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Old 08-21-2012
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CS30 prop size? Vibration

I'm wondering if anybody knows the original supplied prop size for a CS30. (1989 Canadian Sailcraft 30)
I have done a considerable amount of searching and reading on appropriate prop sizes for a 30' boat.
I currently have a 2 bladed LH 15d x 15p x 1 and from what i read this seems to be far larger than normal.
I have a Volvo 2002, 18 hp with a 2.4:1 ratio.
In neutral i can get the motor to 3400 rpm and it is surprisingly quiet and vibration free.
In gear in flat water she will go 7.0 - 7.2 gps knots but the motor only gets to 2700 rpm. I cruise at 2200 rpm getting 6.4 knts (which is hull speed). There is a lot more noise and vibration than i would like.

I've only starting looking a prop size because last winter i replaced a traditional stuffing box with a PSS shaft seal and i have had a rattle in the drivetrain ever since.
I use Mainesail's excellent tutorials for just about every thing i change on the boat. The only thing i didn't follow of his was to change the coupling. The old one was an easy slide back on. Prop shaft to my eye looked good.
Alignment? i replaced the cutlass bearing at the same time but the old one looked in fine shape. I assumed alignment was good if it didnt wear out the old cutlass bearing.
I'm going to get some feeler guages this weekend and check.

Rattle is most prevelant at low rpms, gone at cruising speed.
Rattle was not there with old stuffing box, noise and vibration are about the same at cruising speed.

Is it possible for a too large of prop to setup a vibration that would cause an old coupling to shake and rattle on the shaft.

Fresh water boat, no barnacles. I've put on fins, snorkel and mask and gone underneath. I cannot feel any slop in the rudder, cutlass bearing or zinc (magnesium for us). No damage to prop.

I have my boat almost back to showroom condition and i know this rattle and vibration was not an option.
1989 CS30
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