2003 Johnson ST 55 HP WRL 2 Stroke Commercial Service Manual, PN 5005483, Page 39Get this manual
INSTALLATION AND PREDELIVERY OUTBOARD MOUNTING
OUTBOARD MOUNTING
Optimum boat performance depends on outboard mounting heightOnce the outboard shaft length has been selected for the transom height of the boat, the outboard mounting height must be determined
Using each stern bracket's lower mounting hole as guide, drill two 1132 in(8.7 mm) holes through the transom
IMPORTANT: Verify outboard water pressure is not adversely affected by the mounting height of the outboardGenerally, the anti-ventilation plate of the outboard gearcase must be in alignment with the bottom of the hullConventional V-hulls can typically perform well with the anti-ventilation plate approximately in(25 mm) above the bottom of the hullTest outboard and boat performance at different heights until the best performance is achievedBoats exceeding 50 MPH may benefit from higher outboard heightsConsult the boat manufacturer for performance information and specific outboard mounting height information related to particular boat