11th International Conference on Fast Sea Transportation FAST 2011, Honolulu, Hawaii, USA, September 2011
Maximising Efficiency and Minimising Cost in High Speed Craft
Gary Davidson
1, Tim Roberts
1, Stuart Friezer
2, Michael Davis
3, Neil Bose
4, Giles Thomas
4, Jonathan
Binns
4, and Rob Verbeek
51
Revolution Design, P/L, Hobart, Australia 2
Stuart Friezer Marine P/L, Australia 3
University of Tasmania, Hobart, Australia 4
Australian Maritime College, Australia 5
Wartsila, Netherlands
Fig. 1.General arrangement of INCAT 130m, 1850 tonnes deadweight, medium speed design.
ABSTRACT
To reduce the capital cost of a RORO vessel the minimum structural material weight is desired which would result in a fairly short vessel. In conflict with this is that to reduce operating costs a much longer vessel is normally desired to allow the vessel to operate at low Froude number so resistance is mainly frictional. However, the resistance effect can be more than offset by reduced displacement for a given deadweight. The conventional steel mono-hull RORO vessel is generally very long for this reason but incurs high capital cost as a consequence. Thus the alternative to a long mono-hull RORO is a much shorter but wider wave piercing catamaran that can have both low capital and operating costs. This opens up the possibility of operating at medium speeds where the high overall efficiency of the catamaran hull form and structure enables operation at medium speeds more efficiently than a conventional RORO mono-hull vessel. This paper explores how a large wave-piercing catamaran can be optimised to operate with high deadweight, high efficiency and high manoeuvrability.
KEY WORDS
Efficiency, High speed craft, Wave piercing catamaran, RORO Ferry.
1.0 INTRODUCTION
The current world economic and environmental climate is demanding reductions in fuel consumption and toxic and greenhouse gas emissions. This creates very different design goals to those for existing high speed craft. Very high speeds are no longer environmentally desirable and
moderate speeds, perhaps around 30 knots, are more suitable. To improve efficiency deadweight capacity should be optimised to average loadings rather than providing a very high reserve capacity that is rarely used.
The issue is how can these speeds be achieved in a cost effective and efficient manner? An efficient mono-hull of around 200m can operate at low Froude number. Thus resistance is primarily frictional, but the vessel may be very expensive and unlikely to have shallow draft and the length may well restrict the ports into which it can operate. This size mono-hull will be suitable for long distance routes but for relatively short routes, with a fast turnaround, such a long mono-hull may not be viable. A wave piercing catamaran (WPC) offers a competitive solution. It can provide high deadweight, high manoeuvrability and shallow draft on a much shorter length with reduced capital cost. Vessels approaching the 130m size are already in service, such as the INCAT built 112m vessels that are operating in Japan and the English Channel. Lessons being learnt from these vessels are driving design refinements to be implemented in designs for new larger vessels. The 130m WPC design offers greater efficiency and capacity and is the culmination of current research and design work to be described in this paper.
A major cost for any ferry service is operating at part loads. The unique characteristics of WPC’s resistance means that power can be reduced substantially at lower deadweights whilst maintaining service speed. Catamaran hull form design can be refined by optimising critical hull parameters such as length to breadth ratio and length to displacement
ratio. On a mono-hull vessel this is not normally possible, to the same degree, due to stability issues.
2.0 LENGTH
It is generally held in ship building, that greater length will mean greater and disproportionate capital cost. When looking at the efficiency of a vessel, capital cost must be considered along with all through life costs. However a shorter vessel will generally have a smaller capacity which will reduce its efficiency. To mitigate that reduction in capacity a shorter vessel may increase its height by adding more decks. The conventional steel RORO (of mono-hull form) does this but stability requirements mean that an associated increase in beam is likely. This reduces slenderness, increases resistance and therefore significantly reduces efficiency. To overcome this loss of efficiency a typical efficient RORO will probably be in excess of 200m and this creates problems with capital cost, port facilities and inefficiencies at part load.
