Density Vs Sink Rate

[walkercope]

I'm wondering if anyone's done any tests to come up with a rough relationship between density and sink rates for swimbaits that they could share.

I've finished modeling what will be my first glide so I know the volume and also the density of the resin I'll be pouring, so it's theoretically possible to get it right first try, but I'm not sure what relative density I should shoot for to get a ~1ips sink rate.

I'm aware that shape also plays a big factor in this but the shape should be similar enough between baits that the relative density to sink rate should be extremely similar.  Looking for something like "2% more dense than water gets 1ips"

[Jinxd12]

Generally speaking.

Split your resin calculations in half and add the 2nd half microballons.

If you needed 4 oz of resin per the calculators, you would have 1 oz part A, 1 oz part B and 2oz of microballons. 

With this simple math your bait should be buoyant. You will 99% of the time need to add weight at some part to make the bait sit in the correct orientation in the water at a minimum.

Adding more or less from there increases or decreases your sink rate. 

If you have a 2 piece pour then you have other options for calculations front half vs rear half. 

Hope this helps!

[walkercope]

28 minutes ago, Jinxd12 said:

Generally speaking.

Split your resin calculations in half and add the 2nd half microballons.

If you needed 4 oz of resin per the calculators, you would have 1 oz part A, 1 oz part B and 2oz of microballons. 

With this simple math your bait should be buoyant. You will 99% of the time need to add weight at some part to make the bait sit in the correct orientation in the water at a minimum.

Adding more or less from there increases or decreases your sink rate. 

If you have a 2 piece pour then you have other options for calculations front half vs rear half. 

Hope this helps!

I'm using smooth on featherlite which is 67% as dense as water, so it will float.  I've done the math to see how much ballast I need to achieve a given % difference in density (object mass divided by volume of water displaced).  The problem is I don't know what % increase in total mass of the body compared to mass of the displaced water to achieve the sink rate I want.

[Jinxd12]

thats when tinkering comes into play. have to put the time in with testing. drill a hole in the bottom with a forstner bit and melt some lead...split shots work. 

[walkercope]

Obviously I could spend the time and money to mold a bunch of baits and vary the densities and measure the sink rates and come up with the relationship myself, but someone out there has already done it and has the answer.  My bait will act a bit differently than theirs based on the shape but it will be in the same ballpark.

[danthefisherman]

There are too many variables besides resin density that will affect sink rate (i.e. hydrodynamics, hardware used, ballast configuration, line used, water temp), so there’s no real substitute for trial and error. As mentioned, you can in theory calculate the buoyant force of your material based on the material properties, but doing so would be cumbersome and does not take into account all the factors that are independent of material choice.

Sink rate is one of the easier characteristics of a bait to adjust in my opinion. At least for me, it is harder (but more important) to achieve good balance, swimming action, durability, etc. After I get those right, it’s pretty easy to make a bait sink slower or faster by adding/removing ballast weight or changing micro balloon content (thus the density).

I know you’re asking for something more specific though, so maybe an example of where I start would help. Say I want to make a slow sink multijoint swimmer. I typically start with around 50% micro balloons (by volume) for good durability of the resin and less ballast needed (thus a lighter overall bait). If the bait is thin or unbalanced at 50%, I will bump up that ratio and increase ballast. 100% by volume is my cutoff so as to not make the resin too “foam like” and prone to breaking.

Hope that helps and good luck! If you’re looking to save time and money, bait building is the wrong hobby to get into … speaking of which all the prices for materials has gone up quite a bit so make sure you take advantage of the Black Friday sales! 

[BrierBob]

1 hour ago, walkercope said:

Obviously I could spend the time and money to mold a bunch of baits and vary the densities and measure the sink rates and come up with the relationship myself, but someone out there has already done it and has the answer.  My bait will act a bit differently than theirs based on the shape but it will be in the same ballpark.

Can you Just add weight until you get your desired sink rate?Using Featherlite, Im not sure how to make the resin more dense since the mbs are already mixed in. 

[Jinxd12]

1 hour ago, walkercope said:

Obviously I could spend the time and money to mold a bunch of baits and vary the densities and measure the sink rates and come up with the relationship myself, but someone out there has already done it and has the answer.  My bait will act a bit differently than theirs based on the shape but it will be in the same ballpark.

Bait building is not an easy process. Time and money and materials used like @danthefisherman said will all vary. The water temperature getting colder has me tinkering a little with some of my crank down resin ratios.

Part of the process and the fun part in my opinion is the fine tuning the weight and weight placement. With a forstner bit it is pretty easy to drill a hole and plug and play weights.

