Up a pitch for better GPH/MPH?

[Mordekai]

Right now I'm cruising @ 4000 rpm for around 30-33pmh GPS indicated speed and WOT @ 4800RPM for around 38-40 depending on fuel load on my mercruise 3.0 with alpha1 2.0 drive ratio.

My stock prop is 14.25 x 21, would I gain better GPH for the same MPH if I changed up to a 14.25x 22 or 23?.

I figure losing 200RPM per 1 pitch increase wouldnt be too bad with WOT @ 4600/4400RPM? I'm not really concern about hole shot fyi.

Cliffs:
Engine\Drive: 3.0L Mercruiser w/ Alpha 1 2.0 Ratio
Current Prop: 14.25 x 21
Future Prop: 14.25 x 22 or 23 Pitch
Why: Wanting better gph/mph.

Any thoughts from the prop gurus?

 

[BoomerSooner]

Re: Up a pitch for better GPH/MPH?

Yes, theoretically you will be able to cruise at the same 30-33 mph at a lower rpm with the higher pitched prop. HOWEVER, that is assuming that all other things are constant. Hydrodynamic drag, drive angle, load, etc. The fuel consumption rate will only vary slightly though. If you get a 200 rpm drop to 3800 from your initial 4000, that's only a 2.5% drop in rpm. That doesn't mean you will get a 2.5% decrease in fuel consumption. Too many factors. Are you willing to spend some money on a prop to get a 1-2% decrease in fuel cost. $100 in fuel vs $98 or $99. Not worth it for me, but to each his own.

 

[Mordekai]

Re: Up a pitch for better GPH/MPH?

Well the main reason is that my first/ prop for this boat is getting beat up so I'm thinking by the end of this season I'll need to switch it, and might as well get a little bit more bang for the buck.

I wouldn't just change a prop out of the blue for the minor increase alone above

 

[QC]

Re: Up a pitch for better GPH/MPH?

Since you need a spare, do it. But I'm with BoomerSooner, fuel gain will be minimal and could even go the other way depending on the variables he notes. I will say, generally speaking, if you are above your WOT range now, or at the top, then speed usually increases by bringing it down some. Buttttt, those dang variables still apply . . . Good luck!

 

[Silvertip]

Re: Up a pitch for better GPH/MPH?

I disagree. Fuel consumption is more involved than just the number you get at cruise. Getting on plane reqjuires lots of throttle. If you go up in pitch you must now compensate the loss of hole shot with either more throttle or the same amount of throttle over a longer period of time. Next -- because you decrease RPM at whatever you feel cruise is, does not mean the throttle setting is also decreased. In fact you may find that it requires MORE throttle to maintain that speed because the bigger prop requires more power to twist it. That is exactly why selecting the prop that puts the WOT rpm at or near the upper end of the manufacturers WOT band is so important. If its wrong there it is wrong across the rpm band. The net result is whatever savings you "might" get at cruise is negated by the loss of hole shot and potentially increased throttle at cruise. Plus you now stress the engine when you do water toys or run WOT. Look at it this way. If you were actually successful in getting 2 - 3% better economy, lets say you burned 100 gallons of fuel per month. 3% of that is 3 gallons. Hardly worth stressing and engine over since you saved a grand total of about 10 bucks.

 

[QC]

Re: Up a pitch for better GPH/MPH?

Almost all data supports higher speed and better efficiency (kinda same thing) lower in the WOT range although I do not suggest it for overall flexibility. Also a wider open throttle can result in better fuel efficiency in any application if the RPM is lowered enough to take advantage of the resulting lower pumping losses. I suggested that it could go either way, so not sure what you disagree with.

You have basically determined that all of Detroit, Stuttgart's, Japan's and countless other auto development locations have wasted the last 50 years of lowering RPM for the same load. You should call 'em . . .

I do agree that chasing fuel efficiency in a pleasure craft application is pretty pointless.

 

[steelespike]

Re: Up a pitch for better GPH/MPH?

