ballast tank and weight placement guidelines

[roedj]

To All,

Just to confirm that I'm not way off base here I'd like to present what I believe are the general guidelines for the placement of ballast tanks and weights. Please feel free to whup me upside the head i*** I'm wrong.

These following two rules apply to a boat where the ballast tank is exposed to the outside water - a wet hull boat:

1) The top of the ballast tank should be at the surfaced water line. This keeps the center of buoyancy high as possible and gives us the most "bang for our buck" in terms of raising the boat to the surfaced water line.

2) The center of gravity/mass should be as low as possible to maximize the distance between the center of buoyancy and the center of gravity. This should maximize the righting moment arm.

In a dry hull boat where ballast tanks are inside the dry hull only rule 2) (above) is important. The center of buoyancy is determined by the shape of the dry hull. I'd guess that even if I was building a dry hull boat I'd try to keep all the "dryness" below the surfaced water line. I'd stuff in extra closed cell foam if I needed to raise it more.

In reality, there's no big difference between a wet or a dry hull boat in that in a wet hull boat the ballast tank's outer skin is the same as the dry hull, the WTC being the dry hull with an outer skin attached to it to give the appearance of a submarine.

Let the whacking begin...

Dan

[Slats]

To All,

Just to confirm that I'm not way off base here I'd like to present what I believe are the general guidelines for the placement of ballast tanks and weights. Please feel free to whup me upside the head i*** I'm wrong.

These following two rules apply to a boat where the ballast tank is exposed to the outside water - a wet hull boat:

1) The top of the ballast tank should be at the surfaced water line. This keeps the center of buoyancy high as possible and gives us the most "bang for our buck" in terms of raising the boat to the surfaced water line.

2) The center of gravity/mass should be as low as possible to maximize the distance between the center of buoyancy and the center of gravity. This should maximize the righting moment arm.

In a dry hull boat where ballast tanks are inside the dry hull only rule 2) (above) is important. The center of buoyancy is determined by the shape of the dry hull. I'd guess that even if I was building a dry hull boat I'd try to keep all the "dryness" below the surfaced water line. I'd stuff in extra closed cell foam if I needed to raise it more.

In reality, there's no big difference between a wet or a dry hull boat in that in a wet hull boat the ballast tank's outer skin is the same as the dry hull, the WTC being the dry hull with an outer skin attached to it to give the appearance of a submarine.

Let the whacking begin...

Dan

Dan - whilst the principal of wet and dry subs regarding ballast tank positions can be common - remember that a dry hulled submarine needs corresponding extra weight to offset the excessive bouyance of the dry hull.

That aside: Here is an extract from my Collins build utilising God's methodology on the subject.

Ok to rewind a bit - the ballasting above was primarily done to test my new ballast system, and secondarily check out the needs of this boat. Further ballasting will be required for two reasons, the first is that I want to improve the subs turning circle, and the second is that this boat will go to sea with the new D&E / Caswell torpedo system adding some weight forward. This final ballasting will be done hopefully in a couple of weekends time, just in time for the Australian Sub Regatta at easter, if the torpedo system is not ready to install, I have her ballasted to run without it.

In terms of ballasting a boat, when I first mucked about with subs I was ignorant and stupid, coming from a target world where ballasting simply meant a scale water line. Nowdays I am still learning and have recently re- reviewed David's cabal report on the Alfa (pt7) which is pretty much a tutorial on ballasting a sub as it is specific to the Alfa. See the link on this site....

http://support.caswellplating.com/index.php?_m=downloads&_a=view&parentcategoryid=23&pcid=5&nav=0,5

To quote David on some ballasting basics from this excellent report:
"Center Of Gravity Considerations Where the c.g. falls, longitudinally, on the hull is important for two reasons. First, the c.g. is, disregarding fluid forces on the moving hull, just below the vehicles center of rotation about the vehicles pitch and roll axis, and right on the vehicles yaw axis. The greater the vehicles control surface distance from the center of rotation (moment arms), the greater the force they exert about the axis point. But, put the c.g. too far forward and the boat becomes overly dynamically stable."

