trimaran hull form

Trimaran Performance vs Hull Form

QUESTION: If I build a multihull with straight sides of plywood to make construction easier, how much performance would I lose compared to a more ideal shape?

Now let's compare that to the shape with a semi-circular bottom that has the least wetted surface. Superimposed, the two might look like this (picture on right). Although I might refer to this simple shape as 'a Vee-hull', the shape I prefer actually has a little wider flat bottom in order to provide useful buoyancy lower down - see later. See also the article on relative virtues of flat panel shapes .

Right away, for the same displacement, one can see that the boxy hull has more draft, is narrower at the waterline but will have more underwater (wetted) surface. In practice, the Vee hull is likely to be 10% heavier in construction, but that might only mean say 5% required increase in overall displacement as the deadweight (crews, supplies etc.) could double the dry weight.

Now we need to look at how a boat's resistance varies with its speed and this is much related to its length. About 140 years ago, a William Froude discovered that up to a Speed/Length ratio (SLR)* of about 1, resistance is mostly made up of frictional resistance and in such a case, would be directly proportional to the wetted surface. From a SLR of 1 to about 2 (for a typical multihull), there's an increase in hull resistance due to waves made by the hull through the water, and the wetted surface resistance, although still there, takes a more minor role.

Once over a SLR of about 3.0, the wetted surface is again on the increase (although wave resistance is still significant).  So for different boat lengths, here are the speeds we are talking about.

*SLR = speed (in knots) divided by the square root of Waterline Length (ft)

So, below the speed given for SLR=1 and above the speed given for SLR=3.0, the majority of resistance would be directly affected by the roughly 20% increase in the wetted surface for the Vee (or 15% for the Box shape) and if we add in the 5% weight penalty, this could go to about 24%. ( While these percentages might also apply for speeds well under SLR of 0.5 or over 3.5, they would in fact be somewhat less than that at the SLRs listed, as not all the resistance would be due to surface friction )

But between the two values listed, wave resistance grows to a peak at around SLR=2 (for the average multihull) and at this point, the narrower beam of the Vee hulls could lower wave resistance enough to offset the frictional resistance and therefore be quite efficient in the range between the two speeds listed above for each length.   The box or Vee'd shape would also offer less leeway and that will also help to compensate.

If we widen the hull at the bottom, the sides can become more vertical and this more box-like section can further lower the wave-making compared to the Vee-section we started out with, as it disturbs the passing waves even less.

Of course, there are other aspects to consider too—like having less interior space at the waterline with the V-hull and also, that the V-hull would initially sink about 15% more for each 100 lbs of extra weight loaded on. The extra draft of a Vee hull is sometimes used as a longitudinal keel to resist lateral drift and that 'might' annul the need for a dagger board or centerboard, although deep fins are clearly more efficient for sailing upwind.

But if you're content to sail in the speed range indicated by the table, which is surprisingly broad, and can accept the other compromises, there's definitely a case for using the box hulls and keeping it simple. Outside of that, expect speeds at around 10% slower at the low end and similar at the much higher end beyond SLR of 3.5.

Of course, even 'ideal hulls' are seldom perfectly semi-circular and the total resistance also depends on many other things, such as the hull ends and even air resistance etc., but this gives a general idea of speed performance for such differing hull shapes, assuming all other factors are alike and comparable. On another aspect, the deeper V-hulls will also have more directional stability but in turn, be harder to tack—helpful for long trips but not for short tacking.

True V-hulls are seldom used for the center hull of a trimaran as they offer so little space. However, they have been used for easy-to-build catamarans and trimaran amas, for owners ready to accept the performance sacrifices noted above. However, the more box-hull can be justified for the sake of easy building. and at least offers more foot space than the narrow Vee'd for a main hull.   [Deep, near vertical flat-sided hulls are also drier than Vee'd hulls and have more recently proven to have less wave drag].

Recent tests (2009) on a small prototype trimaran with this Box-hull form and flat bottom, demonstrated that performance can be surprisingly good and some of what is lost through increased wetted surface is indeed made up by the slimmer form. While this may not be true at low speeds (below say 4 kt), the flat of bottom may give enough dynamic lift over at least part of the hull length to offset the theoretically greater surface, and show that the higher speeds of a light trimaran will not be as adversely affected by this box form as one might first think.

