Corrected figures

BCC Forum
1

Thanks to Hudson for pointing out an error I’d made in a formula on my
spreadsheet.

I’ve corrected (I hope) the comparative figures for the BCC - the
indices that changed included Length/Beam and the Comfort Factor (the
formula I’m using is CR=disp / (.65 * (.7 * lwl+.3 * loa) *
beam^1.33)).

The following figures are, I hope, closer to the truth:

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: 2.78
(a measure of fineness of the hull; fine hulls have ratios of 3.0-4.0
and higher but beamier hulls sail flatter and have more interior room)
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): 37.0
(racing designs are less than 20, cruisers 30-40)
Ted Brewer’s Comfort Factor (cruising gear): 45.0
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
Period/Beam (design): 6.96

Cheers

Bil (BCC 116)

Bil Hansen
Post: 5 Lintang Pantai Jerjak, 11700 Sg Nibong, P Pinang, Malaysia

2

what are these formulas for? Isn’t anyone going
sailing? Lyle Hess did all the figuring, you guys
just do the sailing. If there were a better cruising
boat to circumnavigate and endure two cylcones in the
Indian Ocean, I would have chosen another designer.
All the righting moment and other critical things are
in the nice to know category and certainly good for
peace of mind, but there is no peace of mind with
formulas in a cyclone! Either you have a good boat or
you can die.

BTW, New World, his 32’ designe never heeled more than
25 degrees in 30 foot waves for 16 hours straight and
I can promise you I never thought once about formulas,
but I was ever so thankful that Hess did.

Also I know of two BCC’s I met in NZ that had exceeded
theoretical hull speed of your figures: 7.7 for 24
hours without a current. Can you explain that with
formulas? I know I can’t.

Fair winds,
M. Lee Gunter
— Bil Hansen <bilh2001@yahoo.com.au > wrote:

Thanks to Hudson for pointing out an error I’d made
in a formula on my
spreadsheet.

I’ve corrected (I hope) the comparative figures for
the BCC - the
indices that changed included Length/Beam and the
Comfort Factor (the
formula I’m using is CR=disp / (.65 * (.7 * lwl+.3

  • loa) *
    beam^1.33)).

The following figures are, I hope, closer to the
truth:

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: 2.78
(a measure of fineness of the hull; fine hulls have
ratios of 3.0-4.0
and higher but beamier hulls sail flatter and have
more interior room)
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): 37.0
(racing designs are less than 20, cruisers 30-40)
Ted Brewer’s Comfort Factor (cruising gear): 45.0
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
Period/Beam (design): 6.96

Cheers

Bil (BCC 116)

Bil Hansen
Post: 5 Lintang Pantai Jerjak, 11700 Sg Nibong, P
Pinang, Malaysia

Do You Yahoo!?
Yahoo! - Official partner of 2002 FIFA World Cup
http://fifaworldcup.yahoo.com

3

Naturally, I agree that the BCC’s numbers look pretty good, but raw
formulas seem to be just starting points when comparing boats.

LWL, LOD, and LOA are all important but very different measurements
that are not directly comparable from boat to boat. Depending on the
specific shape of the hull, they ought to have a varying impact on
things like speed, velocity, stiffness, etc. It seems to be a
mistake, for example, to assume that a bowsprit has no effect on
stability and “comfort”. Similar complaints can be made about
imprecise terms like “beam”. Even the term “ballast” is meaningless
unless you know where the ballast is located. And the term “Sail
Area” is generally reported at 100%, but boats have a greater or
lesser ability to add (or remove) canvas in differing wind
conditions. Other measurements like Draft are rarely looked at as a
determinant of stability and comfort (surprisingly).

Anyway, the significance of static measurements and of formula
results should not be overstated. For example, I question whether
it’s reasonable to say that Beam/LOD is particularly meaningful,
especially if hulls lower than 0.350 are described as “tender” and
those much higher are described as “stable as a barge”.

Actually, I think that the complicated and unpredictable nature of
how a boat performs is why we should continue to call them “she”
rather than “it”. But that’s another story.

H.

At 12:02 PM +0800 06/07/2002, Bil Hansen wrote:

Thanks to Hudson for pointing out an error I’d made in a formula on my
spreadsheet.

I’ve corrected (I hope) the comparative figures for the BCC - the
indices that changed included Length/Beam and the Comfort Factor (the
formula I’m using is CR=disp / (.65 * (.7 * lwl+.3 * loa) *
beam^1.33)).

The following figures are, I hope, closer to the truth:

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: 2.78
(a measure of fineness of the hull; fine hulls have ratios of 3.0-4.0
and higher but beamier hulls sail flatter and have more interior room)
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): 37.0
(racing designs are less than 20, cruisers 30-40)
Ted Brewer’s Comfort Factor (cruising gear): 45.0
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
Period/Beam (design): 6.96

4

— In bcc@y…, Hudson Barton <hhbv@h…> wrote:

And the term “Sail Area” is generally reported at
100%, but boats have a greater or lesser ability
to add (or remove) canvas

I’m surprised to see a SA/D of almost 19 for the
BCC. Is that based on 100% foretriangle? The
builder’s website doesn’t list I, J, P, and E,
so it’s hard to tell. Does anyone have these
handy? I’m just curious …