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Old 26 February 2013, 05:23   #1
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3 v 4 blade prop

I am running Mirage 19's on my new twin mercury 150 setup on Redbay 7.4, still deciding what props to actually buy as these are on loan. With the 19's i am getting 6200rpm at WOT. The top recommended RPM for these engines is 5800rpm. Even with 7 onboard still over reving. I do run with up to 10 people on board so don't think its worth going up to 23's

I am thinking that four bladed 21's will bring the revs down keep the acceleration and keep the top speed the same. I also hear that fuel economy is better on four blades?

Grateful for any input?
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Old 26 February 2013, 06:44   #2
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I'd try some 21's with three blades first.

As far as I know the reason to go up to 4 blades over 3 is to get more blade area.

This is good for acceleration - if you have the power.
But, more area means more drag which means lower top speed and greater fuel consumption.
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Old 26 February 2013, 07:08   #3
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Quote:
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I'd try some 21's with three blades first.

As far as I know the reason to go up to 4 blades over 3 is to get more blade area.

This is good for acceleration - if you have the power.
But, more area means more drag which means lower top speed and greater fuel consumption.
That's my understanding also. I have Mariner 150 and run 23s can't get up to 6000 so when contact agent they said that I should be running a 19.

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Old 26 February 2013, 13:19   #4
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Im running 17P revolution 4 prop's (4 blade) on twin mercury 150 4 strokes, on a 733 hurricane. Awesome prop, good bite in the rough stuff, changing over from a mirage to the rev4 will drop your rpm's 300-400.
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Old 26 February 2013, 14:09   #5
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Quote:
Originally Posted by redthunder View Post
I am running Mirage 19's on my new twin mercury 150 setup on Redbay 7.4, still deciding what props to actually buy as these are on loan. With the 19's i am getting 6200rpm at WOT. The top recommended RPM for these engines is 5800rpm. Even with 7 onboard still over reving. I do run with up to 10 people on board so don't think its worth going up to 23's

I am thinking that four bladed 21's will bring the revs down keep the acceleration and keep the top speed the same. I also hear that fuel economy is better on four blades?

Grateful for any input?
21" rev4s will be too coarse for you as previously said, 19" shouldn't be far off then you can have the props tweaked if required. Are your twins counter rotating? What speed are you getting at WOT? Are you looking for cruising economy or maximum top end speed?
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Old 26 February 2013, 15:56   #6
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I would say you'd be fine with revolution 4 21's. if you are getting 6200 with 19 mirages then you have plenty to play with as these new 150's have a big top end rev range (5000-5800). In my opinion I'd go with the 21's as they are the best for bite and best suited to these engines with their masses of torque. We did the arctic 22 with 2 of them and 19 rev 4's they're getting 5800 rpm I've got a 21 RH rev 4 if you need one to test. Only one though. Cheers. Peter
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Old 27 February 2013, 06:11   #7
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you can have the props tweaked if required.
Who would you recommend?
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Old 27 February 2013, 10:31   #8
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I've got 19" Rev4s on cr 115 Etecs on BP. They are about 10% thirstier than the 3 blade vipers that were originally fitted. The grip is phenomenal especially in the rough & on tight turns. The vipers were horrid, blowing out & slipping at every chance. I top out at 5750rpm which is exactly ideal WOT for my engines. I'd imagine with the bigger boat & bigger engines you wouldn't be far off the mark with 19s. Are the rev4s available in 1" increments, some are, in which case 20" might be the way to go.
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Old 27 February 2013, 12:56   #9
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Old 28 February 2013, 06:36   #10
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Quote:
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21" rev4s will be too coarse for you as previously said, 19" shouldn't be far off then you can have the props tweaked if required. Are your twins counter rotating? What speed are you getting at WOT? Are you looking for cruising economy or maximum top end speed?
They are counter rotating. Getting 45Knots at 6200 RPM Not holding the throttles open for long as I like my warranty. I am looking for good economy not top speed. They are very good so far with 1.5 litres to the mile at 28 Knots. So I need to drop 400 RPM. I believe you lose 200rpm per prop size up. A guy on another forum said that I was getting 11% slippage which is good.
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