THEYACHT The leading magazine for the design, construction, management, ownership & operation of luxury yachts report Issue 114 June 2010
Catamaran or Monohull? Two are better than one... Whether that is true for hulls is a matter of a number of complex balance of factors such as use type, speed and the type of conditions anticipated. Here James Roy and Alex Meredith Hardy of BMT Nigel Gee Ltd look back over 20 years of design to try to answer that question. Back in the early 90s we designed a 38-metre, 40-knot catamaran yacht. As experts in the field of fast commercial catamaran design we thought at the time that the (then relatively infant) superyacht community would be in awe of this and many more catamarans would follow. The phone didn t exactly ring off the hook and the (large) catamaran yacht has never really gained any significant market share over the last two decades, with only a few examples in existence above 35 metres (the Bannenberg styled Lady Moecca being a well-known example). Compare this with the commercial market where over 40% of the world fleet of high-speed passenger ferries (>25 knots) are catamarans, compared with only 20% as monohulls. The other 40% are made up of other types of hulls such as Hydrofoils, Trimarans etc. Some years later a new owner of the 38-metre catamaran called us to come and see him in the Western Mediterranean. Sitting on the aft deck he told us that he loved the boat but did not feel comfortable berthed stern to with long white monohulls on either side all with raked bows, whilst the bow of his yacht was effectively a right angle. Gesturing to the yachts moored on either side he then gave me my brief, I want my yacht to look like that! He was, of course, pointing at a monohull. How do you begin to compare a monohull and catamaran? Is comparing similar lengths a fair basis for comparison? The catamaran presents a difficult styling challenge, even if done well it is not to the taste of the majority and some of the elements of styling that might soften the boxy plan-form are often conflicting to the catamarans technical abilities. This article will explore and compare, in an unbiased manner, the catamaran v the monohull for a yacht application. Hydrodynamically, the catamaran is a method of reducing wave-making drag. By dividing the total displacement between two hulls the displacementto-length ratio (which is the most influential parameter for wave-making drag) is improved and in turn the wave-making component of the total drag is substantially reduced. The payback is that the wetted surface 102 ISSUE 114
Catamarans & monohulls clockwise from top left: typical commercial catamaran; Tamsen Yachts tatii; Sanlorenzo SD122-FX; 40-metre CAT final profile; Lady Moecca; the launch of an Admiral 42. ISSUE 114 103
area is significantly increased so the frictional component of drag increases. In regions of speed-tolength ratio where wave making drag dominates (moderate to high speed), the catamaran will therefore be at an advantage whilst in areas where frictional drag is dominant (slow speed) catamarans typically exhibit higher overall drag than a comparable monohull. Catamarans also exhibit wave interference effects from the two hulls, which can cause a decrease or increase in drag. These effects tend to only be pronounced at lower speeds and practical concerns often constrain the naval architect s ability to vary the spacing of the hulls once an overall beam is fixed. For a catamaran with any reasonable speed potential the use of aluminium or composite as the main construction material is the only viable choice. With the very high wetted surface area the structural weight fraction is relatively large and steel catamarans are seriously penalised by structural weight. These are of course very generalised statements; the exact balance of comparison will depend heavily on the actual displacement and length values with which the comparison is made. This begs the question: how do you begin to compare a monohull and catamaran? Is comparing similar lengths a fair basis for comparison? On a fixed length basis the catamaran will be significantly heavier; the wider beam resulting in an increase of usable interior volume of approximately 50%. It would, however, be a fairer comparison, especially for a yacht, to compare the catamaran and monohull on a fixed GT basis. Using the 500GT threshold as an example the catamaran could be approximately 40 metres, whilst the monohull some 25 30% longer at around 50 metres; the relatively larger beam of the catamaran means it is effectively the same size as a monohull that is 10 metres longer. Using this as the basis for comparison the catamaran has some work to do as with a shorter waterline it is now disadvantaged from a drag and power perspective. However, running the numbers on some real designs reveals that the benefit the catamaran offers in reducing wave-making drag offsets the penalty of having a shorter waterline. In effect the shorter catamaran matches the longer monohull on a speed v power basis. From an arrangement perspective the principal advantage the catamaran offers is the increased beam, which The catamaran presents a greater challenge from a seakeeping perspective where its ability can be more limited than that of the monohull if not correctly addressed. gives a bigger impression of space both internally and externally offering fabulous entertainment, owner and guest areas. Whilst shorter, the catamaran will achieve the same useable deck area as that of the longer monohull. The drawback is that space below decks, being split between two hulls, is more restricted (by nature of the fact that it is segregated between port and starboard hulls). Back to naval architecture. The catamaran presents a greater challenge from a seakeeping perspective where its ability can be more limited than that of the monohull if not correctly addressed. In general the limiting factor is the wet deck clearance (the distance from the connecting structure between the hulls to the water surface). If this is too low then slamming will begin to occur, necessitating a change in course and speed. Avoidance of this at the design stage requires a healthy wet deck clearance to be adopted, which is undoubtedly at odds with the wishes of the stylist to reduce