[PUP] PPM - Stability Curves (was Let's design the perfect...)

[John Marshall]

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 at yachtsdelivered.com>
To: "Passagemaking Under Power List" 
<passagemaking-under-power at lists.samurai.com>
Sent: Friday, October 31, 2008 12:27 PM
Subject: Re: [PUP] Let's design the perfect passagemaker - Lets start ....


>> .  The boat I am building has a
>> positive righting moment (think that is what it is called).  If knocked 
>> over
>> by "a big one" it will right itself.
>
> 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)
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