STIX and vanishing stability

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

As a part of the construction of my new BCC which has had some hull
modifications I had a complete stability study undertaken by Raymond R
Richards, N.A… At design weight, stability vanishes at about 130
degrees. Fully laden ( 4 people and stores and fuel for 3 weeks)
stability vanishes at 136 degrees
Cabin top buoyancy not included in study so in actuality we are looking
at 140 degrees. Hell if we were Pacific Seacraft we would probably
advertise 145 or more.
Seriously, Ray is both a Naval Architect and degreed Marine Engineer and
he was very happy about stability. Said he felt quite comfortable about
sailing the boat anywhere.
Jim Hiller

-----Original Message-----
From: bilh2001 [mailto:bilh2001@yahoo.com.au ]
Sent: Thursday, June 06, 2002 3:53 AM
To: bcc@yahoogroups.com
Subject: [bcc] STIX and vanishing stability

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

BRISTOL CHANNEL CUTTER OWNERS ASSOCIATION
Mail List Home: http://groups.yahoo.com/group/bcc
BCC Owners Home: http://www.geocities.com/bccowners
Post message: mailto:bcc@yahoogroups.com
Subscribe: mailto:bcc-subscribe@yahoogroups.com
Unsubscribe: mailto:bcc-unsubscribe@yahoogroups.com
List owner: mailto:bcc-owner@yahoogroups.com

Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/

At 7:53 AM +0000 06/06/2002, bilh2001 wrote:

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)

Given that a BCC’s LOD is 28 and Beam is 10.1 approximately, I don’t
understand your calculation.

H.

— Hudson Barton <hhbv@highwinds.com > wrote: > At
7:53 AM +0000 06/06/2002, bilh2001 wrote:

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)

Given that a BCC’s LOD is 28 and Beam is 10.1
approximately, I don’t
understand your calculation.

Thanks Hudson. I’d erred in my spreadsheet and used
LOA (including bowsprit and boomkin) instead of LOD.

Cheers

Bil

http://www.sold.com.au - The Sold.com.au Big Brand Sale

  • New PCs, notebooks, digital cameras, phones and more … Sale ends June 12