All outboard and stern drive using thru hub exhaust propellers
Poor acceleration on hard-to-plane boats.
Solution - Modify the propeller by drilling three 6mm (7/32") (see notes at
bottom) holes "A" thru outer shell positioned as shown in the diagram. If there
is a rib "B" under any of the proposed hole locations, use another propeller.
Properly drilled, the holes will have no affect on top speed, maximum RPM, or ventilation
in turns. Incorrect hole size or location will result in no improvement or in excessive
ventilation, particularly in turns.
Drilling the holes allows exhaust gasses to bleed onto the propeller blades causing
controlled ventilation during acceleration. This allows the motor to turn higher RPM under
acceleration providing more power to plane the boat. Review the Diagnostic Procedures for
additional items that will contribute to poor acceleration.
The boat and motor should be inspected for the following conditions:
· Transom should be at least recommended minimum height for motor installed. · Boat
bottom must be clean and reasonably straight.
· Manual trim models should have angle adjusting rod positioned so motor may be trimmed
down to a negative trim position when accelerating onto plane. If motor will not achieve
negative trim, aluminum transom wedges should be installed between transom and motor.
· Transom and boat bottom should be inspected for deterioration or weakness to assure
they are not deflecting under acceleration.
· Propeller should be checked to assure it is undamaged, the correct type for the
application and approximately the correct pitch.
· Check the boat for weight balance. Many times a slight change in position of fuel
tanks, passenger seating, or other equipment will make an improvement in the boat's
ability to plane. Move weight forward to help the boat plane more quickly.
· Check the engine synchronization linkage. Make sure the timer base is advancing
properly, that the carburetors are opening at the same time, the throttle cam pick-up, the
pick-up timing and full advance timing are correct.
· If the motor's performance seems marginal, check the full throttle RPM using a test
wheel or dynamometer.
· Water test the boat to verify the condition. Use a shop tachometer to verify the full
throttle RPM is correct for the model motor. Boats having acceleration problems should be
propped to the upper end of the recommended operating range.
· To achieve good load carrying capability and top speed, the motor should be near the
maximum rated horsepower of the boat.