inboard vibration/noise problem

[bbar]

Hi,

I hope I'm leaving this thread in the right place. I have a 1964 Chris Craft 27' cabin cruiser. It is single screw with a plywood smooth bottom. Ever since putting a new bottom and different rotation engine in it 8 years ago, i have been chasing a vibration and noise problem. The noise kicks in at about 2800 rpm and gets steadier as speed and rpm increase. It will cycle as I go over wakes but even in calm water is there; very audible, very annoying. If I put trim tabs down, I can get some of it to go away. It will completely go away in a left hand turn, and get worse in a right hand turn. Over the years ,i have checked alignment, changed props, strut and cutlass, shaft, moved rudder to correct offset side for this rotation engine; all with no change. I don't know where to go from here. Any input would be appreciated. I have a video of it that I will try to put on here soon. Motor is left hand rotation, rudder is offset 1 inch off center to starboard. I can feel vibration if I put my hand on rudder box and steering arm, virtually nothing in rest of driveline. Thank you

[Ad Hoc]

Can you clarify the noise and vibration a bit more?

Do you get the noise the same time as the vibration and vice versa?

What is actually vibrating, ie the location(s)...is it the engine girders, the transom, the bulkheads the whole boat etc etc??..can you narrow it down a bit. Can you also give any indication of the amplitude of the vibrating members.

Have you tried to run the engine, clutch out, and rev the engine in flat calm water (harbour) over 2800rpm and does the vibration and noise still occur, for example??

[PAR]

Post a picture of your rudder, I'm fairly sure I know your problem, but a picture will tell for sure. Also see how much slack you have in the gland nut on your rudder port. While you're there check the quadrant, arm, steering for slop, etc.

[bbar]

Thank you for inputs. noise and vib are at the same time. feel most of it in rudder post, a bit in lower transom. noise is a hollow whirring sound that changes when going over wakes, stays steady in fairly calm water. noise and vib are only there under load at speed.

I have used 2 rudders; both stock to this boat. one bigger than other. both do same thing; noise and vib go away on left turn, right turn is worse than straight. Bigger rudder also causes steering wheel to want to turn boat hard right. Rudder box new this year, repacked , tightened to virtually no slop.
Will try to get pics up soon.

other info: shaft is 1 inch stainless steel. Thanks again

[bbar]

Have not been able to post video or photos yet. If you have any thoughts, please let me know as may pull boat this week. thanks

[baeckmo]

You say "motor is left hand rotation...." I take it that it is the prop rotation??? What is the shaft inclination? Has the distance propeller-rudder been altered with the engine/bottom rebuild? With the boat dry, look at the rudder. My guess is that the paint is worn away on its portside near the top!?

If so, you have a tip vortex hitting your rudder. The rising propeller blade (stbd side segment) is working with higher angle of attack than the downgoing side, generating a cavitating vortex, hitting the rudder with blade frequency. In addition there may also be high frequency "blade singing" due to vibrating trailing edges of blades.

Can you compare the old and new props regarding shape?

[Fanie]

If you put a left hand bottom on a right hand motor where are the drive shaft pushing to ? Could it be it's thrusting the wrong way and 'riding' the gear instead of flushing it...? Sorry, kinda guessing, I never opened a left hand motor.

[bbar]

THank you for inputs. Baeckmo- I am using a 12 degree strut so I guess that's the shaft inclination? I believe you are right and the paint has been worn at top portside. Current prop is a michigan djx(d shaped blades), past prop was a more typical elephant ear. Problem happened with both. If my problem is the tip vortex, what is the cure? Thank you

[baeckmo]

What shape is your rudder; is it a plate r. or is it profiled? What is the closest distance between propeller blade trailing edge and rudder leading edge? Rudder chord and balance? Could you describe where the paint has been eroded?

(Sorry to hit you with a lot of quests, when you're waiting for a solution, but they are necessary to find the right approach......)

[bbar]

sorry for delay. Am trying to compile info now. Need boat out for some of the numbers.