I have an 81' Hunter 30, and I just changed the prop from the stock 2 blade (13X13)to a 3 blade machine pitch prop. The diameter is the same but the pitch has been dropped from 13 inches to 11 inches. Thrust is now plentiful, but vibration has increased significantly. Should I worry about the vibration, or is it normal and expected?
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Bob Howie
Prop new or used?
Sometimes, if you use a used prop that was just put on the shaft, it might not be balanced. Also, you might be getting a little compression between the outer edges of the prop and the hull. Even tho it's the same size prop, more blades can make a difference. Best advice is to talk to the prop shop and see what they say. Repropping is part art, but more science, so talk to the pros.
Sometimes, if you use a used prop that was just put on the shaft, it might not be balanced. Also, you might be getting a little compression between the outer edges of the prop and the hull. Even tho it's the same size prop, more blades can make a difference. Best advice is to talk to the prop shop and see what they say. Repropping is part art, but more science, so talk to the pros.
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David
Prop
As Bob wrote talk to the prop shop. Make sure the 3 blade has been balanced. You need at least 1" clearance between the blade and the hull. Also, the added rotating mass of the 3 blade may have made more noticeable a defect in alignment of the drive train or a defective cutlass.
As Bob wrote talk to the prop shop. Make sure the 3 blade has been balanced. You need at least 1" clearance between the blade and the hull. Also, the added rotating mass of the 3 blade may have made more noticeable a defect in alignment of the drive train or a defective cutlass.
more clearance than 1"
David: I believe that the prop guys use 15% of the prop diameter for tip clearance. This would equate to something like 2". So you may be on to something here. I am not sure what a "machine pitch prop" is but Freds problem may have more to do with clearance than anything else.
David: I believe that the prop guys use 15% of the prop diameter for tip clearance. This would equate to something like 2". So you may be on to something here. I am not sure what a "machine pitch prop" is but Freds problem may have more to do with clearance than anything else.
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J. Tesoriero
Should not vibrate
I did the exact same prop upgrade on my 1980 Hunter 30, using a new Campbell Sailor 3-blade ( 13X11). There was no increase in vibration. Check your prop balance and alignment.
I did the exact same prop upgrade on my 1980 Hunter 30, using a new Campbell Sailor 3-blade ( 13X11). There was no increase in vibration. Check your prop balance and alignment.
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David
Steve Dion
Steve, when I worked for a prop shop we used 8% to 20% depending on the application. On sailboats a rough estimate for MINIMUM clearance we used shaft diameter. That is the reason I stated a minimum clearance for Fred of 1" as I assume he has a 1" shaft.
Steve, when I worked for a prop shop we used 8% to 20% depending on the application. On sailboats a rough estimate for MINIMUM clearance we used shaft diameter. That is the reason I stated a minimum clearance for Fred of 1" as I assume he has a 1" shaft.
Not a prop expert!
David: I am no prop expert. This is somewhat science and somewhat voodo. If this is the case then he should not have any problem with clearance. I was told by multiple sources that you need 15% of the blade diameter to prevent cavitation and excess prop wash. My real question is what is a "3 blade machine pitch prop". I have never heard of this terminology
David: I am no prop expert. This is somewhat science and somewhat voodo. If this is the case then he should not have any problem with clearance. I was told by multiple sources that you need 15% of the blade diameter to prevent cavitation and excess prop wash. My real question is what is a "3 blade machine pitch prop". I have never heard of this terminology
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Fred G
Thanks guys, and more info.
Thank you all for your replies. The prop was bought new. I will ask the shopabout the vibration. I still have the old prop as well. I took a lot of photos when the boat was on the hard getting refitted, so I should be able to check the prop clearance roughly with the photos. I was told the type of prop I was getting was a "Machine Pitch" prop. It is a round 3 blade that looks like all the other 3 blades I see on sailboats.
Thank you all for your replies. The prop was bought new. I will ask the shopabout the vibration. I still have the old prop as well. I took a lot of photos when the boat was on the hard getting refitted, so I should be able to check the prop clearance roughly with the photos. I was told the type of prop I was getting was a "Machine Pitch" prop. It is a round 3 blade that looks like all the other 3 blades I see on sailboats.
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David
Prop balance
Fred, even if it was a new prop it must be balanced. Any good prop shop will always balance a new one before delivery.
Fred, even if it was a new prop it must be balanced. Any good prop shop will always balance a new one before delivery.
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