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Archimedes Dunk Test
11 replies to this topic
Posted 25 April 2011 - 01:43 AM
I found this technique here on TU while reading some old posts (something you all should do, some real gems hidden back there). It was originally posted by FOLK, but he hasn’t visited since Dec 2008. I am surprised that the technique has not been mentioned since. Here is a link to the original post: 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.
Posted 25 April 2011 - 04:59 AM
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.
Posted 25 April 2011 - 10:29 AM
Nice work Dave! That's a good simple method that will help guys build better crankbaits.
Posted 25 April 2011 - 05:59 PM
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
Posted 25 April 2011 - 08:15 PM
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.
Posted 26 April 2011 - 01:56 AM
Haha good ole days eh Vodka. I have changed to using an Archimedes flask coupled with a .5ml graduation measuring cylinder.
Posted 26 April 2011 - 02:25 AM
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.
Posted 26 April 2011 - 04:19 AM
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.
Posted 26 April 2011 - 07:47 AM
very good idea. thanks for sharing yet another great idea.
Posted 26 April 2011 - 07:53 AM
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.
Posted 26 April 2011 - 09:57 AM
Would the temperature of the water make any difference for this test?
Posted 26 April 2011 - 11:33 AM
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.