I am replacing my two blade prop with a three blade prop. It requires a 13' prop with an 11 pitch. My prop specialist has a 13X13 and said he can change the pith to 13X11. Is it common to change the pitch of blades or should I keep looking?
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Changing Pitch
This is a common practice when a seller would like to get rid of some dead inventory vs ordering the "right" thing. I've heard of it before. It is easy to do with bronze, but the problem is that the pitch starts at the hub and moves out the blade in a very smooth curve making it easy for water passing over the surface to maintain laminar flow. The pitch is constant from the hub to the tip. When re-pitched, a twist is induced into the blade creating sharper bends. The pitch will no longer be constant over the entire length and may induce cavitation due to the loss of laminar flow over a small portion of the blade. I would ask him to order a new blade for me.
This is a common practice when a seller would like to get rid of some dead inventory vs ordering the "right" thing. I've heard of it before. It is easy to do with bronze, but the problem is that the pitch starts at the hub and moves out the blade in a very smooth curve making it easy for water passing over the surface to maintain laminar flow. The pitch is constant from the hub to the tip. When re-pitched, a twist is induced into the blade creating sharper bends. The pitch will no longer be constant over the entire length and may induce cavitation due to the loss of laminar flow over a small portion of the blade. I would ask him to order a new blade for me.
pitch and dia
my hunter 30 came with a 2 blade 13x10 but previous owner installed a 3 blade 14x12 which gave it some push and a lot of prop walk. Unfortunately , that much prop just loaded down the engine which is not good for diesels. I found a local prop shop that took 2 inches off the dia and it works well. he finished wheel was a 3 blade 12x10 which gave me back my rpm's and still get hull speed. good luck, Charles Creel www.sailingmobile.com
my hunter 30 came with a 2 blade 13x10 but previous owner installed a 3 blade 14x12 which gave it some push and a lot of prop walk. Unfortunately , that much prop just loaded down the engine which is not good for diesels. I found a local prop shop that took 2 inches off the dia and it works well. he finished wheel was a 3 blade 12x10 which gave me back my rpm's and still get hull speed. good luck, Charles Creel www.sailingmobile.com
Props, hulls and engines
This topic came into play yesterday in a Mack Boring seminar, the jist being that prop manufacturors are in business to sell props and engine manufacturers build engines to be sold in boats to achieve specific 'full load' rpms for that specific model/design boat...which a diesel engine needs. Chuck alluded to the down-side of this with his 'loading the engine' reference. Lots of negatives begin to cascade if you can't get your engine to operating rpms because the prop won't let you. Yesterday's take-away information was that if you're considering changing to a prop different than OEM, make sure the engine manufacturer has a chance to provide input prior to purchase. Prop manufacturers are akin to barbers, who if asked "do I need a haircut?" [insert: do I need a new prop] will always answer "Sure you do!" I'll bet Yanmar has a stack of Service alerts on this topic, if that's who built your power plant. Check with them or your local dealer. Good luck w/ your investigation!
This topic came into play yesterday in a Mack Boring seminar, the jist being that prop manufacturors are in business to sell props and engine manufacturers build engines to be sold in boats to achieve specific 'full load' rpms for that specific model/design boat...which a diesel engine needs. Chuck alluded to the down-side of this with his 'loading the engine' reference. Lots of negatives begin to cascade if you can't get your engine to operating rpms because the prop won't let you. Yesterday's take-away information was that if you're considering changing to a prop different than OEM, make sure the engine manufacturer has a chance to provide input prior to purchase. Prop manufacturers are akin to barbers, who if asked "do I need a haircut?" [insert: do I need a new prop] will always answer "Sure you do!" I'll bet Yanmar has a stack of Service alerts on this topic, if that's who built your power plant. Check with them or your local dealer. Good luck w/ your investigation!
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