Vertical or less than that. I mean canted forward at the top motor mount about ten degrees. Reason being, when the motor is under load, the boat will assume an arch shape because the bow is pushing in. The prop is on the end of a long lever and tries to climb up under the boat. My 1980 18' Avon came with a motor mount and it is angled about 20 degrees out at the bottom before you load the motor, then about ten, loaded, then you go full throttle, negative ten, (lower case climbing under boat.) This is a 9.8 merc. four stroke, about 75 pounds. You set the motor up to take wide open throttle, because you might need it. If the outboard is angled up a little at idle, it makes no difference. Prepare to be amazed at the drag a raft has at speed. It is gargantuan. The 18' Avon will barely go upstream into a 2 MPH current with the motor wide open. This is with maybe 1000 pounds on board. On an 18' NRS Kodiak cataraft, maybe a little lighter, it goes three times the speed. Rafts drag a shiteload of water along. Plan for it.