Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015 4.2 INFLUENCE ON SHEAR FORCE
Values of shear force coefficients just above the values of the ITTC line agreed
with findings fellow
investigators [26] who presented shear force coefficients for a high-speed catamaran at Fr = 0.17 – 0.60 and log(Re) = 8.6 – 9.1.
It was found that the shear force was not only influenced by the Reynolds number, but also the Froude number and the displacement. It was seen that the values of Fr = 0.37 are generally closer to the lower of the two ship-model correlation lines, in model-scale as well as in full-scale. The wave elevation along the hull changed for varying Froude numbers and displacements and an influence of the shape of the wetted surface area was assumed. The value for shear force coefficient is generally lower than for a higher displacement and a positive form effect on the shear force was concluded.
4.3 DEMIHULL INTERACTION Using CFD at
The results showed that the change in resistance was not significant for varying demihull separation considering the parameter variation in this study. This agrees with the findings of other researches ([11], [15], [17], [18]) which all presented only minor changes in demihull interference for s/L < 0.2 at Fr < 0.45, but more noticeable changes for larger Froude numbers and larger values of demihull separation value. However, these effects are mainly influenced by the displacement of the hull and the associated wave-making. The slender
hulls under
consideration will have low wave-making characteristics and therefore penalties in interference drag are expected to be small.
4.4 NON-DIMENSIONAL DRAG FORCE
It was demonstrated that a slenderness ratio around 13 for Fr = 0.29, 0.45 resulted in lowest non-dimensional resistance and of 12 for Fr = 0.37. This is somewhat contrary to the recommendation presented in earlier work
5. CONCLUSIONS
This paper reported on a numerical investigation into resistance properties and transport efficiency of large medium-speed catamarans. The vessel
[19] where recommendations for slenderness were L/1/3 = 6 – 7 around Fr = 0.30, L/1/3 = 7 – 9 around Fr = 0.37, and L/1/3 = 8 – 9 around Fr = 0.45. As previously discussed those values were guidelines based on built
monohull ships and optimum values may change due to advances in technology. Therefore, the present work concludes that these guidelines are not applicable for large medium-speed catamarans because the resistance can be reduced by up to 25% at Fr = 0.45 when using demihulls of higher slenderness.
4.5 TRANSPORT EFFICIENCY
The transport efficiency provides an indication of the efficiency to transport payload from a hydrodynamic point of view. Appropriate designs were identified for highest transport efficiency, depending on the speed and loading condition. In addition, the operational profile
The 150 m hull showed best performance at light displacement over the entire speed range, whereas the 170 m performed most advantageously at
the heavy
displacement case and the 190 m hull at the heavy one. At at certain speed ranges other length hulls were capable of providing comparable transport efficiency. However
from L = 110 – 190 m, but draft, demihull beam and overall beam were kept constant. Froude numbers varied from Fr = 0.25 – 0.49 which corresponds to speeds of 16 – 41 knots. Furthermore, three different drafts were considered. The study concluded with proposals
for
design parameters for highest transport efficiency and lowest drag.
Generally transport efficiency was highest for the heavy displacement case and at
certain loading case and speed a vessel length with highest transport efficiency was found.
low speeds. For a Furthermore,
configurations with comparable transport efficiency that was within 5% or 10% of highest achieved value were identified.
length ranged full-scale Reynolds numbers showed
consistent results for a wide range of speeds, slenderness ratios, and drafts, however in future work the validation of this approach
will be focussed and possible
differences between model and full-scale results will be studied. Furthermore the flow around the transom requires a high resolution for partially wetted transoms, compared to other areas around the hull for accurate resistance prediction. A study of flow characteristics around deep square transom will be undertaken.
needs to be taken into consideration as the target speed and load case may vary during the life cycle of the ship. The charts provided in Figure 13 also give information about the suitability in off-design conditions.
From a production cost perspective a short hull may be preferred to minimise material costs, whereas a longer hull may be preferred to minimise the wave-making for reduced environmental impact such as wave wake at the same transport efficiency.
4.6 FUTURE WORK
A cost comparison model for the full life cycle of the cargo vessel needs to be developed to compare the economic and ecological benefits of differently sized large medium-speed catamarans. Building costs, fuel prices, emissions, port duties and operational profiles have to be taken into account to estimate amortisation times of these vessels.
©2015: The Royal Institution of Naval Architects
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