Propeller pitch, diameter and number of blade recommendation

[Michael-Searay]

I have a 2004 3.0 litre Mercruiser sterndrive that the currently has a damaged 4 blade prop on it which I am going to replace. Looking in the prop section of this site it looks like the 3 blade may of been standard and I can seem to find in the manual what pitch and diameter would of been standard.

So I have two questions,

1) what would of been standard
2) what would be best for the following sort of use

lake & river (no ocean), light loads (3-4 light people), waterski so want that initial punch, reasonable top end speed as we cruise a distance to get to our favorite spot.

Thanks, Michael

 

[thumpar]

Re: Propeller pitch, diameter and number of blade recommendation

Each boat is different. Generally the more blades you have the better holeshot you get but loose a tad bit of speed on the top end. You pic the pitch by your WOT. It might take some testing.

Also this should be in the prop section of the site.

 

[Michael-Searay]

Re: Propeller pitch, diameter and number of blade recommendation

Thanks I will re post in that section.

 

[Michael-Searay]

Propeller pitch, diameter and number of blade recommendation

Edit: Threads merged.
iBoats Mod Team.

 

[Rick Stephens]

Re: Propeller pitch, diameter and number of blade recommendation

What pitch is the current 4 blade? And did the engine top out around the correct RPMs at WOT? (your engine specs might state a range like 4200-4600 or 4400-4800 RPM) You want it to be inside those.

If you know that your old 4 blade was inside the operating spec then you can work back to a three blade by adding an inch or two of pitch to it. But nothing wrong with running a 4 blade. I found a 4 blade runs a bit more precise control at slower speeds, definitely accelerates faster under load and maybe loses a teensy bit of top end (maybe, I don't run much wide open). From your description a 4 blade's not a bad choice.

Rick

 

[steelespike]

Re: Propeller pitch, diameter and number of blade recommendation

Your present prop size should appear near the base of the blades or under the prop nut.

 

[Michael-Searay]

Re: Propeller pitch, diameter and number of blade recommendation

Thanks Rick

Present 4 blade is 14.4 x 18 pitch. Engine recommended operating RPM range 4400-4800, but I find anything over 3600 sounds like it is reving way too high, so I have been running around 3000-3200 which seems comfortable for the engine but at this rev range it only maintains around 20-22mph and that is with a light load and not towing anything.

That's why I am trying to find out what would of been the standard prop for this Mercruiser Alpha 3.0L (Serial number OM640000) as perhaps the owner before me has fitted a prop that's pitch is way out and that's why it can't rev to where it should to maintain a bit more speed.

Michael

 

[thumpar]

Re: Propeller pitch, diameter and number of blade recommendation

The prop is about right for the boat. Get it to WOT and see what you are at. That is how you choose a prop. If you are trying to get a bigger prop so you can go faster at lower RPM you are going to be lugging the engine. The normal prop would have been a 3 blade 17 or 19 pitch probably. It depends on the boat.

 

[Rick Stephens]

Re: Propeller pitch, diameter and number of blade recommendation

Is it possible the prop was bent or damaged in the past and vibrating or creating some cavitation? Bends and big chips can do some weird stuff. That could cause the feeling that your engine is revving too high.

But only way to know if a prop is over or under pitched is to run wide open and see what the RPMs do.

Choosing between a 3 and 4 blade is more related to how much bite you get at slower speeds and maybe top end speed.

If I trim my 3.0 perfectly I can just get to 25 MPH at 3200 RPM with a 17 in. 3 blade. With a 15 in. 4 blade best I can do is around 22 MPH at 3200. This is measuring with a GPS. Numbers all fresh in my mind since I ran my prop tests this last summer on a boat I bought mid summer. I prefer the 4 blade, though slower, as it is much easier handler wriggling in and out of bass fishing spots than the three blade. If I was pulling up skiers I would also prefer the 4, but the three would be good choice if I wanted a higher top speed. With each, the important thing is WOT RPM being safely within range. Push the top of the RPM envelope and engine life is probably going down.

Rick

 

[steelespike]

Re: Propeller pitch, diameter and number of blade recommendation

The reason 3000-3200 seems better is because it is around a typical cruising speed.
Any motor should be propped so it can rev freely within its rpm range. Then you can operate it at what ever rpm you prefer.
If you force a motor to operate below its recommended rpm it's actually harder on the motor.Taking longer to plane,fading faster as you add weight and struggling more to maintain steady low cruising speed.
In a typical 18-19 ft boat a 3.0 could be propped with about a 21" prop for speed but with little use for water sports or heavy loads.
A 19 would probably work for water sports and a 17 for heavy loaded water sports.When propping the lower numbers you would
need to watch the lightly loaded rpm.
The sizes would vary depending on hull design and weight.
We need your brand and model boat, dry weight, if possible,wot rpm and gps speed.

