[Alberg30] Positive Stability Range

[George Dinwiddie]

Gordon LACO wrote:
> on 3/5/03 10:19 AM, George Dinwiddie at gdinwiddie at alberg30.org wrote:
> 
>>Back in 1994, Jack Hornor estimated the positive stability range to a
>>bit under 120 degrees.  Judging from the graph he gave out, I'd say
>>about 118 degrees.
>>
> I am surprised at a figure as low as 120... last year I was up to my hips in
> a problem involving a sailing ship whose Limit of positive stability was
> under 100.  In the course of working on that issue I came across material in
> Machaj's "Seaworthiness - the forgotten factor" that indicates "dangerous"
> yachts having numbers under 130, (yachts lost in the Fastnet of '79 are used
> as examples - Grimalkin had an LPS of 125) and yachts of our type - narrow
> with full keels having numbers of 160 degrees and more.   When you say 120
> degrees - what are you refering to?  When I say 160, I mean that I expect
> the boat to still have a righting force that will return it to an upright
> position.

Gord,

You're right; that doesn't sound right.

First of all, the calculation he did were very approximate with a lot of 
untested assumptions.  Also, he scaled the lines from a very small 
drawing.  There could be a lot of error in that.

The diagram I have has a vertical axis labeled "GZ ft" and a horizontal 
axis labeled "Heel degrees."  The graph is labeled "A-30 Curve of 
Righting Arms w/ VCG @ DWL."  The point I picked is where the GZ curve 
crosses zero.  The lowest point in that graph is about 150 degrees.

Pulling out "Desirable and Undesirable Characteristics of Offshore 
Yachts" (http://www.amazon.com/exec/obidos/ASIN/0393033112/alberg30-20), 
I can see that these numbers don't look right.  Also, the "Max GZ" of 
1.352 ft @ 60 degrees seems low compared to the graphs shown in the book.

  - George

-- 
  ----------------------------------------------------------------------
   Hope your road is a long one.                        George Dinwiddie
   May there be many summer mornings when,       gdinwiddie at alberg30.org
   with what pleasure, what joy,                 http://www.Alberg30.org
   you enter harbors you're seeing for the first time;
                                            from 'Ithaka' by C.P. Cavafy
  ----------------------------------------------------------------------


 +======================================================================================+
 |     New England Ropes on Sale;  EXAMPLE: 1/2 inch Sta-Set only 51 cents per foot     |
 | http://www.myaffiliateprogram.com/u/sailnet/t.asp?id=1066&p=store/item.cfm?pid=25719 |
 +======================================================================================+

Public-list mailing list -- Public-list at alberg30.org
http://www.alberg30.org/mailman/listinfo/public-list
To unsubscribe: email to Public-list-request at alberg30.org
Include command "unsubscribe <password>" in subject or body.
Use command "help" for more options.

 1046887543.0