Okay, I see the circuit board now, looks like a relay control board.
So yes I agree, most of the time when a relay starts buzzing it's caused by low voltage (can be caused by a poor ground, corroded connector or a pinched/damaged wire, or even low battery.
But if you have a wiring schematic diagram it's something I don't have access too so hard to say I'd suggest checking the power input while the blower,etc. is turned on and see if voltage drops. Also I'd check the main ground while blower, etc. is turned on and see if there's more than zero volts.
Basically, relays will chatter if there's not enough voltage to hold them in the energized position. The voltage might be fine before you turn on a switch but once the relay energizes the device it's controlling (bilge blower, etc.) begins to draw current and if the connections aren't all good there may be high resistance (or even a short, low impedance, heavy electrical load) that causes the voltage to drop.
If you're blowing fuses, there's probably a low impedance, such as a super-duper cargo chip bilge blower attached, for instance.
BTW, fuses are selected for the size of wire and anticipated load. So if the fuse isn't blowing you know it's large enough and not a problem. But if the fuse does blow it could be too small for the load or, there could be a short or low impedance load drawing more current than the fuse is capable of.
So say for instance you remove a blown 10A fuse and replace it using a 20A fuse, now you're taking a chance the circuit wire gauge might burn and catch on fire in case of a short.
I would guess in your case the 20A fuse you referenced is the main fuse, it more than likely is nearest the main power connector. I say this b/c that's how circuit boards are most often configured, not b/c I'm familiar with the CB you're working with. All of the relay driven devices added together might draw nearly 20A, thus that choice of fuse for main power.
So take your volt meter and measure main power, I'd bet it's that oval 2-pin connector? Measure the power supply voltage there using a couple of diaper pins to back-probe the connector shell and connect your voltmeter to them, make sure not to short the diaper pins together, or if you do you're wearing welding goggles. If you measure 12.5 volts with blower off but drops to less than 11 volts with blower on, the problem is the power supply voltage drops too much to hold the relay in.
Hopefully that makes sense.
So to recap, the switch operates the slave relay, which operates the blower. That's what it looks like to me.
I guess you could've installed 787's but that could be a lot of fun getting all the parts together. Remember, there aren't a ton of these boats still out there and many of us haven't owned that particular model and we don't have the service manual, we're just trying to help based on decades of experience and limited documentation. Fortunately, the circuits aren't particularly complex and it's not a supercomputer.
