
Need help with density of wood.
Started by
Palmetto Balsa
, Jan 06 2008 02:23 AM
35 replies to this topic
#1Posted 06 January 2008  02:23 AM
I have a piece of 2"x4" Balsa Wood, 36". I want to know the weight/board ft. so I can know density of the board I have.
Does anyone know how to do the calculation? The actual size of the wood is 2" x 3 15/16" x 36" and the total weight of the board is 22.5 oz. (1.40625 lbs.). One cut from the board is 2" x 3 15/16" x 9 and weighs an exact 6 oz. This might be a better size to calculate with, but I don't have a clue. If the calculation is easy to explain, I would like to learn it. If you know how to calculate it but the formula is a pain than I would be satisfied with just the answer. #2Posted 06 January 2008  03:09 AM
As it happens, I have just been working on a future thread, of which this is a part of.
To calculate the density of an unknown sample of wood: Chop off a sample piece. Size or shape is not important. However, the larger the sample, the more accurate the data. Attach pure lead to achieve neutral buoyancy (just below the surface). 1. Accurately weigh the sample+lead, in grams. 2. Accurately weigh the lead only, in grams. One gram of water occupies a volume of one centimeter cubed. Therefore, the volume of the sample can be calculated, knowing that the density of pure lead is 11.389gm/cm³. 3. Calculate the volume of the lead by dividing the weight of the lead only by 11.389 4. Calculate the volume of the sample by subtracting the volume of the lead from the weight of the sample+lead weight. 5. Calculate the weight of the sample by subtracting the weight of the lead only from the weight of the sample plus lead. 6. Calculate the density of the sample by dividing the weight of the sample by the volume of the sample. Now I've got a headache, as I suspect anyone else reading this has. But from the information you have given above, I calculate a density of 0.55gm/cm³. To calculate this, convert all your figures to metric (grams and centimeter cubes). Divide the weight by the volume. This figure is very high for balsa, which is normaly about 0.12gm/cm³. #3Posted 06 January 2008  03:46 AM
Vman, you are right, I still have a headache.
Your way of calculating the density seems OK to me, but isn't there a thing to be taken into consideration when going through this process? Wood takes in water, so if you suspend the wood as you say, you will get the neutral buoyancy, but if you wait a little bit more, you will see that the wood goes down, as it takes in more water. So should the wood be sealed somehow or not, before using all your neurons to go through this process? #4Posted 06 January 2008  04:25 AM
I developed this technique for predicting the ballast for a lure, long before I get to the painting stage. I would shape the body, seal it to prevent water ingress and then do the volumetric calculation to obtain the density.
If the test is to be performed on unsealed wood, it would probably be a good idea to weigh the wood sample by itself before the buoyancy test. #5Posted 06 January 2008  04:39 AM
Vman, now I remember that you said somewhere on TU that there is another method to calculate density, which is much simpler than this one. You must have a perfect shaped piece of wood, with all surfaces and lines parallel. You calculate the volume of that piece of wood, you measure its weight, and the density is a ratio between the 2. Isn't it simpler?
#6Posted 06 January 2008  05:01 AM
Yes it is. In fact that is the information that PB has provided above. But as usual, I went over the top.
The beauty of this method is that it can calculate the density of any scrap of wood or any other buoyant material. The method reads complicated, but once you have done it once, it is quite simple. I think the method was worth bringing to the table. The principles used in this method can be applied to other aspects of the lure, such as: Calculating the buoyancy of an existing lure, in order to duplicate the buoyancy characteristics in the next lure. Improving repeatability in production. Calculating the effect of a top coat on the buoyancy. Another example: you want a suspending bait. In the bath tub, you hang the lead to achieve suspension. Drill a hole, fit the lead. But the resulting suspension does not happen. The above method will help explain the error and allow you fix it. #7Posted 06 January 2008  06:52 AM
Vman, I am trying to answer Palmetto's question.
I am deeply confused by the american system of measuring units. I think I would be lost in the US. I had to use the internet to transform the data provided by PB into international units. Please note that the measuring system in inches, ounces, gallons, miles etc, gives me more headaches than you will ever be able to. I took into consideration the following data: 2" = 5.08 cm 3 15/16" = 3.9375" = 10.00125 cm (here I had a little problem. If I look at the figures from a mathematical point of view, I should multiply 3 by 15, then divide the result to 16. The outcome would be 2.8125, so less than 3", but 15/16 is added to 3") 36" = 91.44 cm The volume now 5.08*10.00125*91.44 = 4645.732644 cubic centimeters Mass 22.5 oz = 637.875 grams Density 637.875/4645.732644 = 0.13730342421 grams/cubic centimeters This means it is a very light material. The density of pure water is 1. Now if there is an american way of expressing the density (such as cubic feet/ounces), I don't even know. Maybe you can take it from here. #8Posted 06 January 2008  07:57 AM
Rofish, you are correct. I made a mistake, used the weight from the 36 inch board for the 9 inch weight. Sorry about that.
