My 1986 penn yan 21’ with 4.3 omc has no problem getting up to 4200 rpm’s but seems to drag the transom and doesn’t want to get up on plane. I verified that it has the appropriate prop so I’m wondering could I be getting prop slip or a coupling issue? There’s no noises etc such as bad spline issues but just not enough speed to put the boat on plane. Also considering the SEI conversion to the Merc alpha series but unsure how difficult the swap is. Thanks for any input