40 Amp Block Diode Cathode to Stud 12 Volt Tech help needed

Would be much better if you were to draw put a sketch of what you have and what is connected to what and how.
Many times translation of words into a diagram get mixed messages.

 

A diode will only block when the voltage on one side (cathode) is higher than the other(anode) usually by 0.7v for silicon.
Cannot see how you have wired the negative of the dinghy winch circuit.
or indeed any charging arrangement.

Your perception of current rating of the cable might be in the ball park however, when it comes especially to low voltage and high current over long distance, as is the case with your dinghy winch motor, voltage drop is what determines the size of the cable used.

There are charts available which will give the cable resistance per amp meter to work out minimum cable size.

I had a similar winch arrangement driven from my tow vehicle and with parallel (2) 16sq mm copper conductors up and back and it was barely enough.
In that instance it was a 5 metre trailer sailer however there are other considerations such as gear ratios etc.

 

Sir Capt Salty . . . . .

Was this what you had in mind, wherein the very close proximity of the dinghy battery to its winch plus adequate gauge of wiring, being used, totally takes care of running the dinghy winch.
The Isolation diode keeps the charge on the dinghy balanced up with the main battery, yet is keeping the dinghy battery isolated.
Your charging system of the main battery was not shown . . . .
That hefty of a diode was not needed just for balance charging the dinghy battery, but within the limitations of that diode and the loss of the long wiring run from the main battery, the dinghy motor may additionally be getting just a wee bit of assistance from the main battery.

This is isn't sufficient to worry the anchor winch, but annoyingly does illuminate the anchor winch led light which is a concern as this is the indicator that power is on to the winch (when it may not be) and a safety concern.

You have two different levels of current passing between the LED light when using the anchor winch, versus the dinghy winch.
Concentrate on the black LED wire going to the battery ground . . . it doesn't carry but decades of milliamperes of current.
Then see if your tried procedure of installing 1-2-3 series arranged smaller 1N4007 types of diodes will establish a threshold such that the LED only lights on the heavier anchor activation.
Remember since you are now on the NEG versus POS side, that TOTAL string polarity may need flipped.

DRAWING . . . . . .





73's de Edd
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By the time you can make ends meet, they've moved the damn ends !

 

The stud diode is only useful if you have a split-diode charging arrangement for ALL the batteries. We can only presume this is the case?

If not, the use of the dinghy winch will draw from the dinghy winch battery until such time the dinghy winch battery voltage drops to 0.7V LESS than that supplied by the main winch battery - at which time the diode conducts and 'assists' the dinghy winch. If the volt drop caused by using the wrong cross-sectional area cable is 'high' then this assistance will be 'all the time'.

If the charging arrangement is supplying (wired to) ONLY the main winch battery then the dinghy winch battery will never get fully recharged due to the volt drop across the isolating (stud) diode.

Of more concern is the current draw. A 6mm cable will drop 17% (around 2V) over 12m at 30A so the cables from the dinghy winch battery to the winch itself should be kept REALLY SHORT whilst the CHARGING cable (from the aforementioned split-charge-diode) also needs to be rated accordingly.

It is wise to keep the voltage drop within 10% so a 10mm cable would be preferred although the duty cycle IS small therefore 6mm will work. I always err on the side of caution due to the risk of fire at sea.....

 

I don't follow why you're charging off the winch circuit. Why not off the alternator? Are the winch batteries in the bow? Typically you have the engine running when using either. The genset with associated charger can add additional amps. I'm guessing you raise the dinghy, then go forward and raise the anchor. Do you attempt to do both at the same time?
Remember the alternator must be running at 1500 RPM's or so for rated output. This doesn't necessarily correspond to engine RPMs. Me I put them all in parallel, It doesn't seem possible to have sufficient amps to run these items.
While this is a great site, try https://www.thehulltruth.com/marine-electronics-forum-19/. It is only marine electronics