Density = 0.137 is a much more realistic value! #9Posted 06 January 2008  10:22 AM
PB,
Since a board foot is 12"X12"X1", all you have to do is figure out what a 1"X1"X1" piece weighs, and multiply that by 144, the number of cubic inches in a board foot. If you have scrap, or enough wood, that's the simplest way. In real life, a board foot of lumber sold is based on 12"X12"X3/4", but it's easier to deal with whole numbers. Volume is length times width times height. If you have a calculator, you can get the volume of your sample, divide it by 144 to determine the percent of a board foot your sample is, and then divide the weight of the sample by the percent to find out what the weight of a board foot would be. Just make a table of the woods you use for reference. Actual specific gravity isn't as important as comparative density from one type of wood to another. #10Posted 06 January 2008  11:01 AM
Mark, both specific gravity and density in what ever units are equally useful for determining the relative buoyancy attributes between wood types. But the clear advantage of measuring the density in gm/cm³ is that the density of water is 1gm/cm³. Therefore, you have a direct indication of the buoyancy of the wood. This figure is far more useful. It is an amazing coincidence that 1cm³ of water weighs exactly 1gm. It is almost as if the weight system was arranged specifically for lure design!
#11Posted 06 January 2008  01:43 PM
Vman,
Just shows how deeply fishermen are embedded in every facet of our history. Looks like an Oliver Stone movie in the making. #12Posted 06 January 2008  03:32 PM
Balsa nominal density is 11.2 lbs/cu ft. That's at standard temperature and moisture content, so "your mileage will vary". Nominal water density is 62.4 lbs/cu ft. I don't see the utility in calculating the density of a specific piece of wood since when it is crafted into a crankbait it has steel wire, lip material, undercoating, paint, and clearcoating attached to it. All of these materials increase the effective density of the bait. By how much? Don't know and don't care. What matters is how much ballast it will take to make that composite object do what you want it to. And the EASIEST way I know to get there is to float test the semifinished bait in water that is the correct temperature. If it rises very slowly, that's good. Stick on a Suspend Dot or two when you go fishing. If you hit a warming trend the bait may suspend perfectly. Sometimes, craft just trumps science in the real world.
#13Posted 06 January 2008  04:29 PM
I think I asked 2 separate questions. (by accident)
Q1 what is the density of the wood? Q2 what it the board ft. weight of this wood? What I really wanted to know is, what is the weight of a cubic foot of this piece of balsa. (balsas weight ranges from less that 7lbs to over 20lbs per cubic foot) Well hare is the board foot calculator that can help. http://www.owwm.com/Math/BdFt.asp It looks like I have 2 board feet of wood with my 2x4x36 (rounded up from 2" x 3 15/16" x 36") @ a weight of 22.5oz = 1.40625lbs. If I multiply that weight x 6 that should give me the weight of a cubic foot of my piece of balsa wood. 1.40625 x 6 = 8.4375lbs/qu ft. Is this correct? If so, then the wood I have been working with is very light and soft. The original question I asked of what is the weight of a board foot. A) 1.40625 / 2 = 0.703125. The other question about density. Density of this piece of balsa on average is = 0.137 (Thanks Vman and Rofish) Pure Lead Pb has a density of 11.34 (according to wickipedia) I am just trying to put all of this together so I can try and wrap my mind a little further around the weight placement vs. action issue. Along with a light weight sealer, wood reinforcement method that is easy to work with and still keep the exterior weight at a minimum. #14Posted 06 January 2008  05:11 PM
BobP,
I think it is important to know how much one wood weighs compared to another for lure building, at least for me. I know from my experience with larger lures that having a lighter wood lets me have a lure that's lighter overall, with the same action as a heavier lure, but that is easier for me to cast and work. For my gliders, the choice of pine over douglas fir can mean the difference of an ounce. Poplar is somewhere in the middle, leaning to the heavier side. Lighter woods, again in my experience, allow for a taller lure without too much more weight. On the other hand, for my triple trout imitations and other jointed baits, fir and poplar are great, because I don't have to add too much extra weight to get them to just barely float, while I get the added strength of those woods over pine. I haven't had the courage to tackle smaller balsa cranks yet. I'm having enough fun trying to make the bigger baits correctly, and learning to paint so it doesn't look like I used a roller. #15Posted 06 January 2008  06:10 PM
