Center of gravity.

[Guest]

I am in the planning state of building a 2,1 meter long RC model of the USS OHIO (SSGN 726).

I have been reading about CG (Center of Gravity) for quite a time. Still I can't really make up my mind on where to place the CG. I am planning to use a single ballast tank system with the ballast tank placed exactly at the CG.

Does anyone have some tips on where to "place" the CG to get a nice proportion between turning and stabability.

Any advise will be appreciated.

-Andreas

[greenman407]

Wow thats big! Personally what

Wow thats big! Personally what I would do is assemble the hull with all its parts including the propellor and other heavy items. Then I would get the help of some friends to lift the submarine up and hold it about waist high. Then take a small diameter rope and place around the sub in a cradle (one strand) and move it back and forth around the center area until you find the balance point. Of course your friends need to let go while you are picking it up so that they do not interfere with an accurate estimation of the center of gravity front to rear. When you find the balance point, mark it with a pencil. I would then place the center of the ballast tank a couple of inches forward from that point. This has always worked well for me. Try it and if you want to experiment you can move it from there.

[mike dory]

C.G.

Guy's - The Ohio's class boats are a tough model boat to turn ( what with postage stamp size rudders and 6ft. long) anything that you can do to help in turnning is going to be helpful. Still I'm a fan of keeping the C.G. in the center of the boat, in my opion, it makes the diving planes work better and it helps to keep the boat level. So that leaves us having a bow thruster. Now I have seen this used on two differnt Ohio's and in worked very well indeed. At slow speed with no prop wash over the rudder it's almost a must, just to get away from the dock. Mind you, this is just my opion, which isn't always shared by all.... Best Wishes on your project....Mike Dory

[Guest]

Finding the CG is no

Finding the CG is no problem (I am studieng physics and a physics/math teacher is living next door), I only want some tips on where to place the CG to acchieve optimum turning VS stabability.

Thanks, answers has come quick.

Any scientifique explanations?
I am still planning, and will be able to plave the CG more or less where I want it to be, because I will need to add at least 5 kilos og lead, to make it possible to dive even with static diving system.

-Andreas

[KevinMC]

Hi Andreas,

I'm by no means

Hi Andreas,

I'm by no means an authority on this matter, but I have been looking at it in detail the last little while. I've taken some ideas on stability from the model rocketry community and put them to test on my OSCAR in the hopes of to improving it's turning performance. The jury's still out but from the one surface run I've had so far this year I certainly haven't hurt the performance any.

In principle the optimum balance for turning vs. stability should be achieved when the CG is co-located with the center of hydrodynamic forces on the hull. (In many cases this will be very close to Mike Dory's preferred "mid-boat" location.)

The trick I used to figure out the location of the center of hydrodynamic forces was to make an accurately scaled cardboard cut-out of the side view of the sub and find the balance point of that cut-out. This technique is a tried-and-true method employed by model rocket folk and does a reasonably good job of identifying the center of aerodynamic forces by balancing surface area ahead and aft of the desired center. The CG of the boat should then be located at this same point.

Based on the above technique I've recently shuffled all the equipment in my OSCAR aft by about 4". She's back to being sea-worthy but I'm still retrimming to get her to submerge nicely, and only then will I really be able to tell what kind of impact my tinkering has had. I hope to have more to say about the results soon, YMMV.

[robse]

I've got an 2,1m Ohio..

Hi Andreas.
I have build a 2m 12cm long Ohio (Scale 1:81)
Drop by my web and have a look. www.robse.dk

About the CG]http://www.subcommittee.com/forum/icon_smile.gif[/img]
Let me know if I can be of any assistance, we need to promote sub's here in Denmark.

[Rogue Sub]

Ive done some playing around

Ive done some playing around with the senter of gravity aswell and have found that moving the Center line aft made the boat turn much much better but also destabilized it to the point where its like driving a fighter jet in the water. You really just need to play with it to find what is comfortable with you. Everything in life seems to have a positive for a minus or a ying and yang.

Kevin