# Let Me See If I've Got This Right

## 35 posts in this topic

Been doing some reading in the archives and while I haven't found an exact answer to my question I think I've come up with the answer. Or at least partially. Hoping some of the more enlightened folks here at TU can tell me if I'm on the right track and maybe even expand on the answer.

For the purposes of this question let's assume that two lures could be created with exactly the same physical dimensions. Same size, shape, contours, etc. The only difference would be the types of wood they were built from. Let's call them Lure "A" and Lure "B". Lure "A" will be built out of balsa with a specific gravity of 170kg/cu.m and Lure "B" will be built out of maple with a specific gravity of 755kg/cu.m. We then weight each lure to slowly sink at the exact same rate. Lure "A" would have to have much more weight in it to sink at the same rate as Lure "B". Now here is where it gets tricky for me. Given there is much more weight added to Lure "A" than there is in Lure "B" the added weight in Lure "A" would have to alter the center of gravity much more dramatically than the added weight in the heavier wood of Lure "B". Is this the reason, or at least a major part of it, that the lighter woods produce a livelier action than the heavier woods?

Now please keep in mind when answering this question that I don't have degrees in hydraulics, physics, hydro-dynamics or pretty much anything else. The only diplomas I have are in the fields of "What's Taking You So Long" and "What Do You Mean Your On Fire".

thanks guys,

Ben

Edited by RayburnGuy

##### Share on other sites

Been doing some reading in the archives and while I haven't found an exact answer to my question I think I've come up with the answer. Or at least partially. Hoping some of the more enlightened folks here at TU can tell me if I'm on the right track and maybe even expand on the answer.

Now please keep in mind when answering this question that I don't have degrees in hydraulics, physics, hydro-dynamics or pretty much anything else. The only diplomas I have are in the fields of "What's Taking You So Long" and "What Do You Mean Your On Fire".

thanks guys,

Ben

Hi ben

I've got the exact same question like you and trying to find out the answer but i think the experts here care to share some light to me.

Sorry Ben, cant be more helpfull i/m just the amateur

fajar

##### Share on other sites

Ben, I would pose an alternate scenario. Take your lure A and B and ballast them identically with just the amount of ballast needed to make the balsa bait swim properly. Which one will be livelier? The balsa bait. It's much more buoyant and you are only adding enough ballast to help it swim right. The maple bait will be over-ballasted, very sluggish, and will almost surely sink with the same ballast.

I do think the center of gravity is a big factor, as is the total weight of the bait versus its water displacement (in other words, its buoyancy). Probably some other physics gobbledygook that I can't think of at 2 am (if ever) thrown in. Why can't you use woods that are more dense, ballast them more lightly (if at all), and end up with a lively crankbait that works as well as one of those balsa wonders? I think you can get great action with intermediate density hardwoods like cedar, basswood, etc (probably not maple though!). They can swim just as well and thump just as hard (maybe even harder) but you can't make a fast rising buoyant bait from them like you can with balsa, and the fast rise helps you get deflections and navigate the bait through heavy cover without snagging. Sometimes it's enough to know the effect even if you're too stupid to figure out all the causes.

##### Share on other sites

Sounds like you need a map to the fire extinguisher.

##### Share on other sites

Absolutely nearly right Ben, is the simple answer.

Both lures will weigh the same because the bodies are the same size and sink at the same rate. But the balsa lure will have a lot more ballast in it to achieve this. The ballast, if all fitted in one place, will give you a livelier action. I can only refer back to my weighted stick example for a physical explanation:

Example - take a length of dowel. Attach an ounce of lead to each end. Hold the dowel at the centre and twist your wrist back and forth fast. You will feel the inertia each time you change direction. Now move the lead to the centre of the dowel and repeat the experiment. Much easier to change direction, even though the dowel/lead assembly weighs the same.

Technical information:

Centre of gravity (CoG), this is were you went wrong.

This is merely the balance point. In your example, you talk about the CoG being “dramatically altered”. In fact, the CoG may not have altered at all. Go back to the weighted stick example. With the two weights at the ends of the stick, the balance point is at the centre, therefore the CoG is at the centre. When you move both weights to the centre of the stick, the balance point is still at the centre and so the CoG has not changed. It is the distribution of the weight that has changed and it is this that changes the action. It changes the inertia.

Density units.

