Propeller Modification



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.

Diagnostic Procedure
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.


Although the installation and use of this procedure is well documented, we suggest caution. This is not the panacea for under powered boats. It can cause cavitation and prop blow out in some situations. A little experimentation is in order to achieve the desired results.
The hole size can be increased for certain applications. There have been applications with holes 1/2" and larger. Some have been cut at angles, some square, and some cut into the forward edge of the prop hub itself.
If by chance you do drill too large of a whole, you can retest with an epoxy patch, but once you have determined the correct size, take the prop to a prop shop so they can fill the hole back in with weld. Then re-drill to the correct size.
When testing be sure to try the prop under different speeds and conditions to verify satisfactory and safe operation.


Note: This procedure does not work with engines with thru-transom exhaust.
           It will work with Mercruiser's "Silent Choice System" ,
          if left in the thru-prop under surface mode during take off.

To see another version of this technique, See the PVS System from Mercury.

 

 

 

 

 

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