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Does anybody know the stock prop dimensions on the 424?
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Thank you that sounds about right 🙂
DO you recall if there is much clearance to swing a bigger prop?
On 5/23/08 8:28 AM, Paul Lefebvre wrote :
Mine is marked 20LH13, and if it is like everything else on the boat, it is
30 years old 🙂Paul
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A bigger prop? There is one problem many overlook with that – as the blades get closer to the hull, a low pressure area forms and causes turbulence and cavitation. It will result in a rather annoying rumbling sound when at speed and will cause the propeller to wear. Really.
You'd probably be better re-pitching the prop.
Bob
On Fri, May 23, 2008 at 11:43 AM, Paul Lefebvre < ([email][/email])> wrote:
There is a 4″ clearance to the bottom of the hull (It's easy to measure these things when the boat is on the hard 50 ft from my desk!!)
Paul
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Bob Fine
Fine Software LLC
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In theory, you can select your prop size to be 85% of the distance to the hull. If there’s currently 5″ of clearance with your 20″ prop, you could go as high as 25″ in diameter. In practice, I don’t know how close you can get before you start to get prop noise. At one time I had a two-blade 22″ prop and had no problem. A 20″ prop will start to cavitate at about 1200 RPM. With a 2:1 gear reduction, this translates to about 2400 RPM on the engine. If you go to a bigger prop with the same pitch, you’ll overload the engine. If you try to reduce pitch to get your engine speed back up, you’ll start to cavitate at shaft speeds of about 1000 RPM.
Why are you contemplating a larger prop?
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Rich Carter
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Because I’m looking into diesel electric propulsion, which has more torque.
On 5/23/08 10:10 AM, wrote :
Quote:In theory, you can select your prop size to be 85% of the distance to the hull. If there’s currently 5″ of clearance with your 20″ prop, you could go as high as 25″ in diameter. In practice, I don’t know how close you can get before you start to get prop noise. At one time I had a two-blade 22″ prop and had no problem. A 20″ prop will start to cavitate at about 1200 RPM. With a 2:1 gear reduction, this translates to about 2400 RPM on the engine. If you go to a bigger prop with the same pitch, you’ll overload the engine. If you try to reduce pitch to get your engine speed back up, you’ll start to cavitate at shaft speeds of about 1000 RPM.Why are you contemplating a larger prop?
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Rich Carter
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