SUBMISSION TO NET ZERO, ENERGY & TRANSPORT COMMITTEE From Professor Alf Baird, 20th March 2023 SYNOPSIS OF PAPERS PRESENTED AT THE ‘CATAMARANS FOR INTER-ISLAND NAVIGATION’ SEMINAR Held at Strathclyde University, 12 May 2022 The presentation summarized Dr. Ballantyne’s design and evolution of catamaran ferries since the 1980s to today. This included the development and testing ofContinue reading "AS THE SCOTTISH GOVERNMENT POUR MILLIONS MORE INTO BUILDING THE WRONG FERRIES."



From Professor Alf Baird, 20th March 2023



Held at

Strathclyde University, 12 May 2022

  1. EVOLUTION OF CATAMARAN DESIGN by Dr. Stuart Ballantyne

The presentation summarized Dr. Ballantyne’s design and evolution of catamaran ferries since the 1980s to today. This included the development and testing of catamaran hull designs, materials, propulsion systems, and the gradual scale shift towards larger capacity vessels for cars and trucks. The improved efficiency of catamarans compared with monohulls was illustrated, as was the capability of medium-speed steel-hull catamarans to operate in higher wave conditions, allowing these vessels to replace monohulls on exposed routes. The lower weight and displacement of catamarans was highlighted, which provides for much lower power requirements for a given speed of vessel compared with monohulls. 

The improved seakeeping of the catamaran was demonstrated by various tests based on optimized hull forms, and since confirmed via extensive operational experience over the past three decades. Results of extensive tank-testing also proved the concept and allowed for further optimizing hull forms and bow shape, further improving efficiency levels. A range of standard proven designs of medium-speed Ro-Pax (passenger and car/freight) catamarans are available from 35m to 85m length, with even longer vessels tank-tested. Orders and successful operating experience for vessels of 70m length and more has confirmed the catamaran as a viable and superior alternative to any monohull of similar capacity. Further design refinements have continued to improve the catamaran’s competitiveness and its superior stability and safety features. 

Other operational roles of catamarans were presented, such as tugs, offshore vessels for fish farms and wind energy installations, plus naval and coastguard operations. Recent refinements including foam installation in the lower hulls seek to improve further the survivability of catamarans, which is already vastly superior to monohulls according to research studies (see below). In event of collision, catamaran stability is greatly enhanced by the two hulls, and foam inserts preventing water ingress. Foam inserts and adjustments to sub-division arrangement allows for more car space in the side hulls, further increasing efficiency of this ship type relative to any monohull, the latter also requiring spacious and heavy ballast tanks in the hull to ensure stability.


This research presentation highlighted the lower displacement and much reduced power requirements of catamarans compared to monohulls, for the same payload. Lower displacement helps the catamaran maintain a good payload (vehicles and passengers) and even greater speed with only half the power requirement of a comparable monohull. 

Efficiency indexes and calculations demonstrated the superior efficiency of the catamaran. The catamarans are found to have twice the level of efficiency compared to comparable monohulls in terms of speed/payload/power, and are also superior in terms of deadweight/displacement, what is termed ‘revenue-deadweight’. Furthermore, the catamarans are found to have only half the emissions of CMAL monohull ferries.

New ferry prices for catamarans are between a third and half the price of CMAL monohulls. This is the case across all ferry sizes and capacities, small, medium and large. Larger 100-car capacity monohull ferries are costing CMAL £50m for each vessel ordered in Turkey, whereas the same-capacity catamarans can be secured for around £20m. This implies that 10 x catamarans could have been purchased for the same price as the 4 x ‘Islay Class’ monohulls CMAL has on order in Turkey.

Part of the reason for this price difference is the much lower weight and displacement of catamarans, with less volume of materials and internal space required in building a superior hull form for the same given carrying payload and speed; the CMAL monohulls have a displacement of around 3,000 tonnes compared with the catamarans’ 1,000 tonnes. The catamaran also avoids several expensive added requirements that a monohull needs such as ballast tanks and stabilizers; these items are not required on the catamaran due to its superior inherent natural stability. The monohull also requires twice the engine power, and larger fuel tanks. Industry evidence confirms that catamarans can be built in around half the time of a comparable monohull. This means that a shift to ordering catamarans would not only be much less expensive, the present outdated fleet could be replaced far quicker too.

The annual operating costs of catamarans is half or less that of monohulls. In the case of the larger 100-car capacity vessels the catamaran offers a potential operating cost saving over the ship’s 25-year lifetime in excess of £100m. Lower operating costs are due to the much lower fuel consumption (and lower emissions), reduced depreciation, lower repair and maintenance cost, lower insurance cost, and reduced crew cost of catamarans. In effect this implies a potential operating saving of around £1 billion for CalMac’s 10 x large ferries in its fleet over 25 years if changing to catamarans. Additional savings in operating costs would be expected if smaller ferries were likewise replaced by more efficient catamarans.

