Hi Guys
We have a 2006 V6 4.3 mpi FWC mercruiser with a alpha leg, on an old 21ft offshore glass hull. 105 hours in 10 months, and about to be 100 hour serviced in the new year.
The original prop that came with the engine is the 3 blade black max mercury model. since we go out in relatively rough conditions, and for long distances (35NM each way) we bought a second hand mercury offshore pro stainless steel 4 blade 14 inch diameter, 19 pitch prop in good condition with no evidence of damage. I think this is now sold as the mercury Vensura model.
This prop keeps the boat on the plane at much lower speed than the 3 blade alloy it replaced, and gives 4700rpm at WOT with an average load, so is correctly propped . it is much better in acceleration, staying on the plane and handling in the rough and hence suits us well. The only issue that has come about since fitting this prop is a vibration at low planning speeds of about 15 knots. The previous prop would not keep us on the plane at these speeds, so i am trying to work out what the problem is. The vibration only happens at the lowest possible planning speeds, a small movement of the throttle up reduces the vibe, and another increment of the throttle up gives us about 18 knots, and the vibe is mostly gone. It is not a pulsating, it is just a regular shake that most definately comes from the engine/leg. No vibes at 19 to 38 knots, nor at low speeds off the plane.
We thought the 4 blade prop might need a balance and hence was given to the prop shop, who charged us very little to tell us it needed not much work and was fine.
while an obvious solution is to change props, i don't understand what else but a balanced prop is needed and whatever happens we will want a similar prop that will allow us to hold a low plane speed for the smoother ride it provides in the rough seas. I know i should try and use another similar prop to test if it is entirely due to the prop, but there is not anything readily available, otherwise i would be onto it.
so would anyone be able to shed light on what might be causing the issue, or things to have checked by the dealer at the 100 hour service? Engine alignment? The prop shop mentioned something about the prop shaft... any more suggestions?
All comments will be appreciated.
Thanks
Max
We have a 2006 V6 4.3 mpi FWC mercruiser with a alpha leg, on an old 21ft offshore glass hull. 105 hours in 10 months, and about to be 100 hour serviced in the new year.
The original prop that came with the engine is the 3 blade black max mercury model. since we go out in relatively rough conditions, and for long distances (35NM each way) we bought a second hand mercury offshore pro stainless steel 4 blade 14 inch diameter, 19 pitch prop in good condition with no evidence of damage. I think this is now sold as the mercury Vensura model.
This prop keeps the boat on the plane at much lower speed than the 3 blade alloy it replaced, and gives 4700rpm at WOT with an average load, so is correctly propped . it is much better in acceleration, staying on the plane and handling in the rough and hence suits us well. The only issue that has come about since fitting this prop is a vibration at low planning speeds of about 15 knots. The previous prop would not keep us on the plane at these speeds, so i am trying to work out what the problem is. The vibration only happens at the lowest possible planning speeds, a small movement of the throttle up reduces the vibe, and another increment of the throttle up gives us about 18 knots, and the vibe is mostly gone. It is not a pulsating, it is just a regular shake that most definately comes from the engine/leg. No vibes at 19 to 38 knots, nor at low speeds off the plane.
We thought the 4 blade prop might need a balance and hence was given to the prop shop, who charged us very little to tell us it needed not much work and was fine.
while an obvious solution is to change props, i don't understand what else but a balanced prop is needed and whatever happens we will want a similar prop that will allow us to hold a low plane speed for the smoother ride it provides in the rough seas. I know i should try and use another similar prop to test if it is entirely due to the prop, but there is not anything readily available, otherwise i would be onto it.
so would anyone be able to shed light on what might be causing the issue, or things to have checked by the dealer at the 100 hour service? Engine alignment? The prop shop mentioned something about the prop shaft... any more suggestions?
All comments will be appreciated.
Thanks
Max
