Prop pitch

Airboat propeller discussion.
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jgebhardt
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Prop pitch

Post by jgebhardt » Fri Sep 25, 2015 5:09 pm

I have a DD boat and when I'm running with it at wide open throttle it's only turning about 2400-2500 rpm's but if I'm sitting still (stuck) and hold it wide open for a while (maybe a minute) it will spin up to 2900 rpm's. My question is why will it only spin up that much when it's stuck and is it safe to take some pitch out of it to get closer to 3000 rpm's while cruising at wide upen throttle. I'm running a 72" Whirlwind super snapper two blade with a Vortec 350.

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scottyroll
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Re: Prop pitch

Post by scottyroll » Fri Sep 25, 2015 6:21 pm

Is it carb'd; If so what kind?

Perhaps the secondaries are malfunctioning
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jgebhardt
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Re: Prop pitch

Post by jgebhardt » Fri Sep 25, 2015 8:24 pm

Yes it's carbureted with a Holley 650 with vacuum secondaries. I have a Holley 500 cfm two barrel but I need to rebuild it.

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mojoe
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Re: Prop pitch

Post by mojoe » Fri Sep 25, 2015 10:25 pm

Is it safe to drop pitch and turn Rpm up... Only with a warp. And only a little.

Doubt that pitch is the problem. I had a dd small block and it would rev up almost instantly stuck or not. Big blocks take a little longer, but not a minute. You shouldn't hold the throttle wide open for a minute on any car motor you want to be reliable. Well built 1/4 mile drag motors are held wot for a fraction of that each time and rarely last 100 miles.

I am no expert, so you may want to stop reading now. But it doesn't make sence. Wot is wot. If it takes a minute to hit 3000 it shouldn't matter if you are moving or not. If anything, you should loose rpms at wot when not moving. So I wonder if you are actually holding wot for the same duration while moving. If so, and I have never seen this, but I wonder if your hull/rigging is flexing and the throttle cable is stretched more on the ground then when in the water. That or a gauge/cable issue.

But if throttle position on tha carb is identical, and duration of wot is identical, it should only increase max rpm moving vs sitting still, no matter what is malfunctioning.

Just doesn't make sence to me. Maybe the secondaries, maybe bumping this up will get the attention of someone who knows. Just don't make sence!
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zrackers
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Re: Prop pitch

Post by zrackers » Fri Sep 25, 2015 10:50 pm

Holding a DD boat at WOT for extended periods will not hurt anything as they are limited to 3000 rpm at WOT. A gearbox boat turning 5200 may be a different story. I have a 4.3 L chevy and regularly cruise open water at WOT for hrs at a time. As long as your radiator keeps everything cool, there are no issues.

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keebo
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Re: Prop pitch

Post by keebo » Fri Sep 25, 2015 11:09 pm

Maybe your holding it longer while stuck or your tach is not right.

I have sen several people saying they cruise at wot, run how you like but remember if your wot your out of control. Meaning your options are limited if something happens. The term when in doubt throttle out no longer can help you avoid a disaster if you are already wot. JMO
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Re: Prop pitch

Post by jgebhardt » Fri Sep 25, 2015 11:31 pm

It cruises around 1900 rpm's and maxes out at 2500-2600. I'd like to see it around 2900 maxed out I'm just leary of adjusting the pitch to get it there under normal circumstances and the over revving it when it does whatever it does to make it spin up when it's sitting still.

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keebo
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Re: Prop pitch

Post by keebo » Fri Sep 25, 2015 11:39 pm

Not sure why it would creep up 300 RPM except for time holding it down or tach issue.
I would set it at 2850 on the trailer and not worry about it.
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zrackers
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Re: Prop pitch

Post by zrackers » Sat Sep 26, 2015 11:11 am

I agree cruising at WOT can be tricky. I run wide open sand flats on the AR river in OK though so usually there's no one else around and nothing to hit. My little boat only tops around 35mph as well.

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Deano
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Re: Prop pitch

Post by Deano » Sat Sep 26, 2015 11:36 am

That is just backwards from what you would expect. Running on open water you should get more RPMs when it's drawing cleaner air. This is why they say to set it around 2850 on the trailer to run around 3000 on the water.

Sounds like maybe when you are in the 'oh shit' mode trying not to get stuck, you are pressing on the accelerator harder than when just cruising on the open water. I would check the throttle linkage, throw, springs and throttle plates to confirm that is or is not what's happening.

Nothing wrong with adjusting the prop pitch as you are a long way from over spinning it, but I would be wanting to know why it is not acting in accordance with the laws of physics as we typically see them. And by the way, when I say 'long way from overspinning it' I am meaning the engine, not the prop. The prop has a much more defined and attainable top end than the engine does in this setting.
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speedymoe128
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Re: Prop pitch

Post by speedymoe128 » Mon Sep 28, 2015 11:45 am

I run a 72 wisper tip wide blade I would check you tack with another one an see if they compare and like I was told set your rpms around 2800 bc when you get on plane you will gain rpm wot

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Re: Prop pitch

Post by One Eyed Gator » Mon Sep 28, 2015 2:39 pm

Check make secondaries are opening

Fuel pressure

Float level

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