Tornado Sailor,
The strut is the mid pole tube located 'below' the spin pole that carries the wire that runs the length of the spin pole. This strut carries the vertical load induced by the luff tension of the spinnaker. (The Tornado does not have this for reasons I will discuss later.) This vertical load travels up the strut and passes around the spin pole in the collar and then on up the Pelican Stricker Tube where it is handed off to the forestay bridle wires. This is the vertical load carrying system on ARC boats. By simply varying the length of the Pelican Stricker tube, the spin pole height at the tip can be set anywhere desired, like low for a longer luff spinnaker.
Now we have to handle the horrizontal forces from the spinnaker. A set of 'whisker wires' connecting the end of the spin pole to the bows of the boat or forestay chainplates near the bows will control and carry the spin pole side loads. Now all spinnaker generated forces are taken care of and we can put the pole at any height position desired.
The Tornado does not have the strut and wire below the spin pole. Therefore the vertical loads and the horrizontal loads from the spinnaker must both be carried by the 'whisker wires' connecting the ends of the spin pole to the bows. Since these 'whisker wires' on the Tornado also carry the vertical spinnaker generated loads, another constraint enters the picture. To keep from overloading the spin pole in axial compression, the included angle between the spin pole and whisker wire when looking at this geometry from the side view must be 10 degrees or greater. This sets the min height of the spin pole above the bows for the Tornado. This 10 degree angle is the same minimum angle as is used on diamond wires relative to the axis of the mast. This sets diamond spreader arm length.
Now there is more to this system. The spin pole can be made of a significantly smaller diameter tube if the spin pole is stableized at its mid length point. The stability equation for tall slender columns in axial compression says that the moment of inertia required for the tube to be stable is directly related to the unsupported length of the tube. Therefore if we can stop the tube, spin pole, from moving at mid point, we can use a smaller diameter tube for the spin pole. That is why the spin pole on the ARC boats and Tornado are stabilized at mid point both vertically and side to side.
Good Sailing,
Bill