The Math of Buoyancy

[Eric Roach]

I made a needle recently from a new piece of cherry and added the usual amount of lead to the tail and belly.

I know wood density between pieces can vary, but I was surprised to see the total weight of the finished plug was almost 1/2 oz less than usual. These needles are designed to sink, but this one floated.

Crisis? No. It behaves very similar to what I expect and it took some reluctant fish this morning.

This did make me think about the math of buoyancy...Suppose you have three plug blanks from the same type of wood in exactly equal shape. All have the same volume, but weigh differently because of the differences in density. You know from experience that the plug has the sink rate you like when the finished lure weighs 3.5 oz. If you adjusted the lead weight for each plug blank so the total weight for each equaled 3.5 oz, would you expect the sink rates to be the same?

Thanks for any feedback.

Eric

[numbskull]

Lets try this again. Yes and No.

The volume of the plugs, once submerged, will displace the same amount of water. It is the difference between the weight of this displaced water and the weight of the plugs that is the main determinant of how fast they sink. In general if they weigh the same and the distribution of the weight is similar they will sink at the same rate.

There is another factor, however, which is water resistance as they go down. A 3 oz plug with all its weight in the tail will go down tail first with little water resistance, one with its weight split 50/50 will sink level and present a much greater surface area to slow its descent.

Be aware, also, that another major factor with a needle that determines where it swims is its balance point. The more tail weighted the needle the more it will want to plane to the top. Counter-intuitive until you realize the body of the needle acts as a planing surface during the forward retrieve. Level weighted needles can be retrieved more slowly, but once they get down they are hard to get back up.

[Swimmer]

Geez, I had the displacement computation from an ocean science class. It took up the whole blackboard. Threw it away some time ago I think. If I find it Eric I could really get you going.

[Saltheart]

Pure water (lets neglect salt water , fresh water etc and just say pure water)..When 1 cc of water is displaced , it will push back on the object dispacing it with 1 gm of boyant force. So if the objects density is more than 1gm/cc , the object will sink. If its less than 1gm/cc , the object floats.

So as long as all the lures are the exact same shape , they all have the exact same volume. If you adjust the total weight of the lure to the same weight for all by using lead to compensate for the density differences of the wood (without changing the outside envelope shape (volume) , the lures will all sink at the same rate.

Now yes water resistance plays a role and technically we should be discussing masses and acceleration do to gravity in terms of F=MA instead of weights but all that washes out to the bottom line that if an objects overall specific gravity is greater than 1 then it will sink. (Specific gravity is the density of the object divided by the density of the water..{SG=(D of Object)/(D of water)}

[Pete F.]

And then you can start on fluid dynamics, the other art of the equation.

[BigFish]

Good luck with all that??? Looking to re-invent the wheel?? Make em'....best you can....and fish em'!

[Eric Roach]

Originally Posted by BigFish

...Make em' best you can...

Tryin' -- that's why I posted the question.

Originally Posted by Saltheart

If you adjust the total weight of the lure to the same weight for all by using lead to compensate for the density differences of the wood...the lures will all sink at the same rate.

Thanks for the helpful explanation, this will save me some time.

Originally Posted by numbskull

...water resistance as they go down...The more tail weighted the needle the more it will want to plane to the top...

Thanks, your explanation of this a couple years ago really helped me build waking needles.

In mine, the tail lead and belly lead weigh the same, the reverse-pencil shape ensures it sinks tail-first. The tail is thick enough to plane it to the top, and the belly weight placement ahead of the shoulder tips the lure forward when it breaks the surface so it doesn't come in at too steep an angle.

[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.

[Eric Roach]

H'Islander has/had a needle that sank slowly and level. I think it was AYC with an equal slug of weight in the nose and tail. It didn't cast well, but it came in primarily level & just sub-surface on a crawl. My buddy likes these in shallow water when he wants to get under the chop.

I agree that prototypes are necessary, but if I can find the buoyancy "tipping point" for a shape, I can adjust the lead to get the total weight where I would expect a certain sink rate to occur...The whole idea is to compensate for differing densities between plug blanks.

Will it be perfect? Probably not, but I believe it will be closer than just putting the same amount of lead into each one and expecting them to have the same sink rate --something that's burned me on occasion.
Posted from my iPhone/Mobile device

[Pete F.]

Hint: F--- G---- Needle is one way to get around the comeuppance of needles. Look carefully at one.

[Eric Roach]

Originally Posted by Pete F.

Hint: F--- G---- Needle is one way to get around the comeuppance of needles. Look carefully at one.

Wow, that's a nice design. I'm going to try and get a hold of one for a good look.

[WoodyCT]

Forget all the engineering Eric. ;-)

Try it my way- it works and is easier than messing with a slide rule.

Level slow sinker? Could be done several ways, but You run into issues if you go with too buoyant a wood.

The more buoyant the wood, the more lead you have to put in, thus the heavier the lure. You also need someplace for that lead to go, so you will have a thicker body. There is no place to hide a ton of lead in a skinny lure.

I would start with maple since it will need minimal lead to sink it. Casting distance could be an issue since you won't have a huge tail slug to drag the plug out to the horizon, but I don't see this as a problem most of the time.

Again, drop the engineering, and go with trial and error. ;-)
Posted from my iPhone/Mobile device

[Eric Roach]

I'm going to try this with an existing needle made from cherry. It wakes now because the tail sinks first. I'll experiment by adding weight to the belly and decreasing it in the tail using the method you recommended -- see if I can lighten it overall so it sinks slower.

Yep, it already wobbles on the cast, this won't help, but if I can get it to glide (even walk) while being close to neutrel with this "fish shaped" needle, I'll be happy with it.

Thanks for the feedback.

[WoodyCT]

If you want it to walk put the forward lead as close to the nose as you can.

This will help it sink level too.
Posted from my iPhone/Mobile device

[Eric Roach]

Thank you again, Woody. When I get some time I'll try these suggestions.

[Canalman]

I have a whole system of formulas that I use that are specific to my needle and both darters to combat this problem... it took some hard learning to get it right -- and I SUCK at math -- but, unfortunately, it's pretty important when you're making temperamental plug styles...

All that and I still can't answer your question!