Mono vs Multihull


In the search for increased efficiency the choice of hullform is a critical factor. But what type of hull is best? The basic choice between monohull, catamaran or trimaran is frequently debated with advocates of the multihull claiming increased efficiency over the simple monohull.

However, the answer is more nuanced than it may seem.

Within the commercial and naval marine markets significant research and development has been undertaken over many decades to assess the relative merits of the application of multi-hull platforms to meet the performance requirements of a wide range of vessel types, size and speed ranges.

There are many factors to consider when making technical comparisons between different platform hull types. The precise quantitative comparison of hydrodynamic behavior must also be balanced with analysis of seakeeping and stability, and the qualitative assessment of operational aspects that determine suitability to meet any particular set of functional and performance criteria.

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, with only a few examples in existence above 35 meters. Compare this with the commercial market where the dominant hull type is that of the catamaran.

Some years later a new owner of the 38-metre 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 us our letter, “I want my yacht to look like that!” He was, of course, pointing at a monohull.

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.

Hydrodynamically, any multihull is a method of reducing wave-making drag. By dividing the total displacement between two, three or more hulls the length-to-displacement ratio is improved and in turn the wave-making component of the total drag is substantially reduced. Across much of the speed range of interest the wave making drag is the dominant component, and it is the length-to-displacement ratio which is the most influential variable. It is often cited that the benefit of the multihull is derived from the very slender hulls, as characterized by the visibly low waterline beam of each hull. However, length-to-beam ratio is a second order variable, and it is length in relation to displacement which is the most influential parameter.

The disadvantage to dividing up the displacement is that the wetted surface area is increased, resulting in higher frictional drag. In regions of speed-to-length ratio where wave making drag dominates (moderate to higher speeds), the multihull will therefore be at an advantage whilst in areas where frictional drag is dominant (low and very high speed) multihulls typically exhibit higher overall drag than a comparable mono-hull. Multihulls also exhibit wave interference effects from the component hulls, which can cause a decrease or increase in drag. These effects tend to only be pronounced at lower speeds and heavier displacements, and practical concerns often constrain the naval architect's ability to vary the spacing of the hulls once an overall beam is fixed.

The hydrodynamic merits of multihulls are not the only factor to consider. Multihulls offer planform area benefits over those of a mono-hull. Driven by a greater overall beam relative to a monohull, the multihull offers a larger planform area upon which to deliver the functional aspects of performance that are related to linear dimensions. The drawback is that space below decks, being split between multiple hulls, is more restricted and compartmentalised.

These are of course very generalized 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 multihull? Is comparing similar lengths a fair basis for comparison? When fairly comparing a catamaran, monohull and trimaran we would expect that all three would be at differing lengths, with the trimaran the longest.

On a fixed length basis the multihull will be significantly heavier; for example in the case of a catamaran 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 meters, whilst the monohull some 25–30% longer at around 50 meters; the relatively larger beam of the catamaran means it is effectively the same size as a monohull that is 10 meters 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 a multihull offers is the increased beam, which gives a bigger impression of space both internally and externally offering greater scope for innovation in entertainment, owner and guest areas. While 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.

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 designer 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 to seakeeping ability. 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 stable platform, but a 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 will be much higher. Some guests and crew find such a motion quite uncomfortable.

Due to the very high stability the catamaran will not benefit significantly from active at rest stabilization as a result (zero speed stabilization), but the use of an active ride control system when under way will soften roll (and pitch motions) when there is an adequate turn of speed to the design. 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 meters) 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 meters+ on an 80-metre) effectively limits the number of established yacht builders where such a project can be tackled. 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 greater flexibility: same space, greater flexibility of space and equivalent powering all on a shorter yacht.

All Things Swath

A SWATH hullform (Small Waterplane Area Twin Hull) is effectively a means to reduce stability in a multihull. By making the waterplane area very small, heave motions are greatly reduced as well as increasing the roll period, making a SWATH far more comfortable than a conventional multihull. The waterline beam of a SWATH can appear unfeasibly narrow, perhaps as wide as just one meter. However the improvements a SWATH delivers over a conventional multihull does not come without a cost, and the SWATH platform exhibits a very high drag penalty and a high wake wash relative to a normal multihull or monohull. In trying to strike a balance between these conflicting factors, we have developed the XSS, or eXtreme Semi Swath.

What is Conventional?