 

[Michael-Searay]

Re: Propeller pitch, diameter and number of blade recommendation

Great feedback thanks guys,

Boat is a 2004 SeaRay 180 Bow Rider. 18 ft (5.49 m), 2,100 Lbs (953 kg) don't know the wot rpm and gps speed.

My local Mercury dealer is saying prop needs to be matched to not only to engine but to boat and suggests trying a 3 blade 16 inch 16 pitch prop, does this sound reasonable??? Any harm in trying to see??? Besides to my wallet of course.

Thanks, Michael



lbs advi

 

[Rick Stephens]

Re: Propeller pitch, diameter and number of blade recommendation

I would bet that you need a 19. A 16 is gonna over rev like crazy.

Rule of thumb is going from a 3 to a 4 blade you drop an inch or two of pitch to maintain same WOT RPM. Going the other way, from a 4 to a 3 blade you would go up an inch of pitch to maintain same RPM.

Not many boats in our class end up with a 16 in. pitch. My boat is about the same as yours - 17 foot and 2300 pounds. I run a 17 inch 3 blade or a 15 inch 4 - most others think I am too low and should be using a 19 in. 3 blade. For my hull, which is probably more drag than yours, a 17 gives me a 4600 WOT RPM. (Also possible my motor isn't giving the full 130 or 135 HP)

If you do try that 16 be careful romping on it. If you get any cavitation you're likely to jump past max RPM.

Rick

 

[Michael-Searay]

Re: Propeller pitch, diameter and number of blade recommendation

Thanks Rick, out of interest what diameter is the 17 inch pitch 3 blade that you run?

Michael

 

[Rick Stephens]

Re: Propeller pitch, diameter and number of blade recommendation

This is the 14.5 x 17 three blade.

And this is the 4 blade 14 5/8 x 15.

Rick

 

[jestor68]

Re: Propeller pitch, diameter and number of blade recommendation

The prop that would have come "standard" on your 3.0L powered 18 ft boat is the Mercury Black Max(aluminum) 14.25 X 21 pitch.

Depending on how many hours(wear and tear) are on the motor now, it may not be able to pull a 21 pitch any more; and may require a 19 pitch. If you purchase a Mercury(or Quicksilver) prop, it will be the correct diameter.

As others have said, only a full throttle test run will confirm which pitch is correct today. The rpm range for your motor is 4400-4800 rpm at full throttle.

Given that your motor is 9 years old, I'm estimating that a 14.5 X 19 pitch Mercury prop will be correct.

Except for the the hard core water sports user, a 3 blade prop will give the better overall performance.

 

[Rick Stephens]

Re: Propeller pitch, diameter and number of blade recommendation

Given that your motor is 9 years old, I'm estimating that a 14.5 X 19 pitch Mercury prop will be correct.

I would fully concur with that.


The Michigan 3 blade 14.5 x 17 inch is what was on my boat when I bought it and runs just fine. On a 25 year old boat like mine, with whatever wear and tear the motor has had and the type of use I got it for, the 4 blade is perfect. For general use that Michigan 3 blade is fine.

Based on your 4 blade 18 inch history, a 3 blade 19 inch is probably right in the ball park.

Rick

 

[Michael-Searay]

Re: Propeller pitch, diameter and number of blade recommendation

Thanks guys for your help, I will take your advise and go for the 3 blade 14.5 x 19 pitch Mercury prop. One last thing to muddy the water my Murcury dealer suggested a 3 blade 16 inch 16 pitch as this is clearly 1.5 inch larger diameter but 3 inch lower pitch, do they even each other out and will this be the same as the 14.5 x 19 pitch you are advising on???

 

[thumpar]

Re: Propeller pitch, diameter and number of blade recommendation

The diameter doesn't really make much of a difference. Just because is it bigger around it is still the same pitch in the end.

 

[Rick Stephens]

Re: Propeller pitch, diameter and number of blade recommendation

I don't think a 16 inch dia would even fit in my Alpha 1.

Best of luck on this. There's always a bit of mystery until you actually have a given prop by a given manufacturer in your boat at full speed.

Rick