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Old 12-13-2010
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Prop shaft reducer/bushing?

I need a new prop on my 36' Pearson. The manual states a 17" dia x 12" pitch. I'm switching from a 2-blade to a 3-blade. I don't have a ton of money to 'invest' in a new prop, and I've found a great deal on aprop of these dimensions, but it has the wrong size boring in the prop.

My prop shaft is 1" diameter, the prop I'm looking at has a 1/25" diameter.

I want to ask you experts what you think about a reducer/bushing to make it fit. I spoke to one person at the local chandlery and she thought it was not uncommon, but might be expensive.

I'm curious if it's safe, what that process is, and what I should expect to spend.

Thanks.
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Old 12-14-2010
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I'm no expert in this, but I can tell you that my Pearson 323 has a reducer inside the prop. The guy who tuned my prop in the last haul-out said it was very common. Just a simple sleeve with a slot for the key/shear pin - didn't look that expensive, but we are talking about boats here.....I'd chat to a specialist prop company, who can probably give better advice.
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Old 12-14-2010
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Turns out my prop had a reducer on it made of some sort of plastic. It had broken at some time and caused the prop to be loose on the shaft, with a lot of associated vibration when running. Just a heads up that they might fail eventually. If you sense a ton of new vibration in the engine make sure to check the reducer to see if it stripped:



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Last edited by Beersmith; 12-14-2010 at 08:26 AM.
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Beersmith, could you have a drive sleeve, instead of a reducer? I must admit I have never seen one. but I am told they replace the shear pin/key, in that they hold the propeller snug in normal use, but if you hit something with the prop they are softer than the metal and so break up or slip.

FWIW, my reducer is a simple metal cylinder with a key slot in it. Looked like bronze. 1/25" thick. Stuck out about 1/8" behind the prop.
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Quote:
Originally Posted by paul323 View Post
Beersmith, could you have a drive sleeve, instead of a reducer? I must admit I have never seen one. but I am told they replace the shear pin/key, in that they hold the propeller snug in normal use, but if you hit something with the prop they are softer than the metal and so break up or slip.

FWIW, my reducer is a simple metal cylinder with a key slot in it. Looked like bronze. 1/25" thick. Stuck out about 1/8" behind the prop.
Not sure honestly. I was wondering how I should replace this and figured there must be a more sturdy solution than the plastic one.
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Old 12-14-2010
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Quote:
Originally Posted by Beersmith View Post
Not sure honestly. I was wondering how I should replace this and figured there must be a more sturdy solution than the plastic one.
The reducer is typically made of bronze. Not sure if the OP will find one that will take a 1 1/4" bore down to 1" though. Also, it sounds as if the OP is planning to use a 3-blade prop with the same pitch and diameter as his old 2-blade, which (unless I'm mistaken) would not be the correct size.

Last edited by Fstbttms; 12-14-2010 at 10:40 AM.
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Changing to a 3-blade from a 2-blade

Great point on the change from a 3-blade to a 2-blade. My owners manual doesn't actually specify if its a 2 or 3 blade, but the boat has had a 2-blade on it for as long as the paperwork goes back (1978).

The engine is a Yanmar 2QM20. 20 HP. The tachometer is long gone (on The List for replacement), but the printed RPM ratings are:

Continuous Output: 2600
1 Hr Output: 2800
15 min Output: 3000

It's a 36' Pearson, 1978.

Is this enough to figure the math for a 3-blade prop?
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Prop Size Calculator

For anyone reading this post - I found my own answer. This tool takes the variables that should be at every boat owner's hands (engine and hull manuals), and calculates the prop size and pitch, and gives the option for a 2-blade or a 3-blade propeller calculation. Pitch Calculation.

It looks to be accurate, and I found several prop shops pointing to this page for their calculations.
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Quote:
Originally Posted by Saileric View Post
For anyone reading this post - I found my own answer. This tool takes the variables that should be at every boat owner's hands (engine and hull manuals), and calculates the prop size and pitch, and gives the option for a 2-blade or a 3-blade propeller calculation. Pitch Calculation.

It looks to be accurate, and I found several prop shops pointing to this page for their calculations.
thanks for that link!
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