[PSUBS-MAILIST] sub test

[Sean T. Stevenson via Personal_Submersibles]

A couple of thoughts regarding stability:

Stability in the submerged condition is established by ensuring that CG (center of mass) is vertically below CB (center of buoyancy), such that a righting moment is created when the vessel rolls. In the ABS Rules, the specified minimum distance between CB and CG is 2", which isn't much. While you can assume a fixed position for CG in the light ship condition, a vessel with any combination of stores, fuel, loose equipment and crew aboard will be subject to movement of CG with respect to the vessel as the vessel rolls - particularly as crew weight can be significant with respect to overall vessel weight in small submersibles. As such, it is possible to approximate zero stability (analogous to a cylinder floating on the surface, which has no tendency to right to a preferred orientation) in a submerged vessel merely by allowing crew and stores to move about, unless care is taken in design to entirely preclude the possibility of the CB-CG righting moment to be zero or less.

Transitional stability reduction occurs on submergence because the vessel is transitioning from a condition of form stability to one of mass centroid stability.

In the form-stable condition, the vessel has positive metecentric height due to CB being displaced laterally as the vessel rolls. Even if CG is above CB (as in most surface ships), as the vessel rolls, the shape of the submerged portion of the hull changes to push CB laterally outward, which provides a righting moment against CG to restore the hull to upright. Stability in this condition is techically zero when the vessel is perfectly upright, but rapidly increases with increasing angle of roll up to the point of diminishing stability (roll limit).

As a vessel submerges, a few things happen simultaneously:

1) The form stability is lost, because angle of roll no longer has any substantial impact on the lateral position of the CB, and of course once fully submerged the CB is constrained to the midline of a laterally symmetrical vessel,

2) The portion of the hull previously above the waterline begins to contribute to buoyancy, moving the CB upwards. Unless the vessel was specifically designed to maintain CG below CB at all times (i.e. even while surfaced), this means that the vertical order of CB and CG will exchange places, which implies that there is a point in time during the transition at which the vessel has precisely zero stability.

3) Any gas spaces exposed to sea which are not vented sufficiently rapidly (e.g. main ballast tanks) will exhibit volumetric compression due to hydrostatic pressure, worsening any initial list if there is differential hydrostatic pressure from side to side, because air in the deeper tank(s) will compress to a greater extent.

Consequently, it is recommended in design to ensure that the transition between surfaced and submerged condition can occur as rapidly as possible, such that the vessel's momentum and inertia compensate for the momentary total loss of stability during the transition. The only condition which can preclude this requirement is if the vessel has no reliance on form stability in the surfaced condition (i.e. CG is below CB even when surfaced, regardless of crew movement etc). This is most typical of smaller vessels.

The other thing to consider is damaged stability. While PSubs (typically) only have a single crew compartment, damage to an external ballast tank can represent a condition which could possibly create a roll (or pitch) moment contrary to the design righting moment in the intact condition, and it is incumbent on the designer to consider the effect of e.g. ballast tank(s) blown empty on one side of the vessel only, and to ensure that this both does not roll the vessel past any operational roll angle limit, nor prevent emergency egress from the vessel by not considering the resultant submergence of all exit hatches in the damaged condition, unless some contingency plan is in place to account for the possibility.

Sean

-------- Original Message --------
On Aug. 15, 2020, 09:58, Tim Novak via Personal_Submersibles wrote:

> Looks great, Brian.
> Good photos. A list is not necessarily instability. Perhaps it a trim issue? A little less stability can be expected during submerging and surfacing transition.
> Tim
>
> From: Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org] On Behalf Of Brian Cox via Personal_Submersibles
> Sent: Friday, August 14, 2020 1:42 PM
> To: PSubs <personal_submersibles at psubs.org>
> Subject: [PSUBS-MAILIST] sub test
>
> Hi All,
>
> Just got finished with yet another sub test down at the harbor. Wow ! went extremely well ! This time I felt confident enough to put it in at the launch ramp, sub very level upon floating it off the trailer. I had this giant flotation buoy on the front since I wasn't quite sure what to expect ! ( I've been surprised before) . Everything seemed fine so I untied the big buoy and let the sub sit naturally. It sat very level and nice! Submerged nicely and by the time all the air was expelled I was down. The first initial test was with the big buoy on the front , then untied the buoy and tried again . I had a bit of a list going down that got me a bit spooked so I came back up. But things are looking really good . I probably could have transitioned through that list and gone down all the way but decided to come back up. What I want to do is add a bit more ballast lead and then try again. It's amazing what chopping off 6 feet off the front of your sub will do !!
>
> Sorry no video but I do have a few stills that I will try to get up. But all in all things are finally looking really good.
>
> Brian
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