3.0 HULL FORM DESIGN
The primary aim of hull form design is to reduce drag of the hull, which is made up of two major components, friction and wave making. Wave making is dominated by the length to breadth ratio (L/B) and/or length to displacement ratio, 1 3
Lwl
, where Lwl is waterline length, isVolume of displacement and B is the demi-hull beam. To reduce either ratio the length can be increased or the mass of the vessel reduced but in a mono-hull stability will limit the reduction.
Frictional resistance is a function of wetted surface area (WSA) and so the reduction of WSA becomes a primary design aim. This can be achieved by optimisation of hull shapes or by reducing the mass of the vessel, but this has to be considered against the requirements for stability. At low Froude length numbers, frictional resistance will dominate (Fn < 0.3); above these values wave making will begin to dominate drag especially around Fn = 0.45. One method of reducing the effect of wave making is to make the vessel very long so that it will operate at a low Froude number where friction drag will dominate. The longer vessel will necessarily have a wider beam as is exploited to great effect by long haul vessels, cruise liners, bulk carriers etc, which spend long periods at sea with large cargo capacity. This is clearly not suitable for a RORO that may dock 8 times a day that needs high manoeuvrability and must be compatible with the RORO ports it uses. Therefore a shorter RORO vessel will most likely operate at speeds where wave making is significant and may constitute up to 50% of the resistance budget while friction and other residual components make up the remainder.
So the overall challenge is how to design the most efficient vessel when it cannot be overly long due to capital cost, port facility limitations and manoeuvrability. It must not be too heavy, must have adequate carrying capacity and stability? The resolution of these issues is to be found in the use of multiple hulls. If for example, there are two hulls then the
displacement is spread over two hulls rather than one so the
L/B or 1 3
Lwl
ratios are reduced substantially and thewave making resistance is also reduced. The catamaran configuration can therefore be significantly shorter than the equivalent mono-hull therefore reducing weight. Stability is assured due to the wide hull separation while capacity is enhanced due to the much wider deck space created by the catamaran.
4.0 CONSTRUCTION COST
4.1 Longitudinal bending moment
The maximum longitudinal bending moment of any vessel generally occurs when the wave length is approximately equal to the vessel water-line length (Mansour and Liu 2008). If we consider that the longitudinal bending moment will vary with the square of the length as in general beam theory then as the vessel length increases the bending moment will increase by the square of the length. There are a number of other factors that could come into play here so the length squared function is of course approximate. It will therefore follow that if the longitudinal bending moment increases in proportion to the square of the length then the structural material weight of the vessel will also increase in proportion to the square of the length. Thus there is a good incentive to keep the vessel as short as possible to keep the capital cost and weight to a minimum. Weight has a large effect on the final performance and economics of the RORO vessel. Weight not in the structure can be carried as revenue earning deadweight, while less displacement corresponds directly to less resistance, provided of course there is no increase in waterline beam for stability.
It naturally follows that a wider vessel will incur some increase in weight and hence cost but the experience with Incat WPC has been that cost increases in proportion to the beam rather than to the square of the beam. Therefore it is much more economical to design with a wider beam rather than longer length to achieve lower capital cost. The extra width also gives benefits in sea-keeping (Davidson and Roberts. 2007) and stability as well as a wide deck turning area for trucks. Increasing the overall beam of a catamaran will also increase the hull separation (S), which in turn reduces catamaran interference effects and consequently resistance. In Fig. 2 the square of the length times beam (BOA) versus the waterline length are compared against actual vessel structural material versus waterline length for various WPC from the 78m through to the 112m vessels. The longer vessels (>130m) proposed are extrapolated from those figures. Good correlation is apparent and this is because the scantlings are controlled by geometry inputs meaning that a relatively simple mathematical formula can mimic reality quite well.
Adding more decks to a catamaran, although adding weight, does not create stability issues as it can with a mono-hull. If greater capacity is required then as with beam it will be cheaper to add an extra deck than to go longer. This is especially so with the already wide deck area on a
catamaran as much more capacity will be gained compared with increasing length on a mono-hull hence keeping building costs to a minimum.
