2003 Johnson ST 55 HP WRL 2 Stroke Commercial Service Manual, PN 5005483, Page 38Get this manual
INSTALLATION AND PREDELIVERY TRANSOM MEASURING AND DRILLING
TRANSOM MEASURING AND DRILLING
Inspect Transom Area
Transom Clearances Make certain the transom and splash well area provide adequate outboard clearances Verify the proposed mounting-hole locations will provide adequate clearance for mounting bolts and washersCheck the inside area of the transom for obstructions prior to drilling mounting holesWater Flow Inspect the hull area directly in front of the transomBoat-mounted equipment should not create turbulence in the water flow directly in front of the outboardTurbulence or disruptions in the water flow directly in front of the gearcase will affect engine cooling and propeller performanceAvoid locating outboard centerlines within in(76 mm) of bottom strakes on dual-outboard installations
Transom Heights Make certain the transom height is consistent with the height of the outboard to be installed 19 to 21 in(48.3 to 53.3 cm) transom height uses 20 in(50.8 cm) shaft outboard The shaft length of the outboard being installed should come close to matching the transom height of the boatDetermine transom height by measuring from the top edge of the transom, along the centerlineUse straightedge as reference to extend the bottom of the boatPosition the straightedge along centerlineThe distance from the top edge of the straightedge to the top edge of the transom is the actual transom height