Evan Gatehouse wrote:
Gary wrote:
Evan Gatehouse wrote:
Gary wrote:
I think the stability curves are wrong. If you plug the numbers
into the formulas at US sailing you get a much higher AVS. 110
degrees just doesn't make sense. Not only that but the 460's keel,
although seemingly a little light, is bulbous with the weight quite
low. Something is wrong with that graph.
Gaz
I didn't find the formula at US Sailing's site but...How do you find
AVS without using a sophisticated stability program that knows the
shape of the hull?
The US Sailing formula may give an indication of ultimate stability
(the capsize screening formula) but that is a very simple rule of thumb.
The inclining experiment data, which does determine the VCG of the
real boat, is plugged into a stability program (GHS/Autohydro etc.)
that gives the stability curve.
Robert has retired fairly recently but he should be able to help you
with your questions.
Evan Gatehouse
Try he http://www.sailingusa.info/keelboat.htm
Under sailing calculators in the right find angle of vanishing stability.
The problem with the inclining data is that it only takes hull shape
and weight distribution into account. It doesn't allow for things
like pilothouses or foam filled masts. I still think the curve is
wrong and Tayanas claim of an AVS of 0 is probably correct.
O.k. I'm a polite guy on usenet but you sir are totally wrong and
beginning to **** me off. I'm a naval architect. I do this stuff for a
living.
The stupid calculator only is used to give an ESTIMATE of the AVS.
An inclining experiment establishes the VCG of the boat in a real world
test. It does NOT calculate the AVS. For that you need software that
does take into account the shape of the hull and the pilothouse. GHS
and Autohydro DO that! You could model a foam filled mast as part of the
model if you wished. They are far more trustworthy and are accepted by
USCG for stability calculations of commercail vessels
Here's the formula from US Sailings site. Note that in the last line,
the formula says the AVS = 110 + .....
This says that the AVS is ALWAYS going to be = 110. There are lots of
boats that are 110!
From US Sailings web site:
" This formula gives an estimate of the angle of vanishing stability or
the angle the boat can heel and still right itself. This formula does
not fully take into account the vertical position of the center of
gravity (VCG)."
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as
pounds of displacement on the webpage and converted to cubic meters by
the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64
)*0.0283168
The Beam and Hull Draft in this formula are in meters. These values
are entered in feet on the webpage and are converted to meters before
SSV calculation.
Angle of Vanishing Stability approximately equals
110 + ( 400 / (SSV-10) )
Glad I'm getting to you Nav Arch. You do seem to have a grasp on the
above formulae but fail to explain why the boat has an AVS of 110 in the
graph and yet the designer and builder both claim it has a much greater
AVS. In fact the extremely high AVS of the Tayana Vancouver 460 is
discussed in many places around the net. There is no way the AVS is 110.
If you really are a Nav Arch and do this for a living, then do it for
us. Work out the numbers using your program and the data available and
give us your results (which are still only an estimate). As a Nav Arch
it should only take you a couple minutes. Right????
You also earlier said :
"- the ballast/displacement ratio is overly low, meaning a tender boat
with not enough stability "
A Nav Arch would know that initial stability is more than lots of
ballast. A raft has no ballast and is not tender. Wouldn't you say
that you are jumping to conclusions by not taking into account hull form
and the location of the ballast?
In other articles on the net, they also talk of the boat as "stiff" and
"stands up to her sails well".
Your turn.
Gaz