won't plane

[JohnAvoca]

Boating newbie here. I've got a 2009 Yamaha 2-stroke 60hp and a circa mid 1980's fibreglass haines hunter 445r. The boat, a runabout, is 4.5metres long and solidly built. I've just got the motor fitted and while it runs very nicely the boat won't get up on the plane. Some boating friends suggested it's got the wrong prop - it's a 17. They say a 15 would be better. The boat mechanic suggests the motor might not be in the right spot on the transom. Any ideas anyone?

[boats.net]

Have the mechanic check motor mounting to see if right, check to see if engine is within rpm range of 4500-5500 rpm's, if has low rpm's to drop to a smaller pitch prop.