65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 LwL, m S tr u ctu ral M ate ri al (A lu m in iu m Structural Material actual L^2 x BOA
Fig. 2.Actual structural material mass vs Lwl and Lwl2\ x
BOA vs Lwl
4.2 Three Hulls
Three hulls or more may deliver yet greater benefits but a practical overall beam limit may mean the three hulls would be too close together, giving high interference and so high resistance. When describing a trimaran in this context a trimaran can technically be said to be a vessel with three hulls of similar volume as opposed to a stabilized mono-hull which is sometimes called a trimaran.
4.3 The Aluminium Structure
Another option for reducing weight is to build in a less dense material with a higher strength to weight ratio, such as aluminium. Aluminium does of course cost more per tonne than steel, but generally only half the tonnage is required. Although labour costs seem higher for aluminium on a per tonne basis, only half the tonnage is required and therefore labour costs may be similar or not significantly greater. Most significantly life cycle costs are reduced substantially through reduced powering requirements, reduced fuel consumption and reduced maintenance costs.
As a rough guide to the reduced amount of aluminium required relative to steel, using the DNV rules (DNV, 2010) we have;
Steel f1 = 1.0
where f1 is an allowable stress factor defined in the DNV rules.
Aluminium f1 = 0.9 Aluminium HAZ, f1 = 0.64
where HAZ is the heat affected zone. Steel density = 7850 kg/m3
Aluminium Density = 2661 kg/m3
Mass difference = 1/0.9 x 2661/7850 = 0.3766
That is aluminium has 37.6% of the mass of steel for an equivalent design.
If considering aluminium in the heat affected zone this becomes;
Mass difference = 1/0.64 x 2661/7850 = 0.53
That is aluminium has 53% of the mass of steel for an equivalent design.
Thus we see that typically an aluminium vessel will be significantly lighter than a steel vessel with the consequence that the loads on the vessel will be reduced further reducing vessel mass.
In a typical WPC the weight variation from steel to aluminium will vary depending on scantling rules (minimum thickness) and whether the structure can be considered un-welded or not but a good average is 50% based on our experience. In an aluminium vessel there is no need to add a corrosion allowance as is required in a steel vessel or to paint all surfaces. This not only affects initial cost but reduces ongoing maintenance costs.
One reason that aluminium labour costs in fabrication are higher than steel is that aluminium vessels are generally high speed craft where the structure is optimised for minimum weight as opposed to minimum cost. This leads to light shell plate thickness and close stiffener spacing. This is an investment in a reduction in through life costs as compared to a reduced capital cost steel vessel. This lightweight structure mindset is ideal for the medium speed vessel where maximum efficiency can only be gained by the lightest possible structure giving the lowest installed power.
4.4 Other Costs
The cost of the structural material in a ship is 8-10% of the total capital cost. Machinery and material labour are perhaps the two biggest costs. Fit-out and other non material labour will be similar whether the vessel is steel or aluminium and whether mono or catamaran with a comparable layout. A RORO operating on a short route at medium speed is unlikely to require cabins as it will be operating at sufficient speed to complete the journey in a relatively short time period.
A significant cost factor in a typical RORO vessel is the machinery cost, perhaps 20-30% of the vessel cost. It is therefore desirable to reduce the power required as machinery cost will generally rise as a function of the power installed. Machinery installation labour will reduce with reduced installed power.
A wave piercing catamaran does not require a ballast system which further reduces cost, it is very maneuverable due to hull separation which also reduces the powering requirements for bow thrusters, all of which reduce the demand on power and tankage and ultimately capital and through life costs.
5.0 RESISTANCE
5.1 Resistance components
Fig. 3 shows a typical total resistance curve in terms of resistance force versus Froude number. Resistance always reduces as speed reduces but there are definite humps and hollows in the resistance curve, the major hollow being at around Fn = 0.35 and the major hump at 0.45. These humps
and hollows are exaggerated when looking at resistance coefficients. 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 Fn length Re si st an ce
Fig. 3.