In your text, you chose to quote the density in Kg/cu.m units (kilograms per cubic metre). The big problem when discussing density, is visibility. If I say that my lure body has a density of 1000Kg/cu.m, it is hard to visualize what it would feel like in your hand. But if I quoted that same density as 1gm/cu.cm and you knew that water had a density of 1gm/cu.cm (grams per cubic centimeter) then you would have a clear picture of the density ‘feel’.

So your maple body has a density of 0.755gm/cu.cm, compare this to 1.000gm/cu.cm of water and you can immediately see that maple is roughly three quarters the density of water and you immediately have a ‘feel’ for the numbers. Balsa has a density of 0.17gm/cm.cu, again it is easy to visualize, the balsa being one sixth the density of water.

Occasionally when reading up on density, you will come across ‘specific gravity’ (sg). This just compares the density of an object with the density of water, without having to quote units like gm/cm.cu or Kg/m.cu. So maple has an sg = 0.755, balsa sg = 0.17, lead has an sg = 11.385, D2T has an sg = 1.2, water has an sg = 1.000, sea water sg = 1.025.

Americans like to quote imperial units, Lb/cu.ft (pounds per cubic foot). Personally I have great difficulty visualizing density using these units and immediately reach for my calculator to convert to sg. Example: maple = 47Lb/cu.ft, balsa = 10.5Lb/cu.ft, lead = 709Lb/cu.ft, water = 62.3Lb/cu.ft. Messy numbers in my opinion, but if you use them all the time I guess you get the ‘feel’ for them.

Dave

##### Share on other sites

Hey Dave,

I see what your talking about in your "weighted stick" example. What I failed to mention was that when weighting the two lures the weight would be placed as low in the bait as possible and that the center of the added weight would be in the same place on both lures horizontally. The only difference being that the added weight would be larger in one lure. The weight in the lighter lure would be longer and maybe taller, but in the same position front to back. If the weights were placed as low in the lure as possible it seems as if this would have to definitely effect the center of gravity. And more so in the lure made of lighter wood.

When I looked up the weights of the wood I did a search for "specific gravity of wood". You are indeed right about them using the different weights of wood as compared to water. Are "density" and "specific gravity" referring to the same thing when used to describe the different flotation characteristics of various species of wood? It seems to me that even though the two lures would weigh the same after weighting in the scenario I described it seems to me they would have to have a very notable difference in their action when compared to each other. Partly because of the difference in weight as a percentage of each individual lure and that the weight would be spread out over a greater part of the lighter lure.

Anyway, that's all for me for now. Will definitely be checking back on this tomorrow.

thanks everyone,

Ben

##### Share on other sites

It seems to me that you have a good grasp of what is happening within the lure. Yes, in the lighter body material, the CoG would be dragged downwards by the very dense ballast lead, more so than the heavy material.

Regarding the use of ‘density’ or ‘specific gravity (sg). Yes, they both are referring to the exact same physical property. Density represents the property as a 'weight' of a given 'volume' and therefore must have units e.g. kg/m.cu or gm/cm.cu or Lb/cu.Ft. Specific gravity represents the property as a comparison to waters property and as such is dimensionless and has no units.

As lure designers, we are dealing with bodies in water, so specific gravity is the simplest to understand and most visual method and you do not have to think about weights and volumes to interpret the numbers.

If there is anything you are still not ‘getting’, don’t feel bad, just tell me and I will try to find another way to explain. But it is already clear to me that you understand the whole concept of buoyancy, centre or gravity and inertia.

Dave

##### Share on other sites

It seems to me that you have a good grasp of what is happening within the lure. Yes, in the lighter body material, the CoG would be dragged downwards by the very dense ballast lead, more so than the heavy material.

Regarding the use of ‘density’ or ‘specific gravity (sg). Yes, they both are referring to the exact same physical property. Density represents the property as a 'weight' of a given 'volume' and therefore must have units e.g. kg/m.cu or gm/cm.cu or Lb/cu.Ft. Specific gravity represents the property as a comparison to waters property and as such is dimensionless and has no units.

As lure designers, we are dealing with bodies in water, so specific gravity is the simplest to understand and most visual method and you do not have to think about weights and volumes to interpret the numbers.

If there is anything you are still not ‘getting’, don’t feel bad, just tell me and I will try to find another way to explain. But it is already clear to me that you understand the whole concept of buoyancy, centre or gravity and inertia.