"With the c.g. too far forward the boat is too dynamically stable; the boat is difficult to steer about the yaw and pitch axis; the pitch and yaw rates are low. However, if you position the boats c.g. too far aft the boat becomes either dynamically astable (presenting to righting forces regardless of angle presented to the fluid stream) or unstable (the vehicle reorients and becomes stable at an undesired orientation to the fluid stream), making the boat a total ***** to drive. Good drivers strive for a boat that is astable -- the boat has good pitch and yaw rates, but requires constant attention by the Driver! The center of pressure (c.p.) on a vehicle is the point where the collected fluid forces gather on the hull -- a consequence of the friction produced by the fluid in contact with the vehicles outer envelop. Again, it takes considerable math skills to work out a specific shapes c.p. But, you can work out a
good estimate on paper. For idiots like me you simply make a profile drawing of the vehicle on graph paper and count the number of squares, you then find the longitudinal point on the drawing where the number of squares aft is equal to the number of squares forward, call that your models c.p. You want the models center of gravity (determined easily with a sling on the model) to be a bit forward of or on the c.p. (determined from the graph-paper study). Remember, an over-stabilized boat is a boat that is hard to turn!"

OK so c.p is therefore important, and as David says in his report we usually stick the BT about half way a long and it works out great - most times. That half way long pt is usually indeed a guess, but I wanted to try out, as I don't have a considerable maths brain either, using David's grid guestimate instead. Using herein with permission Steve Batchelador's profile, modified by me, I worked out a good way to grid this method up. - Use a Spreadsheet program - in this case Excel.

First I resized the Excel spreadsheet to have the cells reformatted so that the cells are squares. Next I pasted in the profile I worked on. Note here you don't need to post in a profile in the exact size of the model, you can use a profile in any scale that suits your ability to have it in a workable size on your computer. The profile should be preserve the dimensions of the boat in whatever scale the profile is. That is, be careful not to stretch the image by resizing it as you paste it in.

The profile image you paste in needs to have a grid over the top of it, so you need to make the image transparent, and you need to format the cells the image covers by puting a border on all those grid lines. The result is that you can then have a uniform grid to which count the cells (squares). The easy, most accuarte way of counting is to group sections of whole squares together, and then just multiple the area L x W. For areas containing portions of whole squares, group these together and estimate how many whole squares these equal. Simply count the ones nearest the curves etc too. Add up all the squares within the groups, getting the total squares for the whole profile. Then divide by 2 to get two equal areas of the boat's profile fore and aft . I then start at the stern and work forward, calculating how much area from the stern equates to half of the total. I marke up a demarcation line. At this point I know where the c.p is. To translate this to the model, I then measure the distance from the stern to this line (just hold a ruler up to your computer screen), and work out what percentage that is to the whole length of the profile on screen. In the case of Rankin its 56%, this then is translated to the whole length of my model.



In my case, the c.p was located 15mm further back from where I initially had the centre of my ballast tank. I was able to move the WTC back, by simply removing some excess lexan from the aft compartment.

Having now established where the c.p is you then can with a fully loaded sub (batteries etc) make this the centre of gravity (c.g) , by placing a sling under the sub at the c.p point, and adding trim weights fore and aft unitil the sub is balanced. Obviously I will have to do this exercise (sling and trim), when the torpedo gear arrives. After your happy with the balanced sub around the c.g at the c.p point - you then go back and put the sub in the water triming with foam. Using the sling last night it was a complete eye-openner as to the fact I had too much weight forward.

Thanks to David for sharing this approach via the cabal reports - I have learnt something - and David jump in here if I have got this wrong.

best
J

[roedj]

John,

Thank you very much. I re-read D's cabal report on the Alfa and learned a lot. Also I like the idea of pasting in the hull drawing onto an Excel spreadsheet - very nice.

Thanks again,

Dan

[Slats]

Pleasure Dan, those cabal reports - even if its about a boat that does not interest you - are a treasure trove.
The most valuable commodity I know of is timely accurate information.
Best
J