My 1/35th, 66" long Disney Nautilus

[Subculture]

The Engel nautilus, which is about the same size, only needs 1000ml of ballast, so one of those spheres should have been enough, unless the lay-up is very thick. The Disney nautilus has low freeboard, so doesn't need big tanks.

The empirical method of trimming your boat to neutral buoyancy or just slightly negative, then using foam blocks to get it to the required waterline is the most foolproof method of deciding tank volume.

Using weight, tends to give you larger tanks than necessary unless the hull is styrene or some other unfilled plastic, plus it's difficult to measure as a hull rarely splits dead on the waterline.

[SubDude]

Agreed. I could have used smaller spheres now that I know what I know but fortunately there is quite a bit of space in the Nautilus which makes it more forgiving of my foibles. Without an accurate method to calculate it I guessed on the side of caution. Since this is boat #2 for me and my first using the "Modular Sub System" I know better what to do with boat #3. I learn something new with every progression. I would like to better understand though how to calculate the size of the tanks so that it is not just a SWAG. Is there a general method for determining such?

[Subculture]

I roughly outlined it in my last post here. For more details I'll quote Skip Asay's original recommendation on hosw to do this, hopefully it makes sense to you. I find it very straightforward, it's totally accurate and removes all guesswork.

How to calculate your ballast tank volume


Build your boat to the extent that everything is done except the ballast tank itself. Put it in your test
tank and you'll almost certainly discover that it's light. Add whatever weight is necessary wherever
necessary to get it to JUST submerge - not sink like a rock - and sit as level as possible. Something
like just enough weight to have it float with just the top of the conning tower out of the water and
then add just enough to get it to completely submerge. At that point, a couple of quarters would
probably suffice.
Now add square or rectangular polystyrene foam blocks under the hull until the boat sits at its
proper waterline. Since it's a round bottomed boat, you'll have to start by building a cradle (use
foam) so the foam blocks won't just roll out from under. Make that cradle and the foam blocks you
put under it approximately 4" - 6" long. When the boat is at its proper waterline, determine the
cubic volume of all the foam (including the cradle) by multiplying length X width X thickness.
Then convert that to the volume of a cylinder (3.14 X radius squared X length).
Another benefit to this method is that by sliding the foam fore and aft, you can determine the
EXACT location for the tank.
Yes, when all is said and done, you'll have to remove weight equal to the weight of the ballast tank
itself but that's a small price to pay. Plan ahead when you're adding the weight to begin with - don't
make it all permanent.

Skip Asay

[SubDude]

I roughly outlined it in my last post here. For more details I'll quote Skip Asay's original recommendation on hosw to do this, hopefully it makes sense to you. I find it very straightforward, it's totally accurate and removes all guesswork.

How to calculate your ballast tank volume


Build your boat to the extent that everything is done except the ballast tank itself. Put it in your test
tank and you'll almost certainly discover that it's light. Add whatever weight is necessary wherever
necessary to get it to JUST submerge - not sink like a rock - and sit as level as possible. Something
like just enough weight to have it float with just the top of the conning tower out of the water and
then add just enough to get it to completely submerge. At that point, a couple of quarters would
probably suffice.
Now add square or rectangular polystyrene foam blocks under the hull until the boat sits at its
proper waterline. Since it's a round bottomed boat, you'll have to start by building a cradle (use
foam) so the foam blocks won't just roll out from under. Make that cradle and the foam blocks you
put under it approximately 4" - 6" long. When the boat is at its proper waterline, determine the
cubic volume of all the foam (including the cradle) by multiplying length X width X thickness.
Then convert that to the volume of a cylinder (3.14 X radius squared X length).
Another benefit to this method is that by sliding the foam fore and aft, you can determine the
EXACT location for the tank.
Yes, when all is said and done, you'll have to remove weight equal to the weight of the ballast tank
itself but that's a small price to pay. Plan ahead when you're adding the weight to begin with - don't
make it all permanent.

Skip Asay

Makes sense except the first sentence has me puzzled. If utilizing a sub-driver that is integrated with the ballast tank how do you build it to the extent that everything is done except for the ballast tank? What about the dry compartments?

[SubDude]

The MSD's ballast tank section comes to you over-size. You cut it down to get the floodable volume to the desired amount -- that's the reason Bob had me make them that way, so you can custom size the three Lexan cylinders to suit you're specific need.

Everything below the waterline is compensated for with lead and foam during the initial submerged trimming.

David

Gotcha! I guess I was thinking in terms of a cylinder that wasn't modular like the ones you now make.

[Subculture]

If your tank is too large, you have two choices. Make it smaller or add foam above the waterline, which helps improve stability by raising the centre of buoyancy.