Monohull or Multihull?

Working with small vessel designs means working with two-decimals accuracy.

There is a common misconception that the smaller the vessel, the easier the design work. This is a myth that we are certainly committed to fight, after dealing with more than 150 vessels with lengths between 12m and 15m. Our day-to-day reality shows that designing and optimising a 15m concept can prove way more challenging than working with a 60m design. Given their hypersensitivity to weight distribution or the limited space availability on board, workboats are indeed a true engineering challenge.

A failure to recognise this complexity may lead to appointing a designer who is simply not experienced enough to recognise the possible pitfalls and plan around them. This can lead to under-estimating risks, scope creep or ‘under the radar’ execution when the design work does not start up in a controlled way.



So, what is the perfect hull shape for workboats / multipurpose vessels? The answer, ‘It depends’!

The perfect hull shape would return excellent stability and maneuverability performances, would be able to carry large amounts of cargo and would be as inexpensive as possible to build. Unfortunately, if naval architecture has taught us anything, that thing is the art of compromising.

Starting from the top, in terms of stability multihulls are stiffer and show far superior stability performances at smaller angles of heel (GZ max is usually at around 20-25 deg), however monohulls have a greater range of stability (60-70 deg for monohulls compared to 40-50deg in the case of multihulls). This means that in relatively decent wave heights, the obvious choice is the catamaran, however, in really choppy seas, the catamaran can lose stability quite fast, whilst the monohull is able to right itself after large angles of heel.

Talking maneuverability, catamarans are wider than monohulls so by default, the propulsion lines are farther apart which makes maneuvering them much easier and more precise than monohulls. To match the same maneuverability performances, a monohull would require a bow thruster.

In terms of cargo carrying capabilities, although narrower compared to catamarans, monohulls have greater buoyancy reserves thus allowing for far larger amounts of cargo to be accommodated on board, both above main deck, but also in underdeck compartments.

Cost-wise, with their complicated shapes, catamaran will always be more challenging to build and thus considerably most costly.


The below table shows the parallel between the characteristics of monohulls and catamarans, for identical overall lengths:


So, what’s the best choice?

If the operational profile requires great stability and maneuverability, as with crew transfer or survey vessels, then the catamaran is the way to go. If the owner is looking for a low-cost vessel capable of accommodating large cargos on board, such as fishing vessels, then the monohull is the right option. So, the answer to the opening question truly depends on the owner’s needs and budgets.

GLO Marine has dealt with more than 150 cat and monohull workboats up to date, solving out various engineering challenges from stability to hull optimisation and on-board arrangements, making it one of the most experienced companies in Europe on this vessel segment. The main takeaway from this experience is the fine tuning involved in the concept design stage, looking at sensitivity to weight distribution, the importance of trim in reducing forward resistance and the challenges related to space allocation. All these engineering senses have been trained in 4 years of continuous hands-on experience.