ZO Effect on Two Popular Ski Styles

[dn]

Following the discussion "ZO Analysis", I wanted to address more closely the issue of how ZO may affect different skiing styles.  The two styles I'd like to consider are what I'll nickname efficient and wide-early.  The efficient style is the one I talked about in my articles on Schnitz's web site or the possible line Chris Rossi talks about in "Skiing the Impossible Line".  The wide-early style is the one that so many of use default to, and even prefer to use, because it works so well at longer lines - but at short lines it evolves into the "impossible line".  

One popular version of the wide-early style is to complete your turn, immediately load hard against the boat, and maintain that max load all the way to the 1st wake, then prepare to edge change.  The efficient style recommends lightly loading against the boat and progressively increasing that load to the 1st wake, then prepare to edge change.  The wide-early approach will lead to crossing the boat path several meters up-course of the progressive approach.  It will also give you higher speed at that point.  Both those can be translated into being a lot wider and earlier than with the progressive, efficient approach.  The best the efficient approach can do is get you to the next buoy 'just in time'.

As I said in 'ZO Analysis', I think the way ZO responds to load makes the wide-early approach a lot more difficult than PP or manual driving does.  Like I said there, my understanding of ZO is based on my observations as a skier and driver, and not on detailed measurements of the way ZO responds or on insider knowledge of the way ZO is designed - so I may be off base in some or many respects.  But using my understanding of ZO I created a computer simulation showing how ZO responds to the wide-early loading and the progressive loading of an efficient style.  Refer to the plot I attached (I'm not sure where it will appear relative to the text.)

The time scale goes from when you pass one buoy (0 seconds) to when you reach the next buoy (2.67s later at 55kph).  You cross the boat path somewhere in the vicinity of 1.2-1.3 seconds.  The top graph plots the skier load versus time.  800 pounds of load is what I've heard some say has been measured.  So in the blue plot the load ramps up to 800 and stays there for 300ms.  You'd have to be pretty strong to do this, but I know some of you are.  In the progressive case, the red plot ramps up more slowly and doesn't peak till the 1st wake is crossed,  with the max load there being about 500 pounds.  Then in both cases, as you begin the transition to the preturn, the load drops off by the time you cross the 2nd wake.  (This is based on a 180 pound skier, so if you assume the same skier strength to weight ratio but a lighter or heavier skier, you could just scale all these plots by the ratio of the skier you assume.)

The bottom graph shows the how the boat speed changes in time to each load with ZO in control. This is based on my estimates of throttle response time, engine rpm slew rate, boat interia, etc., and a typical skier setting (B2?).  I'm sure ZO engineers would tell me I've goofed somewhere, but I think as a rough model this is useful.

The blue line is the speed response to the wide-early loading.  Boat slows down 0.6mph, then speeds up and peaks at 0.5mph above the 34.2mph average.  The area below 34.2 equals the area above 34.2, so the boat average doesn't change.  The red line shows the progressive-efficient boat speed.  The slowdown and speedup are only 0.3mph, or half as much as in the wide case. 

I know when I ski wide-early, the boat really seems to fight me and my buoy count drops.  When I load progressively and take the efficient path, the boat response seems benign, and my bouy count climbs.  The difference in boat speeds based on this computer simulation is not as dramaticly different as I was expecting, but maybe it is different enough to explain why it feels so different and effects me so much.  It doesn't take a lot of intereference to mess up a pass.

 

[dn]

can anybody tell me how to make the jpg file appear with the text?

[Jim Neely]

try this to post a picture

http://www.ballofspray.com/vanillaforum/comments.php?DiscussionID=2260&page=1#Item_5

[eleeski]

ZO is here. Tell us the settings we should use for various styles instead of analyzing ZO to death.

An interesting thing I noted while boat judging some savvy high level skiers was that they started at C3 but switched to A1 when the rope got really short. Does this really work or have a sound theoretical basis?

Eric

[rq0013]

I have heard the opposite, that some pros start out with A, B and hten move to C once they get into the shorter lines.

[skidawg]

I can tell a huge difference bt settings at 38/39 off. C is a must, u gotta b picked up earlier to avoid losing speed