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dramone, February 22, 2008
This is one of those things that is on my "to do" list, but just haven't got around to.
You can only really calculate or measure the effect for one particular shaped lure. The information gained cannot be applied to a different shaped lure. This is because the amount of epoxy and its effect on the average density of the lure is directly proportional to the surface area. So a deep, flat, thin body will have a larger surface area to body weight than a short, fat, round body. Therefore, the thin body will require relatively more epoxy and the average density will be affected more.
To my knowledge, no one has actually measured the effect. We know the problem exists and the experienced builder allows for it, without even thinking about it.
When I get to that stage, I will of course be calculating it to death and it will form part of a density spreadsheet. This will enable me to weigh the lure at any stage and I will be able to compensate for any differences with slight adjustments to the ballast.
In the absence of such data, I suggest that you start the study off and calculate the effects on your lure. Do you own an accurate set of digital scales? if not, then the subject is going to grind to a halt very quickly. If you do own or can get hold of the scales and are interested to do a few minutes of testing, let me know and I will write you a method sheet. The method is very simple and will give you the density of your lure. Repeat after each coat of epoxy and you will be able to see the difference that each coat makes, each subsequent coat having a slightly larger effect than the previous, as you rightly pointed out, the volume of the lure increases as does the displacement.
You have already figured most of it out, I hope you take up the challenge.
Long story short, epoxy is denser that wood, so it adds to the overall weight and affects buoyancy. It makes the bait heavier, so it floats less. How much depends on the bait, which epoxy/clear coat, and how many coats you put on. That you can only learn by experience, that is, trying it yourself on the lures you make.
In order to know how a clear coat will affect Buoyancy, you really need to know the Specific Gravity. Water is 1.00. Anything greater than 1.00 is heavier than water & will decrease buoyancy. Anything less than 1.00 will actually make the Crankbait more buoyant.
Devon 2 Ton is 1.17. Adding multiple coats of Devcon will decrease the buoyancy. This is most noticed on Jerk Baits due to the large amount of surface area relative to the volume (Long and Slender). You may not even be able to detect a second coat on a Fat Crankbait.
I don't know the Specific Gravity of any other Clear Coats, however I am sure any hard Clearcoat will be greater than 1.00.
I ended up with a faster sink rate than I planned on my earlier glide lures due to epoxy making the lure less bouyant. I plan a little lighter now and reserve the option of heavier gauge hooks to bring the lure down if needed.
specific gravity is the key as far as I can reason it. it is the denisty as compared to water not wood that you should be thinking about.
I am just about to do a first clear on a lure, so I will weigh it before and after and get you the % increase, apply second coat tomorrow and weigh again - this should be the apx % increase for any lure. Trouble is each coat is more volume in D2T, therefore more weight. pete
I did not under stand the epoxy could make that much difference. So I grabbed a tooth pik and coated one side. When I put it in the water the epoxy side was lower in the water. It was not alot but intresting.
thank you all for your help!
so far, i'm writing things down, to know what to use for each lure (habit, weight/where etc.)
i also estimate the amount of epoxy (experience, trial and error) i'm going to use, and subtract it from the lead. (pretty difficult when foiling with "deep" scales" )
i'm thinking about vodkamans approach (thx Dave ), maybe i'll give it a try.
One coat of D2T typically adds about .57 gr to bass size crankbaits (60 mm long). That's usually not enough to have large effects on floating baits, especially ones made from balsa or other light wood. I also write down the build details and component weights on every batch of baits. It's the only way I can go back and replicate one exactly. Plus you begin to see trends in how the components and build details affect performance as you build more baits and collect their details.
I just did a Google for Devcon 2 ton and found a data sheet. This states the cured density of the epoxy is 1.1gm/cm³.
This means that one centimeter cube of epoxy weighs 1.1 grams. One centimeter cube of water weighs 1 gram. Therefore, the epoxy only adds 0.1 grams to the ballast for every centimeter cube of epoxy.
In real terms, approximately 10% of what you add to the lure counts as ballast weight. For Bobs example, he added 0.57gm of epoxy. Of this, 10% (0.057gm) will count towards ballast. This is a very small number and will effect suspending lures, but its effect on floaters or sinkers is unlikely to have any visible effect.
Well I cleared one (D2t) and and got a 3.16 % increase in weight, so thought just to get an average, I cleared another and got 2.2% increase in weight - like Dave says, hardly worth considering. Sorry for the delay but decided to do the two for science - and running out of Devcon fast. pete
that is exactly the answer i was looking for!! (shame on me that i did not made the brainwork by my self )
i now assume that the extra ballast due to the epoxy is neglectable for most of the lures
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