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.