Determining Volume of Ballast Tank in Dry Hull Boat.

[redboat219]

How does one determine the volume of the ballast tank needed in a small dry hull boat like the one below?

[bob the builder]

Same as a wet hull boat.

A properly designed dry hull has all the mechanisms and watertight compartment below the surfaced waterline. Therefore, you need a tank large enough to displace enough water to lift the surfaced superstructure out of the water (Your upper hull in most cases). Weigh the upper hull, take that weight and convert it to water volume (this is easy if everyone were to get smart and switch to metric, as one cubic centimeter of water weighs almost exactly 1g. If your upper hull weighs 250g, you need 250mL of displacement to offset it. Factor in between 10 and 15% fudge factor, and you'll be after a tank of around 275 or 300mL.) WHICH, by the way, works out to 275 or 300 cubic centimeters! It's so easy!

Through the wonders and infinite wisdom of the US imperial system, if that same upper hull weighs, say, 9oz, then you need to displace 9oz of water (this ONLY works for water, by the way). Most people use ounces interchangeably for volume and weight (*shakes head sadly*)). 9oz of water works out to about 16 cubic inches (using a multiple of 1.805, because... imperial). Dont' forget to add the fudge factor.

I hope that makes sense!


Bob

[Ralph --- SSBN 598]

Nice description, Bob.
I am going to print this out and glue to the inside of my pond tool box so I know where it is all the time.

[redboat219]

Thanks Bob.

Been getting mixed advices.

Somebody suggested I put the boat in the water and put lead on the deck til the boat dives ( sink?). The weight will tell me the volume of the BT I need.

Another suggested I put the whole boat in a container of water and see how much water it displaces when submerged ( Archimedes Principle), again the volume of water displaced= ballast tank volume.

Only you and another chap suggested getting the volume of the upper hull ( above waterline).

[bob the builder]

The first alternative is saying the same thing, just going about it differently.
The second only works if he is talking about putting the boat in the water, floating at surfaced waterline, then pushing it down and noting volume displaced.

What I outlined is the easiest to accomplish with only a scale needed. The second alternative outlined requires a huge tank and some manner of collecting water around it. Messy, awkward and time-consuming.

When I first started, I, too, was obsessed with mathematical calculations down to the gram. At the end of the day you need to realize that building the boat is as much art as it is science. The best way to get a perfect sizing or trimming is trial and error. Start with a tank that is 25% larger than you think you need. Test it. Knock down the size a bit if needed.

Bob