Vibration, etc, in C34MkII with M-35B engine

[seacatc34]

First of all, the M-35B is different than previous C34 engines:  35 HP @ 3000 RPM; displacement is slightly more than the M-35 by about 6 cu in.  In a spec sheet by Universal (Westerbeke), dated 9/97 as best as I can determine, is the following:  "Universal recommends a propeller that will allow the engine to turn 3000 RPM underway at full throttle."
 
 I have a vibration that I've been living with for some time at higher RPM.  A big chunk of it went away after I tightened the engine mounts, but there remains enough to be bothersome.  There was a ding in one of the blades, likely from hitting a lobster trap buoy.  A prop shop did a few tests and found that the prop was unbalanced.  The next test showed that the shaft hole was drilled slightly askew!!  This created a built in wobble and is the source of the vibration.  These tests were done in my presence and explained on the spot.  The wobble was confirmed by computerized measurements at 4 radii on each blade.  The ding was not a significant source of vibration.  Punch line:  This stock Michigan prop is junk and must be replaced.  Now the question:  what to replace it with?  Why the question you might ask?
 Seems the computer recommends a 16X11 prop, given 35 hp, the 3000 RPM target at max throttle, 29.9 ft waterline, 14000 lb cruising displacement, a target speed of 7.25 kts and 1.8:1 gearbox.  Note that the prop determines RPM underway.  Too much prop and you overload the engine so as to max out at below target RPM and causing sooting, and converse.  Next step:  measure end of shaft to hull to confirm (or not) that there's not enough room for a 16 inch prop, allowing for cavitation room.  I'll let you know what I find.
 However, if anyone has any insights I'd much appreciate hearing from you.  In the meantime, if you have a persistent vibration problem consider having your prop checked at a Prop Scan facility.
 
 Mike Roy
 Cat's Paw
 #1373 (1997C34MkII)

[Bob Kuba]

As I sit typing this, my 16" Maxiprop is lying here next to me on the floor, waiting to be boxed up and sent back to PYI Inc. for a "trim" and a re-build.
 
 I bought a C34 earlier this year with this prop installed on the boat. And I can tell you, that 16" is too big for this boat. Cavitation and the resulting vibration become a problem at around 2200 rpm. I've talked at length with the manufacturer of the prop, and they have the ability to re-size the prop to the proper dimensions, and then rebuild and balance it.
 
 I've read on this forum that there should be a 20% clearance between the prop tips and the closest structure on the bottom of the boat. So a 15" prop should have 3 inches of clearance to avoid cavitation and vibration problems. Right now, with this 16" prop, I have approx. 15% clearance and it's not enough.
 
 I'll let you know how the re-build works out.
 
 

[Bob Kuba]

Well today was the big day. And it was worth every penny to have the prop re-sized to 15". It runs as smooth as silk through all RPM ranges. I also increased the pitch to 10 degrees, with a noticable increase in boat speed without any penalty in the M35A engine performance. (From the chart on the PYI website, X and Y translated to K and E.)