The ABS Rules specify that the minimum vessel stability requirements continue to be met under all conditions of jettisoned ballast or drop weights, and inverted surfacing is expressly prohibited.
-Sean
On Sep 15, 2009, Jay K. Jeffries <bottomgun@mindspring.com> wrote:
Frank,
One of the issues that has to be considered with using a very large drop weight is how will the hull float after dropping. Vance can verify but I seem to remember that one of the Perry boats would float upside down once the weights were dropped. Don’t really think this will be an issue with your hull.
Dean has done a great job with his video clips. Looking at his freeboard and guessing at the volume of his conning tower with the cylinder extension aft to equal ~3 ft.3, he only has about 189 lbs. of reserve buoyancy…not much. From what he has previously stated, he was planning on adding further buoyancy materials to help rectify this matter. I am guessing that he is going to have to add a substantial amount to get a safe reserve buoyancy capability and still have a sizeable drop weight. Looking forward to the presentation on syntactic foam at this year’s Conference.
R/Jay
Resepectfully,
Jay K. Jeffries
Andros Is., Bahamas
Save the whales, collect the whole set.
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of ShellyDalg@aol.com
Sent: Tuesday, September 15, 2009 11:53 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Drop Weights
Dean, you mentioned that your drop weight was not very big and the front was a little light.
It started me thinking about the size of my planned drop weight. I've read about a lot of subs and there doesn't seem to be a lot of commonality in percentage of drop weight used for the various sizes of psubs.
I remember the little video about towing the Delta sub and during the submerged part of the video they said it had a 40 pound weight that could be dropped.
Other subs had drop weights that ranged up to 400 pounds.
I wonder if there's a danger in having too big a drop weight. Can it cause problems when it's released?
An advantage to a large drop weight would be that when dropped the sub would have more buoyancy and be able to "tear free" from a minor entanglement like the kelp we have around here.
The disadvantage could be a too rapid ascent and maybe some instability once you reached the surface.
Both of these could be quite dangerous.
The surface instability may not be that critical because at least you're back up where you can be reached by the surface crew and a line can be attached to the sub.
The uncontrolled ascent rate is the thing that I was worrying about.
I had planned on a pretty large drop weight. More like 750 pounds. The drop weight mechanism is designed in a keel shape, and has a bunch of wheel rollers so I can roll the sub around when it's on the floor in the shop, and when launching the sub it can roll on and off the trailer.
The actual lead weights will be shaped to fit inside the steel keel form and bolted in. The total weight can then be adjusted by adding/removing weights but the steel keel mechanism itself weighs about 250 pounds.
The total lead ballast needed to sink the sub is going to be close to 1800 pounds. Most of this is bolted to the floor inside with some of it on a sliding mechanism to adjust trim angle.
These are rough numbers at this point of the build and my float test will determine exactly how much is actually needed depending on what all I put into the interior of the sub.
Adding a passenger, equipment, and later improvements like a manipulator arm or whatever can be compensated with the underfloor weights.
In the model testing I did the sub wobbled during ascent when it was simulated dropping the weight but it stayed level and fairly stable on the surface. It's the wobbling when coming up that concerns me.
Anybody got any thoughts on this?
Frank D.