RE: [PSUBS-MAILIST] Intact Surface Stability

["Alec Smyth" <Asmyth@changepoint.com>]

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. 

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.

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

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. 

cheers,

Alec 

 

-----Original Message-----
From: Cliff Redus [mailto:dr_redus@devtex.net]
Sent: Tuesday, October 01, 2002 9:48 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Intact Surface Stability


Gary
> I have no idea what "Sea State 3" encompasses, but I would think the wave
> height (perhaps wind?).  The height of the hatch above the water would be
> the obvious factor in not taking on water.  But, that is given by ABS, I
> believe.

Table D.1 in the ABS text defines a Sea State of 3 as:
Wind velocity = 16 knots
Average wave height = 2.9 ft
Average 1/10 highest =- 5.8 ft
Significant range of periods = 2.0-8.8 sec
Average period = 4.6 sec
Average wave length = 71 ft
Minimum fetch = 40 nmi ( what is this?)
Minimum duration = 6.6 hrs

Also in ABS Rule 3.19.1a , "the distance from the waterline to the top of
coamings around hatches which may be opened with the unit afloat is to be
not less than 2.5 ft. with the unit upright."

 I would think that the values of, and distances between the CB
> and CG and the "righting" torque they produce would also be a factor.  One
> other factor that might come into this is the rolling Moment of
> Inertia.  To not use dive planes on my sub I have to have the CB and CG
> closer than most.  Thus, the amount of torque to say tilt my sub to 30
> degrees from vertical is considerably less than for most.  However, due to
> the concentration of mass in the hull (18 inch radius), the keel's mass,
> and the sail's mass, my boat takes applied force longer to obtain a
> rotational velocity.  Thus, it does not roll badly in the wake of a
> relatively large wave.  But be careful getting in and out and standing on
> the hatch.  At least get in and out quickly!

I agree, there must be a straight forward dynamic load analysis that one can
make.  I am hoping someone out their that has submitted their boat for ABS
classing will share with us what they submitted.  I am sure this is also in
some marine engineering text.

Cliff

----- Original Message -----
From: "Gary R. Boucher" <protek@shreve.net>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, October 01, 2002 10:08 AM
Subject: Re: [PSUBS-MAILIST] Intact Surface Stability


> Just a thought here...
>
> I have no idea what "Sea State 3" encompasses, but I would think the wave
> height (perhaps wind?).  The height of the hatch above the water would be
> the obvious factor in not taking on water.  But, that is given by ABS, I
> believe.  I would think that the values of, and distances between the CB
> and CG and the "righting" torque they produce would also be a factor.  One
> other factor that might come into this is the rolling Moment of
> Inertia.  To not use dive planes on my sub I have to have the CB and CG
> closer than most.  Thus, the amount of torque to say tilt my sub to 30
> degrees from vertical is considerably less than for most.  However, due to
> the concentration of mass in the hull (18 inch radius), the keel's mass,
> and the sail's mass, my boat takes applied force longer to obtain a
> rotational velocity.  Thus, it does not roll badly in the wake of a
> relatively large wave.  But be careful getting in and out and standing on
> the hatch.  At least get in and out quickly!
>
> I know I am not answering the question.  But, in lieu of any guidance from
> ABS or others with knowledge on this, one might make an intelligent
> argument based on these factors.
>
> Gary Boucher
>
>
>
> At 08:13 PM 9/30/2002, you wrote:
> >ABS Rule 3.19.1a states that "All submersibles are to have at least one
> >hatch.  The unit is to have sufficient intact stability on the surface so
> >that in the worst loading conditions when subjected to a roll expected
under
> >sea state 3 the unit will not take on water through any hatch."
> >
> >Can someone that classed a boat with ABS and met this requirement explain
> >what calculations you can make to meet the roll constraint above?
> >
> >Cliff
>
>