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Re: [PSUBS-MAILIST] FW: sub ops

I get the immediate impression that fore and aft (pitch) stability analysis is probably not being given the same care and attention as the transverse (roll) case in many cases. The same considerations apply to both, and in fact an argument can be made that fore and aft stability is more critical.
In any case, small vessels are inherently prone to significant motions due to both the sea and the movement of occupants within, and accordingly, MBTs should be designed to accommodate the worst case scenario motions (i.e. all possible motions within the point-of-no-return surface stability limits) without the loss of ballast air. For this reason, I am a fan of totally enclosed MBTs. Realistically, though, as PSubbers we are already making concessions to our intended use - apart from Carsten, I don't think anyone is meeting the mininum freeboard requirements of ABS etc. Commercially, PSub size boats like the Nuytco Deepworkers must be hoisted on deck before hatches are opened. At the Vancouver conference, by applying maximum transverse thrust in alternating directions, at the natural frequency of the vessel's roll, I was able to roll the DW to a pretty good angle - and that was still on the hook! (Phil, if you're reading this, I wasn't trying to break it - honest). The point is, it wouldn't take much effort on the part of the occupants to put a surfaced PSub hatch awash, so the question becomes, when determining to what extent you comply with the commercial design guidelines, how much safety do you make inherent to the mechanical design, versus the operating procedures and simple assumption that the vessel occupants are not going to put the vessel at risk?
As an off-the-cuff suggestion, it seems logical to me that a surfaced vessel with MBTs blown should be able to accommodate its design crew complement on the weather deck without significant risk of stability failure, when operating in the design sea state. 

-Sean


On 16/01/2011 5:16 PM, Alan James wrote:

Hi all,
This whole issue of buoyancy seems to have been under rated.
There was Tao Xianglis oil barrel sub that went bow up & had to be dragged from the water. Doug's Argonaut Junior surfaced too quickly & on the backward bounce compressed his ballast air 
& sank to the bottom before he could adjust it. & now Jim's boat.
It's a bit of a wake up call in this area.
Regards Alan

----- Original Message ----- From: "Hugh Fulton" <hc.fulton@gmail.com>
To: <personal_submersibles@psubs.org>
Sent: Monday, January 17, 2011 1:31 PM
Subject: RE: [PSUBS-MAILIST] FW: sub ops



Sean,
Under ABS rules there is a stability check for maximum movement of personnel etc within the confines of a sub. It would be an interesting excercise to have that comparison made on BG or a Kittredge design under 7-10.2 of ASME PVHO. This is for underwater stability but righting moments etc for on top 
with metacentric height etc can be calculated.  This is one of my big
concerns.  I think I remember throwing out that thread on stability some
time last year but there was no interest at the time. I am considering
putting a diaphragm on the base of the MBT so that in the event of a tail or 
nose entanglement blowing of the MBT's will not result in burping at a
severe angle.
After testing of the Comsub I have tried to alter the various components
like an Ali rudder instead of SS as the moment was too great and it was tail heavy on test, also increased drop weight, lighter batteries. However these 
are minor changes to what you are alluding. Hugh




-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Sean T.
Stevenson
Sent: Monday, 17 January 2011 12:41 p.m.
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] FW: sub ops
Dean - check your email settings. Your messages are showing up in a huge font on my system.
As for your comments - I'm sure every designer has their reasons for keeping their designs as light as possible, but it is an interesting thought exercise to consider the actual downsides to incorporating larger main buoyancy tank volumes, in conjunction with additional lead ballast, to increase the righting arm of any given boat for the same freeboard. Lead is relatively cheap, and exterior MBTs would seem to be one of the easiest structural / mechanical retrofits that can be effected. The obvious downside being the necessary power to drive the boat at the same speed, but for most PSub applications (Cliff excepted), we're not racing around down there. Perhaps designing for sufficient stability to support a load on the weather deck is worth the additional displacement? Even without changing displacement and ballast, it might be possible to lower the position of fixed ballast on a given design in order to increase stability? I am curious to know if anyone on the list has, after completing and testing a sub, opted to make modifications in order to change the stability. Comments? 

-Sean





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