Posts: 3508
Location: Elk River, Minnesota | Hiya,
Have you tried changing the hole size for your vents to a smaller hole? What could be happening is you are getting some exhaust coming out the vent holes at the lower RPMs, causing some ventilation of the blades. When this happens, you lose grip on the water, and thus the prop cannot hold the bow up, thus the porpoising. The issue with a vented prop is that at lower speeds, the vents allow exhaust into the fins and you cannot keep it on plane....that is normal. So...when you run a vented prop, you must maintain a certain speed to keep the boat on top of the water. Not always the most efficient system to have, and if you get into a wind where you need the extra thrust but not able to run on plane, a vented prop can be a nightmare.
I am also wondering on your max RPM's for the motor and what are you getting for max RPM's for your normal load. If you are low on max RPM's, then I would say you are over-pitched a touch and the motor in it's mid-range power cannot keep the bow up at any trim angle.
I would start with the vent plugs and see what happens there. Go to a smaller hole for each vent and try out the prop...you might be pleasantly surprised. It is an easy thing to do and not all that expensive.
Steve
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