Essentially, you are correct this is a function of density. To achieve neutral buoyance (i.e.the condition which causing a suspending bait to suspend), the density of the surrounding water (in a volume equal to that of the bait) must be equal to the density of the bait.

Recall (from physics) that:

 

Density = m/V (mass divided by volume)

 

Figuring the mass of of the bait should be easy (just weigh it in grams and convert to kilograms, SI units work much better for these calculations so get water temps in Celsius).

 

Not sure how to accurately measure volume of the bait except to submerge in a graduated cylinder in a fluid of known density, The volume of fluid it displaces (read on the cylinder) is equivalent to the volume of the bait.

 

Not just divide your mass in kg by the volume in Liters and you have the density of the bait.

 

Online, you should be able to find a table of water density versus temperature (most thermodynamics textbooks have them too).

 

Measure the temperature at the depth you would like to fish, find the density of water using the table, then select the exact density bait.

 

A long answer to a short question, but you got my engineer's brain thinking and thought experiments like this are always fun to do.

 

 

....I am a little bit weird, comes with territory of engineer.... 

 

Lol... then this is exactly what I'll be doing for the next week in physics. FYI they must have done something to you in that engineering school because you are starting to sound just like the physics teacher.

That depends on a number of factors - How fast is the water heating up? how much has the 2 degree surface temp increased increased the temperature at 5' in the immediate vicinity surrounding your bait. It beomes a bit of a mess when dealing with transient heating (as the sun does not necessarily heat at an even rate all day). It involves partial differential equations and some magic to get the answer...if you are willing to compensate I am sure I could figure it out for you  ...just kidding.

 

It may be more fun just to set up an experiment to see how much the bait sinks as the water warms, I think some good electronics or a sonar flasher would be able to do the trick. 

And I am still trying to grasp this. Is it only differentiable on a certain interval or time? You lost me.

Lol... then this is exactly what I'll be doing for the next week in physics. FYI they must have done something to you in that engineering school because you are starting to sound just like the physics teacher.

And I am still trying to grasp this. Is it only differentiable on a certain interval or time? You lost me.

 

It is differentiable in both time and direction (mostly just depth, the other directions may be able to be ignored).

 

To really get an accurate answer is just a long process and a lot of things need to be known (unless you have a lot of extra instrumentation on your boat don't know if these variables could easily be known).

 

Basically the last paragraph you quoted is the reason bait companies say that the bait will suspend at 3-5 feet as opposed to saying at exactly 4.5 feet (remember, the type of line you use plays into this as well as it also has density and a specific volume). It's just complicated

It's just complicated

This pretty much sums it up. If you REAAALY wanted to get an idea, you could make some assumptions to use for estimating, but in order to get an accurate answer, you'd have to give yourself one heck of a headache.

Come Spring I'll follow a very slowly sinking jerk bait under my sonar and see if it stops before the bottom... heck, that's really what I was wanting to know, but there's too much ice on the lake now to push my boat out and check.

 

oe

 

(there seems to be more of us Iowegians on this site than I imagined... maybe it's time we dust off the Minniyota jokes!)

That, you could figure out if you knew the density of the lure and the bottom temperature. Water density is highest at 39 degrees celsius (1000 kg/m3), so any bait that was less dense than 39 degree water would act as you say under some circumstances.

There sure do seem to be a lot of Iowans around this time of year. Everybody must be almost overcome with cabin fever by this point.