T
ypical WPC resistance curve versus Fn The resistance coefficients are derived as follows:2
2
1
Resistance
Total
SV
Ct
where Ct = total resistance coefficient
= water density (kg/m3)V = vessel speed (m/s)
S = wetted surface area of hull (m2)
Cf = approximation to the turbulent flat plate frictional
coefficient determined by ITTC78 methods or Grigson’s method, (Bose. 2008)
Cr = residuary coefficient determined by subtracting Cf
(1+k) from Ct,
(1+k) = form factor.
Cr will remain constant for both model scale and full scale
according to Froude scaling whereas Cf will be determined for model scale and full scale depending on the Reynolds number. In Fig. 4 through 6 typical resistance coefficients are shown which were determined by model testing a WPC at varying length to displacement ratios. Cr is predominately due to wave making resistance but does include other components such as transom effects and hull interference (multi hulls). 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 Fn length C* Cr Cf Ct
Fig. 4.Typical WPC resistance coefficients versus Fn for length to displacement ratio 11.68
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 Fn length C* Cr Cf Ct
Fig. 5.Typical WPC resistance coefficients versus Fn for length to displacement ratio 10.99
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 Fn length C* Cr Cf Ct
Fig. 6.Typical WPC resistance coefficients versus Fn for length to displacement ratio 10.31
5.2 Wave Making Resistance
Wave making resistance will vary with the length to displacement ratio, the higher this ratio the lower will be the wave making component. Fig. 4 through 6 show typical WPC resistance coefficients at three different length to displacement ratios. It can be seen that wave making becomes more dominant at the lower length to displacement ratio. The three different length to displacement ratios are for the same given hull separation.
So the answer to lower resistance when at Fn = 0.4 or higher is to keep the length to displacement ratio high which the WPC does very well by splitting the displacement over two hulls and increasing that ratio dramatically.
5.3 Frictional Resistance
Frictional resistance is a major component even for a high speed vessel so wetted surface minimisation is important. As stability is not a concern with the wide hull separation of a catamaran then the hull can be easily optimised for minimal wetted surface area. The theoretical minimum surface area in cross section is a semi-circle or a breadth (individual hull) to draft ratio (Bh/T) of 2. A typical
mono-hull RORO cannot achieve this without sacrificing stability. Typical values of breadth to draft ratio for mono-hulls are between 3.5 and 6 whereas the typical section of a WPC can be optimised with values approaching 2.0.
The catamaran offers many advantages in keeping the length to displacement ratio to a minimum. By dividing the weight between the two hulls the length of the vessel can be
less, which keeps the weight and cost to a minimum. Wide hull separation also provides a large deck area and carrying capacity on a shorter length and high stability so that the hull wetted surface can be minimised.
A further benefit of the INCAT/Revolution Design WPC over conventional catamarans is that the reserve buoyancy in the bow area of conventional catamarans hulls can be trimmed away to improve sea-keeping while the centre bow remains clear of the water in calm conditions and prevents deck diving in following seas. This allows optimisation of the wave piercing catamaran bow shape for resistance and sea-keeping with less compromise in the design to mitigate deck-diving and bow broaching. Bow broaching occurs when a catamaran, in following seas, immerses one bow only, which then acts to rotate the catamaran. The vessel then broaches around the immersed hull bow. The wave piercing catamaran designs have been developed and refined through experience to minimise bow steering effects and broaching. The success of these refinements can be seen in the size, or absence of size, in the WPC skegs compared to other catamarans, therefore reducing appendage resistance.
6.0 WEIGHT SAVING
It is important to keep the weight to a minimum and there are a number of ways to do that:
Using aluminium as the structural material; Use of composite materials in fit-out areas;
Use of lightweight machinery where possible, particularly gas turbines or lighter more power dense diesel engines;
Accurate determination of loads, (Davidson et al. 2011);
Intense design effort to minimise scantlings, (Davidson et al. 2006);
Efficient structural arrangement to minimise discontinuities in load paths, (Davidson et al. 2011);
As discussed previously, compact vessel dimensions to minimise capital cost.
