Washing machine drain shared with deck scuppers

[sailingunity]

I'm installing a small wall mounted washing machine, it's a gravity drain, and happens to be located directly above an already existing pipe for my deck scuppers.

Due to the short run, and the fact that the scuppers would break any siphon, I think this is an application where I don't need to worry about a vented loop... right? I might just put in a flapper valve to prevent the deck scuppers from backing up into the machine if it's raining quite heavily.

Alternatively, I could route the drain to the sink thru-hull which is about 6 feet further away.

But I could be wrong - and might just drain it into the bilge to be safe?