# How to mantain optimal difference

1. Nov 28, 2004

### niko2000

Hi I am thinking about some problem which I would like to write a software application but I just can't come to the right equation I could use to write a function.
Let's have trawl a sphere with a boat. The sphere is tied with a rope thus there is some distance between a boat and sphere. Let's have a depth indicator on boat and let's have an engine of constant speed which is rolling the rope.
The problem:
We would like to maintain constant difference between the sphere depth and sea depth (let's say between 20 and 30 ft).
It seems very easy and maybe it is but I just cant write an equation.
If we imagine that we are fishing with a boat and trawling a fishhook, which is tied on a rope and that rope can be rolled with an engine. We are looking on depth indicator and boat's speed and try to maintain the fishhook at the distance of few feet from ground pushing the engine's button up and down as necessary.
I would be really grateful if anyone could write an equation for this thing or just give some idea for realizing this mathematically.
Regards,
Niko

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