Re: Submarine Ballast Tanks {revisited}

[Ray Keefer <Ray.Keefer@eng.Sun.COM>]

Hi Kiteman,

>        For what it's worth, I just watched part of a show on TLC (The
>Learning Channel}
>called NUCLEAR SUB: SEAWOLF.  I say "part" because I forgot to start the VCR
>when the show came on.   [Oh pooh.]  This show mainly profiled the new USN
>submarine
>"Seawolf".

Almost makes cable worth having. 

>
>        Anyway, they mentioned something I asked here some time back.  They
>showed
>that earlier subs used ballast tanks that ran along the sides of the hull.

Saddle tanks do stick out from the sides. Which creates a larger cross
sectional area which translates into drag. The only advantage to saddle tanks
is that their bouyancy  off to each side aids in surface roll stability. In
other words they are great for submarines which spend most of their time on
the surface.

>Now however,
>subs use tanks at the front and rear (or thereabouts), which allows for
>faster diving
>and/or surfacing.

Submarines that use the tear drop shape, like the American nukes, are much
more stream lined. As a result the ballast tanks are put just about the only
place the pressure hull isn't, in front and in back. 

Being more steamed lined the nukes do go faster for a given amount of power.
However, since the axis of the ballast tanks are centered on the lognituginal
axis of the pressure hull the roll stability on the surface is sluggish at
best.

As for "diving and/or surfacing", nukes spend most of the time at sea
submerged , so they only need to dive or surface once or twice per cruiz. So
they don't need to dive, or submerge, very quickly. I understand it take 4 to
8 minutes to submerge. Part of the reason is that the holes in the bottom of
the ballast tanks are relatively small to  reduce the flow noise at these
holes at higher speeds.

An old diesel-electric boat with saddle tanks spends most of it's life on the
surface. When it had to get submerged it had to do it now. Often in under 30
seconds.

I guess the surfacing speed was simular for both types of ballast tanks.

Regards,
Ray