# Achieving neutral buoyancy

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### #21 carpholeo

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Posted 16 June 2008 - 04:07 PM

Then the material you remove to make room for the weight is not enough to worry about?

### #22 jim45498

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Posted 16 June 2008 - 06:04 PM

Then the material you remove to make room for the weight is not enough to worry about?

The material removed in hardly anything in weight. At least nothing to worry about. It works for me.(My crankbaits are for Bass and may be small compared to some on here.)

Edited by jim45498, 16 June 2008 - 06:09 PM.

### #23 Vodkaman

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Posted 16 June 2008 - 11:38 PM

Jim. It is not just about the weight of the balsa that you drilled out, which is very small. I'll try to explain. Don't worry about the sums, it's the answers that tell the story, but I cannot think of a simpler way to explain about external and internal ballasting.

Consider a balsa body 80mm in length.

By suspending in a jug of water, it is found that 19.4gm of external lead was required to achieve suspension.

The body weighs 3.0gm.

The density of lead is 11.389gm/cm³.

From the above known information, it is now possible to calculate the volume of the body:

Volume of lead = weight / density = 19.4 / 11.389 = 1.7034cm³.

Total weight of body and external lead = 3.0 + 19.4 = 22.4gm.

Because this weight resulted in suspension, then the weight of water displaced by this combination of body and external lead (22.4gm).

1gm of water has a volume of 1cm³.

Therefore, the body + external lead has a volume of 22.4cm³.

Therefore the volume of the body alone = (volume of body + external lead) – (volume of external lead)

Volume of body = 22.4 – 1.7034 = 20.6966 = rounding off to 20.7cm³.

This means that the volume of the body and internal lead has a volume of 20.7cm³ and when suspended will weigh 20.7gm.

But by fitting the external lead to the inside of the lure, the ballasted body now weighs 19.4 + 3.0 = 22.4gm. This is 1.7gm too heavy.

This has not taken into consideration that you have drilled out a cavity for the lead and in doing so, removed buoyant material. This makes the situation slightly worse, as there is less balsa to support the lead.

In addition to this problem, we then slap a coat of D2T over it all, which has a density of 1.17gm/cm³, which is heavier than water.

If this method works for you, I have to assume that you are doing something extra or different, making an allowance somewhere along the line, or I am making a huge error in my thinking somewhere. No one can argue if you say it works for you, but these figures are pretty hard to argue against too.

The purpose of attaining neutral buoyancy is not to demonstrate to the fish how much effort we have put into catching him, but to stop the lure bobbing to the surface or sinking like a stone, when we pause the retrieve. If the lure rises or sinks slowly, surely we have achieved our goal.

The tolerance required for close to true suspension is approximately 1/100 of the weight of the lure. A lure weighing 20gm needs to be built to 0.2gm accuracy. With some planning, this is just about possible. To control suspension to a depth is probably ten times tighter, 1/1000 of the weight of the lure. This is just not possible without specialist equipment, in any case, you would be at the mercy of temperature fluctuations and water purity.

If the tolerance is widened still further to say ±0.5gm. the result would be a very slow rise or fall, which is surely acceptable for the purpose.

I don’t do these calculations every time I build a lure, I only do them once, in a spread sheet. Next time I just do the external lead suspension thingy in a jug of water, weigh the body and lead and type the weights into the spread sheet, which tells me how much lead and how deep to drill my hole. This gets me well within a tolerance of ±0.5gm and works for me.

As a post script. With the information gathered above, it is now possible to calculate the volume of the body alone, by subtracting the volume of external lead from the volume of body + external lead. Simple, no jars or pitchers AND much more accurate.

Edited by Vodkaman, 17 June 2008 - 12:05 AM.

