General Prop Issue

happyfisher

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Dec 28, 2008
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Just purchased an 18' Aluminum (Spectrum) F&S, mostly for fishing. I have a Force('93) 120 w/ 13x19 Alum. Quicksilver. This prop gives great hole shot and top end. I also have a 13x19 SS Quicksilver that I haven't put on and tried yet. Problem is that with the Aluminum on I am easily running around 1/2 to 3/4 throttle to keep in the 4500-5500 RPM range. And does this thing drink the fuel. Running in the 45-5500 range I run around 45MPH. Satisfied with all but the fuel sucking. It seems that I need at least a 21 or 23P range to bring this into proper range. Should I be running in 3/4 to W.O.T. to get the max fuel economy? Or should I be looking at another issue? Such as main carb jetting? Guess that the Total running weight, full fuel is about 15-1700#. Never weighted it.:confused:
 

Bondo

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Apr 17, 2002
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Re: General Prop Issue

Ayuh,...

I'm not sure I understand your Question...

The motor needs to be propped so it Can pull the recommended rpms at wot...
Once that's done,...
If you want to save fuel,... Pull back on the throttle,+ Go Slower....
 

happyfisher

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Dec 28, 2008
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Re: General Prop Issue

Bond-o

Thanks for the reply. What I mean is that with a carb feeding the fuel, there seems to be "sweet spot" of throttle plate (carb) opening and desired throttle range. The design of most carbs in marine use seems to be made this way. I am only running about 50-75% throttle opening, achieving 4500 RPM and good speed/plane 45MPH, but if the throttle opening is too low won't I be using more fuel due to the lack of ideal fuel/air mixing at that ideal spot the manufacturer designed the engine to run at(4500 -5500RPM) at W.O.T.????:confused:
 

a70eliminator

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Sep 9, 2007
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3,762
Re: General Prop Issue

Heck I can answer that one, MORE PITCH if wot spec. is achieved @ 1/2 throttle then your underpropped for sure, 1 change in pitch = about 200 RPM. how that relates to throttle position I do not know, but a higher pitch will definately cause you to apply more throttle to achieve the same RPM. I'm underpropped a tad with my boat like you just pull back the throttle a little when by myself, but with a boat load of kids coolers ect. pulling tubes it's great.
 

steelespike

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Apr 26, 2002
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19,069
Re: General Prop Issue

In order to make a prop decision we need you actual max rpm and speed.
The 120 is will use 12gph(gallons per hour) at wot(wide open throttle)
In my opinion best cruising depends as much on the boat as the motor.Weight distribution as well too much stern weight will require more power to get and stay on plane.
Best cruising speed is usually from just on plane to about 2,000 rpm higher.
Experimenting is the only way to figure without a flow meter.
I'm a little surprised that the Force will push a 19" beyond 5,500in your setup.Is the prop in excellent shape,a tach. and speedo can be off about 10%
I'd try the SS prop it should at least improve performance if not solve rpm.
 

happyfisher

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Dec 28, 2008
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Re: General Prop Issue

Thanks Everyone

I think that the main and most important question I am asking is not being understood. The Question is: Is there any difference in achieving the mfg's recommended operation range (4500-5500 RPM) with the throttle only open 1/2 way or WOT. I mean as applies to fuel economy?

The reason I think there may be, is that the Mfg's always state an operating range and prop sizing achieves that range @WOT. Marine carbs only have several fixed fuel orifices(jets) and the fuel/air mix MAY??? be rich if 45-5500 RPM range is reached when the throttle valve is blocking a significant amount of air from mixing with the fixed fuel flow from the main jet.???:confused:
 

Silvertip

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Sep 22, 2003
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28,771
Re: General Prop Issue

You are misunderstanding the relationship between the prop and "why" the manufacturer specifies a maximum WOT rpm. Since the manufacturer has no idea what boat (runabout, workboat, pontoon, etc) the engine will be installed on, they don't even supply a prop on larger engines. The dealer that sells the boat recommends a prop for the combination that allows the engine to reach it's receommended WOT rpm with an average load. If it can't the engine is lugging (two strokes don't like that) and if it overrevs, that's not good either. So if you were to install a 24P prop on your engine you would seriously overprop the engine, which means you need more throttle, not less under most conditions. I suggest you get your tachometer checked for accuracy as 1/2 throttle at 45 MPH is does not point to accurate numbers. Either the speed is way off or the tach is wrong. With a 19P prop and 120HP 45 mph is a realistic number "at wide open throttle". But not at 1/2 throttle.
 

steelespike

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Apr 26, 2002
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Re: General Prop Issue

Have you observed the throttle plates at "1/2 to 3/4" Throttle?I'm betting they are wide open or as Silvetip says something doesn't add up.
Like a guy skiing behind a 5hp.It don't add up.
 

happyfisher

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Dec 28, 2008
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Re: General Prop Issue

Again Thanks for the feedback

This is still under investigation, but, as I said I just got this boat. I used about 20g of fuel in about 2hrs. Investigation of the carb rebuild that had been done by the marina showed a complete screw up. They had failed to set the floats AND had failed to put the gasket in the bowl between the bowl and main jet tower. That made the main jet useless. So that may be a large part of the fuel issue. Second talking with a friend that sells Rangers and other hi-end boats he clued me in that, while not common, he has seen a number of times where the prop hub starts to fail and will slip as HP and RPM's are increased. I have changed to the 13x19 SS prop so will test everything when the rivers in the area go back in their banks. Will post findings.:confused:
 

Frank Acampora

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Jan 19, 2007
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12,004
Re: General Prop Issue

Because of the design of the carbs and because of the relationship of timing to carb butterfly opening, the "sweet" spot is at about 1/4 BUTTERFLY opening. This does not translate into 1/4 throttle control handle movement. You need to find that yourself.

Note that this engine is the same block and carbs as the old Chrysler engines. These old engines had a collar around the control lever hub. There was a clear area and a painted blue line showed through at "CRUISE." This was the economy setting and corresponded to about 1/4 butterfly opening.

Take the carb covers off and observe the throttle butterflies. You will note that at approximately 1/4 opening the timing is almost fully advanced, giving the best economy. Also, with someone else operating the boat, brace yourself and look into the carbs as the driver opens them for a hole shot: You WILL be amazed at the amount of fuel atomizing in the venturis. When that "finger" on the carb advance cam pushes the roller on the bottom carb down and the butterflies open all the way, those babies like to suck gas.

ADDENDUM: DO NOT PLAY WITH THE JETS. Do not lean the low speed needles in an effort to acheive better fuel economy. Either is a fast way to buy another engine.

This, of course, is predicated on the engine running in the recommended RPM range at full throttle. Running too small a pitch prop is much like running an auto in second gear on the highway: Of course you will use more fuel.

BTW: With my Force 125 running 60 MPH at 5900 RPM, I burn through 15 gallons in something like 2 hours. Actually, that is not too shabby! Given that I am not at 60 ALL that time, it still works out to somewhere around 5 miles per gallon.

If your gear ratio is 1.95 to 1 as the three cylinder engines are, then the top theoretical speed with a 19 pitch prop would be: 46 MPH at 5000 RPM. If the ratio is 1.78 to 1, then the speed would be 50.

As stated above, check your tach and check the speedometer against a GPS. If the numbers are correct, then step up to a 21 pitch. Stainless if you got the money and it's burning a hole in your pocket.
 
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