Prop pitch vs efficient use of gas

[brian3127]

I have a 1975 johnson 40-hp 40el75c and it sits on a 14 tri hull boat use mainly for fishing and maybe cruising around never pull anything
I have been told on this sight that it should be ran at 5000 to 5500 rpm at wot. I have read that it should be ran at 4500 rpm. My boat is running at 3800 rpms. which one is correct? So I think I need a different prop because I'm running to low


Now the carbeurator is set to put a certain amount of gas into the cylinders at wot. Does that mean when my boat is only running at 3800 rpms I'm using more gas than I would be if I would be running at a higher rpm? And of course not getting as much power or speed the motor is designed for

Is there a calculation to figure out how much pitch to get a certain rpm.
I don't know what prop I have on the boat right now

 

[jestor68]

Re: Prop pitch vs efficient use of gas

The full throttle operating range for your motor is 4500-5500 rpm.

At 3800 rpm, you are "lugging" your motor, which places more stress on the internal parts, leading to a shortened life.

Props for that motor are are getting hard to find, since Michigan is the only one I know of making a few sizes.

You'll need to remove the end cap to see the part number stamped on the prop hub to determine what size it is.

I have seen sizes from 14 pitch down to about 10 pitch. You will need a smaller pitch; just what depends on what size is on there now.

The "standard" size OMC prop listed(#0384460) is a 10-1/2 X 13 for that motor. The other OMC size listed (0385955) is a 10-3/4 X 11. The found these at Marine Engine.COM. These are new old stock genuine OMC props for $168. Obviously, the Michigan replacement is cheaper.

I have seen quite a few of those props on E-Bay, as well as a couple of Michigan sizes available here at Iboats.

 

[steelespike]

Re: Prop pitch vs efficient use of gas

Before making any prop changes we need to try to determine why your rpm is so low.To do this we need you to find the prop size
could be near the base of the blades It is written diameter X pitch Pitch being the key number.Then you need to gat us your present rpm and gps speed.
Marine motor fuel use is measured in gallons per hour,gph, a 40 hp will use "about" 4 gph at wot(wide open throttle) Mpg is achieved bay selecting a prop that produces the most speed for a given throttle setting within the rpm range of the motor.This will usually produce a good overall performance.Most prop selections are a compromise and sometimes a specific prop may be selected for water sports or best speed or other specific needs. Mpg for a good setup or an excellent setup would be hard to measure
the difference.

 

[brian3127]

Re: Prop pitch vs efficient use of gas

alright thanks guys i dont know if i will be able to get that info anytime soon going camping this weekend so im going to be busy till the first part of next week

very good info though thanks

 

[brian3127]

Re: Prop pitch vs efficient use of gas

My prop had 8'6 7v stamped on it my boat runs 16 mph (GPS) at 3800 rpms
14 foot tri hull

 

[Frank Acampora]

Re: Prop pitch vs efficient use of gas

Go to ebay and search Bronze propellers. There are a couple of very nice Oakland Johnson props on there for reasonable prices. I think one of them was a 10 1/2 X12 pitch and should probably do well on your engine. You can also find other used props to fit your engine by searching OMC propellers--be prepared though: it is a long list.

 

[brian3127]

Re: Prop pitch vs efficient use of gas

I found my extra prop it is 10 3/4 x 11 I'm running at 4,000 rpm at wot and it moving 16 mpg according to my gps
The prop that was on it I can't read the numbers on it they are scraped up I was running at about 3800 at wot and 16 mph.
So I'm guess the two props are the same. So what should I try next

 

[steelespike]

Re: Prop pitch vs efficient use of gas

I think you have more issues than a prop selection. It would take about a 5" pitch decrease to gain 1,000 rpm.
at 16 mph the first thing I would check is if its running on both cylinders.
Lots of folks don't realize they are on one cylinder except for the lack of power.
Pull one plug wire at a time and see if it will run.

 

[brian3127]

Re: Prop pitch vs efficient use of gas

I did that and it runs like nothing happens when I pull one of them. Then I pull the other one and it runs rough but it stays running
I guess I will go back to the outboard repair section and ask guest ions there. Thanks for the help

 

[steelespike]

Re: Prop pitch vs efficient use of gas

That would be an indication that the cylinder is weak.