Prop help

[DEL1]

Have a 89 16 ft lowe with 2002 mercury 40 2 stroke prop is 10 3\8X14 and maxs out at 3000 rpms. What prop willl get me to 5000rpms with a decent holeshot? Anybody with this boat and motor combination? Thanks.

 

[jestor68]

Re: Prop help

The rule of thumb is 150-200 rpm change per inch of pitch.

Before changing props, I'd verify tach accuracy and motor functioning properly.

 

[DEL1]

Re: Prop help

The rule of thumb is 150-200 rpm change per inch of pitch.

Before changing props, I'd verify tach accuracy and motor functioning properly.

Checked out by mechanic, says to try new prop, maybe 10 3/8x11.

 

[Mi duckdown]

Re: Prop help

I'm with Jester. For one thing that motor should be running about 5500 rpms. something does not add up. a prop change too gain 2500 rpms????

 

[jestor68]

Re: Prop help

Since you haven't mentioned "which" 16 ft Lowe boat you have, I'll throw some darts at the board.

Using the Mercury prop selector(it never lies), using a calculated weight of 1568 lbs; it says you should do about 5250 rpm @ 28 mph, using a 12" pitch prop.

Now if we add 2" of pitch to that estimate(your 14" pitch); the rpm should be reduced by 300-400 rpm; resulting in 4850-4950 rpm. That's what you should expect to see with your 14" pitch prop.

But you're only seeing 3000 rpm.

So; either your tach is wrong, your motor is not performing, your operating weight is more than 1568 lbs, or.........?

The Mercury prop selector tells you what a new healthy motor should be able to do. Your 3000 rpm is off the chart; the minimum on that chart was 4000 rpm given the 14" pitch.

 

[DEL1]

Re: Prop help

Boat has modified v. weight 660+ motor 175 me 200 and fuel 75lbs gear 25lbs= about 1200lbs. Actually motor was checked out by 2 different mechanics. Of course they do not put motor under load. Maybe got to 3600 rpms trimmed up. Never got on plane. Will see if I can pick up a used prop and get a better number. Otherwise you might be right about the motor.

 

[steelespike]

Re: Prop help

If the boat is not planing that would account for an unusually low rpm that can't be used accurately.
Obviously if it won't plane at 1200 lbs its surely not the tach.causing the low reading.
Something is wrong with the setup.
Perhaps the throttle doesn't open all the way.
Maybe the foam under the floor is saturated with water.
perhaps the motor is losing a cylinder under load.
It does seem that at 1200 lbs your 14" prop should work.
Perhaps a little sluggish but planing as it should.

 

[DEL1]

Re: Prop help

Had 89 40 hp evinrude on it before. Did 33 mph and had good hole shot
Don't think it's the boat. Maybe it is dropping a cylinder. How could I test for this ?
Thanks

 

[steelespike]

Re: Prop help

Get a spark checker should jump 3/8 to 7/16 of an inch.
Assuming a 2 cylinder you could unhook one plug wire at a time and ground it
and see if it will run on the remaining cylinder.
remove the plugs and check the condition. Should be brownish/tan and dry.
If it turns out to be the motor I would wonder about your mechanics.

 

[DEL1]

Re: Prop help

Ok I've got it to 4000 Rpms cleaned carb. If I drop down to 10 1/4x 12 should I gain enough to get to 4800?

 

[steelespike]

Re: Prop help

Whats the dry weight of the boat If its a bare utility tiller steer I would guess about 300.
If a floor and console about probably 500 to 700.
A bare utility I think would scoot with a 14" prop about 5000 rpm
The problem is regardless of the boat a 12" prop is only going to addd about 400 rpm.
And a 10" prop is approaching that for a pontoon.
I think you still have some sort of issue.

 

[DEL1]

Re: Prop help

Side console 660 lbs.

 

[pootnic]

Re: Prop help

Did you ever tell us what speed your getting ?
Like mentioned already,it should handle a 14" pitch prop.
Maybe tach...probably motor/carb/fuel issues.
You gained 1000rpms just by cleaning the carbs;sometimes you have to clean them again.
Sure about your boat weight,foam soaked with water?

 

[steelespike]

Re: Prop help

Side console 660 lbs.

You still have issues other than prop selection.
If you go to the mercury marine section check the 40 hp tests
regardless if 2 or 4 stroke The motor should be turning about 5000 with a 12" pitch
What was your speed at 4000?
You should check compression.

 

[DEL1]

Re: Prop help

Compression 120 both cyl. Speed 26 mph trimmed up
Found leak in fuel line connector on engine and cleaned carb. That's how I got
To 4000. Once engine gets to 4000rpms and give full throttle (carb butterfly's open full)
Engine bogs and loses Rpms until I back off throttle.

 

[steelespike]

Re: Prop help

Assuming a 2.33 gear ratio with 10% slip 26 mph calculates to about 5000 rpm.
If the motor bogs at full throttle the timing may be wrong or advancing too far.
Carb could still be too lean.
If the gear ratio is 1.85 rpm would be indeed be 4000.

 

[BobGinCO]

Re: Prop help

That 200 RPM per inch of pitch is a VERY LOOSE rule of thumb.

On my boat, decreasing the pitch by 2" bought me 800 RPM.

 

[DEL1]

Re: Prop help

Gear rattio is 2.00. What would that calculate to. And if I adjusted carb a little richer would that might
Stop it from bogging. Assuming To much air is coming in carb through butterfly's .
Would love 800 more Rpms.

 

[steelespike]

Re: Prop help

Using 4000 rpm,2.00 ratio,14" prop 26 mph produces 2% slip highly unlikely at 4000 rpm.
One or more of our variables is wrong.

 

[steelespike]

Re: Prop help

Using 4000 rpm,2.00 ratio,14" prop 26 mph produces 2% slip highly unlikely at 4000 rpm.
One or more of our variables is wrong.If we change slip to a more reasonable 10% rpm is about 4300.
Jestor suggests that your rpm with a 14" prop should be in the area of 4800-4900
so it would appear you need to solve the bogging issue.