### #24 mark poulson

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Posted 17 June 2008 - 08:09 AM

I would use cold water when trying to achieve suspension. For me, it's in winter/cold water, with suspended jerk bait fishing, that a lure that is actually neutrally buoyant is much more critical. Cold water is more dense, and any lure that suspends in cold water will slowly sink in warmer water. Generally that's not a problem for me, since I use a much more active jerk retrieve in warmer water, as the fish are much more active due to their increased body temperature.
A lure that suspends in warm water will slowly rise in colder water.
I really wouldn't sweat a small amount of rise or sink, though. Unless I'm fishing over flooded brush, where the lure can get snagged, a lure that sinks or rises slowly is still effective.
I guess we'll have to modify that old saying to read, "Close only counts in horseshoes, handgrenades, and suspending baits".

### #25 clemmy

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Posted 17 June 2008 - 08:29 AM

I may have missed someone mentioning this, but couldn't you simply take the mass of lead of given volume, subtract the mass of same volume of applicable wood density to determine effectiveweight of ballast? Ie lead at 11340 kg/m^3 vs say, 14 lbs balsa at 224 kg/m^3?

### #26 mark poulson

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Posted 17 June 2008 - 03:19 PM

Clemmy/Vodkaman,
I call foul! No fair talking in engineer talk. My computer doesn't have an automatic translate feature, and neither does my brain.
Are you saying you can take the volume of the lure, calculate weight of the water it would displace, figure out the weight of the lure minus the ballast holes, calculate how much lead (along with an allowance for the finish) is needed to reach the weight of the displaced water, to achieve neutral buoyancy, and go from there?

### #27 jim45498

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Posted 17 June 2008 - 05:03 PM

Vodkaman, I read and re-read that explanation. I still don't know what I read. But, I have come to a conclusion. Your water in Liverpool is different from my water in Kentucky. I think your test tank has some Vodka mixed in the water. :lolhuh:jk

### #28 mark poulson

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Posted 17 June 2008 - 06:07 PM

### #29 Vodkaman

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Posted 17 June 2008 - 06:29 PM

I will try really hard to come up with a simple explanation. You should know by now that explaining is not my strong point.

Mark, correct.

### #30 Shortlite

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Posted 17 June 2008 - 06:57 PM

VodkaMan, nothing immmersed in water will displace the same WEIGHT of water. It will displace the same VOLUME of water (Archimedes' principle: Eureka!!), so your calculations are a bit off, when you say that 22.4 g of lure displaces 22.4 g of water. Incorrect. You have to measure the volume of water displaced (using change in depth of water in the jug), to find the volume of the lure and external weight. Also, because the lure has a volume of X cubic cm, it will not weigh X g unless it it composed of water. Come on!! And volume is determined by dimensions: the lure has the same volume after you put the lead into it, it just now weighs more due to different density. It is a composite of two materials, in the same physical space. Volume of lure is unchanged.

A very buoyant lure (ie. floating) displaces less water because of its buoyancy. A sinking lure displaces more because it enters the water, and immerses its full body volume. So the principle does not apply perfectly to all lures. A suspender that floats very slowly is still a buoyant object, so your volumetric calculations have to be take with regard to the amount of body IMMERSED in the water. This is the only part of the lure that can displace water. Buoyancy counteracts weight, so only physical displacement can change water level.

The second part of reasoning with regards to internal vs external, removal of material, and changes in buoyancy due to this and paint and topcoating holds water (pardon the pun).

### #31 mark poulson

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Posted 17 June 2008 - 07:22 PM

Grrr... Just what we need, another guy who actually knows what he's talking about!
Welcome into the pool Shortlite.
Maybe you can explain how obnoxious people, like exwives, keep coming back up no matter how much you weight them under. There must be a formula for that.

### #32 Shortlite

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Posted 17 June 2008 - 07:33 PM

Hahaha, thx for the welcome. Now, not advocatingviolence or anything illegal, but the best thing to do is to beat all the air out of 'em (reduces buoyancy) and add lots of external concrete ballast (preferably encased). No formula, just overkill!

Didn't mean to ruffle feathers going into buoyancy and stuff. Maybe them 1/8 oz balsa lures weighted with 1/2 oz of lead really do float/suspend in vodka? Seriously, doing an accurate calculation is tricky. Getting very close is tricky. So just stick the damn thing in water, add the external ballast, and try and get it to suspend. When it comes to actually drilling out and putting the lead in, take away a little of the lead. Also, test with all the hardware fitted to the lure.

