better fuel at cruising?

[countvlad]

hi there,
hswiii could you please run these numbers on you magic program and tell me if its the optimum for my boat/motor... right now i have great hole shot, acceleration and i can also trim the bow up gaining few MPH.... the only thing that i would like to o if possible is increase my MPG or GPH at cruise speed....

right now i'm using 5.5G on a 18Mile trip whch is about 3MPG doing 4500RPMS and 32MPH... just wondering from your experience if i could increase that or thats as good as it gets.... tuneup done last week, motor is in top shape....

here are my numberes: motor is a 1989 120 Johnson Looper

gear 2:1 ration
WOT 5900
prop 15 SS
WOT speed 41

i plan to change the spark plugs and wires this week... do you think a diffrent prop might help fuel and speed whise or this is the best set up so far..? tnx for your time

 

[hwsiii]

Re: better fuel at cruising?

Vlad, I am sorry I just forgot. My apologies, I was working on other things and I have CRS baddd. Give me a little bit and I will have it done.


H

 

[hwsiii]

Re: better fuel at cruising?

Vlad, now I remember why I didn't run it immediately. I need numbers from 3,000 RPM up to WOT in 500 RPM increments with the speed and RPM for each one. We need to know what your prop slip is for different RPM to determine how efficient your prop is running at midrange speeds and RPM. I am pretty sure I know what type of prop you need for maximum cruising efficiency, but without your present numbers I am not sure it is worthwhile buying a new prop for.
I will also tell you after we run the numbers and see where we are I would recommend you ask Dhadley to recommend the best prop for your cruising, as he is years ahead of me in that category.


H

 

[truckermatt]

Re: better fuel at cruising?

i would think that using a 17 pitch prop would gain you some MPG, and would lower you RPMS a little

 

[countvlad]

Re: better fuel at cruising?

Vlad, now I remember why I didn't run it immediately. I need numbers from 3,000 RPM up to WOT in 500 RPM increments with the speed and RPM for each one. We need to know what your prop slip is for different RPM to determine how efficient your prop is running at midrange speeds and RPM. I am pretty sure I know what type of prop you need for maximum cruising efficiency, but without your present numbers I am not sure it is worthwhile buying a new prop for.
I will also tell you after we run the numbers and see where we are I would recommend you ask Dhadley to recommend the best prop for your cruising, as he is years ahead of me in that category.


H

its no problem we all have lives i know the feeling... today im going fishing and i will try to get some numbers from 3000 and up....

i thought about changing the prop but i think the rpms that im turning are pretty good ...

 

[junior1113]

Re: better fuel at cruising?

those are great motors i had a 93' 140. id put a 4 blade 15pitch. being able to be on plane at lower speeds, better mid range fuel ecconomy, hole shot, better reverse. better everything. have you worked setup (motor height) yet.

 

[Dhadley]

Re: better fuel at cruising?

A lot depends on what kind of a hull you have. Different props do different things.

 

[junior1113]

Re: better fuel at cruising?

A lot depends on what kind of a hull you have. Different props do different things.

can you give an example of a setup that would perfom better with a 3 than a 4

 

[Dhadley]

Re: better fuel at cruising?

A 20' Lake & Bay will run faster with a 3 blade TXP than with a 4 blade TXP. However the 4 blade will carry a tournament load slightly better. Some high end bass boats are faster with a 4 blade than with a 3 blade. Some are the other way around.

Some boats have a lot of natural bow lift so they really don't benifit from a prop with a lot of bow lift, in fact more bow lift may hurt. Some props have stern lift, some have overall lift.

 

[junior1113]

Re: better fuel at cruising?

thanks d figured it was only for top end but ss would be needed. i know you would'nt run a 3 thats why i ask.

 

[Dhadley]

Re: better fuel at cruising?

We actually run a 4 blade on our L&B now but we're going to a 3 blade. The Lightning 3 blade has so much bow lift it hurts top speed. The Ultima 4 (like yours) makes the boat skiddish in the mid range (really weird feeling). But the Ultima 4 works perfect on my Hewes. A buddy with a 30' Scarab runs a pair of 3 blade Turbo Vectors and it takes off better than any 4 blade props we've tried.

Ain't this fun??

 

[hwsiii]

Re: better fuel at cruising?

Dhadley, I certainly agree with you, it is science to a point but at the final point it becomes a BLACK ART. LOL

 

[steelespike]

Re: better fuel at cruising?

Unless I'm missing something we are missing what boat you are running.
From my limited experience it appears your running at the wrong speed for best mileage.Typically the best cruising could be just on plane to about 2,000
rpm higher.Usually running near the upper end will produce less mpg but not necessarily.Your GPH will remain pretty constant fore each throttle setting
It's fairly obvious the best speed at a given rpm will produce the best mileage.Your present fuel usage at your selected rpm does seem on the high side of normal.

 

[hwsiii]

Re: better fuel at cruising?

Vlad, can you tell us exactly diameter and pitch as well as the make and model of the propeller you are using.


H