I have exactly that: 24V on a 12V starter motor.
It was done on ealier engines where the original starting system was not capable of turning the engine sufficiently. The Sea Panther is a Ford Kent crossflow block which was designed as a petrol engine. The bottom end is bullet proof & so made a great block for conversion to diesel & marinising. But with the compression ratio doubled, the original 12V starter wasn't capable so the easy fix was to put 24V on it & specify it for limited engagement. i.e. if you can't start after a few seconds, leave it to cool.
Starting has never been a problem on my boat (apart from when I had a blocked filter). BUT, & this is a big BUT, the reliability of the starter motor is seriously affected. Since I've had the boat, I have regularly replaced the starter motor every year. The cost of £30-£40 from the motor factor was worth it. I just found at the start of last year that the cost has rocketed since they are not being refurbished. I'm almost certain that the brushes burn out & these are chaep to buy so should be easy to replace.
If anyone knows of a longer term solution like anyone that would do a rewind to make the original mechanics turn over the engine better on 12V, I'd be interested to find out.
I always wanted to learn to spell "engineer". Now I are wun.