Prop vibration

[DHHong Kong]

Anybody help with this one ??
We have excessive prop vibration on a relatively new build (one year)Schionning Wilderness 56 ft cat with 40 HP Yanmar 3JH4E engines and 2.64 ratio gears. Vibration is only in fwd gear at 1500 RPM (6 kts) and above on both engines and has been there since launch date - no vibration below 1500 in fwd and none in reverse thru the full speed range to 3000 RPM.
The props are two blade folding 19 x 16 Seahawks on 1.25" straight shafts thru skegs.
The max engine speed in fwd gear is 2600, so short of 400 from the rated speed, but this has slowly increasd from 2350 at initial launch to 2600 (9 kts) now that the engines have some hours on them (approx 20 hours). We may be over pitched slightly but the vibration is of greater concern at this time.
Due to total lack of vibration in reverse and at low speeds I am tending to think that possible turbulent water flow over the skegs at speeds above 6 kts may be affecting the bite of the two blade props each time they pass the vertical skeg. Too easy ???
Grateful for any assistance.
Cheers

Paul

[amarinesurveyor]

Paul,
I would suspect that the props might possibly be out of balance. What do the shafts look like when they are spinning, any wobble? has the engine alignment been checked? any play in the cutless bearings? any shaft zincs that are unevenly worn?
Brian

[bill good]

Could it be from the prop over the rudder? Can you lift one rudder (or both)
to see what this does.
Regards Bill Goodward

[DHHong Kong]

Thanks guys - the shafts, cutlass bearings, props, rudder post bearings are all "as new" with only 20 hours on the engines since launch. I have been under to take a look and there is no play anywhere, also no movement or vibration in the shaft log or anything else forward of the shaft, including gearboxes, engines etc when the 1500 plus RPM fwd vibration is going on at the prop ends.
There is approx 80 mm of shaft overhang between the aft cutlass bearings (we have cutlass bearings at each end of the 1500 mm shafts) and the prop hubs that provides room for shaft anodes. Prop tip clearance is approx 90 mm for the 480 mm(19") diameter.
Bill may be close to the answer (though we cannot lift a rudder as they are fixed) as I have talked with Ben Hawke at Seahawk who has suggested possible wash from the blades against the rudders and/or the bottoms of the hulls which are flat right above the props. It is very unlikely to be prop balance or shafts as there is absolutely no vibration at full bore in reverse. The skegs may be suspect as there is a relatively large grounding bulb at the bottom of each skeg which may be causing a vortex as water passes it at higher speeds which is then picked up by each blade tip as it comes around, the blades on these 19" props extending almost the the bottom of the skeg.
It is a bit of a head scratcher but we are taking her out of the water at month end to have a better look at any effects of prop wash and, most likely, to fair the skegs. I'll let you know how we go.

[Pblais]

You might check the shaft alignment. If done well it can be done to a tolerance of .003 inches. It usually is worse at high rpms. It can be far out of alignment and you really couldn't see it. If you could tell by looking it would be really bad.

[DeepFrz]

With cutlass bearings at each end of the shaft, alignment of the engine/transmission is even more critical. In fact I would think you should have a CV joint in the shaft behind the transmission. Maybe you have, but I didn't see it mentioned.

[DHHong Kong]

Thanks for the comments - we are using Centa couplings which are capable of absorbing quite a bit of mis-alignment however the alignments were re-checked and both couplings and shafts can be easily spun by hand so I do not think that alignment is the issue. There is no vibration at the inboard end of the shafts. It is a bit queer but we will probably wait until we come out of the water at month end, fair the lower skegs and look for any propwash signs on the rudders and/or hulls. More later. Thanks again.
Paul

[gramos]

Originally Posted by DHHong Kong

Thanks for the comments - we are using Centa couplings which are capable of absorbing quite a bit of mis-alignment however the alignments were re-checked and both couplings and shafts can be easily spun by hand so I do not think that alignment is the issue. There is no vibration at the inboard end of the shafts. It is a bit queer but we will probably wait until we come out of the water at month end, fair the lower skegs and look for any propwash signs on the rudders and/or hulls. More later. Thanks again.
Paul

try removing the props and carefully weigh each blade / if there is a tiny difference this will cause vibration .

[Esprit Marine]

Hi Paul,

Have your props been calculated according to your precise boat data ? From quick calculations according to Dave Gerr Bp-d method, your propellers will develop 850 pounds of thrust, and should have a minimum DAR of 50% (three blades) to avoid cavitation.

You should ask your propeller manufacturer to calculate maximum allowable blade loading for your propellers.

[DHHong Kong]

Thanks Esprit and gramos - I have been relying on the prop manufacturer, Seahawk, for calculations and they have been quite responsive while insisting that there calcs are correct as they have the boat specs etc. Interesting re the three blades comment as they do not consider cavitation likely at the 1500 RPM level with the engines developing less than 20 BHP at that RPM, also due to experience with other similar cats with similar sized two blade folding props. We may change to a slightly less pitch blade when we come out of the water at end-February to try to recover the 400 "lost" RPM at top end and this may also provide some reduced vibration benefit if cavitation has been present. I will be discussing this with Seahawk again on Monday mornign and will question re the blade loading and area.

I doubt we have a balance problem as there is no vibration in reverse even at 3000 plus RPM, though the props will not be open much in reverse with the boat stationary. There is no vibration when free running at high RPM in reverse as I recall (need to re-check this).

Thanks again guys - more later !!

[Esprit Marine]

Hi Paul,

You're right, cavitation should not occur below 2000 RPM even with a two blade propeller.

[GordMay]

Prop' clearances from:
http://www.cruisersforum.com/gallery...p?i=4649&c=500

[DHHong Kong]

Thanks Gord - Good detail. We are OK on dimensions a and b, approx 2.5x on d and c is OK as the folding props have a very log hub that moves the blade disc way back from the skegs which are faired well except for the bulb at the bottom which is the suspect. I'll record all dimensions when we come out. Not enough time this month to drop the rudders, pull the shafts and shorten dimension d but we'll see where we are at month end. Thanks for the input.
Cheers, Paul