21 replies to this topic
Posted 05 May 2005 - 04:50 AM
Browsing the old posts I realized, that some people have difficulties with determining the weight of ballast, that a lure body will take up.
I describe my way.
First of all I must say, that I have access to a digital laboratory balance, but a good kitchen balance (with at least 1 gram resolution) will serve the same.
First I weigh the raw wooden body. Next, I put a jar of water on the balance, and tare (press zero). Then stick a nail into the tail (or nose) of the lure, submerge it under the water, and read the display. The reading is equal to the weight of the expelled water, or with other words, the total buoyancy of the lure body (you know, Archimedes...). I usually do it after sealing, to avoid the wood absorbing the water, and influence the measurement.
What you only need to do then, is to subtract the ?dry? weight of the body and accessories (trebles, split rings, screw eyes, etc) from the total buoyancy, and you will get the weight of ballast (for making a suspending bait). For floating bait, you should obviously decrease, for sinking bait to increase the ballast. Suspending baits may require fine tuning, as glue, lip, lacquer, etc. can slightly modify the theoretical weight of lead to be added.
I hope it will be of use for some people, and apologize to others, who find it trivial.
Posted 05 May 2005 - 03:38 PM
That is a interesting concept...I'll have to give it a try..Nathan
Posted 05 May 2005 - 04:17 PM
makes you wish you hadn't dropped out of college after a year lol
Posted 05 May 2005 - 06:43 PM
Isnt he the one who sprang from the bathtub and ran through the streets naked screaming Eureka! I found it!......? Maybe not, I went to school in the 70's.
Posted 05 May 2005 - 07:42 PM
So you are saying that by displacing the water with the bait then you are increasing the weight of the jar of water?
Posted 06 May 2005 - 01:46 AM
Yes, the same effect, as if you were added surplus water to that level, what it reached after submerging the bait.
You are literally "pressing down" the balance with the same force, that needed to keep the bait underwater. That force is equal to the buoyancy, or the weight of displaced water either.
Practical, isn't it?
Yes, he was that old Greek guy, your memory serves you properly
Posted 06 May 2005 - 02:18 AM
I believe what folk is doing is actually measuring the "force" to sink the lure completely under water which is = the bouyancy force of lure body = weight of the displaced water = weight needed to sink the lure
Let me try and explain this another way.
After putting the jar of water on the scale & tarring (zeroing the scale). If you put something else on the scale it would read the weight of the object (I'm stating the obvious ). Now, if u use your finger to push down on the scale, your get a reading too --- the force that u applied to the scale. So when u push a lure into the jar or water, you are pushing down on the scale like using ur finger to do so. Only this time it is done indirectly thru the force needed to sink the lure under. To be precise: the force used to push the lure under = the buyoncy force of the lure body. So reading the scale actually give u the weight needed to sink the lure. (Hope I'm not confusing this even more )
Another way to do this is to get yourself a measuring cylinder in mililiter graduation. Fill it with water enough to let u push the whole lure in like folk described. After u push the lure completely under, water level will have risen. Take this reading and minus off the original amount of water in the cylinder. This will give u the water displaced by the lure in mililiters. Roughly, 1ml of freshwater = 1 gram. So if the water level rises 12ml, the weight needed to sink(suspend) the lure will be 12grams.
This method actually can let u calculate how much weight to use to suspend a lure in a certain tempreture & can even be calculated tailored to a certain body of water. It's quite long to explain it but anyone interested can PM me, I'll try my best to explain the method, it's quite simple actually (if you didn't sleep during your physics class in school )
Oops folk beat me to it explaining in fewer words, he must be doing it the same time as I did LOL.
Posted 06 May 2005 - 03:26 AM
It's not easy to express myself in English, so I try to use as few words as possible
As for your measuring cylinder suggestion, it theoretically works, but for the necessary precision of reading you'd need a narrow (or at least narrow necked) cylinder, what you couldn'd press the lure in :-)
Posted 06 May 2005 - 02:43 PM
This is an interesting post. Depending on water temp, it will take more or less force even in a small container? If this is true, WOW and thank you for this post. I like a bait with a slow rise and this will eliminate "some" of the trial and error. I will have a true starting place instead of using an educated guess.
Posted 06 May 2005 - 03:27 PM
It's threads like this that make TU such a special site. I'm glad someone here was paying attention while in school! :oops:
Posted 06 May 2005 - 04:28 PM
Sry Tally, I led u down the wrong lane . Water displaced for a given lure body is the same regardless. So by measuring the amount of displaced water, we can then calculate the weight needed for different tempreture & water composition based on "water density will vary with tempreture & dissolved minerals in it".
