You don’t give us much information. We need some details:

- exactly what is the “fairly hot” motor?
- exactly what prop are you running?
- exactly what battery are you using - brand, type and capacity?
- exactly how long is “running for a bit”?
- what is the timing advance set to?

Most likely you have too much motor for the boat/voltage/type of running you do, or inadequate packs. But more details will help.