Huang and von Kercezek 
The present calibration is in good agreement with references (33 
and 34), 
Patel ?* also found that a Preston tube can be used with 
acceptable accuracy (maximum error of 3 percent) if the pressure 
gradient parameter is limited to the range -0.005 <»/(pu2 ) 
dp/dx <0.01 where dp/dx is the pressure gradient along the flow 
direction and U, = Vtw/p is the shear velocity. The validity of 
using a Preston tube in boundary layers with large crossflows is not 
known. However, it is believed that the crossflow on the present 
ship model is rather small (the crossflow angles calculated are all 
less than 15 deg. ). 
The directional response of the Preston tube has been cal- 
ibrated and is shown in Figure A5, It is not practical to rotate the 
Preston tube flush on the three-dimensional ship hull and therefore 
the Preston tube is not used for measuring the angular position of 
the shear stress vector. The three-tube directional Preston probe, 
as shown in Figure A6, has very good directional response. One 
drawback of the Preston tube and the directional Preston probe is 
that several of them can not be used close together because they 
will not only cause an increase in the ship model resistance but will 
also have an interference effect on the downstream probes (see 
Figure A7). 
LOTS 
