Weight Distribution Scenarios - Is one better than another?

[trout]

Pondering: Is there a better way to distribute your weight and foam?

Assumption: The sub is trimmed well underwater and on the surface. The dot [black] represents center of gravity (CFG) and is in the center despite drawing may be off. Bluish Green is the floatation foam and gray block is weight.

Question: Of the four drawings, which would produce a better balanced sub? or is there one that is even better?

[Slats]

The diagrams above are for cross sectional weight distribution. Also foam is not a weight distribution concept, its a trim concept.

Method 1 I guess is closest to the way I approach all boats, but importantly in you examples - Nothing longitudinal there. AND COG / COP / COR are things to focus on.

My method is always find the COG via profile grid overlay of the profile plans. Find the point of the middle square and plonk the centre of the ballast tank at that point.

J

[trout]

John, why did I know you would correct me? yes it is more of a trim question, but it is just a generalization and like I said in the assumption the boat is trimmed level. Because in each of the examples a sub could statically be trimmed, but performance wise be different right? These are longitudinal views over simplified for example.

When I trimmed my Gato out, I found that I could put a small foam or weight at the extreme ends or a larger piece more towards the center. They both brought the sub to level, but I thought it better to put the weight or foam more towards the extreme ends to smooth out the ride (no science here just speculation on my part) because if the weight was more in the center like #2 the boat would act more like a pendulum. That started me thinking of "Is there a best practices when installing foam and weights?"
Although I do not understand completly all the intricacies of COG / COP / COR i do know it is important.

[vital.spark]

I have a great book, "Submarine Design" by Ulrich Gabler, Third Edition 1985. He did a lot of design work before, during and after the war. The book explains all aspects of Submarine design with very little in the way math so can easily be understood. His firm, IKL is a leading German Firm in Submarine design.

[Slats]

John, why did I know you would correct me? yes it is more of a trim question, but it is just a generalization and like I said in the assumption the boat is trimmed level. Because in each of the examples a sub could statically be trimmed, but performance wise be different right? These are longitudinal views over simplified for example.

When I trimmed my Gato out, I found that I could put a small foam or weight at the extreme ends or a larger piece more towards the center. They both brought the sub to level, but I thought it better to put the weight or foam more towards the extreme ends to smooth out the ride (no science here just speculation on my part) because if the weight was more in the center like #2 the boat would act more like a pendulum. That started me thinking of "Is there a best practices when installing foam and weights?"
Although I do not understand completly all the intricacies of COG / COP / COR i do know it is important.

Hey Tom its all good.
And its a good question. I don't profess have all the answers either.

Unfortunately its a bit of generic question, applied to different combinations of hull's and their WTCs. What I mean by that is forgetting longitudinal stuff, the key cross sectional factor you can't generically fit into the neat little 4 panels you put up is found in specifics of the size and location of the WTC, (missing in your examples but also a factor for stability). Metacentric height / bouyancy and positioning of the WTC are all BIG factors as is the shape of the hull that the WTC its sitting in. For example, putting too big a diameter WTC in the boat can cause its own stability issues that require more fixed ballast to overcome the bouyancy of the WTC down low, which is where you don't want it. That could lead to fixed ballast requirements that purely for space issues see a setup of fixed ballast - low and left and right of centre.
But the basics I find are pretty much what you had in pannel 1 - but other hulls might dictate other setups.

For the ins and outs on COG/ COP / COR, have a look at my Seawolf thread, or better still see if you can dig out David's Cabal reports he did a few years back on the Alfa tunrkey he did for a client.

Best

John

[trout]

John, please forgive me, i was not clear, I was smiling while I typed "Why did I know....." because you are very smart and I like the way you break things down to their components. If something is misstated, you will correct or clarify. Qualities I really like in you!
it is a generic question with so much more going into a sub. You rock my friend, thank you. I have read your excellent Seawolf thread and poured over David's Cabal reports also excellent. In many ways I am a slow learner and do not always (more like mostly) do not follow a linear thought pattern although I continue to try :-)

Myron, I will look up that book. Thank you very much for that recommendation.

