After hearing this puzzler last week, I thought about sending a response warning that reversing polarity to the starter motor would be a b-o-o-o-g-u-s answer, for several reasons. Harold’s right; most starters in service today are series wound and will turn the same direction, regardless of polarity. But newer cars, especially the small ones, are using permanent-magnet starters nowadays to save weight. Permanent magnet DC motors do switch directions if the polarity is switched. And, according to their internet websites, some marine suppliers are selling this new style starter as a cheaper replacement for common series-wound starters. But all the models I saw used solenoid-operated pinion gears, so the pinion gear would have engaged the flywheel (solenoids don’t change direction if polarity is switched). The engine wouldn’t have turned because the pinion has an overunning clutch, but that clutch would have screeched to high heaven.
I worked on small aircraft a number of years; some of the small twins use engines that turn in opposited directions. It’s a safety feature that takes a while to explain. Both engines used the same part-number starter motors, but had different “Bendix” (inertial pinion drive) part numbers. If only the motor was being replaced, the mechanic had to be certain it would turn in the correct direction; if it needed to be changed, he would rotate the back-plate 90 degrees. That would change the brushes’ position on the commutator, reversing polarity only for the armature. Switching either the armature or field polarity will change a series-wound motor’s direction, but switching both at the same time (hooking up backwards, for example) will not change the direction.
Sometimes, a single-engine aircraft owner would would drag his airplane up to our door, complaining that the starter motor would spin, but not engage the engine. We could look at the starter and tell, in most cases, that the owner had taken it to a local auto electrical shop and had it repaired; that glossy, sloppy black spray-paint job on the starter was a dead give-away. The motors were designed by Prestolite, so were the same type as lots of farm equipment, so the shop wouldn’t have necessarily known it was an aircraft starter. But even so, they should have been careful to mark the backplate position before taking the starter apart. The aircraft owner, who had removed and reinstalled the starter himself, not knowing for sure what he was doing, really didn’t have an excuse either.
We would rotate the backplate (usually could do that without removing the starter), look over the installation and fix any other screw-ups, and give the owner a stern lecture that he must call one of us to check his work in the future, or we would turn him in to the feds. I don’t know if the lecture worked 100% of the time.
As Rich’s Toys noted, if the battery was, indeed, charged backwards, everything on electronic on the boat was at risk; the alternator diodes, for sure, would have smoked. If the bilge hadn’t been thoroughly cleared (wonder which direction the bilge fan turned), that alone would have been interesting and eventful. The puzzler submitter apparently didn’t mention how much this cost the marina, but if it really happened, it wasn’t cheap.
I doubt, however, that the battery would have been discharged deeply enough to allow reverse charging. I haven’t seen a battery charger in thirty years without reverse-charging protection; if the boat only had the navigation lights left on overnight and the battery was in decent shape, it would still register 5-8 volts the next day. No “quick-charge” battery charger would charge it in reverse. If the battery was, indeed, stone dead. sulfation would limit its charge rate on a constant-voltage charger; recharging (in reverse) would take 12 hours or more, I bet. It would likley take a constant-current pulse charger to get it started charging.
Telling the whole story on a radio show would have taken a long time; but shortening it up will leave most folks with the mistaken idea that reversing polarity will make a starter motor turn backwards when, in most cases, it won’t.