measurement tether eng

Effort required during self-rescue

Standard configuration TV3E3

Maximal effort ~ 5 pulls at 42% of your weight, during the shortening of the last section.

Holding force between pulls ~ 3.5% of your weight (required to prevent the strap from slipping back).

4-loops variant TV3E3L

Maximal effort ~ 7 pulls at 34% of your weight, during the shortening of the last section.

Holding force between pulls ~ 1 % of your weight (required to prevent the strap from slipping back).

For instance, for a person weighting 73 kg, the pulling effort with TV3E3 is ~30kg and the holding force ~ 2.5 kg, with TV3E3L, the effort is ~25 kg and the holding force below 1 kg.

These values are an empirical estimate of what happens during a 'typical' self-rescue, boat speed 6 knots or less, with the body almost out of the water when the harness loop is shortened.

Details

The maximal effort occurs during the shortening of the last section. This was observed systematically in all our self-rescues, with a boat velocity up to ~ 8.5 knots. This is compatible with two observations:

1) the drag of the water is not higher than the weight because the body is inclined (almost surfing),

2) as the body is lifted above the water, the drag decreases and the unsupported weight increases.

The shortening of a section is realized by means of several strong pulls (42% of weight) which pull out the strap ~50cm each time (arm length). Between consecutive pulls, the person rests and only applies the holding force (3.5% of weight) to prevent the strap from unrolling.

To initiate a pull, it is necessary to overcome the friction of the strap. The force to apply (42% of weight) is the sum of :

1) your unsupported weight divided by 6 (multiplication factor for 3 loops of strap)

2) the friction of the strap (on itself and on the rings, i.e., capstan friction), first static then the kinetic friction.

3) the drag of the water on the immersed parts of your body.

Experimental measurement

Values measured on ground with a digital scale, participants lifting themselves vertically with one section of a harness loop. One participant (72 kg), then 2 participants together (146 kg), repeated measurements with dry and wet strap.

The effort was determined as the minimal force for which the strap started to move (shortening), and the holding force as the minimal force that prevented the strap from slipping backwards.

We could not observe clear differences between dry and wet strap.

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