The influence of ridge is assumed not only limiting the case in which the ice concentration is more than 6, but also including influences against all ice concentration. The comparison for F(C_r) between Canadian Ice Numeral and present method is shown in Figure 2.6.6.

　

Figure 2.6.6 Comparison for F(C_r) between Canadian Ice Numeral and present method

　

I_C

Ice index I_C indicates the influence of the ice strength. Firstly, the mean monthly temperature was calculated by taking the monthly difference of the cold sum. Hence the ice strength highly depends on the temperature and salinity of ice, the ice strength which corresponds to the mean of monthly temperature is defined shown in Table 2.6.4 (Riska ,1996). Utilizing M_CS, I_C is obtained as the equation(2.6.21).

　

Table 2.6.4 The multiplier for ice strength, M_CS

　

　

where, M_CS is a multiplier. After the parametric study by NEWSIM2 in which the bending strength value and the compressive strength value were varied, the value of M_CS was calibrated using the relation between ice index and the ship speed.

　

Example for calculating Ice Index

For example, Ice index： I is calculated as follows in case of the next condition.

Ice thickness：1.0m(MFY)

First year ice concentration：6

Multi year ice concentration：1

Ridge sail height：0.5m

Ridge density：20/km

Monthly average temperature：-5℃

Ice class：Type A

I_A=1×6+1×(-3.5)+2×(10-6-1)=8.5

C_r=0.5×20/5=2.0

I_B=-2×2.0/10×(6+1)=-2.8

I_C=0×6+0×1=0.0

I= I_A+I_B+I_C=8.5-2.8+0.0=5.7

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