Editors Note: For this reason, this simple-to-build form was chosen for the new W17 that has since proven to perform very well indeed. The added resistance at the very low end (say under 4 k) will still be there and will need some imaginative boat trimming and added light-wind sail area to overcome. But for a significant speed range above that, this boat, especially when built to design weight, is proving that the flat underbody surface can indeed offer a very clean running hull with some dynamic lift at higher speeds that some W17 owners are calling 'oiling', as it reportedly feels 'like the boat is running on oil'. Even with the very moderate cruising rig, a speed of 14.9 k has already been recorded (by GPS) in this mode, so this is impressive and promises to offer lots of fun. So for this particular design at least, the high end restriction of a boxy hard chine hull has been overcome by the relatively narrow hull, the flat of bottom and its low-rocker design profile. Compared to a round bilge, the box-hull also offers additional lateral resistance, so the dagger board wetted surface can be slightly reduced for another small speed gain.

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Global optimization of trimaran hull form to get minimum resistance by slender body method

• Technical Paper
• Published: 19 January 2021
• Volume 43 , article number  67 , ( 2021 )

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• Parviz Ghadimi   ORCID: orcid.org/0000-0002-9315-5428 1  

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In the current paper, different geometrical parameters of the trimaran hull ship are investigated to achieve the optimal points of geometry parameters. Considering the fixed displacement volume, the values of the longitudinal center of buoyancy, block coefficient, midsection coefficient as well as the side hull length and position are computed and using Lackenby shift transformation, the geometry is reconstructed during the optimization process. It is then necessary to compute the ship resistance of the reconstructed geometry which is hereby accomplished by slender body method. Subsequently, D-optimal method is used for finding the best parameters to achieving minimum resistance at cruise and sprint speeds. Two strategies are pursued to find optimum value of design variable: trimaran hull transformation and separated hull approach. Generally, a hull optimization process takes huge time and depending on the applied methodology, it might take somewhere between 6 months and 2 years. However, through slender body method and design of experiment study, proposed in this paper, the total time of global optimization process is only 1 week. Meanwhile, 9.1% resistance reduction at cruise speed and 2.24% resistance reduction at sprint speed is achieved. Hence, a successful ship hull optimization with suitable computational time and effort is the novelty of the current work. The conducted optimization indicates that two parameters of longitudinal center of buoyancy and block coefficient have significant effect on the total resistance. Comparison of the original and optimized hull signals the validity and superiority of the proposed optimization strategy, which can be extended to other maritime industrial projects.

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Nazemian, A., Ghadimi, P. Global optimization of trimaran hull form to get minimum resistance by slender body method. J Braz. Soc. Mech. Sci. Eng. 43 , 67 (2021). https://doi.org/10.1007/s40430-020-02791-8

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• PERFORMANCE TOPICS

Optimising Hull Lines for Performance

This article was inspired by a question about the rocker line in the new 8.5m cat Design 256 and I want to stick to the point, so we won’t turn it into a book, but I’ll discuss two issues, hull fineness ratio and some aspects of the rocker profile.

When you manipulate the hull form you’re adjusting the lines in three planes, waterplanes (plan view), buttocks (side view including the keel rocker) and the section shapes. So you need to be aware of how the shapes are changing in the other two planes as you manipulate any one of these three, or all three globally as is now possible with computer modelling.

There are two fundamental constants that you start with and don’t change throughout the process. The big one is the displacement or the amount of buoyancy you need.

If you make the hull finer by narrowing the waterlines you have to increase the draft or make the ends fuller to get back to the required displacement number.

If you flatten the rocker line you have to increase the hull width, fill out the ends, or square up the section shapes rather than having a V or rounded V. 

The other constant is the longitudinal centre of buoyancy. You really can’t do any meaningful shaping of the hull form until you have settled on the these two constants.

A third number that we can plug in as a constant if we want to is the prismatic coefficient which describes bow much volume there is end the ends relative to the cross section shape in the middle of the boat, but in sailing boats this is of less importance compared to other factors. 

The hull lines for Design 256, 8.5m Cat. It's that hump in the rocker line - right under the back of the cabin that brought up the question and is one of the key points discussed here.

