prop pitch

[Bayou Dave]

I have a 1987 Mercruiser 4.3L 175 hp with an Alpha One. The gear ratio is 1.84:1 For normal cruising with probably only 2 in the boat what prop size and pitch should I choose?

 

[sweet addiction]

Re: prop pitch

What is your current prop size and material? What are you looking for out of a new prop...top speed, holeshot, etc.? Are you currently within the recommended wide open throttle(wot) RPM range?

 

[achris]

Re: prop pitch

I have a 1987 Mercruiser 4.3L 175 hp with an Alpha One. The gear ratio is 1.84:1 For normal cruising with probably only 2 in the boat what prop size and pitch should I choose?

You need a propeller that will allow the engine to turn at between 4400rpm and 4800prm at Wide Open Throttle. I would start with a 21" and see how that goes. If the revs are too high, increase the pitch, too low and you need to decrease the pitch. Each 2" pitch changes the revs by about 400.... (Stay towards the top end of that recommended rev range)...

Chris.........

 

[sweet addiction]

Re: prop pitch

^^ Agree. achris maybe you could help with this.... There is also the matter of stainless vs aluminum and how it can affect RPM. I can't remember the numbers exactly or the general rule of thumb, but there is going to be a reduction of RPM when going from aluminum to stainless.

 

[achris]

Re: prop pitch

^^ Agree. achris maybe you could help with this.... There is also the matter of stainless vs aluminum and how it can affect RPM. I can't remember the numbers exactly or the general rule of thumb, but there is going to be a reduction of RPM when going from aluminum to stainless.

As long as the style of prop is the same, there is very little change in revs. I have a stainless "Vengance" and an Ali "Black Max", both 19", and they both run my engine to almost bang on 4600...

If you're going from a Black Max to a Mirage/Laser II/Hi-5/Revolution, different ball-game altogether. They are different style props, and you need to do all the test runs again. No two boats will be affected by the change the same.

 

[Bayou Dave]

Re: prop pitch

Only looking for top speed with an aluminum prop. I don't plan on pulling ski's or boards. Will proably only run WOT occasionally. I don't know what the rpm range it is as I bought the boat in December and have not put it in the water, yet. I will get it in the water on Sunday and will check the rpm range.

 

[jestor68]

Re: prop pitch

Your boat has a dry weight of 2400 lbs.

If you put your numbers through the Mercury prop selector, it comes up with a calculated weight of 3142 lbs and recommends a Black Max 14.25 X 21 prop.

 

[Bayou Dave]

Re: prop pitch

Your boat has a dry weight of 2400 lbs.

If you put your numbers through the Mercury prop selector, it comes up with a calculated weight of 3142 lbs and recommends a Black Max 14.25 X 21 prop.

Thanks jestor68 and all the other members that replied.. that is the anwser I was looking for. Just trying to keep it stock. Dave

 

[achris]

Re: prop pitch

Thanks jestor68 and all the other members that replied.. that is the anwser I was looking for. Just trying to keep it stock. Dave

You need a propeller that will allow the engine to turn at between 4400rpm and 4800prm at Wide Open Throttle. I would start with a 21" and see how that goes. .......

I guess you didn't see this then....