Myles,
You show concern that you will need a "substantial air reserve to blow tanks of such size." Perhaps you should considering using pumps, or do what the U-Boats did...blow it out with the diesel exhaust. This is after you have broken surface, of course.
Unless I am mistaken (which has happend before!), it only takes a small blast of air to bring a submerged boat to the surface, be it a one ton boat or a 20 ton boat. Filling just 1-2% of your full hard tanks with air will bring you up to the surface. Or at least the tip of your boat up to the surface. The question then becomes one of getting rid of the other 98% of the water in the tanks, in order to give you your full freeboard on the surface. That means either (1) blowing with compressed air, (2) pumping overboard, or (3) blowing it out with your diesel engines.
You are considering option #1. That takes a lot of air. Option #2 takes more battery power but much, much less air. If I had to choose between lots of extra air tanks and lots of extra batteries, I would take the batteries. They have more uses than the air does. Option #3 is not usually open to psubbers, as most don't have diesels. But if you do, then you won't need extra air or extra batteries. An extra advantage of this practice, according to WWII German accounts, it that the oily exhaust coats the inside of the ballast tanks and reduces rust buildup while on patrol.
Just a thought,
Doug Farrow
-----Original Message-----
From: Myles Hall <myles.h@sasktel.net>
To: personal_submersibles@psubs.org
Sent: Sat, 29 Oct 2005 16:54:42 -0600
Subject: Re: [PSUBS-MAILIST] Hard / soft ballast tanks.
Bill,
Thanks for that.
I have been giving the design much thought. Have drawn and redrawn a lot of plans.
My plan from the onset was to make the sub U-boat -esque in that I wanted something long and lean, diesel electric, with decent surface performance. I arrived at a length of 30' max from tip to tail, being what I could feasibly transport. I have calculated a pressure hull length of 10' x 36" which would be in the middle of the 30' overall length. The two ends on either side, which would give the sub some hydrodynamics, I was planning on making into for/aft soft ballast tanks out of a fibreglass shape. The "saddle" tanks were going to be compressed air reserviors (following the principle of having your bouyancy up high). I figured I would need substantial air reserve to blow tanks of such size. My bat pods will be mounted low in the floor within the pressure hull but in their own sealed compartments for safety reasons. The fuel cell will also be mounted low with vents and the filler cap accesible from the floor. All the while I am trying to design the sub with the weight down low, and the bouyancy up high. ....then comes the variable ballast tank. My design called for mounting it directly underneath the pressure hull amidships in that I wanted to control just the one tank to obtain neutral bouyancy. I was looking at the design of actual U-boats which also have a similar tank directly underneath the control room amidships. I was working under the assumption that with all the other weight down low, as well as a weighted keel that will be under the tank, that would be sufficient to keep the boat stable, both surface and submerged. The primary function of this variable ballast tank is to compensate the weight lost as a result of the diesel fuel consumption.
That's the basic premise from which I'm working from.