Predicting Resistance and Power
for an Integrated Tug/Barge Using NavCad
A HydroComp Technical Report
Report 120
Product link: NavCad
Predicting
resistance and power for an integrated tug/barge (ITB) unit requires
us to consider the connected "system". However, experience has
shown that the resistance prediction of the system and the power
prediction must be treated differently. The recommendations shown
herein are based on a limited review of actual trial data. There
is very little information available about the hydrodynamics of
ITBs, and much of what is presented below is based on common sense,
validated with a handful of actual sea trials.
Resistance
The approach
here is to model the tug and barge as a single ship system. It
will be necessary to calculate the total length, displacement,
wetted surface and system LCB. Use maximum beam and draft (most
likely from the barge). You can then choose a suitable prediction
method for the new ship system. The illustration below shows this:
You will
want to remember to add appendage drag. (This typically amounts
from 5% to 10% of the barehull drag.) Also, many ITBs have significant
hull and superstructure area, so wind drag may be significant.
Power
prediction
Predicting
power is the same as for any vessel using NavCad, with one exception
 the calculation of the propulsive coefficients (wake fraction,
thrust deduction, relativerotative efficiency) are best modeled
by considering the tug only. Intuitively this makes sense if you
consider the nature of these coefficients. The water flow gets
"reset" with turbulence at the intersection of the tug and barge,
leading us to conclude that it is the tug's geometry that drives
the coefficients. Trial data has suggested that this is the proper
approach.
Our recommended
technique is simply to prepare a second project for the tug only,
run a Shaft Power calculation to predict the propulsive coefficients,
and then manually transfer these coefficients to your integrated
system project. (You can also do a copy/paste for an entry grid
selection in System Analysis Edit>Hull.) Then run Shaft Power
or System Analysis without predicting new coefficients.
