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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.

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