I through wire my carved wooden plugs, and drill out the hole to accept the lead, then seal and test. After that, I reckon the epoxy to seal in the lead, paint and topcoat will add more weight too, so I go for enough lead to make the lure float slowly. That's for a suspender. If I still want it to float, then I reduce the lead I put in some more.

Edited by Shortlite, 17 June 2008 - 07:40 PM.

### #33 mark poulson

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Posted 17 June 2008 - 07:56 PM

Works for me. Both topics.

### #34 Vodkaman

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Posted 18 June 2008 - 12:14 AM

Shortlite. Welcome to TU. No feathers ruffled here. I agree with all that you say, it's all about the volume. I just didn't manage to get it down on paper very well.

I have written another explanation, but the pictures and/or the PDF's are too big and I don't have the tools to fix this problem, as I am in work at the moment. I will re-post this evening.

### #35 LaPala

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Posted 18 June 2008 - 02:04 AM

just stick the damn thing in water

Welcome Shortlite, that's word of wisdom sorely needed in this discussion Sometimes the best course of action is to just do it!! We are lure makers and not engineers so we do not need to over analyze things. After all, the end result from all means is to get the damn lure to suspend:yeah:.

### #36 carpholeo

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Posted 18 June 2008 - 10:26 AM

Whats that saying.... Be careful what you ask for, you just might get it.
My brain hurts,maybe I should just stick to floating baits.

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Posted 18 June 2008 - 11:37 AM

Vodkaman,

Any chance you would want to "share" the spreadsheet your are talking about? Or maybe give us a taste of a similar spreadsheet that we could plug in the variables and it could tell what the weight of the ballast should be? I would be more than happy with a lure that would rise or fall just slightly, no need for it to "hover", most fish do move some, even at rest something is moving.

FERG

### #38 Vodkaman

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Posted 18 June 2008 - 12:22 PM

Ferg, no problem, PM me your e-mail.

This vid is based on my earlier post. It's just a demo, not a teach-in. It may answer a few critics.

There again, it may not.

### #39 Shortlite

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Posted 18 June 2008 - 12:35 PM

Hahaha. I was just pointing out that the calculation was flawed. Background in Engineering and Science; instinct.

Just stick the damn thing in the water!!! Add a little lead. Always going to be trial and error, because (especially with wood) no two pieces will ever be exactly the same. Too many variables. Bottom line in what I was saying is that no lure has the same density as water, because it is a different material. Only water has the density of water. What you get in suspension is an equilibrium of forces. Buoyancy negates weight (which is a force, not a mass: nothing weighs 3 g, it has 3 g of mass).

A composite of wood and lead (lead inside a wood body) has a different density than plain wood or plain lead. The calculations for external ballasting is easier than internal ballasting, for this reason. Complex dimensional shape, placement of the lead, etc. Let's just say that simplifiying the density thing, a 8 cm long cylinder of 1 cm diameter has a volume of 6.284 cubic cm. Internal lead weighting doesn't change its volume, just its mass. So 3g of lure + say 5 g of lead has a total mass of 8 g. Density is mass/volume = 8/6.284 = 1.27 g. Greater than density of water. Full stop. If this lure suspends, it is because it's buoyancy is equal to its weight. 8 g of mass weighs 80 Newtons (force). Mass is scalar, Force is vector quantity. Mass just "is", force "acts" on an object or in a direction.

Just stick the damn thing in the water!!!!! Hahahahahahaha. Spending too much time trying to work that crap out = waste of time = less productivity = less lures made = loss of fishing time = less fish caught!

Git 'er done, git 'er wet, catch some fish!!!

### #40 Shortlite

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Posted 18 June 2008 - 01:17 PM

Nice video. Spot on. Care to share the spreadsheet program? I assume it takes into account lure material, density, etc? Sounds and looks like avery useful tool to get you in the ball park, where you then just adjust a little to get what you want. However, I still stand by my statement that you need to put the fittings, spit rings and hooks on before doing the ballast testing: those extra grams will definitely change the attitude of the lure.