Ohio Class, Ballistic Missile Nuclear Powered Submarine, USS Kentucky SSBN737

Where Does Everything Go, Trimming Considerations?

UPDATE: Current work involves trimming, so data on this page is subject to change within long.

A few things...

There are a few things to keep in mind when getting ready to trim your sub.

First, the positive and negative center of buoyancy need to be aligned on top of one another in order for the sub to align it self without any list,
but the bigger the distance between these two points, the better. If they are right at the same point, there's almost no stability, nor the ability for the sub to upright it self. As a result, I chose to lower the watertight compartments 10mm ( 0.4" ) down "under" the center line of the hull, and towards the keel of the sub.

Second, the positive and negative center of buoyancy need to be in line with the center of the ballast tank. If not, the sub will not dive without developing a list end over end, which would make the sub hard to control when sailing while submarged.

Third, in order for the dive and surface function to be of any value, the surfaced waterline must be above the floatation material. Only exception is if some foam is to compensate for something, only when dived down. (e.g.. if the sail has got positive buoyancy etc.)
If some of the foam is above the surfaced waterline, then the dive function is limited, as the dive function first has to overcome more and more positive buoyancy during the initial dive. You could say, that the weight of the sub is not constant, if some of the foam pops out of the water when surfaced, and that will mess up the equation.

Is the trimming a once-and-for-all thing? No. Trimming is effected by the water temperature (density) and salt level. Just like real life, a ship has a bigger draught in fresh water areas, than in waters with high concentration of salt. Some adjust the trim of their sub before any trip, just to make it absolutely right. If you want that ability, use small lead weights glued on Velcro strips, or similar. 

My actual submarine:

Since early construction, I tried to even the negative and positive buoyancy, and at the very least keep an eye on it, during planning and construction. This table shows the results, and buoyancy for each component:
Item: Weight: (Out of water) Displacement: Bouyancy:
WTC 1 3,19 kg 7,04 lb 2,23 kg 4,90 lb - 0,966 kg - 2,13lb
WTC 2 1,3 kg 2,87 lb 1,672 kg 3,68 lb +0,37 kg +0,81 lb
MBT (*1) 0,458 kg 1 lb 0 to 3,3 kg 0 to 7,27 lb up to +3,3 kg up to +7,27 lb
Air Tank 2,0 kg 4,4 lb 1,661 kg 3,66 lb +0,462 kg +1,02 lb
Fixed ballast 1,67 kg 3,68 lb ? ? -1,67 kg -3,68 lb
Hull (*2) 4,5 kg 9,9 lb (Not avail.) (Not avail.) -4,5 kg -9,9 lb
Floatation material kg lb kg lb +kg +lb
Resulting bouyancy: ( kg) ( lb) kg lb kg lb

(*1) The bouyancy of the MBT is not the result of the displacement minus the weight because the weight of the MBT is considered part of the weight of the hull / ballast.
The top of the bare hull, including sail and missile deck, weights app. 2 kg ( 4,4 lb ), and the lower half including bulkheads 2,5 kg ( 5,5 lb ) in free air. The positive buoyancy of the hull has not yet been determined, so this figure is subject to change later on. When it changes (decreases), the floatation material will be reduced as well.

As you can see, the floatation material only need to displace app. kg / lb of water for the sub to be buoyant neutral.

How to trim:

Testing the center of gravity while the sub is out of the water will most likely show that the point does not match the center of the hull along the length axis. Try to place this point a little in front of the middle, as this will enable the sub to react easier to the rudders and thereby be easier to maneuver. The ballast tank should be in the center of gravity.
When the above is true, continue with the trimming in water.

Getting the sub trimmed and aligned is quite a time-consuming task. Place it in the water, and put rubber bands around it at several places. Now move the inside stuff around to find the location that requires the least amount of floatation foam, and that places the center of gravity in front of the middle of the hull. When this is done, place some foam / lead at the rubber bands, equal volumes at both sides and symmetrical. Ensure that the foam / lead is placed below the surfaced waterline.

