Very interesting and detailed! But the thickness you now list is way more than 76mm.
76 mm is 3 inches, about half of 6.3 inches
2/3:ds of 6.3 inches is about 4.2 inches or 106.6 mm
If you look closely at the examples above you might notice that some are more round than oval, same are more flat than oval. The drawing of the mold for making buoys shows a rather flattened cross section, thickness/width approximately 0.5.
For the "flattish" look I suggest scaling down from 3 inches,
for the "rounded" I suggest 10, 10.5 cm.
Still not close to 15 cm though.
The 76 mm was from memory, seeing the TryggHansa lifebuoys that can be found in various places around Sweden (TryggHansa is an insurance company, maybe they sponsor these buoys as some kind of marketing, it gets their name visible all over the place ....)
To get the correct value we would need to find the measurements used for the lifebuoys used on the ship that is being modeled.
This book might be of interest, US Coast Guard specs for life buoys. The Code of Federal Regulations of the United States of America, volume nr 46 Shipping, parts 150 to 165:
specifically page 97 where the measurements are given and page 98 where the construction is discussed. On page 99 you will find the requirements on buoyancy for the three sizes.
The measurements are as follows for the three specified
sizes: _____30 ____ 24 _and_ 20 inches.
Outside:___ 30 ____ 24 _____ 20
Inside: ____ 17 ____ 13 _____ 11
Elliptical cross section:
Long dia:__ 6.5 ____ 5.5 ____ 4.5
Short dia:__ 3 ______ 3 _____ 3.5
Buoyancy:_ 32 ____ 16.5 ____ 16.5 lbs
The long diameter of the elliptical cross section is the "width" of the ring when seen as a ring (as an o)
the short diameter is the thickness of the ring when seen from the side (as an I). The long diameter x 2 is the difference between the outer and inner diameter.
The smallest size (the 20-incher) is slightly thicker to compensate a little for the reduced volume.
The text says that the life buoys shall have an elliptical cross section. It also defines the minimum thickness of the layers of the used material when the body of the buoy is constructed of layers of segments.
If only two layers is used the minimum thickness is 1.25 inch (for a total of 2.5 inches + the fabric cover). For a three layer construction the minimum thickness is 3/4 inch which results in 9/4 inches or 2.25 inches. Note that this is minimum thicknesses and the cover and glue also adds some thickness.
A buoy which is 3 inches thick (see also the size/shape specifications above) exceeds the requirements on minimum layer thicknesses and is also possible to grasp by most adult hands. A thickness of 15 cm (slightly less than 6 inches) would be too much for my hands. Even 12 cm is uncomfortably thick.
Thickness 3 or 3.5 inches depending on total size, see US Federal Regulations.
Other rules may apply for ships from other nations but I would like to assume that they are more or less similar. The variation in the size and weight of sailors within a navy or merchant marine is more or less the same as the variation between navies. Maybe the IJN had slightly smaller lifebuoys ....
With most of the body floating in the water the lifebuoy only needs to support the head and shoulders.
Over and out