Just my opinion, but the 70hp on the 17' would do well, but I feel the 50 isn't enough for the 18. Talked with a guy that had a 60 yammy on his 18 and said the motor worked all the time to keep it on plane. Said he wished he'd gotten the 90. I'm inclined to agree with him. I feel it's better to have a little extra and keep your foot out of it, than to overwork a smaller motor. Good luck, those 18's are nice. You might contact "The Shootist" on the Tennessee board, he just re-powered his 17 and can tell you first hand.