Our conventional catamaran hull has been optimised to give exceptional performance in calm-water and has been extensively proven at full scale in a wide range of commercial and workboat applications. However, whilst conventional catamarans have a large waterplane area and beam giving a very stable platform this can result in a short roll period often causing uncomfortable motions. For this reason the conventional catamaran hull not gained widespread use in large yacht applications.

What is SWATH?

A SWATH is a ‘Small Waterplane Area Twin Hull’ vessel. As a direct result of its very low waterplane area a SWATH achieves a far lower level of stability than a conventional catamaran, yet maintains the same beam overall thereby giving a more pleasant motion in roll and heave. A SWATH vessel offers unrivalled seakeeping performance in rough waters, however the compromise is a very high resistance penalty and high draught. High powering requirements, a complex hull form geometry and a lack of useable volume within each demi-hull are primary reasons why SWATH technology is often only chosen for very specific applications and is disregarded not only in the development of new yacht concepts, but also the greater majority of projects in the commercial and military sectors.

What is ModCat?

Following our work at the forefront of multihull design we were commissioned by the US Navy to undertake a far reaching research and development project to develop a catamaran hull form with significantly improved seakeeping performance, but with minimal resistance penalty; the semi-SWATH ModCat hull was born. With narrower sections at the waterline, a lower centre of buoyancy and a slender bulb at the bow model tests demonstrated that the vertical accelerations of the ModCat were up to 50% lower than those of a comparable conventional catamaran with only a 5% increase in power required to achieve the same speed. The ModCat hull has subsequently been adopted for the 79m, 55 knot US Navy ‘Sea Fighter’ and 59m fast passenger ferry ‘Betico II’ for operation on very exposed routes in the Pacific Ocean. More recently major oil companies have commissioned designs for ModCat vessels to replace helicopter operations for crew transfer to offshore oil platforms. The advances made in developing ModCat open up the possibility of developing catamaran yachts offering high-speed performance with significantly improved seakeeping ability.

What is Extreme Semi-SWATH (XSS)?

XSS technology effectively closes the gap between ModCat and SWATH technology, enabling further improvement in comfort in high seas not only during transit, but also at zero speed. This has been made possible by taking the ModCat’s semi-SWATH form and making it more extreme, such that the waterplane area is further reduced and the centre of buoyancy is shifted further below the waterline. However, the hullform has been developed to ensure that the volume within each demihull is useable, unlike a true SWATH. The powering requirements of the XSS remain significantly lower than those of a true SWATH. The resulting hullform evolution of ModCat to XSS presents an even more attractive option for yacht-owners looking to take advantage of the catamaran platform for new designs. XSS designs are currently proving themselves in the demanding offshore energy industry and further designs are in build and under development.

The Pentamaran

Hydrodynamically a stabilised monohull platform (also called a trimaran) allows the optimisation of the central hull to be undertaken without the traditional constraints of a conventional mono-hull. On such a platform stability can be added, and indeed finely tuned with the sizing of the sponsons (also frequently termed ‘outriggers’ or ‘amas’). In this regard the stabilised monohull platform can have its stability characteristics (both intact stability and roll stiffness) more closely engineered and tuned than that of a catamaran and even to some degree that of a conventional monohull.

However it is not just intact stability and roll stiffness characteristics that are a driver to the design of trimaran sponsons. Damage stability characteristics play a significant factor. Typically damaged stability requirements will increase the length of the sponsons required to meet post-damage criteria. This increase in sponson size carries a resistance penalty.

During the mid 1990’s Nigel Gee & Associates investigated a variant to the traditional trimaran platform to address this issue. The solution developed was to have two sponsons per side, with only one set on either side being immersed in the static upright condition. Such a configuration allows for far shorter and lower immersion sponsons to be used to achieve the intact stability and roll stiffness characteristics desired, whilst the forward sponsons provide the reserve of buoyancy needed at higher heel angles or in damaged conditions, without any drag associated penalty.

This design solution was termed the Pentamaran. A significant number of designs have been developed and model tested, from smaller (65m 60 knot) high speed fast ferries, large (300m 50 knot) trans-oceanic fast freight ships and high speed sea-lift naval logistics ships and fast combatants. The Pentamaran platform is also the subject of numerous published technical papers in the public domain and international patents.

The Pentamaran has been developed further by BMT in 2020 with a number of variants designed to meet the specific challenges of long range autonomous operations.

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