Hull fineness.

Fine hulls are fast, but only in the higher speed range. There’s a misconception I come across quite a bit that you can add weight and windage and you’ll still be fast as long as your hulls are fine.

Well you won’t be. Your boat will simply sink to find the new state of equilibrium. If your transoms are submerged you’ll have more drag. If your bridge deck is too close to the water you’ll have slamming. Much better to be conservative with your displacement figure in the design stage than overly optimistic.

And fine hulls have more wetted area so you have more drag in light air where friction resistance is the primary drag factor. 

I’ve seen promotional material for catamarans stating that the boat has less wetted area because it has fine hulls. For a given displacement the minimum wetted area is described by a sphere (or a semi sphere in the case of a floating object). The more you stretch it out in length, keeping the displacement constant, the more wetted area you have.

The more you make the section shape into a deep V or a broad U with tight corners, as opposed to a semicircle, the more wetted area you have. Add into the equation finer hulls are slower to tack.

So fine hulls are only an advantage if your boat is light and has enough sail area to ensure you’re travelling at speeds where form resistance is greater than skin resistance.

In my view the advantage of fine hulls is often overrated as it applies to cruising cats.

At the other end of the scale the resistance curve is fairly flat up to about 1:9 which is still quite fast in most conditions. From there the resistance rises steeply as the hull gets fatter and at 1:8 and fatter you’re suffering from some serious form drag.

This is the rocker line isolated from the lines plan above (in blue) and and the red line shows a more moderate rocker line that achieves the same buoyancy and maintains the centre of buoyancy in the same position.  The bow is to the right.

In the image lower right I've squashed it up and increased the height to make the difference in the lines more obvious.

The difference in the two lines is quite subtle, but races are often won or lost by seconds.

Rocker Profile

So if we’re looking for low wetted area we would want a rocker profile that was even and rounded, relatively deep in the middle and rising smoothly to the surface at each end. But this would give us a low prismatic which is not ideal in the higher speed range, and it’s not ideal for pitch damping which in my view is the critical design factor that is often underrated. 

Pitching is slow. It destroys the airflow in your sails and the flow around the hulls, and your performance is suffering from slamming loads.

The single most effective way to counter pitching is with asymmetry in the water planes. You can achieve that in the with a fine bow and broad transom. Or you can achieve it with V sections forward and a flattened U shape aft. Or you can achieve it in the profile view with a very straight run forward and a bump in the aft sections. A flatter rocker line is better for resisting pitching than an evenly curved one with deeper draft in the middle.

The final result is a combination of all three of these factors.

On a cat like Design 256 the weight is concentrated well aft so we need to get buoyancy well aft.

The kink you see in the rocker profile helps to do this. It also helps to keep the rocker straight for most of its length and smooth the water flow exiting the hull aft at higher speeds, possibly promoting some planing effect.

If we had a more even rocker line we would slightly reduce the wetted area, but we would increase the pitching and the water would exit the hull aft at a steeper angle, increasing form drag in the higher speed range.

How much of a bump can you put in there without creating a flow separation, and how damaging would that flow separation be? I really don’t know. The way all of these factors interplay in the various conditions we sail in is very complex.

Ultimately a lot of this work is gut feel nurtured by experience, observing things in nature and most importantly experimenting and trying new ideas.

Is the new Groupama AC45 a breakthrough that will influence the form of racing catamarans into the future? I don’t think anyone has a computer that can answer that. We have to wait and see.

Symmetric and non symmetric water-planes. The blue line with grey fill is the DWL from the design above. As is typical with modern cat hulls the bow is long and fine, the stern is full and rounded. This is the asymmetry that has a damping effect on pitching. The red line on the other hand is more like you would see on a double ended monohull and quite a few multihulls have also used this shape in the past. It's quite symmetric about the pitch axis and does not have good pitch resistance.

The hull lines of the new 8.5m Sports Cat Design 256

Mad Max , Previously Carbon Copy . She was designed in 1997 but she's the current (2016) title holder of the Australian Multihull Chamionships (2 successive years) and the fastest inshore racing boat in Australian waters.