7.0 ALTERNATIVE FUELS: LNG
A further method to significantly reduce running costs is the use of cheaper fuels such as LNG. Current predictions are that the price of distillate fuel will increase as the world economy picks up and demand exceeds supply, but LNG will not due to high world reserves exceeding demand (EIA. 2011).
Unfortunately LNG equipment adds to the capital cost of the vessel and adds extra weight with the required tankage. The cost is offset with lower running costs while the extra weight is split over both demi-hulls for a reduced effect on the length to displacement ratio compared to a mono-hull.
8.0 PROPULSION
Generally high speed craft (HSC) have been propelled by water-jets which have proved to be very efficient at high
speed and offer a number of advantages over propellers including:
No appendage resistance; Shallow draft;
Good manoeuvrability;
High efficiency (at higher speeds).
As the high speed requirement is reduced, in combination with a higher deadweight target for medium speed vessels, then propellers become progressively more competitive. A problem with the application of propellers to the catamaran hull form is that their diameter is large compared to the narrow demi-hulls. Even quad prop installations can end up with propeller blades outboard of the demi-hulls as well as significantly increased draft while single propeller installations have significant draft and gearbox complications (with 4 engines). The 130m vessel operating at 29 knots has been estimate to be slightly more efficient with water-jet propulsion but a 150m vessel operating at 25 knots or less will most likely be propeller driven. If the 130m vessel is to operate at lighter loads and speeds greater than 29 knots, the water-jets gain some more propulsive efficiency (refer to Fig 9), giving a performance benefit over a propeller driven version. Clearly defining the appropriate changeover point between using water-jets and propellers for large catamarans remains a challenge.
9.0 CATAMARAN INTERFERENCE RESISTANCE
The downside to the catamaran configuration as a high efficiency vessel is the added resistance from interference effects. The wave and wake produced by each catamaran demi-hull can combine adversely, mostly in the tunnel. If during model tests the resistance is recorded for a single hull of a catamaran then generally the resistance of the two hulls together is greater than twice that of the single hull, (Molland et al. 2006), (Larssen and Raven. 2010). It must be remembered that the single hull is not a mono-hull (as sometimes referred to in published results) but one hull of a catamaran. This single hull on its own would not have the required stability to act a mono-hull so comparing the resistance characteristics of the single hull against a catamaran is not appropriate. It is merely a comparison made to highlight the interference effects of the catamaran. For a typical mono-hull to have the equivalent carrying capability to a catamaran it would need to be longer and beamier and of greater draft. Beam would increase disproportionately to account for the increased weight (by the square of the length) and to maintain adequate stability. Also catamaran interference is not constant throughout the speed range. It is most significant around the residuary resistance coefficient hump speed (Fn~0.45), so that when designing near these speeds it can be advantageous to minimise the added resistance due to hull interference by increasing hull separation.
10.0 RESIDUARY RESISTANCE
There is a trend for Cr to reduce as hull separation increases and its lowest value is for the single hull, (Molland et al.
1996). There may even be a separation where the resistance of the two hulls is less than twice the resistance of the single hull, (Larsson and Raven. 2010); practically the separation to achieve a resistance reduction may be greater than possible for large WPC designs. Residuary resistance is highest around hump speed (Fn = 0.45), (Molland et al, 1996), approaching similar lower values at higher and lower Froude numbers. Therefore if the operating speed is well away from hump then any adverse effects of hull separation should be minimal.
The interference effects occur in both viscous frictional and residuary resistance components. In viscous frictional resistance it is mainly due to the waterline on the inboard side of the demi-hull being higher than outboard which increases wetted surface area. Cr reduces considerably as the length to displacement ratio increases so a high length to displacement ratio should also reduce the interference effects to a minimum even at Froude numbers close to hump. Investigation of these effects is one of the aims to be explored during a current joint research project between INCAT, Revolution Design, AMC, UTAS, Wartsila & MARIN.
