propeller and "fuel enconomy"

[skperez]

I have a simple question.. or maybe not so simple from all the info i've read. I have recieved conflicting info even from prop manufactures. Is there any one who has tried a prop change to acheive better fuel effiency. I have a 23' seaswirl cuddy cabin with a 225 ficht. I'm swinging a 15 x 17 alum. and am getting a wot of 5950. I have a floscan installed and get best fuel eff. at 4200 rpms at 2.6 nmpg. 2 people, full tank and gear. I was thinking of trying a 14.25 x 19 alum. (I do a lot of shallow water diving and fishing and have heard it's asking for trouble swinging a stainless around all those rocks. Nobody wants to his rocks but are the chances less for serious damage to lowere unit using alum vs stainless) all experts agree I lose about 400 rpms or so, but will gain top speed. Lose rpm and still gain speed? Don't really care too much about top end, but with actual cruise speed. Does anyone know?? Also, I've been told I lose out of hole performance. I have an excellent hole shot, It will be ok to lose some, but will it be night and day? Basically will the 19" give better fuel efficency. thanks

 

[Chris1956]

Re: propeller and "fuel enconomy"

SK- You may not loose top end, or you may not loose much (2-3 MPH tops) top end if you go to the 19P prop. Also it is likely that you will be able to cruise at 3800RPM, at the same speed as the 17P prop, but with better fuel mileage. As a general rule, using a prop with more pitch is like using a higher gear in a car's manual transmission. You get better mileage unless you lug the motor. Also, a boat is like a car with a one-speed transmission. You need to find the optimum gear for power, top speed and economy.

 

[skperez]

Re: propeller and "fuel enconomy"

Thanks for the reply. I think I'm going to try the "19". I have plenty of power and think that prop will work out. Thanks again....

 

[Jack Shellac]

Re: propeller and "fuel enconomy"

Just remember it's more important to keep your motor in the top 1/4 of it's recommended maximum RPM range than it is to gain a little better fuel economy. The long life of the motor will more than offset the gas savings in the long run.<br /> Also, you were told right. You can lose RPM and gain speed with a higher pitch prop. If you go too high on pitch, you start lugging your motor which is not good.<br /> I prefer SS props myself because they perform better, but am not around any rocks. You may want to consider an inexpensive composite prop for use when you're "rocking". Especially if the chance of actually hitting them is high.

 

[skperez]

Re: propeller and "fuel enconomy"

Jack,<br />My motor recommends 5500-6000 rpms. I currently am at 5950 with the 15 x 17 prop. Are you saying it would not be recommended to go below 5875 rpm? I'm told I will lose around 400 rpms with a 2 degree change in my prop? Which would be approx. 5550 rpms or so..

 

[Jack Shellac]

Re: propeller and "fuel enconomy"

That's a lot narrower max range than I am accustomed to seeing. Usually, you get a 1000 RPM range and this is where it's important to stay in the upper quarter. What kind of motor is it? Maybe you can get a better answer on this in the forum for that motor. In any event, I think I'd want to be closer to 6000 than to 5500. I have a friend who runs a outboard repair shop and he insists that you want a prop that will let you turn slightly over the max RPM. Don't run it there though. He's basing his recommendations for the long life of the motor and nothing else.

 

[skperez]

Re: propeller and "fuel enconomy"

My outboard is an evinrude 225 ficht (2001). Phoned dealer and told me 5500 - 6000. I also checked a few prop sites which state the same info. I'm all about saving a buck on gas. However if there is even a thought the engine may not last as long or problems could arise from turning a little under recommended rpm, it would definitely not be worth it. The cost of these engines at $15000+ would have to be an awful lot of fuel savings.

 

[Jack Shellac]

Re: propeller and "fuel enconomy"

Now you're talking. How much gas could you have bought with 15 G's. Same goes for trying to save a few dollars per gallon on cheap oil or a few cents per gallon on off-brand gas, as far as I'm concerned.

 

[LubeDude]

Re: propeller and "fuel enconomy"

A 14.25 X 19 stainless should be perfect. Id put it in the 5,600 RPM range. MHO.