Trolling Depths for salmon? How much Line out on the counter for each depth?

So, I've only had limited success trolling for salmon in the ocean so far. In the last few outings, I've marked several salmon at depths from 15' to 150', and I'm always wondering if I have enough line out to get to the depth the fish are cruising out.  My method so far was purely guessing, and hoping I'm close to a 45 degree down angle, but both have been pure guesses.

In an effort to dial in the right depth, I consulted my math wiz teenage daughter, who said, " this is easy dad, it's the Pythagorean Thrum A squared plus B squared = C squared!"  I said that was a long time ago when I learned that one.

Long story short, she made me a little chart that I thought might help some of us rookies.  Here's a copy of the chart.  I shrunk it down and have taped it to the face of my fish finder with some clear tape for easy access on the water.

Hope this helps me and the rest of us put some salmon in the net.

Nice chart!  I always remember it is about 10ft out for 7ft of depth.

This of course depends on the angle of the line in the water, which is a function of speed and weight. 

Finally, we get to use trigonometry in the real world!!

Depth (ft) = sin(angle) x  line out (ft)
where the "angle" is the angle parallel to the water (i.e. flat-lining = 0 deg, straight up and down = 90 deg).

Just so happens the sin(45 deg) = 0.7071 ... which is the basis of the above chart.
sin(0) = 0
sin(30) = 0.5
sin(60) = 0.8660
sin(90) = 1.0

It was too hard to calculate the drag of the gear so here's my observational approach logging depth on the meter when the diver hits bottom and line out. Actually found an incredibly strong linear relationship.

Gear - 50lb braid, 7oz deep six, flasher, spoon
Speed ~ 2knts

Depth = .46 * Line Out + 4.26

workhard your chart is more what I've always approximated with - half the line out for depth @ 45 deg.

The Pythagorean doesn't really account for all of the line bow and whatnot so I've felt the straight math to be a bit optimistic...

This of course depends on the angle of the line in the water, which is a function of speed and weight. 

Finally, we get to use trigonometry in the real world!!

Depth (ft) = sin(angle) x  line out (ft)
where the "angle" is the angle parallel to the water (i.e. flat-lining = 0 deg, straight up and down = 90 deg).

Just so happens the sin(45 deg) = 0.7071 ... which is the basis of the above chart.
sin(0) = 0
sin(30) = 0.5
sin(60) = 0.8660
sin(90) = 1.0

This is the way to do it, IMO. I worked it out without the sin function, so you win !

I can remember 3 numbers: .5, .7, and .85

There's nothing magic about 45 degrees to the fish. It's just the easiest angle to eyeball.

But what does matter to the fish is speed. If you were to have your weight out and decide to pick your speed up from 1.5 to 2.5 kts, the line angle changes, but that doesn't matter. You have a new angle, and need to recalculate to get your depth. Pick the closest of the angles and bam ! you have your depth. You don't need to bring everything up and put more weight on, unless you can't maintain 30deg at your chosen speed.

I will also say that in my opinion, guessing deep is better than guessing shallow. Because if the angle is less than you guess, you will be a little less deep, and the fish can see up, not down.

Thanks for hurting my brain! Can’t we just use dynamite?


Get a buddy to follow you with a good FF and call out your depths?

Im not a down rigger guy but I would think that some breakpoint exists in which down rigger gear becomes more effective than trolling  100+ feet of line out. My guess is that around 60 feet depth is that breakpoint? Maybe other people troll deeper??

All of my coho this season have been 18-25 feet of line out with 6oz. This weekend I am going to troll deeper to put a chinook on the tag. How deep? I dont know.

Well, I thought I was onto something with the chart, and it's going to be way more accurate than I was before. 

What I did learn from all of the charts, graphs and math is that I haven't been letting out nearly enough line to reach the depth I was aiming for.  I guess there are way too many variables to truly have one way to figure out what's accurate each and every time.

Cosmo, like most people around here I am way into using the 45 degree angle while trolling for salmon, whether I am fishing right down near the floor or for suspended fish in deep water. I match the lead I need for the trolling speed I feel is right for whichever fishery I am in and maintain the line angle rather than worrying about my actual speed on the ground. This seems to be what a lot of people do too, and it is a great way to know exactly how deep your gear is.

When I started fishing in the ocean, I did the same for awhile and noticed two things: I wasn't catching as many fish as the people I was with and they were leaving me in the dust while I was putting along watching my line angle. I figured I would probably need 20 oz of lead or more to do a 45 degree angle and go as fast as they were going.

After noticing this, I decided to make the ocean the only place where I completely ditch the 45 degree angle rule while I troll way faster than I am used to. Now I never have a 45 degree angle in the ocean, If I think the fish are a little deeper, I will fish more lead with more line out, but the line is always flatter than 45 just because of the high speed. As soon as I did this, I started catching WAY more ocean salmon.

I think the higher trolling speed requirements are possibly the reason for the prolific use of divers in the ocean.
I hope this helps somebody, and thanks to those who showed me the way.