Prop for part throttle mileage help.

[mjhoward]

Newbie here. I have a 90 model 19' runabout with a OMC 4.3 (185hp?) and a stock replacement 14 1/4 x19 pitch aluminum prop. I mainly spend most of my time just on a good plane and cruising in the 3000 rpm range and ~30mph.
I'm looking for what will give me the most efficiency at this mph with little concern for top speed. I don't know if more pitch is what I need and how much or is a 4 blade the way to go and what pitch?
I'd like it to plane out a little faster as well for tubing and skiing but my #1 concern is fuel mileage at cruising speed with this $4/gal mess.
I'd like to lower my rpm for the mph and wondering if a pretty aggressive prop is the answer.

Thanks.

 

[steelespike]

Re: Prop for part throttle mileage help.

First; Boat motors are rated in gallons per hour (gph)at wide open throttle.(wot)10% of the rated horse power yours is about 18 gph.The idea is to get the most speed at the least gph.The boat must operate within its max rated rpm range.The best pratical cruising speed can be as little as just on plane to up to about 2,000 rpm above.I'm not an expert in prop technology but it is my feeling a 4 blade may provide the best cruising speed mileage.Also trim tabs/smart tabs would help.Boat still needs to operate within wot rpm rating.A flow meter and gps are an excellent investment as well.Easy to see actual gph and speed.A Stainless prop will also help mileage how long to make up the cost??.At over $4 maybe not as long as we might think.
If your present prop is near the top rpm a 19" 4 blade may be a good start.
You may get more info from the pros.

 

[mjhoward]

Re: Prop for part throttle mileage help.

Thanks for the input. LOL! at the gph vs mpg. I just plot out on a map about how far I went on the water and top it off when I pull out for the day. I'll have to check in on a flowmeter.

 

[45Auto]

Re: Prop for part throttle mileage help.

From Steelespike:

The idea is to get the most speed at the least gph

This is commonly called "best MPG" ......................

Never have figured out why some people go all around in circles to keep from calling it miles per gallon.

This test is a newer 19' runabout, but it will give you an idea what to expect.

My experience has been that going up 2" on prop pitch will give slightly better cruise economy but cost you a little acceleration and top speed if your boat is currently propped correctly to put it in it's optimum WOT (Wide Open Throttle) RPM range.

 

[steelespike]

Re: Prop for part throttle mileage help.

Actually"best mpg" on many boats Is usually around 4 to 8 mph. (displacement speed)the report comes close at 1,000,@ 4.7 mph.probably right on at about 1,200 I read about a 16ft bio diesel launch that approached 30 mpg at dislacement speed.
Best "practical" mpg is at or above planing speed.Many folks believe just planing when it can be signifigantly higher.Pretty tough to figure "best mpg" without mentioning gph.There are plenty of boaters that don't know they can measure gph and figure "best mpg" nor is it imeadiately obvious that simply boating at a low gph may not be the "best mpg"

 

[Silvertip]

Re: Prop for part throttle mileage help.

Nobody has answered the question so I will. Changing props to get best cruise MPG (whatever that means to you) also means you are very likely over propping the boat. Keep in mind that if the prop is wrong at WOT it is also wrong throughout the rpm band. Prop the boat correctly so it can reach its recommended WOT rpm and fuel economy will be the best it can be. Decreasing pitch from there increases fuel consumption. Increasing pitch causes the engine to lug which requires more throttle at hole shot so any gains that may be realized are eaten up during hole shot and anything higher than cruise.

 

[steelespike]

Re: Prop for part throttle mileage help.

Nobody has answered the question so I will. Changing props to get best cruise MPG (whatever that means to you) also means you are very likely over propping the boat. Keep in mind that if the prop is wrong at WOT it is also wrong throughout the rpm band. Prop the boat correctly so it can reach its recommended WOT rpm and fuel economy will be the best it can be. Decreasing pitch from there increases fuel consumption. Increasing pitch causes the engine to lug which requires more throttle at hole shot so any gains that may be realized are eaten up during hole shot and anything higher than cruise.

