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measuring pregates-is my calculation correct?


alex38
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We're adding pregates to a course and I have a rope to measure with but I'm curious (not smart-just curious) if I can measure the distance from entrance gates in seconds, or TIME the placement of the anchors. I scratched out some math and my boat putts at a GPS displayed speed of exactly 5.0 MPH so I'm figuring if I put an anchor on the edge of my swim deck, drive STRAIGHT through the gates and hit a timer right when the anchor goes through and drop it at exactly 24.5 seconds that will be 180 feet or 55 meters. I hope I'm right cuz I want to try it, but if not, will enjoy hearing what is incorrect, and if it's possible!!

 

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The math looks OK... Might get you close... Likely the anchor would not fall exactly vertical and probably not at the correct width (interior boat guide width.) If your interior boat guides have been placed at or about 1.15m. or at least are all the same width, you can 'eyeball' from water level and get within an inch probably. I would still measure the 180'/55m., or have it surveyed by a TC or someone who can do a single, two or three point survey. For practice it would mostly make little difference. In the event you want to hold a record tournament, and the TC surveys your course, the homologation calculator uses the pregates to compute the course baseline- so it's nice to have them fairly close, and save resurveys and recalculations when the C/L and interior buoy locations look 'warped' until the pregates get set in correct locations. Probably TMI :-) ...
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Cool, and definitely NOT TMI @dusty, super interesting, thanks. I will probably do it then remeasure, maybe even time it and drop then retime it to see how close the anchor fell then measure, thanks for the feedback.
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A lot,easier to buy some no stretch rope or thin Galvenized wire with a little extra length, mark off the distance starting a few feet in and add 4 or more floats to it, have two guys in the water holding each end and drop a weighted line in at the end in line with the boat guides.

 

That is cheap, fast, and fairly accurate.

 

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I loop (slip knot) a measured rope over a gate ball then putter slowly to the end of the 55M line. I drop the brick sub buoy and ball in as close as possible then hop in the water and move to exact position. When done I drive to a more distant position and eyeball the line up. Make changes as necessary. Repeat for other side. If its windy anchor your boat unless you like to swim.
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Good tips from AB and thager. Especially about using at least two people at each end of tape/rope/wire. If you can't stand up easily though... it is nigh impossible to pull the tape tight enough to get a good measurement. You can use a decent compass to find centerline direction, and 90 degreees to that and offset to the shore to measure the 180 ft.(in C/L direction), then back 90 degrees into the water to boat guide line. You can get within about 1 -2 feet pretty easily- good enough for practice generally. FYI a whole lot of courses were surveyed in as lakes were built, before there were 55m. buoys... They got added later after the lakes were filled. Usually... fairly close... When we began surveying the local courses we noticed the alignment being torqued around by the pregate baseline, and discovered how the homologation program made the offset calculations. (Some of those anchors were/are an absolute %#$&* to move.) A really fine project for a crappy non ski day, or when your hands can't take another set! Beer or other analgesic to counteract banging your shins while 'walking' with anchors is recommended :-)

YMMV!

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I've put them in with 300ft tape. If the course is going to be surveyed, make sure they line up with the boat guides not end gates. The survey is very sensitive to the width and alignment of of the 55's. They can throw the imaginary centerline created in the survey off to one side and the whole thing will look really off. The 55m distance has tolerance of .275m or almost a foot.

 

 

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To clarify what @thompjs said the tolerance is +/- .275m which means +/- almost a foot (or a two foot tolerance). So getting the 'length' right is fairly straight forward alignment to the boat guides a bit more critical especially if its going to be surveyed.
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We had a project in Vermont a couple of seasons ago to replace a floating course with a bottom-anchored one. Keeping it in essentially the same place. Floater didn't have pregates. At one end, the view was blocked by trees. So, we made up a 55m (about 180 1/2 ft) temporary measuring line, clipped a buoy onto one end, and slowly dragged it with the boat, dropping the anchors when the buoy passed the exit gate. Then, aligned them manually with diving (shallow and good visibility), and re-checked with the line.

 

I could go to another location where both endgate and new pregate can be seen with my survey gear, and get them right on, but for the time being they were good enough, and we never got to that part. Not a tournament site; was on a public lake.

 

Back in the Olde Daze, it was common to check/fix slalom courses with 3 measuring lines and 3 people in the water with vests on, doing triangulation to the skier buoys. Good bit of skill and calm water required.

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@AB funny. I'm +/- a boat length on my starts anyway depending if my contacts have cleared or my shorts are straightened. And some days, my skirt is up over my head and in my eyes so the 55's are the least of my problems. Truth be told we put three -15 ropes together along with a made-up 5'-0 section. Then attached our ski handles to either end so the swimmers would have something decent to hang onto. After the swimmers were in the water and the line extended the boat idled over and dropped the buoys in. The boat then backed off a ways and put the educated eyeball on the buoy placement and the swimmers did a little anchor adjusting.
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For Alex38. The GPS units available will not be precise enough. The ones used by surveyors and the miltary could do it maybe- but as I recall it takes a long time and lots of satellite 'looks' to resolve the position. No longer in that industry, but one vendor claimed resolution within a sphere 10 cm. virtually anywhere... maybe not so hot in 259m. but pretty tight between L.A. and N.Y.
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So I tried "timing" my 1 existing pregate at exactly 5.0 mph. It seems far like 28 seconds, and my calculations show it should be 24.5 which would explain alot of crappy skiing!! I come up with 7.3 feet per second at 5.0 mph that makes my 1 gate waaaay off.

 

I think.

 

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