Page 20 - WestlawnMasthead23_Sept12.pub

The Masthead
Planing Craft Analysis: Static vs. Dynamic Trim
-
Continued
Sept. 2012 Page 20
Again, as with the hydrostatic analysis, it is assumed that
the CG remains fixed to the hull geometry at all times, as
well as the Thrust line.
Figure 4 shows the “ideal” Savitsky hull, and the forces con-
sidered by this method for the most general case of analy-
sis.
As we can see in Figure 4, the system of forces considered
by the Savitsky method is slightly different from the one
shown previously in Figure 3. However, the system of forces
in Figure 4 can easily be resolved into components parallel
to those in Figure 3. Once the equilibrium condition is found,
both representations are equivalent and the Trim angle is
the same as well.
We can see here that, apart from craft’s Weight, the Savit-
sky method considers (in fact predicts) the resultant of the
Pressure forces acting Normal to the hull bottom, the Vis-
cous Drag acting along the hull’s bottom (both of them act-
ing within the wetted section of the hull), and it also consid-
ers the direction of the Propulsion Thrust, usually the shaft
angle for the Propeller thrust.
One “key” aspect of the Savitsky method to keep in mind is
that the hull under analysis is not the “actual” hull (i.e., the
3
D modeled hull in Rhino). Instead, the Savitsky method
creates an “equivalent” prismatic V-type hull, with a con-
stant deadrise angle and chine beam over the entire length
of the hull. Even with this simplification the method is very
reliable, if used for the analysis of similar hull types, i.e.,
single monohedron hulls with nearly constant deadrise ex-
tending, at least, over the wetted portion of the hull at run-
ning speed. Some corrections for warped hulls are available
in the implementation of the Savitsky method used in Or-
ca3D.
Figure 5 shows a correlation between the ideal Savitsky hull
and our analysis case.
For further details of the Savitsky Method, please see Refer-
ence 1 and/or Reference 2.
Conclusion
Coming back to the initial question, we can understand now
why the “Static” and the “Running” trim of a planing boat
are not directly tied to each other, since each of them are
the consequence of a different type of problem and analysis.
However, we have also shown that this is just a simple inter-
pretation of such an answer, since, as explained and shown
in the previous sections, it is now easy to identify Weight
and CG as common factors that influence both the “Static”
and “Dynamic” trim calculations.
But, again, remember that the Savitsky method doesn’t take
into consideration anything about the “Static” attitude of the
hull when it’s at rest. It simply has no means to know any-
thing about the hull in this condition. The Savitsky method
only attempts to bring us a “snapshot” of the hull, and the
main forces acting over it, when the craft is running at the
steady” speed of analysis.
Since the implementation of the Savitsky method in Orca3D
gets shape information from the hull in the "as-modeled"
condition, it is good practice to have the model oriented so
that stations give a good representation of the deadrise. In a
hull without rocker in the keel, this simply means having the
keel parallel to the longitudinal axis.
References:
1)
Lewis, E.V., “Principles of Naval Architecture, Vol. II: Resistance
and Propulsion,” SNAME 1988
2)
Savitsky, D.,
NAME
Marine Technology, Oct 1964
3)
Orca3D Help file, in Hydrostatics & Stability/Output.
Learn about Westlawn student
pricing on Orca3D software on page 6.