Alan,
All boats have positive righting moment. If they did not they they would
immediately turn turn turtle.

The issue is how much righting moment to resist turning turtle, AND,
At what point does the righting moment vanish, also known as the point
of vanishing stability. The graph of this is a curve and is known as the
curve of vanishing stability.

There are rough rules of thumb for the desirable number of degrees
rolling before this point is reached. A bluewater boat, sail or
otherwise should have at least 120 degrees and 140 is better.

Such a vessel when inverted will tend to not return upright within the
cone of 40-60 off of vertical degrees and to stay inverted until wave
action causes it to roll beyond that cone.

In other words, if a boat rolls over in a flat calm sea, it may not ever
roll back upright. If the sea is rough and it rolls over, the greater
the chances are that it will roll back upright. The chances of this
happening are increased if it has a large number of degrees of ultimate
upright stability as the cone in which it will be trying to escape the
upside down mode will be small.

All this ignores the possibilities of down flooding occurring or the
contents changing position, while inverted.

Mike

Capt. Mike Maurice
Tigard Oregon(Near Portland)

o;? o;? o;? o;?
Hey Mike -- right you are.  You made me go and search for Kasten's
Hydrostatic Report and check the large angle stability analysis and the
stability curve.

My boat has no region of negative stability.  In other words, it is fully
self-righting.

As you noted, the calculations ignore down flooding issues, so we'll keep
the doors closed in heavy weather!

Alan

-------Original Message-------

From: Mike Maurice
Date: 10/31/2008 3:27:33 PM
To: Passagemaking Under Power List
Subject: Re: [PUP] Let's design the perfect passagemaker - Lets start ....

over

Alan,
All boats have positive righting moment. If they did not they they would
immediately turn turn turtle.

The issue is how much righting moment to resist turning turtle, AND, At what
point does the righting moment vanish, also known as the point of vanishing
stability. The graph of this is a curve and is known as the Curve of
vanishing stability.

There are rough rules of thumb for the desirable number of degrees rolling
before this point is reached. A bluewater boat, sail or otherwise should
have at least 120 degrees and 140 is better.

Mike,
These graphs of vanishing stability aren't generally available from the
major production manufacturers. At least I can't get them from Nordhavn.

Do you have access to such things for popular boats, or know how they 

can be calculated with the kind of data we routinely have available for
production trawlers? Are such calculations usually accurate, or does this
take tank testing (which nobody is going to do, except on a model).

The reason I ask is that we'd probably want to make sure that the 

stability curves for our PPM was equal or better than most production boats.

Last point, I'm told that you need 1/2" to 3/4" thick tempered glass in 

all windows (the bigger the windows, the more challenging) in a very stout
metal frame to keep them from bursting inward while inverted. Dashew
calculated 3/4" (19mm) for his boat to ensure adequate margin -- he has
really big windows -- which is a fair bit of weight and cost. Nordhavns use
12mm (1/2") tempered glass in generally smaller windows, which they claim is
adequate. Not sure about other production boats.

Obviously, if the windows break while inverted, the stability curve is a 

non-issue -- you're going down fast and upside down.

Is it reasonable to design a boat in this price range that can take a 

roll and survive without giving up most of the window space? Or is this
overkill? Clearly, sailboats can do it, but they have teensy little windows.

The other argument I hear is that after a roll, a power boat would be 

inoperable due to tremendous damage to machinery and equipment. You might be
afloat, but probably disabled. (Can the engine hang inverted from its
mounts?)  Any thoughts on this?

John Marshall
Serendipity - Nordhavn 55
Sequim Bay, Wa

----- Original Message -----
From: "Mike Maurice" mikem@yachtsdelivered.com
To: "Passagemaking Under Power List"
passagemaking-under-power@lists.samurai.com
Sent: Friday, October 31, 2008 12:27 PM
Subject: Re: [PUP] Let's design the perfect passagemaker - Lets start ....

These stability numbers are easy to calculate using a computer program.
I have been using DelftShip. You have to create a table of offsets to
input to the computer, but after that it will generate most of the
numbers with little more effort.

I used this on a sailboat for which I had the drawings. The generated
numbers that came from the computer generally matched what I expected
after examining the designers notes.

The table of offsets is not as hard to infer from the actual built hull
as you might expect. You can easily measure off some crude offsets with
a tape measure. Input them to the program and then smooth the displayed
lines to match what you see in reality.

As for the thickness of the windows. Big windows means thick ones to
withstand impact. Frankly, I have real doubts about big windows being
very realistic.

Mike

Capt. Mike Maurice
Tigard Oregon(Near Portland)