carpholeo

Achieving neutral buoyancy

62 posts in this topic

I made Swimbait recently from blank of unknown wood, just by luck it wound up having neutral buoyancy. Its a great bait and I would like to make more of them. However,in the future, I would like to rely on more than just luck.

So when planning A suspending wooden swimbait, what are some of the things I need to do to wind up achieving neutral buoyancy.

Edited by carpholeo

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I use a pitcher of water and put it inside a cooking pot. Make sure the pitcher is full to the top. Put the bait in the pitcher of water and submerge it. Weigh the water that spilled out of the pitcher into the pot.

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yhats a tricky neat way from fish devil. i have experienced big changes in water temps to buoyancy factors.

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Google 'Archimedes Principle' ---------- not 'Screw'. pete

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http://www.tackleunderground.com/forum/hard-baits/13095-how-estimate-weight-needed.html

This link covers a method of calculating volume by weighing a suspended body.

http://www.tackleunderground.com/forum/hard-baits/12275-how-does-clearcoat-affect-bouyancy-lure.html

This link discusses the effect of a D2T top coat.

http://www.tackleunderground.com/forum/soft-plastics/10027-density-materials.html

This link discusses other methods of calculating volume, like the pitchers and pans or jars and tubes, basically as stated above by fish devil.

Both the above mentioned methods of calculating volume work, but the displacement method, using pitchers and pans, has accuracy problems. The smaller diameter the pitcher, the more accurate the result.

For example, if the pitcher was the size of a bucket, you could almost submerge the lure without spilling, under normal meniscus tension. If the bucket was so full as to make spilling inevitable (forced meniscus tension), once the meniscus breaks and the water flows, it will not settle until it reaches its normal tension, thus giving a false high value of the weight.

The following thoughts are not meant to put anyone off the idea of calculating ballast, in fact that is the direction I am heading myself. But it is meant to highlight the issues that effect the results. It is far more involved than the puzzle seems initially.

Knowing the volume of your lure immediately gives you a figure to work with. It tells you how much the lure must weigh to be a suspender, provided you work in metric units. If you work in imperial units, you will get bogged down with incompatible units.

Example metric. If a lure has a volume of 23 cm³ or 23 ml, its suspended weight will be 23 grams.

Example imperial. If a lure has a volume of 1 inch cube or 0.554 US fluid ozs, its suspended weight will be 0.3197 ozs.

It took me about 30 minutes, a couple of web searches, two large vodkas and a lot of number crunching to arrive at the US numbers above. Inconvenient to say the least. Conversely, using metric units, you do not need a calculator to convert volume to weight.

But none of the above, or your question tackle the real problem. That is, you want to fit the ballast before the top coat. But D2T is heavier than water and will sink the lure. D2T has a density of 1.17gm/cm³. This means that D2T is 17% heavier than water.

I fear that things are even more complicated than that. Think of it another way. Consider a lure (80mm length) of volume 23 cm³ with hardware and without top coat, requiring 1 gram of D2T.

1 gram of D2T has a volume of (1 / 1.17) 0.855 cm³.

Adding the D2T increases the lure volume by 0.855 to 23.855 cm³ and also the suspended weight to 23.855 grams.

The weight of the lure has increased by 1 gram to 24 grams.

Therefore, the D2T has added (24.0 – 23.855) 0.145 grams and the lure sinks.

But it gets worse! You drill the hole and fit the correct ballast and weigh the lure, just to check. The weight is wrong. The fact is that when you drilled the ballast hole, you removed some weight of body material. The result is light by this weight. Small but significant. My experience is that the range of tolerance for a suspender is approximately 1/100 of the weight of the lure, so for a 20 gram lure, accuracy of less than 0.2 grams is required.

I will be putting all of these thoughts to a practical test soon, with the intention of producing a comprehensive spread sheet that will greatly simplify weighting suspenders.

All this theory and calculation is well and good, but there is nothing wrong with good old trial and error. Hands on experience with suspenders can be quite pleasurable.

I believe weighted stick-on dots can be purchased for the purpose of fine tuning suspenders. Another solution to fine tuning: Approximately 20mm of Φ1.05mm solder weighs 0.1 grams. So if you can get your finished lure very close to suspension, then by gluing the solder to the rear belly of the lure, the solder can be snipped to fine tune the suspension. The trailing solder also adds a little realism to the lure!

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Water displacement isn't the best method for accurately measuring buoyancy.

Example: any piece of matter, lets say, 2 boxes, both measuring 1 cubic inch.

one filled with air, one filled with lead.

Both will displace the same amount of water, but the buoyancy of each is obviously much different.

