Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun
8. ASSESSMENT AGAINST WIND TUNNEL DATA
The ‘reef and flat’, ‘twist and ease’ and ‘power’ models have
been implemented in FS-Equilibrium and
performance predictions carried out for true wind speeds (VT) of 4, 6, 8 and 10m/s over the range of apparent wind angles tested
in wind tunnel. The and ease’ models.
describes the de-powering in the wind tunnel accurately and is used as the benchmark for the ‘reef and flat’ and ‘twist
Figure 7 shows the lift
coefficient (CL) obtained from the three different models plotted against the effective wind angle (βeff). For VT of 4m/s the sails are not depowered for any βeff and the CL of the three models is identical since the model descriptions only vary in the way the de-powering is modelled. CL is increasingly reduced with increasing VT and it can be seen that the ‘twist and ease’ model shows better agreement with the wind tunnel data expressed by the ‘power’ model.
Figure 8 shows that modelling the drag coefficient (CD) accurately when de-powering the sails is difficult. The ‘twist and ease’ model improves the agreement with the
‘power’ model, in particular at small βeff where modelling CD accurately has a big influence on the predicted performance.
A similar trend can be seen for the reduction in centre of effort height (zCoE) in Figure 9 where the ‘twist and ease’ model agrees significantly better with the ‘power’ model at small βeff where zCoE has a large influence on the performance. The ‘reef and flat’ model reduces zCoE insufficiently and at VT of 6m/s zCoE is not reduced at all. With increasing βeff both CD and zCoE have less influence
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 15 30 β45607590 eff [°]
Figure 8: CD for four VT from VPP calculations using the ‘power’, ’reef and flat’ and ‘twist and ease’ models
B-18
3.5 4
3 0 1530 βeff [°] 45 6075 90
Figure 9: zCoE for four VT from VPP calculations using the ‘power’, ’reef and flat’ and ‘twist and ease’ models
©2010: The Royal Institution of Naval Architects VT=10m/s power
VT=10m/s reef & flat VT=10m/s twist & ease VT=8m/s power
VT=8m/s reef & flat VT=8m/s twist & ease
VT=6m/s power
VT=6m/s reef & flat VT=6m/s twist & ease VT=4m/s power
VT=4m/s reef & flat VT=4m/s twist & ease
1.2 ‘power’ model 2.8 VT=10m/s power 2.4
VT=10m/s reef & flat VT=10m/s twist & ease VT=8m/s power
VT=8m/s reef & flat VT=8m/s twist & ease
2 VT=6m/s power
VT=6m/s reef & flat VT=6m/s twist & ease VT=4m/s power
VT=4m/s reef & flat VT=4m/s twist & ease
1.6
0.8
0.4
0 0 15
304560 βeff [°]
7590
Figure 7: CL for four VT from VPP calculations using the ‘power’, ’reef and flat’ and ‘twist and ease’ models
on the yacht’s performance.
In Figure 10 the difference in predicted boat speed (VS) of the ‘reef and flat’ and ‘twist and ease’ models relative to the ‘power’ model is shown. While the ‘reef and flat’ model under predicts VS by up to 4%, the deviation with the ‘twist and ease’ model is only ± 1%.
7.5 8
6.5 7
5.5 6
4.5 5
VT=10m/s power
VT=10m/s reef & flat VT=10m/s twist & ease VT=8m/s power
VT=8m/s reef & flat VT=8m/s twist & ease
VT=6m/s power
VT=6m/s reef & flat VT=6m/s twist & ease VT=4m/s power
VT=4m/s reef & flat VT=4m/s twist & ease
CD [-]
zCoE [m]
CL [-]
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