prop diameter

[sharkin95]

I have read that a 15 1/4 x 19 pitch Mercury Mirage prop runs a lower rpm because of the diameter. I have a IMP Sportsman 220 4.3 v6 alpha 1 o/d 3400 lbs. should I go to a Mirage 15 1/4 x 17 to achieve 4800 rpms

 

[alldodge]

I have read that a 15 1/4 x 19 pitch Mercury Mirage prop runs a lower rpm because of the diameter. I have a IMP Sportsman 220 4.3 v6 alpha 1 o/d 3200 lbs. should I go to a Mirage 15 1/4 x 17 to achieve 4800 rpms

You didn't mention what your rpm is now. Rule of thumb is 200 rpm increase with each degree of pitch decrease

 

[sharkin95]

I understand about the lower pitch or higher pitch let me redue the question. is there a difference between a mirage 15 1/4 x 19 compared to a say a Vengeance 14 1/4 x 19 in rpms

 

[Bondo]

I understand about the lower pitch or higher pitch let me redue the question. is there a difference between a mirage 15 1/4 x 19 compared to a say a Vengeance 14 1/4 x 19 in rpms

Ayuh,.... Probably not, but I bet there's a big difference in blade area,.....

 

[sharkin95]

thanks Bondo so pitch wouldn't matter but larger blade area will be a bit slower on top end?

 

[achris]

Mirages are designed for big displacement engines, with lots of torque. They also produce some serious bow lift. The 200rev/in rule is for props of the same family, like a 17" Vengeance and a 19" Vengeance. Once you change prop style you have to start the testing and tuning again. There is no rule regarding changing from say a Vengeance to a Mirage.

I had a customer many years ago who binned a couple of Yam 70s for a Merc 135. To help with the process we lent him a handful of props, including a couple of Mirage. To get the right revs from the engine he needed a 13" mirage, but in the smaller diameter props 17" was closer...

Pitch for pitch, the bigger blade area of the Mirage will cause it to have a much lower WOT engine speed.

Why are you looking to change? If you have a 19" Vengeance pulling too many revs, change to a 21" Vengeance. Your V6 will not appreciate a Mirage prop... You need an engine with a lot more mid-range torque.... And that's from first-hand experience...

Chris......

 

[sharkin95]

my boat is 3400 lbs I figured a larger diameter prop will get me on plane faster and hold better in rough seas with better mid range fuel economy.I know trying to sling a 15 1/4 x 19 mirage might be a load on the engine but I think she can handle it, that's why I was asking if a 15 1/2 x 17 might be a better choice and still achieve the same results

 

[achris]

What are you currently running and what performance are you getting?

I have roughly the same weight boat and a 19" Vengeance gets me right in the middle of the rev range, but I run 2" shorter (17") and the boat performs so much better. And as I have a fuel flow meter I can also tell you that regardless of the prop, speed for speed, the fuel consumption is exactly the same. Lowering the WOT revs will not use less fuel. All it will do is load up the engine more. The engine will use what it will use. The best way to use the least fuel is to ensure the hull's clean, the prop's in good condition, and the engine's in perfect tune...

Chris....

 

[tpenfield]

Both diameter and pitch contribute to how much water a propeller wants to move per revolution. So, in that regard, yes expect the smaller diameter propeller to move less water and needing less power, therefore a given powered engine will turn the smaller prop faster.

As to what that means for your boat, the rpm and speed that result, that is difficult to say exactly. The sort of thing you need to try to be sure. Expect the smaller diameter prop to spin faster, much like the lower pitch prop would.