Re: what are these used for ?
I had the same question on my boat as to what the bottom knob is for. After trial and error and taking the think of and looking at it, here is what it is......it is cruise control.
Now I know that you are probably laughing, but on my boat, this knob turns several turns. It tightens down a bolt which puts pressure agains the throttle. If you lossen this knob, you should notice that your throttle moves freely. If you tighten it down, you should notice that your throttle is stiffer. When you are moving at speed, you may notice that your throttle tends to creep down. Tightening this knob will help hold it in place... alas.... cruise control.