[PUP] Scelenes vs Nordhavns

Tue Apr 3 11:29:16 EDT 2007

Bob:

You'd have to define "a knockdown". All boats have a "limit of positive
stability" (LPS) -- an angle of heel after which the boat loses its righting
moment and will tend to go on and roll over. Even sailboats have such a
limit, though it tends to be much larger on sailboats.

For example, my previous sailboat, a 43' J/130 had an LPS of about 130
degrees. That means that even if the mast was 40 degrees underwater, the
boat would still have righting moment to pull it back upright. After that,
the boat will probably finish going over. However, with a very high LPS, the
chances are, the boat will go right on over and right itself again because
either momentum or another wave will tend to roll the boat on over until it
reaches that positive stability limit again and right itself.

Unfortunately, very few, if any trawler style boats have LPS numbers even
approaching the values for most sailboats. I've never seen the number
published for Nordhavns, Krogens, Selens, etc.. My suspicion is that the
number is more like 45 to 60 degrees for both.

Metacentric height is one measure of stability that some of the
manufacturers do publish--you should be able to ask for the number if it
isn't published. For a definition of Metacentric height, see:
http://www.selenetrawlers.com/2008/iindex.php?O=Page&F=AU&S=4
(scroll down this page about 2/3 of the way down).

A larger metacentric height means more righting moment, and a higher limit
of positive stability, though I don't know of any direct way to convert
metacentric height to a limit of positive stability.

For the Selene 53, for example, the web site lists the metacentric height
(GM) as 4' 10-5". This means that the center of gravity of the boat is
almost 5' below the metacenter (the "hinge point" around which the boat
rolls). That's pretty good. I looked on the Nordhavn site, but couldn't find
metacentric height specified there. Perhaps it's on printed spec sheets.

Of course, it's important to recognize that we owners affect the stability
of our boats by how we load them as well. Heavy dinghys stored up high on
flybridge decks have a substantial influence as well (they obviously raise
the center of gravity, which reduces metacentric height).

The only way to make a trawler have a high LPS value is to lower its center
of gravity--either add a LOT more ballast, or more likely reduce the big
comfortable saloon/pilothouse/flybridge area. If you look at Dashew's boat 
"Wind Horse" or the Diesel Duck designs, both of them have made this
tradeoff. The A/B ratio, which should also be specified by manufacturers, is
a rough measure of this. It's the ratio of area above the waterline to the
area below the waterline. The lower the A/B ratio, the lower the area of the
structure above the water. Notice that this is not a measure of the weight
above the water--just the area. On the Selene 53, the A/B ratio (again from
the web site) is 2.53.

So.....I hope that helps and not just "muddies" the water.

Mark

-----Original Message-----
From: passagemaking-under-power-bounces@lists.samurai.com
[mailto:passagemaking-under-power-bounces@lists.samurai.com] On Behalf Of
bob england
Sent: Tuesday, April 03, 2007 7:09 AM
To: Passagemaking Under Power List
Subject: Re: [PUP] Scelenes vs Nordhavns

Can either of these types of boats, and similar voyaging boats handle a
knockdown and recover. How about a barrell roll. I'm not being sarcastic, I
want to know the facts, and, I really like both of them, they just look
awesome !> Date: Sat, 31 Mar 2007 07:17:27 +0200> From: mygoleen@gmail.com>
To: passagemaking-under-power@lists.samurai.com> Subject: Re: [PUP] Scelenes
vs Nordhavns> > Hi Bob (and all),> > You wrote:"Nordhavn has done this
brilliantly in their marketing and> advertising campaigns."> > Yes, they
certainly do and I feel that after we purchase a boat, it is our>
responsibility too to keep her in good shape and visiting the 'engine room'>
often and not only when things goes wrong, is a MUST!> > You said:"One needs
to either be able to fix them, or do without"> > Absolutely! On Goleen we
keep
our engine room not only sparkling clean, if> possible, but with an eye on
everything at all times to spot signs of wear> and tear. OF COURSE, THINGS
CAN
GO WRONG NO MATTER, AND NOT ONLY ON THE> ENGINE ROOM!> > On passages, we
visit
the engine room very often and Chris measure> temperatures, search places
for
possible leaks, fuel levels, fuel filters> and so on.> I hardly even hear
music on board when we are on a passage because I like to> hear if the
engine
'sound' is the same or if it changed in any way, which is> one of the first
signs of trouble ahead, if it changes.> I do appreciate that it is difficult
for any of us, and even for the> 'engeneer's types' to know it all, but one
need to be prepared to take> action and do something on the spot and
sometimes, FAST.> > We had problems before that we didn't know how to fix it
immediately so, we> opened the manual and read' or made a phone call and
listened.> If one is prepared mentally, physically and emotionally for this
kind of> event to happen, it should be fine, and THAT is why I keep banging
on
the> key on this forum that talking about HOW we cope ( and not ONLY about
'WHAT> WE DO') in adversity is important too. In the end it will be our
mental> state that will save the technical issues (and ourselves) when 'push
comes> to shave', in MY opinion.> > Just to illustrate:> On Yachting World,
April edition, 'Run down at Night', pg 80/85, Clark Beek> wrote about his
experience of his 40ft ketch Condesa been run down by> a freighter:> > >
Pure
Panic paragraph:> *"Let me state for the record that I am not proud of what
I
did. This was> one act of pure panic.*> *...I did a freestyle sprint at a
speed only possible when that sort of> adrenalin is pumping through one's
veins, tumble in a crashing bow wave,> then sprinted again. Ian (*his crew*)
would have been right behind me,> following my stupid lead, but because of
Hilary (his* other crew was stuck> inside the vessel*) he missed his chance
at
being the second human in> history to bodysurf a freighter's bow wave."*> >
And he goes on: *" My most important piece of advice is this: plan to be an>
idiot. I always considered myself a pretty cool customer under fire, but>
when
life and vessel were on the line I found myself functioning at about 10> per
cent of mental and physical capability. It made me re-evaluate some of> my
emergency procedures omboard because they are too complicated for a> rattle
mind."*> **> You see what I mean?> > All the best and keep learning,> >
Sonaia
Hermida> NH 5729> > > On 3/30/07, thataway4@cox.net <thataway4@cox.net>
wrote:> >> > My impression looking at 2 or 3 year old Nordhavn's vs the same
age Selene> > is that the Nordhavn ages bettter.> >> > There are some design
features including seating on the flying bridge,> > engine room access which
I
like better on the Nordhavn.> >> > Any new boat will have problems. I spent
over 3 months getting issues> > repaired on a new boat last year. Further
more, with time many systems on> > boats will either wear or fail. One needs
to either be able to fix them, or> > do without. If one looks at the
Nordhavn
40 around the world--there were a> > number of issues, if one studies all of
the reports from the boat and> > various people aboard.> >> > I also need to
clarify a word which may have caused some> > confusion: Scheme: " A
systematic
program for obtaining some objective" is> > Webster's first
defination--Nordhavn has done this brilliantly in their> > marketing and
advertising campaigns.> >> > Bob Austin> >
_______________________________________________> >> > Passagemaking Under
Power and PUP are trademarks of Water World> > Productions, formerly known
as
Trawler World Productions.> >> > To be removed from the PUP list send an
email
with the> > subject "unsubscribe" (no quotes) to the link below:> >> >
mailto:passagemaking-under-power-request@lists.samurai.com> >> >
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and PUP are trademarks of Water World Productions, formerly known as Trawler
World Productions.> > To be removed from the PUP list send an email with
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