the overall profile line of the yacht. Unfortunately there is little room for compromise here; some changes in hull shape to adopt more SWATH (Small Water plane Twin Hull) like section shapes can help, but the wet deck clearance remains a fundamental. Some proponents of the wave piercer designs will claim that the adoption of a third central bow will fully alleviate this problem, but this is not the whole answer. The third bow does, however, make the styling slightly less of a challenge. The high beam of the catamaran will also lead to a very stiff roll period. This means that roll angles are likely to be significantly lower than those found on a monohull but that roll velocities and accelerations can be higher. Some guests and crew find such a motion quite uncomfortable. The catamaran will not benefit significantly from active at rest stabilisation as a result, but the use of an active ride control system when under way will soften roll (and pitch) motions. Such systems are in common use in commercial designs and if correctly specified are very effective. The need to maintain a healthy wet deck clearance would lead one to conclude that the very large catamaran (>50 metres) begins to make more sense as the impact on the profile height will produce a design of more balanced proportions. Additionally for very large yachts that have a higher speed requirement the catamaran becomes a serious contender. For an 80-metre requiring speeds upwards of 25 knots the catamaran is a good hull choice. However, building such a yacht is difficult as the large beam (18 metres+ on an 80-metre) effectively limits the number of established yacht builders where such a project can be tackled (and berths it can occupy Ed). The catamaran also makes a lot of sense for the moderately sized (500GT) medium to high-speed yacht, or the so-called sport yacht genre (typically the 30 50-metre, 30-knot yacht). Being on the 500GT limit these yachts are always pushing the boundary of available space and for these applications the catamaran platform can, in our opinion, offer 104 ISSUE 114
greater flexibility: same space, greater flexibility of space and equivalent powering all on a shorter yacht. We believe that the key is not to try to drive the styling towards established monohull profiles but to work with what you have. So will the phone ring off the hook with enquiries for catamarans? We suspect not but designers are starting to explore the use of the hull form in greater numbers. As with everything in the superyacht business, watch this space... James Roy Alex Meredith Hardy BMT Nigel Gee Ltd To comment on this article, email issue114@synfo.com with subject: Catamaran or Monohull MLC 2006 Design firms have been criticised for burying their heads in the sand over MLC 2006 or believing that the yard will deal with that. Redman Whiteley Dixon were the exception and participated in the MCA Working Group. Here Justin Redman gives his view. We consider the MLC 2006 to be a subject that has somewhat crept up on our industry and as a result, this important issue is now having to be dealt with almost retrospectively, never a good thing As a studio, we are the only independent yacht design office represented on the consultative committee at the MCA and as such we are helping to shape the response and future direction of the implementation of the MLC in the yachting industry. We importantly feel that the majority of the MLC should be embraced by our industry as it covers mostly the welfare and social well-being of the crew. However, the only major sticking point from the yacht industry s point of view, and this is the only point, is the onerous crew cabin floor area requirements. All of the rest of the document has been accepted by the industry representatives that also serve on the consultative committee with ourselves. From a design standpoint, the crew cabin area requirements will render a large swathe of the new yacht market with unviable yachts for use for their intended purpose as a private yacht. The areas required will mean having to use a large percentage of the floor area aft, traditionally the realm of the guests, to accommodate the crew accommodation; thus removing the appeal of a new yacht to future owners and charterers, due to the lack of guest accommodation. We have been engaged upon a design process with De Vries and Royal Huisman to raise the standards of appointment and finish for crew areas in the design projects that we have been involved with. We feel very strongly that this is an important part of the design of yachts, in order to attract and retain the best crew for our owners and provide an environment for the crew to live and work in, in which they will feel comfortable and at home. It is becoming rather self evident that the legislators at the ILO who drafted the MLC document do not understand the situation that they have created and have no comprehension of effect of the MLC regulations upon the yacht sector. Various organisations within the largeyacht sector and the MCA consultative committee are now engaged upon a process of trying to educate the ILO about the impact that the MLC will have upon our industry. There now appears that there may be a way forward in the short term, but as an industry we must all work together with the ILO and the bodies that represent our industry in finding an acceptable solution for us all. In the long term we could hope to get an amendment to the MLC 2006 regulations. However, due to the extraordinarily slow process this may take 10 15 years to come into being and in reality is unlikely. What we do not really want is a permanent exemption, as all the rest of the regulations are fair and should be embraced as being good for the long term welfare for yacht crew. In the meantime we have to design to the requirements of the MLC document for our forthcoming projects that will start sometime in late 2011/12. However, with the movable date of when the regulations will come into force and combined with their current onerous nature, these are creating unfortunate uncertainty and possible delays in the defining of new build projects, which in the current climate none of us need Justin Redman Redman Whiteley Dixon To comment on this article, email issue114@synfo.com with subject: MLC 2006 ISSUE 114 105