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HYDRODYNAMIC STUDIES ON A TRIMARAN HULL FORM

2013, The Institution of Engineers, India Sep 2013.

It is expected that, there will be large demand for trimaran hull forms to meet multi-mission defense requirements of world in the recent future. Multi-mission requirements include a platform that is fast, agile & versatile with multirole capability such as anti-submarine warfare, surface surveillance, mine countermeasures, littoral and deep sea combat capability with modular mission payloads etc. Hydrodynamically these platforms possess powering and seakeeping advantages, shallow draft operability and excellent maneuverability. Hydrodynamic model tests and CFD studies were carried out to design and evaluate the performance of a 3000 tonne trimaran configuration at NSTL. A brief description of the hydrodynamic activities carried out is highlighted in the present paper.

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International conference on ship and offshore technology, IIT Kharagpur, Dec 2015.

Kareem Khan

Future surface combatant designs are focused on multirole mission capabilities and unconventional efficient propulsion systems. Concepts in naval design and related systems introduced by several nations in the past two decades shows great amount of advancements from the traditional platform operation criteria such as speed, shock & noise reduction, infrared signature to futuristic demand criteria like flexible mission capability, alternative propulsion architecture, higher degree of stealth, longer duration of loitering capability with smaller crew and cleaner emissions. Few of such platforms are already in operation and many more are under design/construction phase by the world navies. Mission effectiveness of the platform largely depends on two major entities i.e. hull form and propulsor configuration. It has been reviewed that most of advanced marine vehicles rely on unconventional propulsion such as waterjets & podded propulsors for effective platform performance. Studies have been carried out at Naval Science & Technological Laboratory (NSTL), India on a trimaran with waterjet propulsion for hydrodynamic performance evaluation at concept level. The present paper highlights the hydrodynamic studies on trimaran with waterjets carried out at NSTL.

Stefano Brizzolara

Proceeding of Marine Safety and Maritime Installation (MSMI 2018)

Bagiyo Suwasono

Ships and Offshore Structures

Lawrence Doctors

Ship technology research

Rainald Lohner

HSMV 2008 8th Symposium on High Speed Marine Vehicles

Igor Mizine

Muhammad Iqbal

Research, in order to breakdown the resistance components of trimaran hull form, has been carried out worldwide. Almost all of the work is focused on the resistance investigation of trimaran configuration in deep sea condition. None of the research has formulated the estimation of trimaran resistance components into certain equation such as for catamaran. The current work is concentrated on the investigation of the effect of water depth, namely deep, medium and shallow waters, into the total resistance of the trimaran configuration. Experimental investigation using ITS model were carried out at a towing tank at various hull separation (S/L = 0.2, 0.3 and 0.4) and various speeds or Froude Numbers. Special consideration is given to medium and shallow water depth because the Froude Numbers are based on water depth and not based on ship length such as in deep water condition. CFD investigation using a CFD code called Tdyn, is also conducted in order to validate the results of the experimental investigation. The results of the experimental test and CFD analysis are in good agreement and comparative studies with other published data strengthen the findings. Keywords: trimaran, resistance, model test, CFD.

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catamaran and trimaran hull form

Discussion in ' Multihulls ' started by samh , Mar 8, 2005 .

samh Junior Member

Can someone provide me with a starting place for designing catamaran and trimaran hulls as far as Cp and LCB? Also the displacement distribution in a trimaran between the amas and vaka (? I think I have the terminology right for the main hull and the floats.)  

Skippy Senior Member

For the tri, it looks like ama size is usually 100-200% of total boat displacement. The main hull of course would be bigger and designed for low wetted surface, planing, cabin space, and other considerations. If you're also asking about actual displacement when in the water, the bottoms of the amas usually sit right at the surface when the boat is dead level, taking zero displacement.  

Doug Lord Guest

tri or cat There can be important considerations based on the size of your boat and the intended use; can you fill in a little more info?  

general coefficients of form I think Larson and Elliason give .54 as a reasonable, general starting Cp for a monohull. I was wondering if there is a same sort of target for catamarans and trimarans? Same with the multihull equivalent of a monohull's lcb of .535. S  

I think Cp is usually higher for a multi than a mono, for both better pitch stability and higher speeds. John Teale has for a mono, V/L ........ Cp 1.15 ...... .55 1.25 ...... .6 1.5 ........ .65 A cat especially should be higher Cp for the same V/L.  

