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PUP PPM *Stability*

SE
Scott E. Bulger
Sat, Nov 1, 2008 2:37 AM

Perhaps this is better as a subset of a broader DESIGN category, but since
its generated significant comment, lets discuss it, and use this topic as
a way to further define how we will discuss the issue and reach conclusion.

First, lets start with a broad statement of the issue:

Stability:  A Passagemaking vessel is likely to encounter conditions that
are significantly more challenging than boats operating closer to local
harbors and ports.  Distance from port, fast moving hemispherical weather
systems or rapidly developing (not forecasted) weather systems may be
encountered at sea, where there is no opportunity to seek refuge.  Therefore
we need to define the stability requirements the vessel will be designed
around.  Rather than re-invent the wheel,  I would propose we research what
standards exist for various levels of seagoing classifications for
commercial vessels.  Understanding what international design standards are
applied to commercial vessels will enable us to assess the applicability of
those standards to our target market, and select the most appropriate level
of stability, then incorporate that into the design standard.

Roll Over Survivability:  Obviously a vessel can be constructed to survive
complete 360 degree roll overs and continue operations.  USCG Lifesaving
boats are a good example of this capability.  However, significant
compromises and cost are incurred by holding a vessel accountable to this
degree of stability or roll over survivability.  Perhaps we should ask the
following questions:

a.        Are there ANY current production Passagemaking trawlers that will
survive a 360 roll over and be able to continue operations?

b.      Are there ANY current sailing vessels that achieve this objective?
Most capsizes I have seen result in loss of the sailing rig, but perhaps the
vessel can continue under power?  Are there standards that describe the
components necessary to achieve this design goal?

c.      As a group, do we believe it is a REQUIREMENT to have a vessel
survive a capsize in an operational state?

As far as Im concerned Id summarize my feeling this way:  If the vessel
can be designed to a high degree of stability at a reasonable cost I would
rather put money into that effort than to put funds into survivability that
might cost 2 or 3 times more than making the boat more stable.  In other
words Id rather put in a pound of prevention than 10 pounds of cure.  Our
only experience with significantly rough conditions was our broaching in
Nicaragua.  We attempted to enter a harbor but the channel has moved about =
mile along the beach.  We were caught by a set of significantly higher than
average period waves and the boat broached.  As the bow buried in the
trough, the stern swung parallel to the trough and the boat healed over to
about 45 to 50 degrees.  As the wave moved under us the boat snapped back
from the roll and we easily turned back out to sea before the second and
third waves arrived.  Had we been another few hundred yards inshore and the
wave had been fully breaking rather than just foaming on top I believe the
boat would have capsized.  Regardless this demonstrates to us that a high
degree of roll stability is desirable.  At what point it becomes an effort
of diminishing returns is not clear, I just dont have enough experience.

I would like to go read what Dashew  has contributed on the subject, because
he clearly has decades of experience to draw from.

In  summary, comments on the degree of stability needed, and the amount of
effort that should be expended on roll survivability should be contributed
and summarized to reach a consensus.

Perhaps this is better as a subset of a broader DESIGN category, but since its generated significant comment, lets discuss it, and use this topic as a way to further define how we will discuss the issue and reach conclusion. First, lets start with a broad statement of the issue: Stability: A Passagemaking vessel is likely to encounter conditions that are significantly more challenging than boats operating closer to local harbors and ports. Distance from port, fast moving hemispherical weather systems or rapidly developing (not forecasted) weather systems may be encountered at sea, where there is no opportunity to seek refuge. Therefore we need to define the stability requirements the vessel will be designed around. Rather than re-invent the wheel, I would propose we research what standards exist for various levels of seagoing classifications for commercial vessels. Understanding what international design standards are applied to commercial vessels will enable us to assess the applicability of those standards to our target market, and select the most appropriate level of stability, then incorporate that into the design standard. Roll Over Survivability: Obviously a vessel can be constructed to survive complete 360 degree roll overs and continue operations. USCG Lifesaving boats are a good example of this capability. However, significant compromises and cost are incurred by holding a vessel accountable to this degree of stability or roll over survivability. Perhaps we should ask the following questions: a. Are there ANY current production Passagemaking trawlers that will survive a 360 roll over and be able to continue operations? b. Are there ANY current sailing vessels that achieve this objective? Most capsizes I have seen result in loss of the sailing rig, but perhaps the vessel can continue under power? Are there standards that describe the components necessary to achieve this design goal? c. As a group, do we believe it is a REQUIREMENT to have a vessel survive a capsize in an operational state? As far as Im concerned Id summarize my feeling this way: If the vessel can be designed to a high degree of stability at a reasonable cost I would rather put money into that effort than to put funds into survivability that might cost 2 or 3 times more than making the boat more stable. In other words Id rather put in a pound of prevention than 10 pounds of cure. Our only experience with significantly rough conditions was our broaching in Nicaragua. We attempted to enter a harbor but the channel has moved about = mile along the beach. We were caught by a set of significantly higher than average period waves and the boat broached. As the bow buried in the trough, the stern swung parallel to the trough and the boat healed over to about 45 to 50 degrees. As the wave moved under us the boat snapped back from the roll and we easily turned back out to sea before the second and third waves arrived. Had we been another few hundred yards inshore and the wave had been fully breaking rather than just foaming on top I believe the boat would have capsized. Regardless this demonstrates to us that a high degree of roll stability is desirable. At what point it becomes an effort of diminishing returns is not clear, I just dont have enough experience. I would like to go read what Dashew has contributed on the subject, because he clearly has decades of experience to draw from. In summary, comments on the degree of stability needed, and the amount of effort that should be expended on roll survivability should be contributed and summarized to reach a consensus.
MT
Mark Tilden
Sat, Nov 1, 2008 7:27 AM

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"

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"