Vodkaman

Archimedes Dunk Test

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I found this technique here on http://www.tackleund...h__1#entry33351

In theory, the technique is used to find the volume of a lure, but in our application, it also tell us what the final weight of the lure needs to be for neutral buoyancy in fresh water. The down side for you guys, is that you have to work in grams, NOT ounces.

The Archimedes dunk test:

1 - Fill a beaker with enough water to completely submerge your lure, without touching sides or bottom.

2 - Place beaker of water on scales.

3 - Set the scales to measure grams and zero.

4 - Hold the nose eye with long nose pliers and submerge in the water, upto the eye. Leave the hooks on for this.

5 - Read off the scale and write the number down.

Because one gram of water has a volume of one centimeter cube, the number written down represents the volume of the lure in centimeter cubes.

The number also represents what the final lure should weigh (in grams) if you are going for neutral buoyancy. If you want a floater, just add less ballast.

This test can be done as early as the first seal of the body, to get an early indication of the final weight of the lure, or right at the end of the build process. If you want to tweak the final lure to get neutral, fit all the hooks etc and test. This will indicate the size of the slug you need to add.

Another application is, if you want to copy a lure and get the same amount of float. Weigh the original (with hooks) and divide by the dunk test weight. This will give you a ratio. When you build your copy, ballast to achieve the same ratio.

I have been using this technique for a few months now and it is quick, very accurate and simple. I recommend that you at least give it a try out.

Dave

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Thanks Dave. You've just made my lure building life a lot easier. I've been hanging different size weights from the hooks to try and determine how much ballast was needed. This will be so much easier and quicker. I've copied your text and printed it out so it can be hung at my work area until this tired, old brain can remember it.

thanks again,

Ben

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Nice work Dave! That's a good simple method that will help guys build better crankbaits.

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Thanks again dave that is about the easiest way to get the guessing out of how to neutra a bait.

I was going to use a Archimedes death ray to dispose of my bad baits but that I cant seem to get working :blink:

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The method also works if you add too much lead and end up with a sinker. The method will indicate how much lead to drill out.

Dave

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Haha good ole days eh Vodka. I have changed to using an Archimedes flask coupled with a .5ml graduation measuring cylinder.

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Haha good ole days eh Vodka. I have changed to using an Archimedes flask coupled with a .5ml graduation measuring cylinder.

LP, when I first started messing about with Archy's ideas, I too tried the water displacement method. I used two jars, connected with clear plastic tubing. Place the lure in one jar, close, then transfer the water and measure off the volume on the tube.

The method was messy, awkward an a little inconsistent. It needed constant re-graduation. For these reasons, I never posted it. You should try this one, it is way more accurate than displacement. The problem that I had with the displacement was the water that got left behind in the first jar or attached to the lure when I removed it, thus requiring constant re-graduation.

I removed lead from a sinking swimbait tail section this morning. After the adjustment, 2mm of the tail sticks out of the water. That is accuracy of 0.1ml.

Dave

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LP, when I first started messing about with Archy's ideas, I too tried the water displacement method. I used two jars, connected with clear plastic tubing. Place the lure in one jar, close, then transfer the water and measure off the volume on the tube.

The method was messy, awkward an a little inconsistent. It needed constant re-graduation. For these reasons, I never posted it. You should try this one, it is way more accurate than displacement. The problem that I had with the displacement was the water that got left behind in the first jar or attached to the lure when I removed it, thus requiring constant re-graduation.

I removed lead from a sinking swimbait tail section this morning. After the adjustment, 2mm of the tail sticks out of the water. That is accuracy of 0.1ml.

Dave

Check out (google) the Archy flask, dip and collect the displaced water in the graduated cylinder for measurement is pretty accurate, no water is stuck anywhere else.

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I typically use the same method as Ben, hanging egg sinkers and split shot from the hook tines until I get the lure to sit or fall like I want it to.

I'm intrigued by your method, Dave, and I'm going to try it next bait I make.

Thanks for posting it.

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Would the temperature of the water make any difference for this test?

Not enough that would register on a 0.1 gram scale. I'll try to give you some numbers without getting techy:

A 28 gram (1 ounce) lure set up for neutral buoyancy at 20C (68F).

If placed in water at 0C it will float and need 0.046 grams (0.0016 ounces) extra ballast to achieve neutral.

If placed in water at 30C it will sink and need 0.072 grams (0.0025 ounces) less ballast to achieve neutral.

Both these results are beyond the capabilities of a 0.1 gram scale. But it is obviously a good idea to test at a temperature close to what you will be using.

If you want actual neutral, be prepared to spend a lot of time with suspend dots or pin head size shavings of lead and temperature will make a difference at this level. But it is my opinion that we only need to get near neutral for a decent effect. The difference between a floater and a sinker is about 1/200th of the weight of the lure, so for a 28 gram (1 ounce) crank, the difference is 0.15 grams (0.0053 ounces), which is in the range of the gram scale.