The presentation discussed CMAL’s (and CalMac and Transport Scotland) previously stated objections to the ordering of catamarans for use in the Scottish state ferry system. These objections, it was argued, should be regarded as ‘fallacies’ put forward by CMAL, i.e. spurious objections which have no substance in fact. They principally include CMAL ‘concerns’ related to the catamaran’s perceived reliability, seakeeping, port access, deadweight, cabins, and regulations. The operational experience of catamarans in Orkney and in many other places around the world demonstrate that these ‘concerns’ are totally unfounded in practice. They are merely ‘fallacies’ that have been invented by CMAL and others (e.g. CalMac, RMT) in order to justify the ongoing immense public sector commitment in ordering far more expensive, yet less efficient monohull ferries.

Moreover, CMAL’s vessel procurement process intentionally restricts bids to just monohulls, with only the dimensions of monohulls being specified in the tender documents. This specifically excludes catamarans from the process because catamarans have different dimensions to monohulls in terms of length, beam and draft. The vessel procurement process should instead focus primarily on the required carrying capacity and speed of the required ferries, thus opening up the tender to superior and more efficient design options such as catamarans. The lower power/lower emission catamaran is likely to win on environmental and emissions criteria, as well as lower capital and operating costs, and on superior safety (see final presentation below), plus reduced recycling costs at the end of the useful life for what is a lower-displacement ship.

The presentation concluded by noting that catamaran ferries were successfully operating for numerous ferry companies around the world, including in Scotland, all of whom had moved away from monohulls and had not gone back to monohulls once they made the shift to catamarans. It was proposed that the Scottish Government should likewise move rapidly to procure catamarans offering lower costs and lower emissions and more rapid delivery than monohulls, with a very high level of certainty as to their effective and efficient operation on routes in Scotland.

  • DAMAGE STABILITY & SAFETY OF CATAMARAN PASSENGER VESSELS by Professor Dracos Vassalos and Dr. Donald Paterson

The presentation gave an overview of damage stability and flooding risk in ships, also providing key definitions and reference to relevant legislation. The focus then moved to Ro-Ro passenger ferries. The analysis presented the results of studies comparing monohull and catamarans in terms of damage stability and relevant legislation.

Each year there are fatalities aboard ships around the world. Damage stability is a key feature to consider in ferry safety analyses. The inherent stability of catamarans (two hulls) offers significant potential benefits for safer inter-island ferry operations. 

The weight of the hull and submerged element (draft) is a key feature; monohulls are heavier and have a deeper draft than monohulls hence are more susceptible to damage. Where integrity is lost or seriously damaged the added weight of a single hull vessel will tend to hasten sinking. The risk of such events can be reduced by use of a multihull such as a catamaran.

The presentation gave results of research of a 75m catamaran (100 cars/400 passengers), evaluating the vessel in accordance with IMO Regulation SOLAS 2020 Ch.II-1, looking at the existing vessel and also the vessel with intended foam additions in the hulls. Low windage profile of the catamaran is an important advantage, relative to the higher monohull that is subject to higher windage, this affecting the rolling motion as well as berthing capability in high winds. 

The lower weight of the catamaran vessel relative to payload favours the catamaran over a monohull. Results indicated that the catamaran has a 48% lower Lightweight/Deadweight ratio on average relative to the monohull.

Comparing the GM margins (static stability measure of a floating structure) of the catamaran with the monohull, “the margins are 24X that of the average monohull design”. This is described as “remarkable and something that simply cannot be achieved using monohull designs”. Assessment of the vessel was made under loaded conditions. The catamaran achieved an attained index of 0.939 “which demonstrates a remarkable degree of damage stability”. 

A comparison was made with a number of European operating monohull ferries. It was found that the catamaran’s Attained index is 14% higher than that of comparable monohull designs, whilst the vessel’s “PLL (potential loss of life) is 82% less than that of average monohull designs”.

A further assessment was made for the catamaran with added foam installation thus giving even greater stability in any damaged condition. The results indicated “the vessel possesses a staggering Attained index of 0.996, in other words suggesting a survival probability of 99.6% given all foreseeable damage scenarios”. The researchers concluded that “this is truly remarkable, and certainly unprecedented in even the most modern monohull designs”.

The Attained index is found to be 20% larger than that of equivalent monohull designs, whilst the PLL (potential loss of life) level is “quite astounding” and far superior to the range of monohull designs considered. 

The study reflected that risk of Ro-Ro passenger ferries due to vehicle deck access are over ten times greater than cruise ships. This presents a strong ‘safety-based’ argument to consider alternative safer designs such as catamarans. The 75m catamaran in the study “achieves a much higher level (of safety) than cruise vessels engaged in international voyages”, and far superior to comparable monohull ferries.

Given that catamaran designs are also several times cheaper to build and considerably cheaper to operate, this creates a credible challenge to those continuing to resist their introduction to inter-island services to replace monohull ferries.


Attached is my paper to the Holyrood Net Zero committee giving my summary of last May’s catamaran seminar presentations.. Earlier I sent the three presentations to the committee, but they don’t appoint a specialist advisor and were clearly unable or unwilling to analyse the material in the presentations themselves. 

I have previously been appointed as ‘specialist advisor’ to select committees at Westminster, Stormont and Tynewald, also EC and OECD, in various maritime investigations, but Holyrood seem to think the clerks can do the job!


No need. Alf’s says it all!

I am, as always



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