Dave

You guys have all got to be bald . all them brains has to be pushing your hair out by the roots. Bet I never build a crank bait. I'm so confused.

##### Share on other sites

You guys have all got to be bald . all them brains has to be pushing your hair out by the roots. Bet I never build a crank bait. I'm so confused.

ROFLMAO yes I am shiney polished bald. I found that the hair roots were feeding off my thought power.

You do not have to think about this stuff to make successful baits, instincts and common sense are enough. Personally, I enjoy all this techy stuff, most of which I learned after joining TU. I had to learn it, because no one could answer the questions that I had. A few other members have also followed me down this road and found it interesting and helpful in understanding what is going on. But some of the best lure designers in the world, who are members of TU, don't give two hoots about specific gravity and nor should they.

Dave

##### Share on other sites

I'm not a bald person, but I'm sure I'll become so if I tried to understand all these calculations:

http://en.wikipedia.org/wiki/Specific_gravity

I have used trial and error. Less time consuming.

##### Share on other sites

I'm not a bald person, but I'm sure I'll become so if I tried to understand all these calculations:

http://en.wikipedia....pecific_gravity

I have used trial and error. Less time consuming.

If you tried brewing beer without measuring specific gravity, you would soon lose all your friends.

Dave

Edited by Vodkaman

##### Share on other sites

If you tried brewing beer without measuring specific gravity, you would soon lose all your friends.

Dave

So.....is the COG different for a 10oz. glass of light beer than for a 10oz. glass of stout, or dark ale?

Does this account for the loss of equilibrium after the consumtion of enough beer?

Does each affect the bladder differently?

##### Share on other sites

So.....is the COG different for a 10oz. glass of light beer than for a 10oz. glass of stout, or dark ale?

Does this account for the loss of equilibrium after the consumtion of enough beer?

Does each affect the bladder differently?

I would expect it's all the same formula. One in three out; lol.

www.novalures.com

##### Share on other sites

I would expect it's all the same formula. One in three out; lol.

www.novalures.com

If they gave astronauts going to Mars a quart of beer, and a quart of coffee, they'd never have to worry about water.

Amazing how that works.

##### Share on other sites

Sorry to disappoint you toadfrog, but I've still got all my hair. Although it has been gray for over 20 years. And I'm pretty sure it wasn't the thought process that turned it gray. It might have had something to do with chasing the ladies for a lot of years.

@rofish

I don't understand all the "calculations" either rofish. And I certainly don't have the capability of sitting down with a calculator or a computer and designing a crankbait. A lot of folks here at TU can talk about the "X" and "Y" axis and how different physical properties effect the "roll" and "yaw" of a lure, but I might as well be reading a flight instruction manual for the space shuttle. I'm just trying to make sure I have the basic information as to how some of the more obvious components effect the way a lure works.

@ Dave

Thanks for clearing that up for me. I knew that "density" had to do with a given size versus weight and that "specific gravity" always seemed to be the weight of something as compared to water, but wasn't 100% sure they were essentially the same thing.

And I really like your explanation about the "weighted stick". I read that quite some time ago, but it seems my memory is suffering more and more as each day passes.

I think I'm starting to understand a little better what everyone is talking about when they speak of "weight placement" and how it effects the action of a lure. So let me make sure I've got this right. If a lure is weighted with the least amount of weight possible to make it swim right and as close to it's natural CoG (I keep going back to CoG because not sure what else to call it) it will have the least amount of effect on the lure. By moving the weight up or down you start effecting the vertical axis of the lure and by moving the weight forward or backward you start effecting the horizontal axis. Thus increasing or decreasing the speed, amount and rate at which the lure moves around it's given axis. (isn't this referred to as "cycles")

There was something else I wanted to ask, but my memory is playing tricks on me so I'm sure I'll be back when it starts working again.

thanks again guys,

Ben

Edited by RayburnGuy

##### Share on other sites

I think I'm starting to understand a little better what everyone is talking about when they speak of "weight placement" and how it effects the action of a lure. So let me make sure I've got this right. If a lure is weighted with the least amount of weight possible to make it swim right and as close to it's natural CoG (I keep going back to CoG because not sure what else to call it) it will have the least amount of effect on the lure. By moving the weight up or down you start effecting the vertical axis of the lure and by moving the weight forward or backward you start effecting the horizontal axis. Thus increasing or decreasing the speed, amount and rate at which the lure moves around it's given axis. (isn't this referred to as "cycles")

