[PUP] PUP PPM *Stability*

[Mark Tilden]

Scott & Group:

There is a standard that has been around for many years that defines the
requirements for sailboats in open-ocean races. The standard is set by the
"Pacific International Yachting Association". While this standard is by no
means directly applicable to Passagemaking Trawlers, it does define some
very useful criteria for open ocean safety--both in terms of basic design
and construction, as well as equipment requirements. The standard breaks
down the requirements into five categories from the most protected in-shore
kinds of events (Category 4) to long-distance open-ocean types of events,
like the West Marine Pacific Cup--San Francisco to Hawaii (Category 0).

For example, it defines an advisable limit of positive stability of 120
degrees for boats in category 0 or 1 events, and requires no less than 105
degrees.

As several others have mentioned, I tried to find LPS numbers for various
trawlers and couldn't find them published for any of the major production
builders.

You might take a look at this document for reference in discussion of the
requirements for a "perfect" passage maker:
http://www.ussailing.net/piya/PIYA/2008%20PIYA%20Certificate.pdf

Having entered two Pacific Cup races, we used this standard extensively as
the guide for preparing a boat for an open-ocean passage, and these races
require detailed pre-race inspections against this standard. Again, the
standard is written for sailboats, but I think there is useful information
in the standard relevant to this discussion.

Relative to full roll-overs: Sailboats definitely have surprisingly
frequently survived full inversions without loosing their rigs. I'm
skeptical that most any production trawler could (much less be internally
prepared for it), but that's just a gut feeling--not based on any real
objective data. I think the biggest risk would be flooding due to failure of
the large windows.

I'm also a bit skeptical of an earlier post that said (if I understood it
correctly) that calculating the LPS of a production boat is relatively
straightforward with hull offsets. LPS is extremely dependent not only on
hull shape, but also on center of gravity and metacentric height, which are
heavily influenced by weight distribution in the boat.

There is a measurement process that sailboats use to predict LPS that
involves adding weight near the gunwales and measuring heel angles, along
with hull shape calculations. This measurement technique was developed for
one of the velocity prediction programs developed at MIT and adapted for
handicapping sailboats under the IMS (International Measurement System). I
don't see how a calculation of LPS could be accurate without good
information about weight distribution in the boat.
 
Capt. Mark Tilden
Selene 59: "Koinonia"