Originally Posted by Cheechako
The shaft will not become loose when you unbolt the coupling. It is very true that , although you may be out of alignment, it isn't necessarily the vibration problem. My 47 footer had a Nanni/Mercedes engine that at about 1500 rpm
had a harsh vibration throughout the boat
. It was not an alignment, bearing or motor
mount problem. Nature of the beast I guess. I just avoided that rpm
, but you have to pass through it going up and down.
Your problem sounds like a new one though, and hopefully it is that alignment.
Same thing with my boat
. Alignment is perfect.
Smooth until 2200-2600 where it's unbearable. Of course, that's exactly the range in which I usually wish to motor
The stern mounted barbecue
rattles so much it seems to threaten to jump overboard
Above that RPM range, smmoooooth again.
I used my Fluke ScopeMeter and an old speaker as a microphone just above the prop in the stern compartment deck
. I measured the frequency of the vibration and proved it was resonance of ihull because the vibrarion was 2x the propshaft rpm which I measured by the tachometer divided by the transmission
ratio. The 2x is because it's a two blade folding prop.
A friend suggested pouring concrete into a hole drilled in the aft deck
Yeah right ! Concrete in the stern of a multihull
? I'd rather deal with the vibration.