twin screw brushless/lipo set up now has wierd problem please help!

[pmdevlin]

Hi all,

really don't know if I am in the right place, but help needed before I go wildly spending money on the wrong things. Ok, its not one of your fast type racing boats, but the "normal" rc boat sites will not understand what I have. Its a 4 foot ply built scale model of a 1950's Vosper Crashtender (Fireboat), please don't lose interest at this point, its pretty special, it runs direct drive (propshafts go straight into the motors) 800kv twin motors, each motor has a turnigy 60amp reversing esc, and runs off 2 x 2 cell 5000mah lipo. DX6i tx, with eagle tree data logging and propshop 3 blade 50mm cleavers its good for 23mph, lame compared to your boats, but its well over scale speed, and is great fun.

So here is the story, its been running ballistic, last time out, I use a low voltage setting of 3.2v per cell on the esc's, and always run with each pack having its own voltage alarm, so no chance of over discharge. Shafts are oil lubed, everything runs nice and smooth, the shafts have 4 bearings, plus the motor bearing, so you get a picture of what we have. I really look after it!

Took it out to sail, with balanced packs, and right away I get a low volt alarm and a slow boat, with one of the escs shutting down. Back to base I charge and balance all packs, everything is fine. I check the settings on the esc's (by the beeps, real pain!) and triple check all is ok, back out to the lake, after a very short run it happens again, the problem is by the time the boat is in the batts have recovered and I don't know which has gone down, infact they have recovered pretty much to full charge going by the digital displays on the voltage alarms, The eagle tree is saying I had a very high amp draw, surprisingly not at peak speed, but quite low speed. Nothing tight or fouling. I had borrowed 2 4 cell 3000 mah packs so fitted them, knowing there is nothing wrong with these, same problem, almost like something is sucking the juice momentarily from the packs, causing the low voltage protection to kick in, again when the boat is back in the cells are showing almost fully charged.

Back home, I charge my packs, and discharge under load on a twin motor rc plane, they kick in to 3.2v within 20 seconds of each other, so I am thinking its not the batteries, although they are a no name brand, nothing special. Back to lake, I throttle real slow, and find as soon as I hit max throttle same problem, one or both of the esc's goes into low voltage shutdown. I check throttle range, all is ok, and decide to replae one of each of the 2 cell lipos with some 3 cell 5000mah packs, so now 5 cells per motor, everything seems to run fine, boat is fast.

SO..... (Are you still here!) its a real big job to change the motors, as the whole set up is very specific, the boat has only run a few times, so its not old. I could try another pair of esc's, I don't think its the batts, as I tried two other 4 cell sets that I know are good, and it was the same propb. When I put an extra cell in per motor, it appeared to be ok, but I am worried I just over volted through the actual problem, I really need a logical step by step diagnosis, I am in danger of running all over the place and cant afford to just buy things blindly, qalso work commitments mean I don't get a lot of time to keep testing things.

Any ideas!! (couple of pics so you can see the offending setup!)

Thanks