Tongue weights at which sway has occurred?

Jato was good enough to remind me that he had already shared his sway data. His 177 had dual batteries and a single propane on the tongue and a full fresh water tank aft when he experience significant sway at only 40 mph. I back calculated his tongue weight to be right about 10% at that point. 

Offgrid, Yes to all questions.

Offgrid, when we bought our 177 home in March 2011, it was in the midst of a tremendous snow storm, snow on the roadbeds averaged anywhere from 4-8" except when we crossed the 5 mile long Mackinac Bridge where it had been freshly plowed.  At that time we had a 2 12v group 24 batteries, with a propane tank on the tongue and empty fw tank.  No sway encountered.  Fast forward to May of 2011 we have added an extra propane tank on the tongue and have filled our 30 gallon aft fw tank; had to turn around after 15 miles as we could go no faster than about 40 mph without severe sway.  The TV at that time was an '08 Explorer.  After that we purchased an anti-sway bar link, problem solved.  Since then we have duplicated the above scenerio with and without a full fw tank of water.  Scale shows 350 lbs. tongue weight with an empty fw tank, haven't weighed with a full tank of water.  Regardless, over the 7+ years we have owned the 177, the following vehicles have towed it: '94 F-150 4x4, '11 F-150 4x4 and '13 Explorer and currently all 3 are still used to tow it.  The interesting thing here is that with the above 3 vehicles I have towed our 177 with and without a full tank of water of speeds up to 65 mph and have experienced zero sway, even with head and cross winds up to 15 mph.  The newer Explorer has a wider track than the older one, hence more stable.  Also with the pick-ups, much longer wheel base, again more stability noted. Nevertheless outside of when I did these tests using these 3 vehicles I always hook up the anti-sway bar link when traveling regardless of when vehicle I am towing it with.

Thanks GlueGuy. I've never had any sway either, even with my relatively light TV and sway control disabled, but with a 550-570 lb (15%) tongue wt outbound and maybe 12% inbound I wouldn't expect any. But I'd like to reduce that figure (I was surprised at first it was that high) and that's why I'm looking for a minimum safe tongue wt (preferably based on actual experience rather than just generally accepted practice). 

Thanks mcarter, great info. And is the fw tank in the 178 just behind the axle like in lostagain's 172? Yours  being empty I can easily run the numbers starting from FR's specs if you don't already know your actual weights then.  Did you have a single full propane tank and a battery on the tongue?