SmartJack,
Just finished reading all 8 pages of this thread! Awesome work! I'm getting ready to embark on a similar journey to lower my CYA significantly! If you find yourself near a post office, go ahead and ship that tarp overnight to Cincinnati
Wish me luck!
It is a MUCH easier process than it appears to be. If you decide to go this route, don't worry about it too much, as you will find how easy it is. I wish we lived closer, because I would have no issue letting you use my tarp, and then this would be a no-brainier for you to do.
But, before you try to go this route, let me hit on a few pros and cons. I plan to cover these in the video I am doing about this, but I have not had time to do this yet. So, the tarp works REALLY well, and from and ease of use, and time issue, I would recommend it. The con is the cost. I needed a 40 x 60 tarp to do my pool. Based on the size you list, you could probably get by with a 30 x 50. I found a 40 x 60 online for only $80, but the kicker can be the shipping. That 40 x 60 tarp weighs 60 pounds, so depending on how far it ships, shipping can range from $20 to $60. That is pricey for some. Now, you can weigh that cost against the cost of filling a pool, and that varies by how you will be filling. Truck method, well water, or city water. Depending on your cost per gallon, based on the method chosen, you can decide if the tarp saves you any money.
So, let's say you can only replace 40% of your water, in order to protect your liner. If you are starting at a CYA of 225, it would take 4 partial drains to get you to a CYA of 48. So, doing four 40% drains on a 21k pool means you would go through 33,000 gallons. Or, using the tarp, you could replace 80% of your water at once, getting you to a CYA of 44, and only need 17,000 gallons of water. So, does the cost of the tarp save you more money than the additional 16,000 gallons of water? Then, there is also the time factor. 1 drain at 80% is way faster than doing 4 drains and refills. I didn't care about the cost, and went this route for less time. I didn't mind spending the money for the tarp, verses all the time to drain and refill. Another cost to consider is the hose to drain with. I ran my hose 150 feet, and it was just a garden hose, so drain time took a very long time. I could have drained way faster, it I used a larger hose, but that was also a additional expense. Since I was filling with a garden hose, and draining at the same time, I didn't need to go that much faster.
So, since I didn't care about the cost at the time I decided to do this, I didn't look at lower costs to acquire a tarp, like seeing if I could rent one. So, I would look in your area, and see if that is possible. If you are like me, and just want to go the fast route, you can buy a 30 x 50 tarp at Home Depot for about $120. It would be cheaper online, but then you add the shipping.
Good luck with whatever you decide to do. If you go the tarp route, I will be with you to help you through it.