I posted this on Facebook, but I wanted to share here. This seems to be one of the top three or four questions I get. I am interested in your thoughts to see if I missed anything or if there are other suggestions.
Estimate Ballast Tank Size
A common question is “How to figure the size of a ballast tank?”
There are several ways to estimate a ballast tank size. I will list several ways, but you will need to try them and see what works for you.
Archimedes is said to have gotten into a bath and noticed the water rise. This discovery led to what is known as the Archimedes Principle. Out of that came ways to figure buoyancy and to accurately measure the volume of objects.
So, you created a new sub hull, how big does the ballast tank need to be? You will only be concerned for what is above the waterline.
Fill a bucket to the brim with water, dunk the parts that will be above the waterline, capture all the water that comes out, that water is the volume you need. You got to love metric measurements, 1 gram of water is equal to 1 cc of volume. Weight the water, then figure out the volume (I.e. That seems like a lot of work and you are not done. We have the volume, but not material the hull is made from. Wood is different than lead. Fortunately a plastic model like Revell’s 1/72 Gato or Mobius 1/72 Skipjack has a specific gravity about equal to water meaning you do not need to adjust your ballast size. If your hull was lead, it would be at least 5 times that of water, a ballast tank would need to be 5 times larger.
That seems like a lot of work, is there another way?
Of course, suspend the submarine with fishing line and attach it to a fish scale (it will need to be accurate). Submerge the submarine (you may need to add weight to get it submerged) and record the weight. Lift the sub to waterline and record that weight. The difference is the mass of water needed to lift your sub. Again, 1 gram of weight will equal 1 cc of volume.
Not happy with that way? How about this…..
float the sub at the waterline with some stability weight and foam, then add weight at the COG / mid point for the ballast tank. The amount of weight needed to submerge the sub to just the point of the sail to go under water or close to neutral balance is the weight you convert to the ballast needed to support surface trim. 1 gram of weight = 1 cc of volume
What more choices?
I simply weigh all the parts above waterline and use that to figure a ballast tank size. It is not the most accurate, it can result in a tank too big. It is better to have extra than not enough.
More?
Find someone that has built a successful working sub made of the same materials you have and ask what size they have used.
What if it is 3D printed?
In some programs, maybe all, it can calculate the volume of your sub. If it is split at the waterline, then it can tell you the volume that will be used to calculate the volume of the ballast tank.
For the suggestions offered, add 10 to 20% to your size. It is easy to add weight if it is too large, but harder to get to waterline if it is too small a ballast tank.
Finally trial and error. Your expertise will develop a knack to guesstimate. David Merriman has built so many subs that he can pretty much eyeball the size needed.
I have not used each one of these, but others I know, that build great subs, have. You will need to try one that works for you.
If you have a better way, please share!
Estimate Ballast Tank Size
A common question is “How to figure the size of a ballast tank?”
There are several ways to estimate a ballast tank size. I will list several ways, but you will need to try them and see what works for you.
Archimedes is said to have gotten into a bath and noticed the water rise. This discovery led to what is known as the Archimedes Principle. Out of that came ways to figure buoyancy and to accurately measure the volume of objects.
So, you created a new sub hull, how big does the ballast tank need to be? You will only be concerned for what is above the waterline.
Fill a bucket to the brim with water, dunk the parts that will be above the waterline, capture all the water that comes out, that water is the volume you need. You got to love metric measurements, 1 gram of water is equal to 1 cc of volume. Weight the water, then figure out the volume (I.e. That seems like a lot of work and you are not done. We have the volume, but not material the hull is made from. Wood is different than lead. Fortunately a plastic model like Revell’s 1/72 Gato or Mobius 1/72 Skipjack has a specific gravity about equal to water meaning you do not need to adjust your ballast size. If your hull was lead, it would be at least 5 times that of water, a ballast tank would need to be 5 times larger.
That seems like a lot of work, is there another way?
Of course, suspend the submarine with fishing line and attach it to a fish scale (it will need to be accurate). Submerge the submarine (you may need to add weight to get it submerged) and record the weight. Lift the sub to waterline and record that weight. The difference is the mass of water needed to lift your sub. Again, 1 gram of weight will equal 1 cc of volume.
Not happy with that way? How about this…..
float the sub at the waterline with some stability weight and foam, then add weight at the COG / mid point for the ballast tank. The amount of weight needed to submerge the sub to just the point of the sail to go under water or close to neutral balance is the weight you convert to the ballast needed to support surface trim. 1 gram of weight = 1 cc of volume
What more choices?
I simply weigh all the parts above waterline and use that to figure a ballast tank size. It is not the most accurate, it can result in a tank too big. It is better to have extra than not enough.
More?
Find someone that has built a successful working sub made of the same materials you have and ask what size they have used.
What if it is 3D printed?
In some programs, maybe all, it can calculate the volume of your sub. If it is split at the waterline, then it can tell you the volume that will be used to calculate the volume of the ballast tank.
For the suggestions offered, add 10 to 20% to your size. It is easy to add weight if it is too large, but harder to get to waterline if it is too small a ballast tank.
Finally trial and error. Your expertise will develop a knack to guesstimate. David Merriman has built so many subs that he can pretty much eyeball the size needed.
I have not used each one of these, but others I know, that build great subs, have. You will need to try one that works for you.
If you have a better way, please share!
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