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props rule of thumb...

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Old 05-22-2010, 07:52 AM
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Default props rule of thumb...

just trying to figure out what props gives what speed...

Pretty much every stock engines runs plus or minus 5000 rpm, right?

from my experience with small props, a 3 blade mirage 21 runs 59-61 mph. a mirage 23 runs 62-65 mph. a 25 runs 66-69 mph... pretty much what I saw.

can we also say, a 4 blades 26 runs 73-75 mph?
a 28 would run...76-79 mph ?
a 30 runs... 80 to 84?

I ran a 47 fountain with 32's at 92 gps...

I'm I pretty much on it with props and speed? of course 1.5 ratio.
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Old 05-22-2010, 12:08 PM
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For the most part I'd say you are pretty close. Stepped hulls are more efficient and will run a little faster due to lower propslip percentages and cat hulls are even more efficient.

There is also the diffence in what rpm the engine is designed to run. It is best to prop them at the upper end of the recommended WOT range. Going from a 7.4 to 500efi, that would be a 500-600rpm difference.
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Old 05-22-2010, 04:24 PM
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My boat runs excellent with the labbed 24P Bravo 1s; quick and effortless to plane, good economical cruise, and strong mid-range acceleration. However, it seems to hit north of 5000+RPM too easy as I have quite a bit more to go on the throttles. I am a little nervous of over-revving my 454 Mags. Speed at 5000-5200RPM is in the low-mid 70mph range depending on load.

I have thought about trying a 25 or 26P, would this help? Then again it runs so good everywhere else in the powerband that I feel maybe I should just leave it alone.

Thoughts?
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Old 05-22-2010, 11:37 PM
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I just got back from the lake today and hit 82 @ 4250 and 26pitch 4 blade KME, Cleaver type. She was in one heck of a porpoise at that speed but think as soon as I get a bit more time, and a bit braver I might be able to run it out and see if the porpoise goes away. Think that 90+ is in the works!
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Old 05-23-2010, 12:50 AM
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Originally Posted by BowenCT View Post
My boat runs excellent with the labbed 24P Bravo 1s; quick and effortless to plane, good economical cruise, and strong mid-range acceleration. However, it seems to hit north of 5000+RPM too easy as I have quite a bit more to go on the throttles. I am a little nervous of over-revving my 454 Mags. Speed at 5000-5200RPM is in the low-mid 70mph range depending on load.

I have thought about trying a 25 or 26P, would this help? Then again it runs so good everywhere else in the powerband that I feel maybe I should just leave it alone.

Thoughts?
What is the actual WOT rpm that you hit???? You can spin a 454mag to 5200rpms with no problem. I spun mine 454mag with a bigger cam to 5400rpms or so. Are your speeds off a GPS???? Also do you have 1.5 drives because if you do your numbers are pretty much too good to be true.

1.5 gears and 71mph at 5100rpms is 7% slip is which is pretty much too good to be true on a straight bottom Formula. Even Fastechs don't have that low of slip.
1.36 gears would make the slip number more realistic.
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Old 05-23-2010, 08:22 AM
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What is the actual WOT rpm that you hit???? You can spin a 454mag to 5200rpms with no problem. I spun mine 454mag with a bigger cam to 5400rpms or so. Are your speeds off a GPS???? Also do you have 1.5 drives because if you do your numbers are pretty much too good to be true.

1.5 gears and 71mph at 5100rpms is 7% slip is which is pretty much too good to be true on a straight bottom Formula. Even Fastechs don't have that low of slip.
1.36 gears would make the slip number more realistic.
Never have pushed the sticks wide open to find out. I would say there is at least 2-3 notches to go in the throttles. If I had to guess, I'd think it would hit 5600RPM easy.

For the sake of this exercise, lets assume numbers are GPS and I will confirm next week or so. Boat has been GPS'd, but it's been some time since she was run in the upper end. Drive ratio is 1:5. Let me add, the motors are making a little bit better power than stock 365HP Mags.

How is slip percentage calculated? If 7% is unrealistic, what number would be considered realistic?
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Old 05-23-2010, 09:08 AM
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rule of thumb is to buy 36 pitch and keep upgrading motors untill you can spin them @ 8k
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Old 05-23-2010, 09:22 AM
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rule of thumb is to buy 36 pitch and keep upgrading motors untill you can spin them @ 8k
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Old 05-23-2010, 01:04 PM
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Never have pushed the sticks wide open to find out. I would say there is at least 2-3 notches to go in the throttles. If I had to guess, I'd think it would hit 5600RPM easy.

For the sake of this exercise, lets assume numbers are GPS and I will confirm next week or so. Boat has been GPS'd, but it's been some time since she was run in the upper end. Drive ratio is 1:5. Let me add, the motors are making a little bit better power than stock 365HP Mags.

How is slip percentage calculated? If 7% is unrealistic, what number would be considered realistic?
For a non stepped V bottom pleasure boat, 10-12% is about as good as it gets. Here is a link to a propslip calculator. For Bravo 1 4 blades, you need to deduct .5 -1 inch of pitch, since the stamped pitch is rated high.
http://go-fast.com/Prop_Slip_Calculator.htm
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