2003 Johnson ST 6/8 HP 4 Stroke Outboards Service Manual, PN 5005471, Page 36Get this manual
INSTALLATION AND PREDEUVERY TRANSOM HEIGHT AND SHAFT LENGTH
TRANSOM HEIGHT AND SHAFT LENGTH
The transom height dimension listed in TECHNI CAL DATA in SPECIAL TOOLS AND SERVICE SPECIFICATIONS is the height of the boat transom required by the outboardThis dimension is measured at the transom centerline, perpendicular to the bottom of the boat
The resulting position of the antiventilation plate must be no higher than the boat's bottom and no lower than in (5.1 cm) below itIf the resulting position of the antiventilation plate is above or below the to in(0 to 5.1 cm) range, the engine shaft length must be changed or the boat's transom must be modifiedIf you install and operate the outboard: Lower than recommendedSerious powerhead damage could result from water entry into the lower engine cover or overloading from an under-revving propeller at wide open throttle Higher than recommendedSerious powerhead damage could result from inadequate cooling or an over-revving propellerPropeller ventilation (slippage) often occurs, resulting in poor boat performance1Transom height 2Antiventilation plate 3Final positionto in(0 to 51 mm)