Dave

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I know this is a really old thread, but i wanted to bump it to the top because i found it extremely useful. 

Vodkaman thanks for taking the time (6+ years ago) to write this up. Being a new hard bait builder this is the best bit of information i have read. 

Edited by rui
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Vman and I have yapped about this some. It's a good way to get and idea of what you need. I have one that I made from a bucket and pvc pipe three or so years ago, Then I made one from a clear plastic fish tank and pvc pipe.

I really don't use the tank for that anymore. I use it to watch the way the lure sinks or floats. I have given up on a true suspending lure. Vman and many other told me about why this is hard to do, this is why.

I was testing some lures two years ago that would rise just a little. The weather got colder, the water got much cooler. I went back and the same lure sunk. The people explained that there are several conditions that can change the suspending lure. So I created most lures to float slowly. Unless someone wants a sinking bait.

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I keep reading about how colder water makes floaters suspend, but, somehow, it runs counter to what my mind tells me.

Cold water is more dense, so the bait displaces less water, and should float.

I always think back to how steel floats on molten lead.

Any help understanding this would be appreciated.

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Another variant on that method (credit Hand Made Fisherman).

make a container with a long spout.

1. fill it with water until the water spills out the spout.

2. dip the lure and capture the over spill in a previously weighed small container

3. determine the weight of the displaced water .

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 As water cools it gets more dense.  A bait that suspends in warm water would float in cold water because it would be less dense compared to the increased density of the cold water.   Conversely, a bait that suspends in cold water will sink in warm water.

I often see statements that suggest exactly the opposite is happening.   But if you read Dave’s last post you’ll see a finite example of how it works.  When you get down to practical building, the sink or float differences are pretty tiny.  I think the logical thing to do is build the bait to suspend in the warmest water you will fish it and use Suspendots or lead tape to add weight to the lure so it will continue to suspend in the coldest water you fish.

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I'm sorry I did have that backwards. I had to go back and read what I posted. Never like adding dots on baits.

Unless trolling in open water, is about it for sinking baits for me. When I'm in structure I want it to bump but then the bait can float above the structure rather than getting hung up. Also to me a bait moving even if it's just floating to the surface is better than setting in place.

JMO,

Dale

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I worked on the water displacement method before developing the 'dunk test', which incidentally I discovered here on TU, in an old thread. I rigged up jars with connecting pipes but no matter how hard I tried, I just could not achieve the accuracy that I required for small lures. The method works well for large objects as the error becomes less significant with size.

I now own a gram scale that has an accuracy of 0.01g. With my old 0.1g scale I could get very close to suspension, but the new scale will nail it every time.

Suspension is a lot more complex than an accurate scale though. You have to take into account the paint and top coat, hooks and eyes. The 'dunk test' can only put you in the right ball park, the rest is up to you.

Dave

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Mark,I've done lots of testing to see what happens in various temperatures with ultra slow sink/float and 'neutral ' density gliders. I don't have the results to hand but can probably find them if interested, thing is , as bob kinda alluded to- the difference in lead ballast between each stage, i.e  float,suspend and sink is tiny . I tank tested using 3 temperatures of water ,lets say very cold,medium and tepid . I also chilled and heated the lure ,very cold, medium and warmish . The lure reacted differently with each combination, this showed me that if you manage to achieve neutral suspended density it will only do so under the exact temperature conditions when it was tested. The chances of encountering the precise temperature of lure and water again are practically zero.  At one point I managed to truly suspend a big glider midwater which was pretty weird looking , almost certain the water had temperature layered and lure sunk in top layer till it suspended /floated on bottom layer. When I mixed the water it sunk . In practice a lure ballasted to sink ultra slowly in a test tank can float when fishing ,which for a slow sinking glider is no good , hence the exact figures from my tests are meaningless - a sinking lure has to sink regardless of conditions !  Dale I get what saying about preferring a crank to rise but for many species a lure that stops abruptly and near suspends in a fishes face can be very effective. ..

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Sorry dave, posted at same time. Your right, there are so many variables , using archimedes on an unfinished lure will only give an approximation ,which For some lure types may be enough. ..

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I usually shoot for a slow rising bait, and then use my line choice, either mono or fluoro, to get it to either rise faster or suspend/slow sink.

I now fish the CA Delta, so suspending baits are not effective for me.  I need them to rise on the pause, or they get hung up in the weeds.

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I like to fish in cool weather which is Nov-Dec here in N.C.  And one of my most productive presentations is a jerkbait that sinks slowly by the tail.  It mimics the shad that are cold shocked.  They slowly sink tail first and struggle to get to the surface where the water is warmer, and bass are attracted to this easy meal.

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Bob p, that's a method I use a lot on fry feeding trout and other predators over here, we call it fishing on the drop and I make patterns designed purely to be taken on the slow descent when predators crash into fry shoals and then pick off the stunned victims. 

Edited by gliders

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