How to measure angle of depression of a fishing line ?

[Stephen Tashiro]

This is an eccentric and specialized question, but I'm always surprised at the variety of expertise I find on the Creek, so here goes:

Is there a good way to measure the angle of depression of a fishing line that is trolling for fish? This would be on a boat that is moving only a few knots per hour towing a light fishing line roughly 15 to 40 ft in length. Does a line like this move through the water roughly in the shape of a straight line or does it noticeably bend?

I myself am not a fisherman. However, on a physics forum (physicsforums dot com), I'm in a discussion with a poster called harlyhar who wants an equation to predict the amount of line needed to troll at a certain depth with a device which is (or is similar to) a Lite Bite Slide Diver. He has data for various settings of the device but hasn't tested all the possible combinations and worries that it is too much work to do so. My thought is that if he (or a fisherman) had a device to measure the angle of depression of the troll line and knew the length of the line, he could predict the depth it was trolling. I suppose the standard device for measuring an angle of depression is the clinometer. But I don't know it it is practical to use one for this purpose. I don't know the details of how the line is attached to whatever it is attached to on the boat and I don't know how unsteady sport fishing boats are when they are trolling.

I have never seen a slide diver device in person or gone trolling for fish. So anyone who needs more information to answer my question will do as well consulting the web as asking me. You can find info on the Lite Bite. The thread on the forum is in Physics Forums > Mathematics > Calculus & Analysis > Help with finding an equation for a set of data

[Brian Kent]

Since that looks like something that would offer some resistance, bringing the bait or lure down and away, I would say "pretty straight". But that is not a physics answer. It is pretty complicated since the line will not be attached at 2 points, but at 3. The rod, the Little Bite Side Diver" and the lure. Then you have the drag on the line itself, since it seems these are used with 40-60 lb test line, for some big fish.

I could not tackle that equation at all. But it would be "pretty straight".

[Stephen Tashiro]

I appreciate the answer, Brian. I'm looking for practical answers or guesses, not equations.

[Larry Edgerton]

I have done a lot of trolling and there are several factors at work. Speed as you mentioned, the downward pull the lure is capable of generating, and the drag on the line as well as the angle of attack. With a lure that is a deep diver the angle of attack is steeper than a shallow diver so there is more water drag on the line pushing it back up than say with a Salmon setup that is long lined and is at a shallow angle.

If you take any lure, say a Rapalla, as you let it out it will dive until the downward pull of the dive fin on the lure is overcome by the water resistance on the line. At that point it will start coming back up until it will eventually come all the way up to the surface. Add to this the factor of the line weight, that being the heavier the line the more resistance and it would be tough to calculate. I am sure there are other factors such as water temp, wind on the line above water, etc. that factor in as well.

All of that considered, some days we are just burning gas. I like to think of it as a nice boat ride.

Larry

[ray hampton]

I think that one point had not been mentioned and this is the height of the boat plus the length of the fisher rod

[Brian Kent]

The Angle of Depression is: Amount Spent / # of Trout Caught.

If I spend $80 on boat rentals, $46 on a license, $14 for an extra rod (so I can catch twice as many trout), $80 for a rod and reel, $15 for new line, $35 for lures and bait divided by the two trout caught (because it was not my worse day) = Angle of Depression of 135°.

I don't know how to fit in the Angle of Elevation from sitting on Big Bear Lake, looking at the mountains all day, and the theological lesson that when I pray for fish, God protects the fish.

The two angles together bring me out to "Level or Above".

[Lee Schierer]

There is a book you can buy that gives the trolling depth for various lures based on speed and line size. Check Bass Pro Shops. The line actually forms a curve. I think it may resemble a parabola, or catenary (spelling?) but I'm not sure. There are a lot of factors to consider such as line diameter, line surface texture, lure pull weight, boat speed, etc.

[Brian Vaughn]

Lee,

Don't forget water viscosity, water temperature (Affects density!), air pressure, salt/fresh water, water turbulence (Waves), outrigger length (Depending on how far out the outriggers are set, the aeration from the propellers can definately cause a change in water density), and lots more. Not to mention if you're using monofilament, the amount of tension will cause it to stretch, changing its diameter. And if it's an older, sun-exposed line, it would tend to stretch less, as that's more brittle.

Stephen, the point is, "pretty straight" is about the best anyone is going to come up with. There's too many factors to give an exact equation or answer. Plus, seeing as fish have the ability to swim up and down in the water, I don't see how there's any problem being off by a half inch. Not only that, I can say with vague certainty that what you use for bait makes much more difference than the elevation of your hook, within a reasonable amount. You're not going to catch a flounder with a surface popper, but you can catch them anywhere from right on the bottom to a couple of feet off the bottom, and it doesn't matter.

[dave toney]

If there is an element of depression involved in fishing you ain't doing it right!
Dave

[Dan Hintz]

I'll give a thumbs up to Vaughn's post, but you could probably get pretty close with first-order info. Lucky for you, plenty of first-order info to work with... the trick would be measuring it. For example, if you know the weight of the line, the length between rod tip and water interface (taking into account gravity effects creating the curve downward... the weight can be used here), the speed of the boat, etc., you can determine how much force is pulling on the line right at the water interface. Adding in water resistance, the weight of the bait, and the force on the line at the water interface calculated previously, etc. you can determine the angle the line is likely making under the water. Knowing the length under the water (total line length minus what's above the water), determining your troll depth is simple geometry.

[William Payer]

Since the diver is being pulled against the water ( the lip or wing of the diver creates that fdrag) the line from the rod tip to the diver can be assumed to be essentially straight. Likewiser the lure exerts its own resistance to being pulled underwater, so the line from the diver to the lure is again straight and usually a short length so it can be assumed to be horizontal inthe water column.
If you are not using a diving device, then the lip of the lure serves to make the lure dive and the line is essentially straight once again.
Having trolled extensively for years the factors involved at length of line out, type of lure or diving device, diameter of line (water resistance) and the type of line (mono-filament or lead core etc) The book sold at Bass Pro is written by Mark Romaneck, a walleye professional fisherman. It was based on actual in water tests using standard lures ( about 40 or so different ones tested) various diameter lines, and actual measurement of lure depth by a person in a scuba outfit.

You can male your own calculations by letting out a designated length of line ( line counter reels are best for this) with the terminal tackle attached, and at a known trolling speed, in water where you know the depth (fish finder will give depth) Let out line gradually , maintaining same speed, until the tackle hits bottom. You now know how deep the lure runs at that line length and speed. Change depth by moving to different area of lake and do again. Repeat until you have a pretty good idea on how much line is required to get the bait to various depths, given that trolling speed. Remember in bodies of water with currents, trolling with the current will cause your speed (relative to the bottom of the lake) to increase, and your lure will run higher in the water column, against the current will slow your speed, and the bait will run deeper.
Its really not rocker science, it just takes time and patience to record enough data to be useful,
.

[anthony wall]

after reading this thread i think its time to give up fishing