Thats what I call a volt drop

[Croftie]

I fitted a bowthruster to my old 32ft boat, dedicated battery in the bow, It was very effective. New (to me) boat has a bow and stern thruster, both supplied by the engine start batteries which I fitted new end of last yesr. 2 X 130 amp in parallel. The batteries are more or less central in the boat (42ft loa) and I never thought the trusters were very effective. Today I fitted another battery 70 ah 800 CCA up front for the bow thruster using 50mm2 cables, as short as possible .

This supplies power in addition to the old setup. What a difference. All batteries charged and showing 12.7v (surface charge removed). Test1 operated the bow thruster with just the old setup which by the look of it only had 50-70 mm2 cables over a good length of the boat and only 8v at the motor! Test 2, switched in the 2nd new front battery and now under load showing 11v at the motor. Now to do the same to the stern thruster..............

 

[Deleted User YDKXO]

Well done, I think you've done the right thing irrespective of whether the thruster performance has improved. A couple of weeks ago, I was backing into my berth when both engines just stopped dead leaving us drifting towards the concrete quay. I couldnt restart the engines from the flybridge helm so I rushed downstairs and tried to restart them there but no luck. By this time my SWMBO had saved the day and some expensive grp repairs to our bathing platform with a couple of well placed fenders. Eventually I found that by switching the ignitions off and then on again I could restart the engines but then the engines stopped again when I was doing some manoeuvering to get the boat into its final mooring position but by this time we had lines to the quay so it didnt matter too much

So what caused it? I got my engine technician in to check for any fault codes and he found none but he did ask whether my bow and stern thrusters were powered by the engine start batteries. I didnt know but I called in an electrician to check my engine batteries and he found 2 out of 4 were completely dead but nothing else wrong but he also confirmed that both thrusters were indeed powered by the engine batteries. He suggested that when I had used one or other thruster during my mooring manoeuvre, that had pulled down the voltage of the two remaining operating batteries to such an extent that the engine management system had cut both engines

Anyway, the electrician replaced all 4 engine batteries and now I'm back on the boat for our summer holiday. So far the engines have behaved impeccably so it seems he was right about the thrusters and the batteries. One thing I have done is operate both thrusters whilst looking at the engine batteries voltmeter. I have a 24V system and I was shocked that operation of one or other thrusters pulls the battery voltage down by 4-5V and thats with brand new batteries. No wonder operating the thrusters with 2 dead batteries pulled the voltage down so much it stopped the engines. So what you are doing by installing separate batteries for your thrusters is an excellent move in more than one way

 

[burgundyben]

Very interesting stuff.