Gallons per hour at wot is fairly constant. Any marine motor will use about 10% of its hp in fuel at wot.ie: a 140 will use "about" 14 gph at wot. whether pushing a house boat or a runabout.Obviously the faster boat will get better mpg.
Very roughly speaking the faster per throttle setting the better the mileage.
The best cruising speed could be from just on plane to about 600 rpm higher.
The best way to check mpg is with a flow meter.On a 4 stroke a vacume gauge
can also help track ideal rpm to speed.

 

[QC]

Re: Up a pitch for better GPH/MPH?

Yeah, I think we all agree we're splitting hairs here. The OP points out that he is above the WOT range, he also notes he needs a prop. So going up one pitch level seems prudent to me.

 

[Mordekai]

Re: Up a pitch for better GPH/MPH?

Well went out tonight and at wot with full load of fuel and ice I was still at 4900 so I think by increasing one pitch I should knock it down by 250 rpm?

Here is a pic to share with you all at the time of posting. Talking about live data huh

 

[Mordekai]

Re: Up a pitch for better GPH/MPH?

Seems like the selection for a 14.25x22 pitch seems to be limited to 4 blade props only and all in SS.

21 then 23 seems to be extremely popular.

 

[180shabah]

Re: Up a pitch for better GPH/MPH?

WOT range is 4400-4800 and you are currently at 4800-4900, so at to just barely over the top end.

As noted, going to a 22" means SS and big bucks and a change to 23" will drop you to the bottom of your WOT range(everything else being equal). neither option sounds very appealing to me.

If I were in your situation I would be focusing on two props.

21" 3 blade Hustler - VERY aggressive for an aluminum prop, and has a ventilated hub, but has a reputation as being somewhat unpredictable. When it "works" it is awsome, when it doesn't, well..... It is my personal belief(no substantiation) that the problems are due to mis marking of props and thus not getting the pitch that is expected.

21" 4 blade amita - all of the advantages of a 4 blader, including "more efficient" cruising, stringer hole shot etc. Typically not quite as fast on the top end as the Hustler.

That black max that came on your boat, IMHO, is one of the worst props on the market. Either of the above should make a huge difference in performance and may or may not lower you WOT rpm slightly. Both are also available right here at iboats for about $100

 

[Mordekai]

Re: Up a pitch for better GPH/MPH?

The prop helper thingy here on iboat suggested going down 1 pitch if going from a 3 to 4 blade, will it hold in my case?

Also what does "aggressive" means when it comes to the hustler? Im guessing it means that it maybe 21 pitch but it acts like 22 or 23?

Thanks

 

[45Auto]

Re: Up a pitch for better GPH/MPH?

Next -- because you decrease RPM at whatever you feel cruise is, does not mean the throttle setting is also decreased. In fact you may find that it requires MORE throttle to maintain that speed because the bigger prop requires more power to twist it. That is exactly why selecting the prop that puts the WOT rpm at or near the upper end of the manufacturers WOT band is so important. If its wrong there it is wrong across the rpm band. The net result is whatever savings you "might" get at cruise is negated by the loss of hole shot and potentially increased throttle at cruise.

It would be humorous to see your explanation of why Toyota went to a larger displacement Atkinson cycle (instead of the common Otto cycle) engine to increase fuel mileage in the new Prius.

 

[Silvertip]

Re: Up a pitch for better GPH/MPH?

I'm not indicating lowering rpm "always" results in poorer economy but it certainly can and more than likely will. This gent wants better economy at cruise. Fine - raise the pitch and you might actually see slightly better economy at cruise. But as I said, that does not take into account what may be an overriding factor of very poor hole shot and decreased economy at the top end and in fact may loose a considerable amount of top end speed. So where is the "overall" savings? Prop the boat where it is supposed to be and economy will be as good as it gets. Picking one spot in the rpm band is foolhardy since adding one person can screw up the performance so again you lose. Unless you are burning a lot more gas than the average boater, a 3% savings in fuel is not worth losing performance or portential damage by lugging an engine.

 

[180shabah]

Re: Up a pitch for better GPH/MPH?

The prop helper thingy here on iboat suggested going down 1 pitch if going from a 3 to 4 blade, will it hold in my case?

Ignore that for the amita - it is designed to swap "pitch for pitch" with most 3 blade designs.

Also what does "aggressive" means when it comes to the hustler? Im guessing it means that it maybe 21 pitch but it acts like 22 or 23?