Thanks. Also found http://www.tedbrewer.com/yachtdesign.html which illuminated some concepts that I didn't really understand. S  

Thanks for that link, it's a good summary. Brewer's book is nice. It really gives you a feel for hull shape, and you don't have to be an expert to read it.  

dougfrolich Senior Member

Longitudonal stability is an important concern for multihulls, consider as the apparent wind angle increases, the trimming moment produced from the rig will start to approach the heeling moment in magnitude, if the trimming moment exceeds the heeling moment to early the boat will be suseptible of pitchpole. For trimarans, by placing the LCB of the ama forward of the LCG, trimming moment can be balanced. To figure how far forward; choose the crit AWA and AWS, find the trimming moment divide that by the sailing displacement, the result is the distance forward the LCB should be from the LCG ( at the chosen AWA,AWS ). Any hydrodynamic lift generated fwd of the LCG, from canted foils or bottom shape for example would reduce the distance. So basically it is a ballancing act. For your hull shape just make them as long and narrow you can for your displacement, choose a sectional shape with minimum wetted surface, and include plenty of reserve bouyancy so you dont bury the leeward bow. For more info check out Design of a 40 foot Multihull for Offshore Racing by James Antrim availible thru SNAME.  

Ron Cook Junior Member

Cats and Tris I have alot of experience with both having over 50,000 ocean mile on them. If a beginer designer makes mistake with design of a monohull it may not be a good boat but if you make mistakes with mulltihulls they will bite you bad. I don't want to put you off your project. I have participated in the design of a number of mulltihulls and I have built some. it was great rewarding fun. Be warned that monohulls have nothing in coimmon with mullties. So done use the ratios from one to help design the other it is apples and oranges. The cp for the average mullti is .60 to .65 mullti's don't suffer in light wind the way monos do with high cp. On a tri the float should be at 150% or higher with its center of boyancy around 12 1/2% forward of the center of B for for the main hull which is commonly around 56% aft. Good luck with your project, I love mulltihulls But be warned that if you screw them up they will bite you. Ron P.S. Is there aspell checker in this posting system?  

DaveB Senior Member

Hi, Interesting to hear the numbers... wasn't sure how they'd compare... I'm workin' on a powercat right now with a Cp of 0.63... Lucky I guess... What do you mean when you say: Ron Cook said: On a tri the float should be at 150% or higher.... Click to expand...

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tri float 150% The float on a tri should be designed to carry 150 % or higher of the total crafts displacement without putting the deck under. The shape of the float on a cruising tri shuold be more v;d then on a racer. The reason is for motion comfort and less pounding which can rattle the the boat inclulding your teeth. Ron  

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TWO HIGH SPEED TRIMARAN FERRIES TOGETHER HIGHLIGHT AUSTAL DESIGN TECHNOLOGY AND AUSTRALIAN SHIPBUILDING CAPABILITY

MEDIA RELEASE

17 JULY 2020

Austal Australia has celebrated the company’s success in high-speed trimaran technology and Australian shipbuilding capability following a successful side-by-side ‘sprint’ by the 118 metre ‘Bajamar Express’ and 83 metre ‘Queen Beetle’ trimaran ferries off the coast of Perth, Western Australia.

As Bajamar Express (Austal Hull 394) was departing Australia on her delivery voyage to the Canary Islands, she was joined by Queen Beetle (Austal Hull 396) undergoing sea trials, in an historic moment capturing two Austal trimaran ferries.

The two trimarans, designed and constructed by Austal Australia, are the latest designs of a proven hull form first developed for Fred. Olsen Express’ 127 metre Benchijigua Express in 2005.

Austal Chief Executive Officer David Singleton said it was a very proud moment for everyone at Austal Australia and supply chain partners to see two trimarans out on the water together.

“Our shipyards, supply chain partners and of course our customers are thrilled to see the results of all our hard work, showcased in these two impressive trimarans,” Mr Singleton said.

“Seeing Bajamar Express side-by-side with Queen Beetle really does highlight Austal’s success in developing the trimaran hull as an effective high-speed commercial maritime transport solution.

