Spinnaker (spelled that way) are, as someone mentioned, a fixed length - that being the j dimension which is essentially from the leading edge of the mast to the stem head fitting where the forestay attaches. This is dictated by racing rules or class rules for one design racers. Whisker poles are not fixed lengths and are adjustable to accommodate the size of the headsail used, or to accommodate the amount of sail that is furled or unfurled on headsail furling system that is designed to be partially furled (not all are designed with that in mind). Also, a spinnaker pole will have attachment points (usually bridles) so that there is up and down adjustment of the pole at the outboard end. Typically when flying a spinnaker, you want both corners of the sail at the same level (going down wind), so with a spinnaker pole topping lift you can raise or lower the outer end to match the corner of the sail that has the sheet attached. The sheet that is attached to the pole end of the sail is called an after guy. The down haul will keep the pole from skying, or unexpectedly raising so the out board end heads up to the sky out of control. Another note is that when you attach the jaw to the mast eye, you should attach the jaw opening pin facing up. If you have it facing down and a big gust hits, and you don't have the downhaul affixed, the pole will "sky" and the leverage of the jaw end facing down will bend the eye on the mast or the track. If the jaw is facing up, then that won't happen and you will live another day without having to install a new mast eye. I might also add that in most cases (not all) the spinnaker pole should be parallel to the water and so with a track on the mast, you can adjust the inboard end up or down too. But, there's more to flying a kite at different wind angles and wind velocities. TMI? Sorry. I get carried away sometimes.