11.0 OTHER CONSIDERATIONS 11.1 Optimal Speed range
It can be seen from Fig. 4 through 6 that there are some distinct humps and hollows in both the Cr and Ct curves. The hump and hollows can be seen in the resistance curves and it may be wise to try and operate in one of these hollows when in service, Larsson and Raven (2010) recommends such a practice. It is always the case that if speed is reduced less power will be required as can be seen in Fig. 3. There has to be a minimum speed that a vessel needs to operate at to maintain a suitable and economic overall timetable. The final design speed will depend on the operator and will be a compromise between capital cost and operational requirements. The most opportune hollow to operate at is around Fn = 0.33 to 0.37, from a resistance point of view. Therefore a longer vessel or slower operating speed may be required to maximise efficiency and minimise running costs.
100 110 120 130 140 150 160 170 180 190 23 24 25 26 27 28 29 Speed, knots lw l, m 0.35 0.33 0.37
Fig. 7.
Waterline lengths to achieve Fn = 0.35 at average
operating speedFig. 7 shows the waterline length to achieve Fn = 0.33 to 0.37 at various operating speeds. Practically it will not always be the case that the optimal speed is in the range between Fn = 0.33 and 0.37. It is a compromise. For example at a length of 186m (for a 27-29 knots service
speed), the vessel weight will be very high (due to vessel structural weight increasing with the square of the length) giving high frictional resistance (due to high wetted surface area) and high installed power driving up machinery weight and capital cost. However as an example the 130m WPC vessel will be able to operate at 29 knots with 1850 tonnes DWT at 80% MCR, with installed power of 4 x 9100 kW. There will be a compromise based around actual required deadweight, deck area, operating speed and capital cost. Port facilities and water depth will also come into the overall economic equation.
11.2 Shallow Water
With shallow water depth, a fast ferry often has the opportunity to transition into the supercritical zone where resistance is reduced. However this requires the vessel to overcome a significant hump in the resistance curve as shown in Fig. 8. The medium speed vessel may operate in the sub-critical region where shallow water effects are likely to increase the vessel resistance and the vessel is unlikely to have sufficient power to get over the resistance hump into the higher speed range where lower resistance is possible. The only way to mitigate this is to increase length to displacement ratio as this reduces wave making resistance and hump effects. Overall the factors that are important for normal operation of a vessel to maintain maximum efficiency are more important in shallow water. Predicted speeds for the 130m in shallow water (30m), based on model tests are shown in Fig. 9 and as can be seen the speed loss compared to deep water is minimal. Operation at optimum Froude number will be even more important for shallow water as the humps are more exaggerated than in deep water. 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 Froude Number C r, ( *10 00 ) 11.68 Deep Water 10.31 Deep Water 11.68 Shallow Water 10.31 Shallow Water
Fig. 8.Shallow water effect on Cr
12.0 PART LOAD OPERATIONS
To obtain maximum efficiency for a mono-hull vessel it is usual to make it very long (and hence heavy) so that it can operate at a very low Froude number (perhaps < 0.3) to keep wave making resistance to a minimum. Wetted surface (and
B/T ratio) may be optimal at maximum deadweight but at
part loads, during non peak service or out of peak season, the wetted surface will be increasing out of proportion to the deadweight carried. Even operating lightly loaded there is still a very substantial ship to be propelled with low efficiency. In the case of the WPC design, part load wetted
surface still remains close to optimal down to much lower loadings. This can be seen clearly in Fig. 9 where MCR can be reduced dramatically at lower dead-weights to maintain the same service speed as full loaded. At 1200 tonnes DWT only 60% MCR will be required to operate at 29 knots. The vessel lightship is very low due to short length, low installed power and the use of lightweight materials (aluminium). For maximum efficiency deadweight capacity may be based on average vessel loadings as opposed to rare maximum loadings. The WPC medium speed vessel may be able to operate more regular trips at peak loading times due to its fast turnaround and medium speeds as opposed to slow speeds on conventional RORO vessels. This will make a significant difference to real through life costing compared to costings based on maximum deadweight.
