The inherent problem with aerial pictures is the fact they don't show enough detail to determine exactly how much ground work is involved. On a job this large, how much trimming and or edging is required is a huge part of the equation. Will you have to edge or trim along each drive? Trim around every tree, every time? The fence along the front, is it stone? Can you use herbicide along it, or will they require trimming? What about the right away along the road? Is it easy to mow, or a deep ditch?
Here is my advice, for what it is worth, and how I would approach this. I would look at the job as three separate properties, as it appears in the photos to be basically divided that way anyway. Pretend there are three different owners, and you are being asked bid on only one at a time. The section with what I assume to be the main house (irrigated lawn?), is likely to require more detail work. The north end has what appears to be a wide open field which should mow quickly. It is easier to determine the time required on each section, than it is to look at the property as a whole.
As for the actual servicing of the property, I see a couple of options. One option, if you choose to do this with one helper, is to mow it as 3 properties. Start on one end, one person mowing, the other trimming. When the guy mowing gets done, hop on, as needed, another trimmer or edger or blower, and finish up, then head to the next portion, trading places every time. A couple of advantages to this are less boredom and the fact if you get rained out, etc, it won't look nearly as bad as if you have partially mowed the whole tract. That is one problem with larger jobs. You can always play catch-up on small places, but if you get behind on a large one, it is difficult to get enough time ahead to finish all of it at once.
If you have two helpers, you could utilize the Walker for what it does best. Let one guy mow the areas around the improvements with the Walker while another trims the outlying areas, and the third person starts covering ground with the faster Z. When the guy on the Walker gets done mowing around each home, he could then trim and do the detail work required in the localized area he just mowed, then move to the next area of improvements. If he gets done first, which he probably would, then he could assist with the rest of the trimming.
To me, the key to this deal is how long it will take. I can see the possibility of the actual mowing time as being as little as 8 or 9 hours. I can also see it taking as long as 14 or 15 hours, depending upon how rough the ground is, and how quickly the grass grows, if this is going to be an EOW mow. Everyone tends to talk about how quickly they can mow. I have 60 inch Gravely's, and, as nearly as I can determine, my AVERAGE time to cut an acre is 30 to 40 minutes. The number of properties I can mow flat out (approximately 13 mph) and get a decent cut is practically non-existant. In the real world, 6 to 10 mph mowing speed is more common, but I'm also more often dealing with northern grasses, predominantly fescue. If they aren't particulary concerned with the quality of cut on the open areas, and the ground is smooth, you might be able to cut full throttle. If you can't, it is going to significantly increase the time required to mow this large a property.
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