Surprise! We were motoring at 6 knots in light air, when we heard some different sounds from our engine room, and the boat started slowing to a stop...dead in the water. We put a call out to the Baja fleet asking for assistance. Within minutes, boats were offering assistance: anywhere from advice, to help on board, to offering to tow us over 100 miles!!! A large catamaran pulled up and tied us along side them. The skipper came over and we delved into the problem. First, the prop was still on the boat, and not fouled by anything. Second, the shaft was free to spin forward and back while the transmission and engine were turned off. The engine purred, so it had to be the transmission. Looking at the tranny, we didn't see anything obvious on the outside. But when we looked at the dip stick, it was low on tranny fluid (ATF). We added some, then added some more, then added some more: we added 3 quarts!! to a tranny with a 4 quart capacity. So it was up to the dip stick mark. We put down new absorbent pads to catch any drips, spray, etc. -- none seen. The question was where did the three quarts of ATF go? We pulled out the manual that had a parts explosion that showed there is an exhaust valve on the top of the tranny. It may be to ease built-in pressure while running for long periods. There was a drip line coming off of it. We had been in a series of seaways where the boat was rocking fairly hard, and that the ATF may have been escaping. So we have been motoring since then. We have stopped twice to check the ATF level. So far so good!