700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Speed, kts DWT , t o n n e s 4 x 9 MW Deep 4 x 8 MW Deep 4 x 7 MW Deep Fn=0.45 4 x 9 MW Shallow 4 x 8 MW Shallow Fn=0.50 4 x 6 MW Deep 4 x 5 MW Deep 4 x 7 MW Shallow
Fig. 9.Deep and shallow water speed vs DWT for the 130m vessel at varying MCR
13.0 CONCLUSIONS
Modern Wave Piercing Catamaran designs offer significant opportunity to maximise efficiency and reduce through life costs when compared to mono-hull or mono-hull type vessels.
It has been shown that length is the main driver for structural material weight reduction in a vessel, so the shorter vessel will be lighter and have reduced capital cost. The use of aluminium can reduce the structural material weight of a vessel by half or more.
The lighter vessel will have higher length to displacement ratio and lower wetted surface area which reduce both wave-making and frictional resistance. The WPC design will increase length to displacement ratio and hence reduce wave making resistance by distributing the weight over two hulls rather than one. The WPC design can also make possible increases of overall beam and height without detrimental effects on stability. Hull interference effects in a catamaran can be minimised with an increase in length to displacement ratio and greater hull separation. Further a WPC design can have optimal hull shape for minimal wetted surface area to minimise frictional resistance. Reduced powering requirements due to light weight and efficient hull design reduce operating and capital cost. Even though aluminium has higher purchase costs and higher labour rates than steel, the aluminium vessel will only require half or less the material of a steel vessel by
weight. Since a WPC design can be much shorter the outcome may be that reduced material quantity is required and that building labour costs are lower. When consideration is given to other factors such as reduced painting requirements, corrosion allowance, elimination of ballast requirements and reduced bow thrusters requirements the WPC design will offer a much reduced capital cost.
A WPC offers a very large deck area for manoeuvring large trucks, high initial stability in harbour (no ballast required), good sea-keeping, (Davidson and Roberts. 2007), very good manoeuvrability due to wide separation of thrust and as stated here very high efficiency.
This gives a quick turnaround time which puts less pressure on the actual journey time so making it possible to operate at medium speed in the optimal speed range for that vessel. In the future it would be attractive to develop larger but perhaps not longer designs. A relatively shorter vessel will be advantageous in terms of cost and efficiency. Therefore to increase deadweight a vessel will be wider and taller, but there will be practical limits to beam and height depending on shore facilities.
Currently INCAT and Revolution Design have been researching designs based on a 150m double deck WPC with 40m beam and around 4800 tonnes DWT and a larger vessel with 60m beam to carry 10,000 tonnes DWT. Results are shown in table 1. Results for the 130m have been validated but the others are preliminary subject to tank test results. Two speeds are given for each vessel, a slow speed to compare vessel Transport Efficiency (T.E.) against current RORO technology and a faster medium speed to show what is possible as speed is increased. Huge reductions in power can be realised by operating at lower speeds. Costs are indicative and are based on an increase or decrease in material weight and installed power, (for accurate prices the builder should of course be consulted). In table 2 the 130m is compared against a 138.2m mono-hull with 1400 tonnes DWT, the 150m against a 214m mono-hull with 5000 tonnes DWT and the 160m+ against a 210m mono-hull with 10,000 tonnes DWT.
14.0 FUTURE WORK
To operate in the medium speed range is likely to require different hull shapes than when operating at high speeds. Funded through an Australian Research Council Linkage Project “Powering Optimisation of large Medium Speed Multi-hulls” Incat, Revolution Design, AMC, UTAS, Wartsila and MARIN are investigating the most appropriate hull forms and propulsion configurations for the medium speed range. Factors that have not been important in high speed vessels may now become important such as LCB position, hull separation, B/T and transom immersion. As speeds reduce, propellers could become more efficient than water-jets but for the size and speed of vessel considered here the water-jet is still favoured unless the vessel size increases substantially. Operation of water-jets in the upper medium speed range demands that thrust and flow
characteristics need to be accurately determined for these conditions.
