Instruments get fried when starting the engine.

OK, have you ever thought about PWM (pulse width modulate) the thruster so that it does a slow turn-on then?

Since, I'm no boater, how long is the minimum time the thruster stays on?

A pwm would be expensive and complicated, you'd need something that resembles a forkl lift truck motor controller.

Yes you cant really switch off all equipment at the helm, even when you just need to start the engine.

If the engines battery is permanantly connected to the engines alty then load dumping wont wreck the alty, however you still might get a spike if you switch in and out other batteries, but at least you might be able to isolate the spike from the starer getting to the domestic and helm supply - you'd have to remember to switch back on though, unless you had a voltage controlled relay that automatically closes when the alty fires up.

If you supply the helm from its own battery, esp a small one then this can be isloated when the engine is started, all your gear will run from it happliy for long enough, and a small battery will not cause monster spikes when switched in and out through a charge controller, which could be automatic - nothing to remember.
You have the added advantage during a major screwup engine and electrics wise you have helm electric at least for a while.

I think your situation has eveolved and sort of got you into a bit of an issue, you need a sort out, these spikey thingies will also be going into all your expensive navitronics, its out of my league size wise however I think you can get 12/24/36v marine supply conditioners, however you might need a bank loan to buy one.

K.I.S.S - the typical duty is on for say 5 to 10 seconds and up to 20 of those blips in a 5 minute period. That is what I have seen from other boaters anyway as I have yet to install and try mine.

dr pepper - Basically any combination of batts is possible as we have 3 off 2 way switches which are the classic marine ones with OFF, 1, 2, BOTH plus several simple connet banks switches. In 20 years we have stopped and started everything in every combination possible except this last episode when all the batts and all the chargers were ganged together. So we know the system works fine unganged but has problems ganged.

Now when we add the bowthruster we can also connect things in the 'best' way but as you say we may forget to reset it. However the entry to a marina is always a time of hightened awareness and so double checking everything is normal.

As a cleptomaniac I found an old car stereo power supply lead in the 'box' which seems to have mH and uF in it plus diodes so maybe some reverse engineering on this and build some more for inserting in line with sensitive navtronics.

Also contacted the bowthruster suppliers and asked the questions but they did not seem to understand, basically they want to sell a unit whatever and the other stuff is your problem. One thing they did do was recommend a 55Ah battery to supply this meaty beast which seems tad on the small size.

A car radio is a good source for a fat choke, you'll find a differential choke in an old pc power supply.

Sounds about right a 55ah battery on a 600a load, wouldnt last long, if I dont get on with things smartly my 110ah goes dead on my little thruster.

Well here is a thing. Just started the engine and the instruments are working again. What is this - some sort of self healing electronics!!!

Nothing else changed so the mystery gets worse by the day. Elf's, some kind of nano robot wow who knows but now I am really confused.

If your 110AH runs out smartly i feel the 55Ah is a bit stingy and hope to use the pair of 700AH to get 1400AH but they have a low CCA value which I must now check.

All I wanted was a simple bowthruster but now I have research coming out of ears. But I suppose it is only fair to it properly.

OP295 self healing - nice to know.

The randomness might come from the point at which you cut power to the motor, if it just so happens that the commutator is disconnected, or the current through the motor hasnt had time to build up to a sufficient level ofter commutation has taken place then switch off might not create so much noise.
If your situation stays sorted out for several operations then the problem has gone away.

Power surveys are good for power afloat.

The capacity of the battery is how log it can run the motor and the cranking amps is how much current it can dump in one go, sure you allready know that, that said cranking amps for a 55 can only go so far.

If the motor was ac rather than dc (which larger motors are becoming more like this) then the problem could be sorted out within the controller as commutation is electronic rather than carbon brushes.

In the UK boats have to get a safety cert every 4 years, I'm expecting every time for the inspector to kick off when he notices that by dc brushed motor that creates plenty of sparks is only a few feet from the gas storage bunker, the bunker is sealed from the bilges but it still worries me a little.

Don't knock "self-healing electronics". It does exist and there is a physical phenomena for it. Typically happens with CMOS IC's. Letting them sit for a long time without power will heal them. Removing power and shorting the battery contacts will heal them faster. I've repaired a few things that way: An HP calculator and a bike computer for someone else.

Nope wasnt knocking that, must be some property of inverse mode fets or something.
Just saying that the noise situation is random and doesnt necessarily happen every time.