[KevinMc]

Pondering: Is there a better way to distribute your weight and foam?

Hi Tom!

I've had the same thoughts many times as I think there are significant differences. Firstly, I don't know what the "right" answer is (if there really is such a thing) but I will weigh in with a few thougths on setups with a "centralized weight model" (ex 1 & 2) versus a "distributed weight model" (ex 3 & 4). Statically it makes negligible difference since the variation in gravity with boat angle just isn't relevant over the distances we're talking. Dynamically, distributing the weights over a large distance creates difference that should be considered. The distributed weight model will have a higher angular momentum than the centralized model, so it takes more energy to start it moving and then to stop it again. This is relevant because it means your dive planes and pitch controller (that for personal reasons I certainly hope is an ADF2 :wink:) will have to work much harder to return things to level when faced with a disturbance in a distributed weight model. For that reason, I always try to set up my boats (and recommend to others when I'm asked) to keep the weight centralized as much as practically possible.

[trout]

Thank you Kevin! Great insight and more food for thought. You and John make good points that there may not be a right answer.

[He Who Shall Not Be Named]

Tom,

In a heavy sea we would fill the forward and after trim tanks as much as possible to produce long moments of inertia between each end of the boat and our center of rotation (pretty much where the c.g. is, under the trailing edge of the sail). That mass would, to a degree, damp out pitching and yawing forces from a following sea. This may seem counter intuitive, but by putting the foam as close to the c.g. as you can you produce the longer mass moments (water is denser than foam, duh!) that damp out the external forces that lift/dump/push left and right the stern and bow. But, this makes the boat a bit less responsive to rudder, stern and fairwater/bow planes.

Put the foam at the extreme ends and you reduce the mass moment arms, increasing the pitch and yaw rates of the boat (induced through outside forces or control surface forces). Like and ice-skater spinning faster as he pulls in his arms -- he reduces the mass moment arms about his spin axis.

Damn! ... you post a great question here, Tom! ... think you're pretty smart, don'tcha!

I used to stand BCP watches as, Chief of the Watch underway. And you had to know this stuff. Also, as I qualified on the TRUTTA, I stood many a manifold watch, so moved my share of trim water.

In the Navy (in those days, anyway) they wanted you to not only know how to do something, they demanded that you knew why and the consequences of what you were doing. We may have been a bunch of knuckle-dragging goons, but we knew our ****!

M

[trout]

That's it! I think my thinking on this was really to understand the consequences more than the best answer...... unless there is a better answer. Your description makes sense and helps me understand.
Me smart? Forrest Gump said " Stupid is as stupid does", I do a lot of stupid, so no, not smart just inquisitive. If I was really smart I would have known the answer :-)

[He Who Shall Not Be Named]

Hot stoves are the best teachers, Tom!

M

[trout]

HAHA I would add knives, CA, hammers, and most power tools to my list. Forgot to add molten metal, yeah that is a good teacher too!

[KevinMc]

Ooo, ooooo! Can we add high explosives to the list??? (Or at least rockets??? Really, they qualify! I use 'em in my boat!)

[alad61]

Ahh I think I see some light down yonder tunnel...

In my Robbe Seawolf, which is a dynamic diver, I don't have the heavy brick battery that it recommends. Instead I used a 7.2 stick battery which obviously is no where near as heavy, so I had to add weight at the bow and stern to get it to the right trim to dive, re diagram No4. Let me tell you it is as much hard work to drive as the Seaview. Even though I have a good throw on the rudder and planes, it still takes an acre to turn and either using the forward planes, which I have on a servo, or letting the rear ones alone so the ADF2 can do it's thing stretches the nerves to near ruin trying to keep it at PD... So much so that in our murky ponds I don't run it as often as I'd like. Which leads me to think, especially after reading here that I may indeed need to either put in a pump and ballast tank or to keep it dynamic replace the stick battery with a gel cell brick...

[HardRock]

Sir, I salute you. Explain, demonstrate, Imitate is a damn fine credo (with the emphasis on "explain") and I'm damn sure that you did know your ****!