VM  I don't think it is any coincidence, not here any way. Metrication It is the best thing to happen here since Federation  I still (after 30 years) I try and convert mm's to inches. 1 cubic metre of water weighs 1 ton, how simple is that for a dumb fireman. although I know it would be like turning a ship around (was here)  You guys should come out of the dark ages and make life simpler, for all of us foreigners. pete
#16Posted 06 January 2008  07:58 PM
I'll have to agree with you on that one. I remember 30 years ago we tried to convert. It never caught on and now we measure some things in metrics and others in what ever else we call. I wish we could get everything converted and this would all be a lot easier. Milk in Gallons, quarts, pints. Soda in Liters, ml, and Ounces. And then there is a fluid ounce and an ounce in weight. Gallons, Quarts, Pints, Cups, Table Spoons, and Tea Spoons. Help #17Posted 06 January 2008  08:34 PM
PB
If the board you have is actually a full 2"x4" x36 " you have 2 board feet there simply weigh your board and divide by 2 for your answer. No need to complicate the heck out of things A standard board foot equals 144 cubic inches. weighing a sample and doing basic grade school math will provide you with the answer #18Posted 06 January 2008  09:06 PM
The density of lead is 11.34 gm/cu cm, or 708 lbs/cu ft. For me, lbs/cu ft are just numbers that are easier to remember. The standard nominal weights for the 3 woods I use the most are:
Balsa 11.2 lbs/cu ft (.9364 lbs/bd ft) Paulownia 16 lbs/cu ft (1.333 lbs/bd ft) Basswood 23 lbs/cu ft (1.9167 lbs/bd ft) Palmetto Balsa, if your board is 8.4375 lbs/cu ft, you have very light balsa. Mark, I completely agree about relative wood weights. I'll build a diving swimbait from basswood. For a big wakebait, I prefer paulownia since it's much lighter. Both woods are durable so fitting hinges is not an exercise in futility and you can expect them to hold up. FYI, the standard densities of the 3 woods you mentioned are: Douglas Fir  31.2 lbs/cu ft Yellow Poplar  26.2 White Pine  21.8 #19Posted 06 January 2008  11:26 PM
You know, when they converted construction lumber from 1 5/8"X 3 5/8" to 1 1/2"X 3 1/2" for s4s (surface four sides) 2X4s, I had to relearn how to do layout. It wasn't that hard, except when we got mixed loads of lumber with both dimensions during the transition. But it made things simpler. Not better, simpler.
Of course, metric is simpler. So what? Think of using our "antiquated" English system as a mental challenge. Enjoy the challenge. Thinking is a dying art, with computers, calculators, and tivo. Your mind is like any other muscle. Use it or lose it. We revel in our ability to come up with original designs, baits, and solutions to lure design problems, yet we whine about how complicated our system is. Rejoice in it. There's plenty of time to have it "simpler" when you're dead. In the mean time, look for the challenge. When I die, I don't want to be in pristine condition, because everything was easier. I want to slide into that last base like I was "rode hard, and put away wet"! Unfortunately, I've already lost my mind, but there's hope for the rest of you. ) #20Posted 07 January 2008  01:16 AM
Good post Mark.
The problem of switching from pounds to Kilo's and inches to centimeters, is not about the numbers. If it was, everyone would change over in a flash. There cannot be any arguement that metric, base 10 units are much easier and meaningful than imperial, multi base units. Hell, the base number changes within the one subject, example: ounces, pounds, stones are the imperial standard measurement of weight, I need not say more. The problem is 'the FEEL'. When you change over, you loose the 'feel' for a measurement. Example, 14st to me or 200lb to an American means a sedentary life style with a few too many burgers. It takes a long while to 'feel' that 90Kg means the same. This uncomfortable position of not being in control of the numbers is why the change is resisted. If a conscious effort is made to change or it is forced upon you, the change happens very quickly. The secret is not to mix and match. When I post, I usually post both units, placing the imperial in brackets. But some subjects just don't belong in imperial, the numbers are just too cumbersome. Density is one of them. If you don't want to change, I can respect that and will rejuvinate my efforts to publish in both units. 