What needs to be measured is the amount of force needed to hold the lure submerged.

I think Lapala touched on a technique a few years ago, but not 100% sure.

I would think you could use a string to pull the bait under & measure the force using a postal scale or similar, but to do that you need the string to re-emerge out of the surface, which could be done using a pulley mounted in the bottom of the container.

then again I'm not a physicist, so I don't know if using a pulley will lighten the load, I know using a series of pulleys does indeed lighten the force needed.

And as woodieb8, skeeter & others have noted, water temperature will affect buoyancy to a degree, how much would be another great experiment.

Good topic to get the minds gears crankin.

You drill the hole and fit the correct ballast and weigh the lure, just to check. The weight is wrong. The fact is that when you drilled the ballast hole, you removed some weight of body material. The result is light by this weight. Small but significant. My experience is that the range of tolerance for a suspender is approximately 1/100 of the weight of the lure, so for a 20 gram lure, accuracy of less than 0.2 grams is required.[/color']

An easy fix would be to:

  1. Attain the additional weight needed.
  2. Drill cavity for necessary added weight.
  3. Weigh the bait after drilling cavity
  4. Subtract that weight from the total needed weight to achieve = buoyancy.
  5. Add that sum back into the lure.

Agreed, epoxy topcoats, hardware, hooks, etc. should be included.

Paint IMO would be negligible.

Still, the biggest gorilla in the room is the fact that water temperature affects buoyancy to some yet unknown degree AND line type/weight will be a factor, and water pressure may be factor also.

All 3 may be minuscule, but might be a consideration to look at.

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Jerry, everything you state is correct of course. The best we can do is get it as close as possible and fine tune at the waters edge. Every day the same body of water will vary due to temperature.

Another thought, if you use a quick release for attaching the line, this too should be included in the lures hardware.

Regarding the buoyancy fix. True, the calculation is an iterative one. The answer has to be fed back into the question a few times to hone in to the real answer. Spreadsheets are very good for this application.

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Companies, make & market neutral buoyancy all the time & for most intents & purposes they are close.

Like you, I'm just trying to get all the fundamentals out there for brain candy & I'd love to see a spreadsheet to help alleviate the trial & error.

I think one (a formula/spreadsheet) could be produced to get most baits very close & acceptable to remain in the preferred strike zone.

Water displacement can be used as a primer for determining needed weight, I just didn't want people getting perturbed if they used it as a single method for determining weight.

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Okay guys, take 10 minutes for a potty break and an alcohol run and we'll take up how drag impacts wiggle and wobble of a bait - Just kidding.

A great thread but it requires to much brain power for a Saturday morning when the grass needs to be mowed...

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:lol: @ Spike......... and Pete........ just "got" your post :teef:

I forgot the simple fact that objects don't weigh the same above & below the surface.

Some good reading:

Pressure

Edited by redg8r

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The most direct method I know to get a suspending wooden bait is a float test. Completely finish the bait but without any ballast. Attach the treble hooks. Float the bait in water that is the same temp you will fish in. Hang lead on the trebles until you get a slow rise, then use the lead for the ballast and patch the holes. If you fish it in water that's warmer than the test water, it will sink. In water colder, it will rise. It's best to build for a very slow rise which can be corrected by adding some Suspend Strips or lead wire on the trebles. The next bait you build in the same pattern will be close if you use the same batch of wood and the same build details.

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Boats float because they weigh less than the water they displace. Steel or wood, it's the same idea.

Probably a good way to measure how much weight of water your lure displaces is to float it in a graduated glass jar, and mark the water level when it's floating. Then push it down until it's just barely covered by the water and mark that level.

Weigh the amount of water that the level rose, and it will tell you how much your lure needs to weigh to achieve neutral buoyancy and "suspend". I put that in quotes because, in reality, the chances of truly suspending are minute. If you put a suspending jerkbait in a pool, and pull it down to 4', it will eventually fall or rise. And it will act differently at different times of the day, depending on both water temperature and barometric pressure.

And a slowly sinking lure that's retrieved faster will stay up near or at the surface.

Another factor in suspending is line. A lure that sinks with 8lb fluorocarbon line will rise with 20lb monofilament.

And a truly neutrally buoyant lure, for me, is only really important in the winter, when the bass take a while to come up close enough to the lure that they'll hit it out of reaction on the next twitch. Even then, as long as the lure gets down to their level, and then either rises or falls slowly, they'll come up and check it out. They can't help it, they're curious creatures.

If you want a perfectly suspending lure, like David said, take some suspend dots or strips to the lake with you, and fiddle around all day. Not only will you lose fishing time, you run the risk of ruining, or, at best, altering the action of that lure.