REFERENCES
Bose, N, (2008). Marine Powering Prediction and Propulsors, SNAME.
Davidson, G.W., Roberts, T., Thomas, G.A., (2006), Global and slam loads for a large Wave Piercing Catamaran Design, Australian Journal of Mechanical Engineering, Vol 3, No 2.
Davidson, G.W., Roberts, T., (2007). Sea-keeping of Wave Piercing Catamarans, The Naval Architect, RINA..
Davidson, G.W., Roberts, T., Thomas, G.A., Bose, N., Davis, M.R., Verbeek, R., (2011) 130m Wave Piercing Catamaran: A new Energy Efficient Multi-hull Operating at Critical Speeds, International Conference on Marine High Speed Vessels, RINA, Fremantle, Australia.
DNV, July 2010 Rules for High Speed, light Craft and Naval Surface Craft – Standard Contents.
Mansour, A., Liu, D., (2008) Strength of Ships and Ocean Structures, the PNA Series, SNAME.
Molland, A., Wellicome, J., Couser, P. (1996) Resistance Experiments on a systematic Series of High Speed Displacement Catamaran Forms: Variation of Length-Displacement Ratio and Breadth-Draught Ratio, Transactions of The Royal Institution of Naval Architects, v138, 55-68.
Larsson, L., Raven, H.C., (2010) Ship Resistance and Flow, the PNA Series, SNAME.
EIA, U.S. Energy Information Administration, (2011), http://www.eia.doe.gov/steo/
ACKNOWLEDGEMENTS
This investigation has been supported by INCAT Tasmania, Revolution Design, Australian Maritime College, University of Tasmania and the Australian Research Council through an Australian Research Council Linkage Project “Powering optimisation of large Medium Speed Multi-Hulls”.
Table 1 Current and future concepts for medium speed vessels
LOA BOA No of truck decks DWT, tonnes Speed, knots Froude No. Operating Power, MW ** Cost Delta to 112m Cost Delta to equivalent Mono RORO, 130m 30 1 1850 29 0.43 4 x 7.3 1.12 1.0 130m 30 1 1850 22.5 0.33 4 x 2.875 0.99 0.842 150m 40m 2 4,800 24.5 0.34 4 x 7.2 1.86 0.72 150m 40m 2 4,800 22 0.30 4 x 5.2 1.8 0.7 160 - 180m* 60m 2 10,000 25 0.33 - 0.31 4 x 11 2.7 0.76 160 - 180m* 60m 2 10,000 22 0.29 -0.27 4 x 6.25 2.56 0.72
* Final length to be determined after thorough resistance testing as it will be a trade off on wave making resistance versus wetted surface area.
**Operating power is less than installed power for optimal fuel consumption and reliability. *** Transport Efficiency = DWT x Speed/Power, 112m T.E. is 1.25 at overload (1450 tonnes, 28 knots)
Table 2 Transport Efficiency compared against similar conventional RORO
TYPE LOA BOA No of
truck decks DWT, tonnes Speed, knots Froude No. Operating Power, MW Transport Efficiency WPC 130m 30 1 1850 29 0.43 4 x 7.3 1.84 WPC 130m 30 1 1850 22.5 0.33 4 x 2.875 3.54 Conventional mono-hull 138.2 ~30 2 1400 27 ~0.39 4 x 7.92 1.19 WPC 150m 40m 2 4,800 24.5 0.34 4 x 7.2 4.1 WPC 150m 40m 2 4,800 22 0.30 4 x 5.2 5.1 Conventional mono-hull 213 31.4 3 5,000 22 ~0.25 4 x 8.4 3.3 WPC 160 - 180m 60m 2 10,000 25 0.33 - 0.31 4 x 11 5.68 WPC 160 - 180m 60m 2 10,000 22 0.29 -0.27 4 x 6.25 8.8 Conventional mono-hull 209 31.2 3 10,000 22 ~0.25 4 x 7.8 8.25