“Following the delivery of Bajamar Express to Fred. Olsen Express, we have a further nine trimarans under construction or scheduled at the company’s shipyards around the world; and Austal remains the only shipbuilder designing, constructing and sustaining large high speed trimaran ferries, globally.”

Austal’s trimaran hull form offer ferry operators greater flexibility in vessel design configuration for vehicle and passenger capacity, while delivering a more comfortable and enjoyable journey for passengers.

The enhanced seakeeping of the trimaran hull, coupled with Austal’s MOTION CONTROL SYSTEM and MARINELINK-Smart program ensure a smoother, more stable ride for passengers and crew in the most challenging of sea states.

When it commences services in the Canary Islands in August 2020, Bajamar Express will transport up to 1,100 passengers and 276 cars at cruising speeds of 38 knots; while the distinctively red-painted Queen Beetle will be able to transport 502 passengers across 2 passenger decks, at speeds of up to 37 knots on JR Kyushu Jet Ferry’s Fukuoka, Japan – Busan, South Korea route (schedule pending).

More images and further information on both vessels are available on the Austal website www.austal.com .

To view the footage of the two trimarans underway off the Perth, Western Australia coastline please visit Austal’s YouTube channel - https://youtu.be/joo0FkgOUF8

Austal Australia farewelled Bajamar Express with a stunning water salute, provided by Svitzer Australia’s tugboats Svitzer Albatross and Svitzer Harrier (Image: Austal)

Austal Australia has celebrated two high speed trimaran ferries travelling together, for the very first time, off the coast of Perth, Western Australia with the departure of Bajamar Express. The 118 metre ferry for Fred.Olsen Express was joined by the 83 metre trimaran Queen Beetle, for a short but impressive sprint along the coast (Images: Austal Australia)

Austal Media Contact:

Cameron Morse

+61 433 886 871

[email protected]

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BMT launches the next generation hull-form – the ‘Pentamaran’

Designed to meet the specific challenges of long range autonomous operations

• bmt launches the next generation hull-form – the ‘Pentamaran’

21 April 2020

Leading the way in multi-hull applications, BMT has released details of its next generation ‘Pentamaran’ platform for autonomous applications. Offering a myriad of applications for defence and commercial innovators, these innovative vessels may be custom configured for military, patrol, intelligence surveillance and reconnaissance (ISR), anti-submarine warfare (ASW) and hydrographic survey work.

The design is the latest from the BMT’s team of expert naval architects and engineers who have been at the forefront of innovative hull design for 34 years. The Pentamaran has been designed to reduce drag as much as possible and tests have proven it offers significant improvements compared to conventional hull forms such as mono-hulls, catamarans and trimaran.

The vessel features a very slender central hull and two smaller hulls or ‘sponsons’ on either side. The sponsons are set one behind the other and when the vessel is operating on flat water, the forward sponsons are not submerged, as they provide roll stability effect in waves only. Compared to a trimaran there is less volume permanently immersed and therefore less resistance through the water.

Martin Bissuel, Business Sector Lead for Specialised Ship Design at BMT comments:

“Our team have carried out extensive work on this. The data gathered through extensive towing tank testing is very compelling. For applications where fuel economy matters, the Pentamaran hull form is more efficient than conventional full forms, which means that using the same engines and the same amount of fuel, it will go further than any other, making it an ideal candidate for autonomous applications. Looking at it from a distance it may resemble a trimaran but that’s where the similarities end.

“The arrangement and careful positioning of the four sponsons makes all the difference. The forward sponsons stay above the water, and only come into action when the vessel rolls, so not only the drag is reduced, but the sea keeping characteristics are improved. Compared to a trimaran hull form, lateral accelerations are lower, reducing g-loadings on the structure as well as the antennae and sensors on deck. The wide deck offers a large working area for multi-role capabilities. It can accommodate payloads or interface with other systems such as unmanned air vehicles.” added Mr Bissuel.

A key consideration, when a vessel is operating autonomously for long periods of time, is the reliability of the propulsion setup which is essential to sustained operational readiness. Our engineers have therefore integrated multiple independent power sources to increase reliability as well as survivability.

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