I suggested that a 4 blade might deliver better cruise mileage reminding that the boat still needs to operate within its max recommended rpm.

 

[danfrompgh]

Re: Prop for part throttle mileage help.

Silvertip said it all. You have to be propped right for WOT, everything else will fall into place. From everything I've read all boats have their sweet spot for gph/mpg. But remember, throw everthing out the window when you you have the kids dragging behind ya on a tube and just $$mile! They are!

 

[dirtyoldman]

Re: Prop for part throttle mileage help.

THis is off topic, but I couldn't care less about mileage. Most of my time on the lake is not spent getting toa destination, jst cruising. If I go slower it takes longer, and I get less GPH, which means less $$ spent overall.

That may not apply to some of you that are ont he big lakes and have somewhere to actually go.

 

[mjhoward]

Re: Prop for part throttle mileage help.

Thanks for everyone's replies. And yes, I do a lot of cruising on larger lakes. Cruise all day and not see it all. I have 2 props that I aquired with the boat. A stock alum. 14.25x19 and a stainless Viper 14.25x21. I ran the stainless last year and tried the alum this year and I did loose ~3-4 mph with the new stock prop. The stainless has been cut down a little and re-worked due to damage otherwise I wouldn't consider another one. I don't remember my wot rpm from last year but I'll do some swapping around again and see what I like.

I like the idea of the 4 blade as well. I'm pretty sure, from what i've read, that the trim tabs may do all I need. I'll get some of them soon.

 

[mjhoward]

Re: Prop for part throttle mileage help.

From Steelespike:


This is commonly called "best MPG" ......................

Never have figured out why some people go all around in circles to keep from calling it miles per gallon.

This test is a newer 19' runabout, but it will give you an idea what to expect.

My experience has been that going up 2" on prop pitch will give slightly better cruise economy but cost you a little acceleration and top speed if your boat is currently propped correctly to put it in it's optimum WOT (Wide Open Throttle) RPM range.

Thanks, interesting info.

 

[mjhoward]

Re: Prop for part throttle mileage help.

Update- I got some Smart Tabs and they solved 50% of my problems. The boat jumps right up on plane now with no laboring to plane at WOT. I did talk to the previous owner and found the prop to be worked on 5 times! He must have like the stumps and slews! Well, this 14.25 SS prop is now measuring 13.5". I think I have found my biggest problem. I'll be getting a new SS prop and report back with results.

 

[Silvertip]

Re: Prop for part throttle mileage help.

45Auto -- I know we've had this discussion before but for the folks who have not thought about the MPG/GPH thing very much, I will add my side of the discussion for them to chew on. MPG (miles per gallon) is a good number to know for your boat. What RPM or speed and load that number is achieved is also necessry for it to be useful. Also, unless you have a GPS or your boats instruments are capable of calculating that number "real time" you have no idea until the trip is over what the "actual MPG" turned out to be. A very inexpensive fuel flow meter that calculates fuel used (GPH or Gallons Per Hour) in a number of ways such as instantaneous, average, fuel remaining, fuel used since last reset, etc., helps to quickly find the best cruise speed or RPM for the current load and water conditions. That monitor can also tell you within seconds after you reached your desired cruise speed or rpm what the current fuel burn rate is. If you know that in the past, your engine burned 8 GPH with the same load you currently have and it is currently burning 10 or 12 GPH that something is amiss and you can return to the dock to determine the issue. It's as simple as that. Either way, you can determine what MPG you are getting. It is simply a matter of doing it "real time" or after the fact.

 

[mjhoward]

Re: Prop for part throttle mileage help.

Update: I just installed a 14"x21 pitch Solas HR Titan 4 blade and WOW! It planes with half throttle and cruise RPM at 30mph dropped from 3300 to 2900-3000. And that was with 3 more people in the boat than with the previous data! I didn't have a chance to open it up due to the amount of people and little one on board but i'll find out soon how the top end does.