Aggressive means, progressive pitch, high rake, cupped blades and vented hub. Some of this you rarely see on aluminum props. The vented hub improves hole shot IF you have thru hub exhaust by increasing prop slip at lower rpms. Progressive pitch, higher rake and the cupping all help performance in the midrange and upper revs.

 

[45Auto]

Re: Up a pitch for better GPH/MPH?

I'm not indicating lowering rpm "always" results in poorer economy but it certainly can and more than likely will.

Wrong on that one. You really believe that all the automotive manufacturers put overdrives in their cars so they'll get WORSE gas mileage????

This gent wants better economy at cruise. Fine - raise the pitch and you might actually see slightly better economy at cruise.

Right about that one.

But as I said, that does not take into account what may be an overriding factor of very poor hole shot and decreased economy at the top end and in fact may loose a considerable amount of top end speed.

At least there is a very slight possibility you could be correct here. But on the other hand, the hole shot difference may be negligible (maybe go from 3.6 to 3.8 seconds), his economy could increase at top speed since he's already propped at the top of his RPM range, and his speed could actually increase. Without knowing where his power curve is currently located with respect to the drag curve of his boat there's no way anyone can say for sure. Either scenario could be correct.

So where is the "overall" savings?

Pretty much everywhere throughout the boat's operating range.

Prop the boat where it is supposed to be and economy will be as good as it gets.

Not true. You can keep it within the manufacturer's operating range and prop for best holeshot, highest top speed, or best fuel economy. You'll only get one of the three. Depends on which is the most important to you. There's not going to be a huge difference in any one of them, but for some reason lots of people seem to be real concerned over a couple of percentage points in fuel mileage or speed in a boat.

Picking one spot in the rpm band is foolhardy since adding one person can screw up the performance so again you lose.

Yep, that's why you have more than one prop. You need a spare anyway, might as well get some use out of it rather than just carrying it as dead weight on the boat.

 

[elkhunter338]

Re: Up a pitch for better GPH/MPH?

Have you considered a pirranah prop with replacement blades and different pitch blades.

 

[Mordekai]

Re: Up a pitch for better GPH/MPH?

Ignore that for the amita - it is designed to swap "pitch for pitch" with most 3 blade designs.



Aggressive means, progressive pitch, high rake, cupped blades and vented hub. Some of this you rarely see on aluminum props. The vented hub improves hole shot IF you have thru hub exhaust by increasing prop slip at lower rpms. Progressive pitch, higher rake and the cupping all help performance in the midrange and upper revs.

I see, thank you

I'm leaning towards the hustler for higher top end but the 4 blade is really tempting me due to better at the mid range. The black max you refer to is whet come with alpha one drives by default I assume?

 

[Frank Acampora]

Re: Up a pitch for better GPH/MPH?

You fellows are comparing apples to oranges when you use the analogy of an auto engine. An auto engine almost never cruises at 3/4 to full throttle and is lightly loaded except during acceleration, and unloaded during coast--boats never coast and at cruise engines are always heavily loaded.

I think a better analogy would be a running person. Running uses more calories per hour than walking. So if you run a specific distance you use a lot of calories for a short time. If you walk, you use fewer calories for a longer time. --Net expenditure is the same amount of calories to go the same distance.

Same thing with prop pitch--roughly the same amount of fuel to go the same distance. OK! I know this is overly simplified so don't get on my case.

 

[180shabah]

Re: Up a pitch for better GPH/MPH?

The black max you refer to is whet come with alpha one drives by default I assume?

Yup,BlackMax is mercs "standard" prop....

Same thing with prop pitch--roughly the same amount of fuel to go the same distance. OK! I know this is overly simplified so don't get on my case.

Or to really keep it simple.

it takes Xhp to go Ymph and Yhp takes Zfuel to create.

RPM is not a direct part of either equation. Every engine has a "sweet spot" where it more efficiently converts that fuel to power, but lets face it, with the average small black the difference in engine efficiency between 3000 and 3400 is so small that the real work difference is negligable.

I will argue that th OP's money wouldbe better spent on a floscan to "tune" his right arm. He'll get a better ROI than swapping props. Of course he won't get any of the other benefits of swapping to a GOOD prop....