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# Transom Expansion

## How to Model the Expansion of a Cylindrical Transom

### A practical application of Copy Snakes

#### Model cyl_transom_expansion.ms2

by Reinhard Siegel

For ease of construction many boats feature a transom which is a portion of a straight circular cylinder. The model cyl_transom_expansion.ms2 shows how to construct the expanded shape of the transom within MultiSurf.

The Translation Surface *transom_0* is the basis surface of the cylindrical transom. Its generator is the Arc *c1*, guiding curve is the Line *l1*. The control points of both curves are defined in the 3-point Frame *F1*, so that one can tilt and move the transom without changing its shape.

The hull and deck are intersected by the surface *transom_0* in the Intersection Snakes *n1* and *n3*, which in turn are projected onto *transom_0* as Projected Snakes *n2* and *n4*.

The expansion of the transom basis surface is the Translation Surface *s0*, whose generator is the Line *l2* (end point is Point *pt3*), the guiding curve is again Line *l1*. The length of Line *l2* must be equal to the length of the Arc *c1*. Since *s0* is in the XZ plane of the Frame *F1*, the Z-coordinate of Point *pt3* (end point of Line *l2*) simply corresponds to the length of *c1*. This length is displayed in Tools/ Mass Properties.

When the arc is changed, for example to increase the camber of the transom, the position of *pt3* must be adjusted manually.

To avoid this, the simple Formula *f1:*

f1 = ARCLEN (c1, 0, 1)

which calculates the length of Arc *c1* is used for the Z-coordinate of *pt3*. In this way it is guaranteed. that *s0* always is just as long as Arc *c1*.

In order to get the developed shape of the transom on *s0*, the two Projected Snakes *n2* and *n4* are copied onto *s0* as Copy Snake entities.

In this way you get without a specialized program the true outline of the circular cylinder transom.

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