The Math of Buoyancy

[WoodyCT]

If the density exceeds that of the fluid it is emerged in then it will sink because it has negative buoyancy.

And vice versa.

Try this. I do it with gliders to get them to stay level in the water.

Shape the blank, through drill and drill hook swivel hole, and seal. Temporary wire it with no tail weight.
Put on the hooks and hardware you will use on it.
Tape on various tail weight/belly weight combos and float/sink test until You find the result you want.
Remove the weights and tape, find the mass of them, and then finish the plug after everything dries.

Then, once you get a feel for it you can do them from memory, or your records.

I just did this with a needle body turned backwards. It is like a super skinny spook/needle hybrid that just barely floats.
Posted from my iPhone/Mobile device

[Eric Roach]

Thanks for the pratical advice, Woody -- I appreciate it.

A friend aked me if I could build a needle that just barely sank, but sank as level as possible...I was wondering how I was going to figure the weight.

Do you have a picture of the needle you are describing?

[BigFish]

Even with all the calculation mumbo jumbo each plugs wood density will vary. Still you will need to make several prototypes to come close to achieving the desired sink rate or lack there of. It all comes down to prototypes and testing more than calculations of water displacement and blah, blah, blah. Any current, moving water, incoming tides and outgoing tides will affect such a plugs sink ratio so it will be a tough one to nail.