Peterborough Gemini 4.3 merc prop

Samgodbout

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Feb 16, 2017
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Hi ! i'm a new member on the Iboats forum. I have make a lot of Search on this forum before asking my question.
Peterborough Gemini 1988, open deck, 18,5 ft.
4.3 mercruiser Thunderbolt ignition 175hp
Alpha one ( dont know the ratio and my boat is store for winter )
Now, I have a Vortex aluminium 4 Blades prop, 18 pitch, 14.5 diameter.
ALone ; max speed 38mph, 4800-4900 rpm
With 6 people ; max speed really not fast, 4400 rpm.
. Which prop in aluminium I need to Buy to have the more speed ? I need aluminium for my Lake. Thank you !
My actual prop is perfect for Wakeboard and holeshot, but really not for crusing.
 
Last edited:

Sea Rider

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Sep 20, 2008
Messages
12,345
Better check a prop selector and write all those specs in. Should provide a starter prop which will need to be water tested as currently loaded to check if achieves at least middle to max wot rpm range. You like boating with 6 up all the time, most of the time, occasionally. Weight is an important factor which varies combo's water performance.

Happy Boating
 

Samgodbout

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Joined
Feb 16, 2017
Messages
3
hi Ses Rider. We are everytime minimum 4 people on the boat. I wanted tout Buy 3 Blades 21 pitch Quicksilver black Diamonds, but im affraid to lost too Much rpm. "On paper" i lost 200rpm per pitch, so I Will top top 4400 rpm alone. But with 6 people, 4000rpm.. this range of rpm is good or not
 

Sea Rider

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Sep 20, 2008
Messages
12,345
One thing is boating at full hammer down lightly loaded, say solo boating other history while heavily loaded. Paper and water trials rpm can differ much. What's the min-max wot rpm factory stated for that engine ? Ideal is to run at least middle rpm as usually loaded. It it were my same case would dial a prop that achieves max wot rpm with 6 up, with just 4 throtttle less while checking tach not to over rev. Going full rpm will assure better hole shot with slight less top speed.

Happy Boating
 

Bondo

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Apr 17, 2002
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70,468
hi Ses Rider. We are everytime minimum 4 people on the boat. I wanted tout Buy 3 Blades 21 pitch Quicksilver black Diamonds, but im affraid to lost too Much rpm. "On paper" i lost 200rpm per pitch, so I Will top top 4400 rpm alone. But with 6 people, 4000rpm.. this range of rpm is good or not

Ayuh,... Carrin' those kinda loads, look to a 19", as the 18" ya got now, puts yer rpms 'bout where they oughta be,....

Prop to the top of the rpms with a light load, so ya ain't luggin' the motor with a full load,...
4000 rpms is too low,... it's luggin' the motor,...
 

Samgodbout

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Joined
Feb 16, 2017
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I Will try to find ? place where I can buy and return de prop if its not good. On paper, 19 Will ne perfect I think, but if I can try the 2 prop IT would be the best.
 

Bondo

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Apr 17, 2002
Messages
70,468
Ayuh,... The 21" prop is goin' the wrong way,...
I'm guessin' the 19" Might be to big,...
A 17" will get yer rpms up to where they oughta be,...
 
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