Understanding compressed air.

[tsenecal]

Mylo,

you don't need 30% of

Mylo,

you don't need 30% of the sub's dry weight in variable flotation.

the only flotation you need is the weight of the material above the waterline. that depends entirely on the hull you are modeling.
my model has a very low surfaced profile. its variable flotation is only 5% of its total weight.

the weight of all that material in your case might in fact be 30% of the weight of the empty fiberglass hull, but not 30% of the dry weight of the entire boat.

dry weight of the entire boat includes everything. the Hull, the WTC, the contents of the WTC, the batteries, the keel weight, the flotation foam... etc.

Tim

[tsenecal]

Mylo,

missed an important point in

Mylo,

missed an important point in your reply to jeff.

quoting from your reply]
What I seem confused about is, I was under the impression that whatever the displacement of the sub's WTC was, the total dry weight of the sub was going to have to be equal to this in order, basically, for it to be heavy enough to pull the WTC under water (running submerged).
[/quote]

at first glance what you are saying makes sense, but remember, the WTC only needs to be big enough to contain the gadgets you want to stay dry. most of those gadgets have weight. by the time you get your WTC full of gadgets, it will weigh enough to counteract most of the weight of that displaced water all on its own. and if it doesn't, well, then, you built your WTC way too big...

that is the secret to the wet hull designs that most of our current models depend on.

if I were to build my alvin as a dry hull model, it would have to weigh 60 lbs just to sit at its water line. and it would need a 2 litre ballast tank to compensate for the huge honking sail that it has on top.

but since it isn't, it needs about 60 cubic inches of foam to keep its svelte 20lb watermelon shaped hull from dropping to the bottom of the pond like a rock, and it only needs a 500ml ballast tank to displace a 1/8" thick ring of fibreglass that is 6"x8"x4" (and looks like the sail of an Alvin)

Tim

[mylo]

Tim,

In other words (man,

[color=#000000]Tim,

In other words (man, quite the conversation), the idea is to get your WTC to displace roughly what the expected total dry weight of the sub is going to be. Given the nature of the materials involved, even a 1]

[skip asay]

Mylo -

A more important (and

Mylo -

A more important (and appropriate) definition of KISS is Keep It Simple, Stupid. That’s the version I use.

First, forget about the boat itself. Determine what components you will be placing inside the WTC. Motors, electronics, batteries, etc. Then determine how large the WTC must be to hold all that suff.....comfortably. Don’t forget that when building a larger than normal size boat, most of what goes in the WTC is still pretty much standard size, ie motors, servos, receiver, etc. Essentially, what this means is that a WTC which is appropriately sized for a 1/25 Type VII doesn't necessarily have to be larger than one sized for a 1/32 Type VII. Of course, battery size (if it will be dry) can change this but only slightly.

The WTC, plus any foam you may use (ugh, I hate using foam) will supply the basic buoyancy the boat requires. Understand that that statement refers to approximately neutral buoyancy. The ballast tank then supplies the required buoyancy to bring the boat up to waterline level. As has been previously stated, the ballast tank holds enough weight in the form of water to offset the weight of everything ABOVE the waterline which is pretty much just “skin” (the thickness of hull, deck, conning tower, etc.). Note that the top of the WTC should be AT the waterline but not above it.

Virtually every boat I have ever built or been involved in building has still required lead in the keel.

I think that your original numbers are way more than what you will find after you’ve gotten the boat in the water.

Hope this helps.

Skip Asay

[mylo]

Skip,

Anything helps. From

[color=#000000]Skip,

Anything helps. From the get go, I was estimating the sub at being around 1]

[tsenecal]

mylo,

The answer to your question

mylo,

The answer to your question about compressed air is simple]http://www.subcommittee.com/forum/icon_razz.gif[/img]



Edited By tsenecal on 1126718238

[mylo]

hee hee.

hee hee.

[skip asay]

Mylo -

While external pressure has

Mylo -

While external pressure has an effect, it's not what you think. When air is introduced into a submerged submarine's ballast tank, it will start pushing water out. The boat will then start to rise since buoyancy has been increased as a result of the reduction in weight equal to that water that's been pushed out. As it's rising, external pressure decreases which then allows the air in the ballast tank to expand pushing even more water out. And so on until there's no more water left to push out.

Skip Asay

[mylo]

Skip,

Yep, that makes sense.

Skip,

Yep, that makes sense. So, for a ballast tank of X cu in, do I need X cu in of air, which can be compressed into 1/10x cu in of space if at 150psi ?....I guess this is ultimately the question I am asking.

Myles.

[gotland]

Hi Mylo,

As You say, the

[color=#000000]Hi Mylo,

As You say, the principle system is very easy.

Metric Example]

[gotland]

Sorry, I meant pressure multiplied

Sorry, I meant pressure multiplied with volume!!!!

Gotland

[skip asay]

Gotland -

I stand properly chastised.

Gotland -

I stand properly chastised. I think that was another senior moment!

Skip Asay

[don prince]

OK; Skip and Gotland you

OK; Skip and Gotland you two gentlemen have me confused... I'm a senior and it's not a moment, perhaps hours!

Your posting about Weight Vs. Volume has me confused. My assumption is brass is heavier than resin. For example; let's consider two OTW U-boats. If one was built with the resin tower, then the total weight would be less than the total weight of my U-boat with the brass tower. However, the volume of my brass tower would be less than that of a resin tower U-boat.

Both boats have the same waterline, but the one with the brass tower would have to lift the greater weight. The trim on both U-boats is set by a certain amount of lift from the flotation foam and the lift provided by the WTC (which contains the ballast tank).

Q1. Which U-boat has to take on more water to get the tower top at water level?

Q2. Where/How does the tower volume enter into this equation?

Q3. What questions are there that I'm not enlightened enough to ask?

Regards,
Don_



Edited By Don Prince on 1127089941

[yabbie1]

A1. The one with the

A1. The one with the resin tower would have to take on more water because its displacement (i.e. volume) is greater. In other words, as you try and submerge it when you dive, it generates greater buoyancy which must be overcome with more ballast tank volume.

A2. Refer above

The weight of the tower certainly affects stabilty, and a heavier tower will require greater bouyancy from the submerged part of the hull to achieve the same surfaced water line (achieved by having more foam). Once those factors are taken into account, however, only the total displacement of the above surface water line components determines how much ballast needs to be taken on in order to dive. Equal thickness superstructures, one of foam and one of depleted uranium, would require the same ballast tank volume.

At least that's how it seems to me.

[mylo]

Yab,

The plot thickens.

I

Yab,

The plot thickens.

I was under the impression that the volume of the ballast tank has nothing to do with diving the sub, and everything to do with keeping the sub bouyant on the surface. The size of the ballast tank determining how much of the sub will be above the water line.

So....

In my opinion of the resin / brass tower eg. ....the sub with the brass tower would require the larger ballast tank because the weight of the brass tower is greater, therefore more ballast lift is required to get it above the water line.

Either sub, whether it have a resin, brass, foam, or depleted uranium tower, should be very close to neutrally bouyant when their respective ballast tanks are full. It is the relation between weight and displacement that has to be tinkered with to get this to happen and not ballast tank size.

Mylo