6-10 hp would be fine, but it would really help if you could find something with a high thrust prop (for a low speed boat)

I wouldn't be at all stressed about taking a backup for a day trip like Mandurah - Bunbury. Its not that far really, there are few traps to fall into and sailing will be a lot more pleasant anyway.

Stonedpirate,

Believe you have just joined the Top Hat site

www.tophatyachts.com/

You will find all the necessary info on that site.
This subject has been much discussed.

"benzo said: 6-10 hp would be fine, but it would really help if you could find something with a high thrust prop (for a low speed boat)"

You will find that this is about right.

Better to get a 4 stroke engine [less fuel used and quieter] than a 2 stroke.

Oh, and remember Top Hats are driven by their foresails, so make sure they are in good condition.