Then dive a foot or so, and observe the trim again. Redo the first step, and try again, until it is trimmed both above and below the surface. This is important, so do take the time and caution to do it right. Important is also that the center of buoyancy is above center of gravity, so the sub won't roll. For the same reason, the final position of floatation foam should be at the sides (Between hull and WTC's), right below the surfaced waterline, but it should not extend all the way to the bottom. This should give your sub two forces of positive buoyancy at work, far apart, giving vertical stability and prevent rolls.

Please see image on the right.

are not mounted in the foam, but in brackets made in inch. plastic. Also notice that the foam should be glued to the top half of the hull, thus aiding maintenance etc. when the sub is taken apart.

It might be necessary to place some floatation foam inside the hull compensating for the sail. This can be tested by observing the hull when fully diving the sub. If the bow aligns it self higher than the aft end, then you will have to add some positive buoyancy aft of the center, and above the surfaced waterline. This way this added buoyancy will not get into play while surfaced (and not needed), but it will get into the equation when needed during a dive (when the sail is submerged).

When all of this is achieved secure the components Important information!!, and connect everything.

Fitting things inside the hull & trimming :

All of the WTC's, MBT, the air system, wires, tubes, shafts and rods etc. needs to be secured inside the hull, and the submarine needs to be perfectly trimmed. Here I'll just show you how I did the mechanical mounting and secure fix of these components, and the floatation foam etc.

First test: The initial test and trimming in the wet dock went somewhat ok. No leaks detected.
Issues: When the surfaced trim and righting is ok, then diving is not possible. When diving is possible, then the surfaced trim is all wrong.
It seams that the top / missile deck is too heavy, so tests continue by trying to move foam up, so that the bottom half will hang from the top, not carry it. As it is now, the bottom floats on it's own right where it should.. correct depth and all, but when the top comes on, it all goes bad.
In test two I'll try and move all positive buoyancy up into the top half, so the top will not mess up the self righting. Second, I need to determine how little foam is allowed for diving to be possible. (Eg. What waterline will that give me..)

Second test: Two metal bars was put in the very bottom as fixed weight, correcting the issue regarding self righting. More flotation foam was placed to archive right surfaced water level. Then, by reducing the surfaced water level in the MBT's, I found the exact "point" where surfacing was possible, as well as diving, and where I had an acceptable surfaced water line.
Conclusion: The sub can now righting it self very well, it can dive and surface, and the water line is fine. Now's it's back to the dry dock to fit all the components at their final location. :-)

Third test: The idea with two independent MBT's proved a) difficult to control and b) absolutely not needed. The two MBS's were therefore scrapped, and a new, central MBT was made. Because the dual MBT-idea required a bit of electronics, then the majority of this was scrapped as well. In general: It was a complete rebuild of the inside stuff, but it resulted in a system much more stabile, and simple.
Read more about test three here.

First four pictures are from test 1, the rest are from the process of mounting modules.


Preparing water test!!

Surfaced trim close to ok...

Waterline marks close

It allmost there.. but can't dive..

Painting with anti-rust..

The mounted main weights.

Notice the drain tunnel.

Air tank fitted....

Drain ports for the MBT's.

Drain ports exiting at the center

Working on brackets.

Re-doing bulkheads.. (view)

and fitting them.

Module, held in place!

All wires made, now testing..

It ALL worked!

WTC2 & old MBT2 + bow thrust
 <==   The heavy WTC2 is off-center by 10mm downwards, thus helping upright stability.

The red caps cover the not yet fitted air vents. (Goes to top half)

Fitting towed array tubes..
See more.. 

Linkage complete

Aft rudder light fitted

Notice the hour counter..

Air ports fitted to lower half

Cuts in upper half to match

The MBT in place..


Click images to enlarge.

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