The issue is electrical as the windlass spins freely. There is a DC bus under the forward berth. Both the forward battery and the stern house batteries are connected to this bus. The windlass and the thruster are the only loads coming off of this bus. The windlass manual says that it uses a 100 amp motor. The manual says that the appropriate wiring is 2/0 for runs up to 50 ft. The cable run from the bow battery and the 2 stern house batteries to the forward bus is 4/0. The cable run from the forward bus to the windlass (about 10 ft.) is 2/0. So the cabling is oversize for the application. We were able to recreate the problem at the dock by unwrapping the chain from the gypsy and running the windlass. So with no chain or anchor weight the motor would initially run and full speed then, after 30 seconds or so, start to run slower and slower until I could almost stop it by grabbing it with my hand. We tried this with all 3 batteries connected, with just the forward battery and with just the 2 aft batteries. In all cases the engine was running. The engine alternator was putting out about 14.2 V. and was putting out about 25 A. When the windlass was running the alternator output jumped to about 52 A. The voltage at the forward bus was about 13.3 V and would drop to around 12.1 V with the windlass running. The windlass motor was almost too hot to touch after these tests. I took the motor out and removed and inspected all of the brushes. They looked ok to me. After reconnecting to shore power the charger did jump into bulk charging at about 110 A for the first 10 seconds or so. This suggests to me that a lot of energy was pulled from these batteries. I did disconnect all of the connections (Pos & Neg) at the forward bus and cleaned all of the contact surfaces. I did not test again after doing this. I also did not check the voltage drop along the positive and negative cables from the bus to the windlass as I was out of time I am thinking that the motor may be on its last legs. The windlass motor is a 1200 W motor. We have had no trouble lifting the anchor and chain for the 4 years we have had the boat. This is the same windlass they have been putting on these boats for years and I have never heard of any complaints with lifting power. I can recreate the slowing down without any chain attached so don’t see a need to add the weight at this point. I know it is not mechanical as the gypsy spins for 2-3 revolutions after I release the switch. I will take the motor to an alternator shop and see what they can find. Any other suggestions?
As you described in your email, both the wiring and the battery voltages at the bow positive distribution seem quiet adequate to run the windlass. It would be interesting to know the actual amperage used by the windlass. If you have a clamp-on meter, I would suggest getting a reading when the windlass is working hard. For certainty, have you measure the voltage at the windlass itself? It’d be interesting to know the voltage when the windlass is at rest and when the windlass is running.
It worries me that the windlass is getting hot just from spinning. Considering that there is no load on the windlass and considering the available voltage. Once you’ve ruled a steady power source to the windlass, I would suggest bringing it to an alternator/starter shop so they can have a look at the motor.