is visibly obvious that the upward lift of the bow will have little effect on the pitching of the main hull. This can also be shown mathematically.
Catamaran and Adastra outrigger bow on view.
The catamaran on the other hand has two bows which are more buoyant than the single bow of the trimaran as the length to beam ratio of the trimaran is in the range of 17 to 20 as opposed to 10 for both of the catamaran hulls.
In the image below it is again visibly clear that the very long thin main hull bow of Adastra will cut through the waves rather than lift upwards. The amount of lift induced by the bows will be significantly less than any of the vessels shown.
LDL monohull, PJ supersport, and Adastra bow on views.
The other factor of induced motion in pitching is the way the wave will lift the stern. So even if the bows of the vessel are lengthened and made narrow, the stern could have too much volume and hence lift the aft end of the vessel and dip the bow. Therefore it is essential to keep the centre of gravity far enough forward to allow the stern to work with the bow to reduce rather than increase pitching.
The long thin main hull of Adastra has the least tendency to lift in a wave of any vessel because of the relatively thin sections forward and aft. In seas with a wavelength up to approximately 70% of the waterline length and a typical maximum height 3 metres she will slice straight through the waves with very little pitching. The outrigger volume is balanced so that they are just large enough to create sufficient roll stability but slender enough to cut through the waves and thereby induce very little pitching motion into the boat. As the wavelengths get longer she will start to follow the wave surface, but with an easy motion which can be further controlled by choosing the best speed and angle to the waves for the conditions.
The image below shows how large the bows of a semi displacement vessel are compared to the LDL, the catamaran and the trimaran. The reason that this does not cause as much pitching as would appear to be the case from the hull form, is because the pitching moment of inertia is high, and the large weight of the vessel damps the movement.
Semi displacement yacht shows volume of bow.
If we refer to the bow on views of the vessels in the previous sections, it will be visually apparent that the hull of Adastra, with the near vertical hull sides and very narrow long thin shape will be lifted by the waves the least of any hull shown. This can be shown mathematically by calculating the rate of increase of volume with hull immersion, and comparing that for different vessels. But the mathematics only shows what the images reveal.
The narrow beam of the main hull will allow waves to rise further up the hull sides without lifting the vessel. Wider hull vessels will heave much more than thin hulls. In heavier wider hulls some of the effect of heave will be counteracted by the forward momentum and this will reduce heave, but in a relatively light vessel like Adastra the long thin hull and the forward momentum combined will control heave.
Surging is the tendency of the vessel to accelerate and decelerate fore and aft. In order to achieve fuel efficiency the vessel needs to be as light as possible. As the boat becomes lighter its ability to drive through waves without decelerating decreases. By making the hull very long and thin, this effect is reduced. Thereby reducing surge to a minimum.
A long thin hull with a skeg aft has very good directional stability. This will reduce yawing to a minimum. The most serious form of yawing is broaching. Broaching is caused by the pressure on the bow slowing the boat, while at the same time the overtaking wave pushes the stern around from aft. The long thin bow of the trimaran cuts more easily through the water reducing this breaking effect on the bow. The lower drag of the hull allows it to move more easily forwards as the wave pushes from aft, and the long hull will track in a straight line down the wave face. This effect has been proven many times in large racing multihulls.
The movement of Adastra sideways in waves will be similar to a long narrow monohull vessel. In normal conditions swaying will not be a significant cause of discomfort in either of these types of vessel. In large breaking waves the ability of the vessel to drift sideways when struck by a breaking crest can be an important factor in dissipating the energy of the wave impact.
Inertia of forward movement
With all vessels the effect of forward movement has a gyroscopic effect (like riding a bicycle), and that creates more inertia and reduces sudden accelerations. Due to all of the points discussed so far Adastra is more easily driven than the other vessels considered in this article, so she can move faster through waves than vessels with a wider hull. This enables her to use inertia created by forward momentum to dampen induced accelerations still further, and gives the captain of the vessel a wider range of speeds to choose in order to achieve minimum pitching.
The search for greater fuel efficiency appears to be limited in the monohull form by the difficulty of creating a seakindly vessel with narrow beam. The solution to use small outriggers to reduce roll on a LDL monohull, still does not make the vessel as efficient as a power trimaran.
Power trimarans can be made to be very seakindly, provided the full benefits inherent in the three hull configuration are taken into account at the design stage, and careful attention is paid to the effect of mass distribution and hull shape on the inertias and driving forces induced by waves.
Very different hull forms are possible in a trimaran compared to monohulls which creates new possibilities for efficiency and comfort at sea.
Adastra has proven to be extraordinarily fuel efficient and in the words of her owners is also proving to be "a good sea boat, and a joy to be aboard".