[walkercope]

@danthefisherman I guess it's a bit misleading to just say "density" of the bait - what I'm calling the density of the bait (I'm making a two piece glide), is the mass of the whole body, including resin, screw eyes, ballast, hook, split ring, swivel, and anything else in the body, divided by the volume of water it displaces (for each of the two bodies individually).  So this covers most things you mentioned.  It's reasonable to account for all of these things in CAD since I can model and assign material properties to each piece and combine into a body.

Also, I guess I'm trying to achieve the proper ballast and buoyancy with an internal harness that will get poured into each bait and located via mold features, hopefully so that I can just use featherlite without mixing in extra microbaloons (which I believe are pre-mixed into featherlite)

If you're carving a master and molding it, then the iterative process you guys are describing seems like the only option but since I've already got my bait modeled in CAD, it tells me the mass, volume, center of mass etc for each individual body so I think I can have CAD do the heavy lifting for me.

Edited November 22, 2022 by walkercope

[danthefisherman]

1 hour ago, walkercope said:

@danthefisherman I guess it's a bit misleading to just say "density" of the bait - what I'm calling the density of the bait (I'm making a two piece glide), is the mass of the whole body, including resin, screw eyes, ballast, hook, split ring, swivel, and anything else in the body, divided by the volume of water it displaces (for each of the two bodies individually).  So this covers most things you mentioned.  It's reasonable to account for all of these things in CAD since I can model and assign material properties to each piece and combine into a body.

Also, I guess I'm trying to achieve the proper ballast and buoyancy with an internal harness that will get poured into each bait and located via mold features, hopefully so that I can just use featherlite without mixing in extra microbaloons (which I believe are pre-mixed into featherlite)

If you're carving a master and molding it, then the iterative process you guys are describing seems like the only option but since I've already got my bait modeled in CAD, it tells me the mass, volume, center of mass etc for each individual body so I think I can have CAD do the heavy lifting for me.

Gotcha. I have an engineering background and some CAD experience myself, but like you mentioned I prefer the more “organic” practice of building and grinding through R&D by hand.

I don’t remember much from my fluid mechanics course, but if you think about it there are essentially three main forces acting on your bait (disregarding your line and other factors): the force of gravity, buoyant force, and a retarding “drag” force. The first two are easy to calculate as constant values and can essentially equate to a smaller effective weight by combining them. Similar to wind resistance, the drag will be proportional to the bait’s velocity through the water. This is where hydrodynamics comes into play, but perhaps you can use some sort of CAD flow simulator/analysis in this area. The bait will reach a certain “terminal velocity” after the sum of the buoyant force and drag force equals the gravitational force. Presto, there’s your sink rate.

Interesting discussion and I like where your mind is going, but I’ll stick to adding another split shot or two 

Happy building!

Edited November 22, 2022 by danthefisherman

[walkercope]

Yeah I'm an engineer too. 

I'm not sure that it'll be ok to combine the gravity force and the buoyant force though because the center of the buoyant force needs to be vertical above the center of the gravity force pulling down.  

If I had some flow analysis software that would be fun to try but I'm not about to pay 30k for a license lol.

[BrierBob]

57 minutes ago, walkercope said:

Yeah I'm an engineer too. 

I'm not sure that it'll be ok to combine the gravity force and the buoyant force though because the center of the buoyant force needs to be vertical above the center of the gravity force pulling down.  

If I had some flow analysis software that would be fun to try but I'm not about to pay 30k for a license lol.

what clear coat are you going to use?

[Jinxd12]

Seems like you need to try a different website to "give" you your answers. We are all just a bunch of cavemen here that do our "own" heavy lifting by adding and subtracting the weight with our hands!

[danthefisherman]

56 minutes ago, walkercope said:

Yeah I'm an engineer too. 

I'm not sure that it'll be ok to combine the gravity force and the buoyant force though because the center of the buoyant force needs to be vertical above the center of the gravity force pulling down.  

If I had some flow analysis software that would be fun to try but I'm not about to pay 30k for a license lol.

Nice! It shouldn’t matter the center of gravity vs the center of volume. If they are offset laterally shouldn’t it just cause the bait to rotate until they are aligned? A well-balanced bait should have a center of mass bellow the center of volume. My senior project was to design, fabricate, and test kayak outriggers so I’ve put a fair amount of thought into this haha.

[walkercope]

31 minutes ago, BrierBob said:

what clear coat are you going to use?

Oh man I'm not even thinking about paint or clear coats yet.  If you have any suggestions I'd love to hear.