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Old 04 August 2012, 12:03   #1
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87 mariner 75hp

Hello
I am running a 15ft rib with mark III tubes. Hurricane sealed hull. It has a mariner 75hp long shaft on it. It's Load varies and I'm looking for a good all round prop, forgiving! Camping with 7 people and gear and tubbing as well as 2 ppl island hopping

Any suggestions would be great. Rpms now are approx 4200 at 28mph in plane.

Thanks I'm advance
Darren

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Old 04 August 2012, 12:28   #2
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More info. It definately is not near over reving when full throttle. 2 stroke. Slow to get up when loaded and stands vertical when running solo.

Thanks
Darren
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Old 04 August 2012, 13:13   #3
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prop size

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More info. It definately is not near over reving when full throttle. 2 stroke. Slow to get up when loaded and stands vertical when running solo.

Thanks
Darren
You should pull about 5000 to 5500 rpm at full throttle.

I would have thought you need a 17 inch pitch.

Good luck
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Old 04 August 2012, 18:29   #4
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Thanks. Now I have to ask the difference between diameter and pitch? And is stainless prop worth it?
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Old 04 August 2012, 21:16   #5
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Now I have to ask the difference between diameter and pitch? And is stainless prop worth it?
Diameter is straightforward; it's the measurement across the prop tip to tip. Pitch is the "angle" of the blades; measured in how far the prop will travel through a solid medium in one revolution. Both pitch and diameter will affect the WOT rpm, bot lowering rpm as they increase. Typically, you'll lose roughly 150-250 rpm per inch of pitch increase, and 100 rpm for a quarter inch of diameter, assuming all else remains equal (which they never do. Ballpark numbers but can be way off.)

Stainless add another level of complexity: blade shapes are usually cupped (reducing rpm vs uncupped for a given blade shape), they are heavier (takes more to spin them), and they deform less (aluminums tend to lose a bit of pitch while running.) Generally, you'll drop an inch or two going from aluminum to stainless assuming the same diameter is used.

My personal feeling is that you should have two (or more) props: A short prop for heavy loads, and a longer one for running light. And a spare in case you hit something (and tools and hardware to change it out.)

jky
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Old 05 August 2012, 17:22   #6
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Thanks guys. I got a Rubex hub and a 3 blade aluminum 13.25x 17. Then work my way to a 15 pitch if needed. Too many rocks in the shallows for a stainless I think.

Thanks again.
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