Practical Ship Hydrodynamics
Practical Ship Hydrodynamics
Practical Ship Hydrodynamics
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Resistance and propulsion 81<br />
are not properly scaled up to the real ship. Appendages may be tested<br />
separately and often the resistance of the appendages is scaled separately and<br />
added in a prediction for the full-scale ship. Unfortunately, this procedure<br />
does not account for interaction between hull and appendages and also<br />
introduces considerable error margins. Fortunately, most ships have only a<br />
few appendages and errors in estimating their resistance can be accepted. For<br />
unconventional ships with many and complex appendages, the difficulties<br />
in estimating the resistance of the appendages properly leads to a larger<br />
margin of uncertainty for the global full-scale prediction.<br />
Schneekluth and Bertram (1998) compile some data from shipbuilding<br />
experience: properly arranged bilge keels contribute only 1–2% to the total<br />
resistance of ships. However, trim and ship motions in seastates increase the<br />
resistance more than for ships without bilge keels. Thus, in evaluation of<br />
model tests, a much higher increase of resistance should be made for ships<br />
in ballast condition. Bow thrusters, if properly designed and located, do<br />
not significantly increase resistance. Transverse thrusters in the aftbody may<br />
increase resistance by 1–6%. Shaft brackets and bossings increase resistance<br />
by 5–12%. For twin-screw ships with long propeller shafts, the resistance<br />
increase may be more than 20%. Rudders increase resistance little ⊲¾1%⊳ if<br />
in the neutral position and improve propulsion. But moderate rudder angles<br />
may increase resistance already by 2–6%.<br />
ž Shallow water<br />
Shallow water increases friction resistance and usually also wave resistance.<br />
Near the critical depth Froude number Fnh D V/ p gH D 1, where H is the<br />
water depth, the resistance is strongly increased. Figure 3.10 allows one<br />
to estimate the speed loss for weak shallow-water influence. The figure<br />
√Am /H<br />
0.9<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0.1%<br />
1%<br />
10%<br />
9<br />
8<br />
7<br />
6<br />
5<br />
4<br />
3<br />
2<br />
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8<br />
V 2 /(g.H)<br />
Figure 3.10 Percentage loss of speed in shallow water (Lackenby (1963))
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