Yes, BUT, when deciding where to fit the ballast, you have to take into consideration all the other hardware fitted, particularly the rear hook and split ring. A size 4 hook and ring weighs 1.3 grams. Consider a 100mm lure, balanced in the centre, which has 5 grams of ballast. Taking moments (mathematics), considering just the rear hook on its own, the ballast has to move 13mm forward in order to keep the balance point in the same place. Of course, the weight of the lip and belly hook will help counter this, but because of the distance to the tail hook, it has the biggest impact on the balance point or CoG. So generally, the ballast has to go slightly forward of the natural CoG of the body, in order to achieve a level floating attitude.

Sorry took so long to reply, my CoG got severely displaced last night and a few brain cells got burned. Just about recovered now.

Dave

Edited by Vodkaman

##### Share on other sites

Hey Dave,

I really appreciate all the help. And the patience as well. Sometimes I'm not the easiest person to explain something to. I will no doubt be back with more questions as I delve deeper into the actual building of a lure.

And my condolences to your brain cells. May they rest in peace.

Ben

##### Share on other sites

Hey Dave,

I really appreciate all the help. And the patience as well. Sometimes I'm not the easiest person to explain something to. I will no doubt be back with more questions as I delve deeper into the actual building of a lure.

And my condolences to your brain cells. May they rest in peace.

Ben

Sounds like a farewell toast is in order!

##### Share on other sites

Sounds like a farewell toast is in order!

No way! I am staying home tonight and read some technical manuals.

Dave

##### Share on other sites

No way! I am staying home tonight and read some technical manuals.

Dave

Dave, Dave, Dave,

##### Share on other sites

As always Dave/Ben, fascinating stuff, I’m still struggling with a formula where you can weigh a blank and calculate the precise ballast weight to make it neutral when finished, sooooo many variables.

Something I think much neglected, is the vertical rather then horizontal ‘fight’ between buoyancy and ballast. One of the all time greatest lures ‘Rapala Original/CD’ (IMHO), and one which has been accounting for large numbers of Trout around here lately, uses this to get that enticingly subtle action that many makers have copied, these lures exploit the buoyancy of balsa with a slight taper on the sides, relatively little ballast (wire, hooks and rings), and a tiny bib to get that irresistible sway that fish can’t seem to resist. If this lure was made of a heavier timber and ballasted the same it would probably be useless and in all likelihood roll over.

Just some food for thought??

Dave how is the dust collector going??- got mine near to acceptable------

Pete.

##### Share on other sites

No way! I am staying home tonight and read some technical manuals.

Dave

awww Dave,

You've gone and let me down. I never figured you for a quitter.

Ben

##### Share on other sites

Hey Pete,

It's funny that you should post that about Rapala's lures when you did. I just finished an "autopsy" on an old Rapala a few minutes ago and made note of the very things you described in your post. Not sure what they seal their lures with, but it seemed to be a pretty substantial type of plastic shell. White in color and seemingly about as thick as a piece of printer paper. I was surprised at how thin (around .030") and soft the through wire was. I would have thought it would have been much stiffer.

By the way, how have you been and did you have any luck on your last fishing trip?

take care,

Ben

##### Share on other sites

In the example of the balsa, if you think about it, you need a whole bunch of weight to offset all that light wood. You end up with a fight between the weight wanting to control and sink the lure and the very light wood wanting to make the lure float. This wrestling match is what creates the lively lure.

In the case of the heavy wood, you need far less weight to make it sink so there isn't such a struggle between the more dense wood and smaller volume lead needed to make it sink and so you lose some of the action.

Nevertheless, it has been my experience that you can't assume this will always hold true. Find a lure shape and size you like then build the lure out of several woods and see what you like the best. You should also experiment with the placement of the lead, a small change in the location of the lead can make a huge difference in lure action.

You can put all the math you want into lure building and in the end you will still find it necessary to experiment.

Jed

##### Share on other sites

You can put all the math you want into lure building and in the end you will still find it necessary to experiment.

This is so true. Fortunately, this experimenting and prototyping, is my favorite part of lure design. I am just off to my friends pool, to test a handful this afternoon.

Dave

Edited by Vodkaman