Do you know the Stability Index Numeral (often abbreviated as STIX) or

the Angle of Vanishing Stability for a Bristol Channel Cutter?

I assume that the ‘easy way’ to find the Angle of Vanishing Stability

is to connect a strain gauge to the masthead and to pull sideways

while taking readings every 5 degrees or so. But I’m not v keen on

trying it myself, at least not today or tomorrow, especially for the

readings between 90 degrees and 130 degrees.

I guess that the Stability Index Numeral involves calculations from

design figures, but I don’t know the formula.

The STIX and the Angle of Vanishing Stability are required under the

European Union’s new Recreational Craft Directive for vessels sold in

Europe. Apart from that, they’re of academic interest.

I have collected other figures and indices for a BCC, which I list

below. Where there are figures noted as ‘design’ and ‘cruising gear’,

the latter are based on my measurements and calculations of BCC 116

with gear on board, while the former are based on the standard design,

unloaded. BCC 116 has slightly less ballast than the standard Sam L

Morse factory build, to compensate for gear fixed on board, and so

your experience may vary. Some figures and indices have parenthetical

notes in the line below them. None of these figures are useful, of

course, unless you just want to talk about and compare boats.

Ballast/Displacement (design): 0.33

Ballast/Displacement (cruising gear): 0.25

Displacement/Length (design): 345

(medium displacement)

Displacement/Length (cruising gear): 417

(heavy displacement)

Sail Area/Displacement (design): 18.63

Sail Area/Displacement (cruising gear): 16.35

Hull speed Vhull: 6.87 knots

Velocity ratio Vmax/Vhull: 1.07

LOD/Beam: 3.73

(a measure of fineness of the hull; fine hulls with ratios of 3.0-4.0

and higher have easier motion)

Beam/LOD: 0.359

(a measure of form stability: hulls lower than 0.350 are tender; those

much higher are as stable as a barge)

Ted Brewer’s Comfort Factor (design): 33.5

(racing designs are less than 20, cruisers 30-40)

Ted Brewer’s Comfort Factor (cruising gear): 40.6

Capsize Risk Screen (design): 1.62

(above 2.0 is undesirable for offshore cruising)

Capsize Risk Screen (cruising displacement): 1.52

Roll Acceleration (design): 0.04937g

(above 0.06g is undesirable for offshore cruising; malaise starts at

0.1g, 0.18g is intolerable)

Roll Acceleration (cruising displacement): 0.03585g

Roll Period (design): 3.9 seconds

(less than 4 seconds = stiff, more than 8 secs = tender)

Roll Period (timed, with cruising gear): 3.8 secs

Roll Period/Beam (design): 6.96

Cheers

Bil

BCC 116