There are special high thrust versions of 9.9's they have a bigger diameter prop. I have a yamaha T9.9 electric start with trim on an 18 foot and its the cats ass. Top speed about 6mph and idle speed of around 1mph.
Some things to think about. If you are going to be running with the kicker tipped up in big waves with lots of pounding with a manual tilt motor the assembly can break from bouncing around. That is why I went with electric trim. You will have to meaure to see if you need a short, long or extra long shaft so the kicker prop will sit below the bottom of the boat (if it is mounted right on the transom) One time I tried a short shaft on a 17foot lund when I realy needed an extra long shaft and found that turning sucked big time and the thing would just cavitate if you gave it much throttle due to the prop not sitting below the bottom of the boat.
A 6hp would also do the trick for your 18foot boat..
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