Re: S2 7.9
Rhys - you ask good questions.
I have a Nissan 4-stroke 6 hp EXTRA long shaft on my 7.9. Weighs 65 pounds and does the job. I've motored almost all the way across Lake Erie with it - a good twenty five miles - at five knots boat speed, half throttle, no problem. That is in very light (or no) air. The 6 hp does not use much fuel. I only have a 2.5 gallon gas tank which is plenty.
If it's blowing, you won't be motoring. And you really need the EXTRA long shaft (25 inches, I think) for the few times that you are caught out in big wind and waves - a shorter shaft motor will cavitate like crazy as the prop pops up out of the waves. No outboard (including a 9 hp) is going to work very well directly into good-sized waves and wind.