So, in conclusion class (I've always wanted to say that :lol:), in practice, a lure that's close is all you can hope for, and all you really ever need.

Edited by mark poulson
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Maybe it would be simpler to make them sinking with an air bladder built in!! (just like the fish do). Some of those lures you guys build would accommodate a 'small' condom. I am just visualizing guys on the bank with thermometers, blowing up their lures for the day (Nitrogen would be better).

Remember the submarines we had when kids, that took a puck of bicarb soda which made CO2 gas, up she came and down she went. A small air bag and a syringe, may be the solution (until the water heated up). A lot of this is in jest, but this is another angle that fish use for neutral buoyancy, and you can not beat mother nature for ingenuity. This, I feel has at the very least, got Dave thinking, (he has probably already tried it). Love this brainstorming stuff, and all these practical solutions offered. pete

Edited by hazmail

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Pete, there was a lure that does the exact opposite. It's floating and u inject water thru a rubber nipple to get it to slow float or suspend. Forgot what brand but it's pretty old. Maybe someone still remembers it.

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Norman Baits made it. It was a Jerkbait style lure and had a syringe in the package. Add water then you would just plug the hole with a stopper.

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How about the worm blower, plastic bottle with a hollow needle to inject air in worm to make it float

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Thanks for all the extremely informative,lengthy and confusing replies to my question. With all the tutorials,theories and excellent advise from this thread, I now feel that I can dive right into my next suspending bait project with the utmost confidence and enthusiasm, and then fail miserably.

But seriously, I will take into account all that I've read here and document my progress as I fumble through it.

A few things I have to my advantage is that I have already made the lure I want to recreate and being A catfish replica, it has lots of accessories attached to it that could be weight adjusted for balance. And the real biggie it that I'll be using it in shallow water that the temp doesn't vary all that much.

Thanks again everyone

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This may do nothing more than cloud the waters even more, but...... I usedto SCUBA dive a lot, and would use my flotation vest to overcome the weight of the belt in order to achieve neutral bouancy. This was great for reef diving or wreck diving. The whole time underwater I had to keep adjusting the air in vest as I would be either too light or too heavy at any given time. In actuallity I was never completely neutrally bouant, just on the very edge a lot of the time, then had to adjust. I do believe that NEUTRAL bouancy is a thing that can be achieved in one place, one time and that is it. Take same perameters to another place/time and you are only close. Lots of variables involved (depth, temp., water movement, etc.). If you can come close in your tests you have done a great job indeed. Just my .02.

David

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Errrr David, I need to butt in here ;), no offense intended about ur diving.

The whole time underwater I had to keep adjusting the air in vest as I would be either too light or too heavy at any given time.
What is happening is you might be diving over-weighted. When over-weighted, you have to put a lot more air into your buoyancy compensator to achieve neutral buoyancy. More air in BC = more change in air volume for every change of depth that needs you to correct the amount of air to achieve neutral buoyancy. And in this case you need to adjust a bigger volume of air too. By trimming the weight you wear to just enough (fine tune with shot weight belt instead of the chunks) you could negate the need to constantly fiddle with adjusting the air in your BC when you stay at a relatively constant depth; because the air needed to trim for neutral buoyancy will be less; less air in BC = less air volume change = less adjustment needed. The other cause would be yo-yo diving where you constantly change depth up or down. So if none of the 2 scenario is involved, you do not really need to adjust your neutral buoyancy at "any given time" it is only needed when depth change is significant enough to affect buoyancy. Buoyancy change is also relatively less and less significant as you go deeper (Boyle's Law) Sorry for the discourse here guys.

Anyway applying the same to lures, we have so little volume of air in a lure (especially a wooden lure) the change in pressure that compresses the air and causes reduced buoyancy is close to negligible at depth. (even less if the lure attains deeper depth. Opposite happens if a lure is floating up, reduction of pressure allows air to expand, buoyancy increase.). As we would be designing a suspending lure to suspend at a fixed depth that the lure is designed to dive to, the air compression buoyancy factor can be safely ignored. Line diameter, line weight does affect how deep the lure will dive to. Temp effect on buoyancy do have to be considered. A suspending lure in winter does not suspend in summer :wink:.

So ideally all suspending lure should come with a note that says, "This lure suspends at X depth, with Y water temp and designed depth of lure is achieved with Z diameter AA type line with an average cast of XX feet." I think that about covers most of the suspending issues :whistle:.

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I put my hardware on the bait, then tie a string to belly hook and add weight to string. I push the crankbait under water by hand (a couple feet deep). When I get the slow rise that I am looking for, I put this weight in the belly.

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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

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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

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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". :yay:

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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?

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