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Question about Prop pitch and size.

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Old 07-08-2007, 09:31 PM
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Default Question about Prop pitch and size.

Hey Guys, I Just got my boat in the water a few times this year and I had a question about the prop I am or should be running. My boat is a 21' wellcraft Elite with a 300hp 350. I am running a 19p 15-1/2 Mirage prop. The boat comes out of the water strong and fast and plans much faster than my Frineds 271 Fastech, and she tops out at about 54mph. My question is what would make the top end go up a little more? Which way should I go with pitch and size? Thanks.
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Old 07-08-2007, 09:46 PM
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Try a 21 picth prop.
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Old 07-09-2007, 12:01 AM
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So a Higher pitch like a 21 would get my top speed up some?
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Old 07-09-2007, 09:40 AM
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350 mags should run in the 46-4800 range wot by my mercury paperwork ( mpi's a little higher like48-5000). If you are running in that range your prop selection is right. If you are over reving or hitting the rev limiter on the mpi go up in pitch. I run 23p laser2s on my carb mags , my bud runs the 23p mirage pluses on tha mpi mags.
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Old 07-09-2007, 09:59 PM
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I am running a 350 with a comp marine cam (218/218 with a 110LSA) and Vortec heads with a 750 Carb. It turns about 4500-4700 with the 19P. The power range on the cam says to 5500.
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Old 07-11-2007, 12:39 AM
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Does the size of the prop (15-1/2inch) have anything to do with it. I know most 19 are about 14-1/2.
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Old 07-11-2007, 05:53 AM
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Yes it could. A larger diameter prop also has more blade area. I run a 21" LaserII. While the LaserII prop is the same pitch as the Mirage I had before the blades are angled back more ( more rake), has smaller blades, and it has a smaller diameter. The increase in rake helps to carry the bow better at speed with less drive trim. The smaller blades have less area and the smaller diameter allows the engine to spin up faster which gets me out of the hole quicker without losing top end speed like a lower pitch would.

If you do swap props an increase in pitch from 19" to 21" will drop engine RPM by 200.
General rule of thumb....every 1" change in pitch equals a 100 rpm change.

And props for out drives are listed in odd pitch numbers (17", 19", 21", etc). Props for outboards are in even pitch increments. The two are not interchangeable.
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Old 07-12-2007, 09:41 PM
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Thanks Guys. Anyone know what the rev limiter is on the Thunder Volt 4 ignition?
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Old 07-14-2007, 12:15 PM
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Does the size of the prop have anything to do with it? My prop is a 15 1/2, where most 19p I have seen are around 14.
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Old 07-16-2007, 06:10 AM
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It might.
The bigger the diameter, the more blade area, the bigger bite the blades have on water, the more strain that's on the engine, the slower the boat gets out of the hole.

Switching from a 21" to a 19" pitch of the same model prop (Mirage to Mirage) you will see a difference in time to plane but you will drop top end speed. If you went to a Mirage Plus with cupped blades then the prop will act like it's 1" larger in pitch.

Selecting a prop is a tough decision. I spent lots of time reading eveything I could on props. First thing I had to decide is what I needed the prop to do. I needed to get on plane quicker and I felt the boat needed too much drive trim to carry the bow. I knew a drop in pitch would speed up the time to plane but it would drop my top speed. I also knew a change in blade diameter would make a differance and an increase in blade rake would help carry the bow better. I searched and found a prop in the same pitch as I was running but with steeper rake, smaller blades, and a slightly smaller diameter...the Laser II.
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