Re: [PSUBS-MAILIST] Intact Surface Stability

["Cliff Redus" <dr_redus@devtex.net>]

Alec,


> Cliff,
>
> I've got just such texts at home and will see if I can provide you with
something in the evening. The nutshell version is there >are two factors
involved in keeping water out of the hatch; freeboard and roll stability.

Great.  Also can you site the reference text if you find the source you are
looking for.
>
> For a surface vessel, the calculation of roll stability involves something
called the "metacentric height".  When a surface >vessel rolls, it submerges
a greater portion of its hull on the side its rolling to, and this produces
a righting moment. Therefore >surface vessels can have their CB below their
CG yet still be stable. In the case of a submarine, however, we're generally
>speaking of cylindrical hulls. As a cylindrical hull does not vary its
lateral displacement upon rolling, it produces no righting >moment.
Therefore for a cylindrical submarine the calculation will be reduced to a
simple matter of how far below the CB >you can get your CG.

I have a spreadsheet like everyone else that calculates CB and CG.  The CB
calculation also takes into account lost buoyancy associated with a portion
of the sail that is out of the water when surfaced. For my boat I have two
1-atm battery pods located just below the CG.  As such, in a significant
roll on the surface, one of the pods would begin to surface thus producing
a righting moment.

The problem I see with CG-CB and metacentric height approach your talking
about is that it is a static calculation. The table in ABS rules describing
sea state is talking wave height, periods and wind velocity.  This leads me
to think a dynamic stability analysis is called for.
>
> Which brings up something interesting. I know your design has a "flying"
intent, so presumably you are trying to keep CG >and CB very close in the
submerged condition. This means you'd need to do something to improve
surface stability per >ABS. I can't remember where you were putting your
ballast tanks. If they're on the sides, then they'd help.

My goal was to stay just within ABS rules on minimum distance between the CG
and CB in order not to make moment required to roll to high but yet have the
required static stability.

>And are you still thinking of articulating the battery pods to drop the CG?

I am using a +/- 4 inch longitudinal shift of the batteries in the pod to
shift my CG longitudinally.  This give me a +/- 18 deg pitch submerged. I am
not shifting the batteries in heave direction.
>
> I haven't done one of those stability calculations since college, and I
haven't done them at all for Solo since it hasn't a hatch. But I've still
got my books, so more later.

Thanks

Cliff