3 bladed prop

[lane parent]

I have a 1986 catalina 36, which has a 25 hp westebeke diesel. It has a 1 inch shaft with a 2 bladed fixed prop. I am experiencing a lot of vibration from 2000 rpm and on since I owned the boat 2 yrs now. The engine was replaced before I bought it, but I am not sure if the prop had or would have to be re-pitched. The strut is good and the cutlass bearing is new and tight.I have had the shaft checked for allingnment and everything seems to be ok.
It has been suggested that a three bladed fixed prop would probably solve the problem.
Question-has anyone else had this issue?
Question-what size and pitch should the prop be?

 

[Stu Jackson]

I suggest you ask at the C36 Association website: http://www.c36ia.com/forums/forumdisplay.php?f=3

 

[Don Guillette]

These are just random thoughts. I wonder if the vibration you are referring to is actually cavatation? When the prop is too large for the boat and there is not enough clearance between the prop and the hull cavatation results. Don't ask me for a technical explanation because I don't know what causes cavation - someone told me it was air bubbles that form when the blade passes beneath the hull but I don't know.

What size prop is on the boat now? Your diver can tell you and he can also measure the clearance.

Also, I wonder if the prop might be out of balance?

I don't think a 3 blade is going to solve your problem. Before you invest in a fix you should determine the cause of the vibration. If it is cavatation, a 3 blade prop of the same size you have now will just make the vibration worse.

Please let us know what you find to be the cause of the problem.

 

[curley]

Same happened to me on a Southern Cross 31, 25hp Westerbeke. I changed to a 3 blade and the difference was night and day.

 

[wireland]

try this

Did you look at your engine mounts?

 

[Maine Sail]

Some thoughts..

Motor Mounts - Any worn, broken or loose?
Bent Shaft - shaft could need truing
Alignment - alignment between shaft coupling and transmission output flange should be adjusted to .003-.004"
Bent or nicked prop - Has the prop hit anything?
Weeds or floating line - Did you dive on the prop to make sure you did not pick something up?
Worn Cutlass - Should be no side to side or up down play and the shaft should pass though it perfectly centered.
Loose Strut or crooked?

There are just to many variables to assume it's because it is a two blade prop. There are plenty of two blade props that don't vibrate. Check the above list and work through it in an elimination type of manner..