Here's an online calculator for water tempreture based on tempreture & dissolved minerals in it: http://www.csgnetwor...2odenscalc.html
Posted 06 May 2005 - 10:58 PM
Yes, what Tally says, this method gives you a pretty good starting point. It's true, that the density of water depends on the amount of dissolved materials, and temperature, but the change comes somewhere in the 3-th or 4-th decimal figure, or, with other words, 0,01-0,1%. You may need to fine tune your (suspending) lure to the actual conditions, by attaching a small piece of thin wire to the belly hook hanger. Slight changes in the density of water generally don't make any diference with a slow sinking or rising lure (provided you don't change from freshwater to saltwater fishing with the same gear )
Posted 09 May 2005 - 12:07 PM
This may a silly question but is this not a simple math question
If you know the average density of the wood you are using cannot everything else be calculated from there.
For example the average density for Poplar is rated at .5gms/cm cubed
if the finished lure body without any additions (finished blank only) comes out at 60 gms the sum would be 60 /.5 = 120 this represents the cubic capacity of the bait , if you multiply this number by the density of water (1) then you arrive with 120 if you then take the 120 and subtract the original weight the bait (60) you arrive at 60 this is the maximum weight you should add to the lure. screws eyes bill and even epoxy should be included taken away from the 60gms to calculate the lead to be added
Posted 10 May 2005 - 01:22 AM
You are right, this is another way. Good approach, if the quality of the raw material is homogeneous enough.
If you want to be really fussy with the calculation, you can take into account, that the metal accessories (trebles, rings) also displace water, so their - say -"effective weight" is less, when sinked under the water
Posted 11 May 2005 - 02:38 PM
I tried the water displacement vs dry weight procedure and didn't get the right weight to suspend my crankbait. I weighed the sanded/drilled blank at .14 oz. The water displacement weight using the tare adjustment on my scale was .33 oz. The bill, line tie, hook hangers, trebles and split rings totaled .09 oz. Paint and epoxy clearcoat were .03 oz. I figure the ballast required to suspend as: .33 - .14 - .09 - .03 = .07 oz ballast. The finished bait weighed .33 oz as expected but it was a medium fast riser, not a suspender. Where did I go wrong?
Two possibilities I can think of:
1) My ballast is rosin core lead electrical solder. Could the rosin evaporate during installation and reduce the ballast weight?
2) Should the line tie and hook hangers NOT be counted in the formula since they are inside the body on the finished crank? They weighed .04 oz. If not counted, the ballast would have been .11 oz, for a total crank weight of .37 oz - which is right on the money for suspending this type crankbait in my experience.
Archaemedes won't answer emails. Maybe one of you can suggest where I went astray? Thanks!
Posted 11 May 2005 - 03:06 PM
Archemedes is out back by my pool sucking a brewski. He said to continue adding wt until you get the suspension you desire to see how off the method is. :idea: How much did the displacement change after adding the bill and hardware? : You added wt but you also added displacement, too. :!:
Arch said if that doesn't do it, check to see if the scale is calibrated correctly.
Posted 11 May 2005 - 03:17 PM
Bob, first let be the first to tell you, this formula is way over my head. Guessing that we are talking about a crank and not a rogue type lure, a lure that weighs .33 ounces, made of wood is not going to suspend. Looking at what you did, I think you should have added the .14 and .33
This is just and educated guess, but I would say you will wind up around 1/2 ounce to suspend and depending on the wood and size of the lure, it would not surprize me if it needed to be heavier.
I may be thinking all wrong here and one of the guys will give us their thoughts on this.
Posted 11 May 2005 - 09:34 PM
Well, this is a flat shallow bass crank that is 2.5" long and 1/3" wide, with a G-10 "circuitboard" lip. Kiln dried basswood. Similar baits I've built were 1/4" longer and suspended at .38 oz "ready to fish" weight. After reading the displacement weighting posts, thought I'd try it to see how it works. Had I guestimated the ballast required, I'd have gone with .11 oz of lead for a finished weight of .37 oz - which I'm sure would have been within .01 oz of correct to suspend. But I thought "what the heck, better living through science". I still want to use the technique if I can get it to work because I think it will be useful for crankbait designs that I have no experience with. But right now I'm dubious.
Posted 12 May 2005 - 08:07 AM
OK, I had to just add one more thing to the discussion. Since the force used to sink the bait is not reeeaaly equal exactly to the buoyant force that floats it, the only exact way to figure the exact weight for neutral buoyancy is to figure the specific gravity of the bait and then adjust the weight or mass to be equal to the equivalent volume of water. That is to weigh a volume of water and then use the bait to displace the exact volume of water that will just immerse the bait. Then with the weight of the bait known, you can figure the adjustment needed to get the bait at the exact same density as the surrounding water. But as so many before me have stated; water density varies with temperature and elevation. Sooo be as exact as you can with your measurements, include all hardware and expect a little added weight for epoxy. Keep in mind that I have only tested this technique on odd shaped musky gliders with no lip and added the weight after final clearcoat leaving the ugly lead plugs visible. It does however work for developmental designs. I do find it just a starting point and usually go a little heavy and drill it till it suspends. Woods from piece to piece and so do not have consistent densities You still have to fine tune it to get it perfect.