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Old 01-26-2018, 06:11 AM   #1
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Default 3 blade to 4 blade prop

Has anyone ever switch from a 3 blade to 4 blade prop. Was looking for some info on selecting right pitch and diameter any help would be appreciated!
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Old 01-26-2018, 11:25 AM   #2
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Welcome aboard

It would be helpful to know what boat, engine and outdrive you have.

I recommend using quicksilver prop calculator that can be found on line.
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Old 01-26-2018, 01:34 PM   #3
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https://www.mercurymarine.com/en/us/...or/#!/step-one

If you go up in the number of blades, you need to go down in the pitch.

What is your goal? The mercruiser gasoline engines all have a max rpm of 4400 - 4800 rpm's. At the very least you would need to know what the max rpm is Wide Open Throttle (WOT)


I agree with Mike, you really need the following information to move forward:

1) Max RPM at Wide Open Throttle?
2) Engine size?
3) Carb or Fuel injected?
4) Outdrive model
5) Outdrive gear ratio
6) Boat stats(Length, width, weight)

I'd run through the calculator and see what you come up with.
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Old 01-26-2018, 05:20 PM   #4
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97 2100 Sr2 5.7efi alpha one gen2 with 1:47 gear currently SS vengeance 14 3/4 x 19p 3 blade looking for a prop in 4 blade that matches for better handling at mid range speed
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Old 01-26-2018, 08:45 PM   #5
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Quote:
Originally Posted by Bsw View Post
97 2100 Sr2 5.7efi alpha one gen2 with 1:47 gear currently SS vengeance 14 3/4 x 19p 3 blade looking for a prop in 4 blade that matches for better handling at mid range speed

What is the current WOT?

Try using the calculator that shrew posted the link for. I would think the prop you have with the engine/drive package would already be good. A 4 blade will help bottom end and may keep you on plane at a lower rpm. Top end will suffer unless you drop pitch.
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Old 02-03-2018, 05:26 PM   #6
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I have an SR3 1800 with the 4.3 Merc. I went from a 19" pitch 3 blade to a 17" four, and the change was great. Most boats usually suffer from too much weight (people) and not enough HP, so you don't get out of the hole, and can't stay on plane. I love the performance with the change, boat feels so much lighter, quicker. I'm really not all that into top end speed.
I think I used the prop calculator before I dropped the money.
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Old 02-05-2018, 01:29 PM   #7
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Quote:
Originally Posted by Bsw View Post
97 2100 Sr2 5.7efi alpha one gen2 with 1:47 gear currently SS vengeance 14 3/4 x 19p 3 blade looking for a prop in 4 blade that matches for better handling at mid range speed
A boats and cars are similar in at least one aspect. The engine and transmission don't contribute to the 'handling'. They dictate acceleration, speed and performance.

Not quite sure what 'handling' translates to in this case. Quicker hole-shot (time to get on plane)? higher speeds at lower rpm's? Faster top speeds?
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