Gph at various rpm...thoughts?

lapl0007

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Aug 15, 2024
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So, the rule of thumb is 10% of hp is WOT gph usage. I have an 83' Mercury 115hp with I'm told is the year they changed to different hp rating (at the prop rather than crank?) making my 115 more like a 140...meaning I "should" see about 14 gph at WOT.
With all that said, does anyone have a breakdown at various rpms what the gph "should" be.
I know, I know...there are a lot of factors: hull type, prop, wind, temp, weight, current...but just some crude numbers would be greatly appreciated...
WOT = 14 gph
4500 = 12?
4000 = 9?
3500 = 7?
3000 = 5.5?
Thanks
 

lapl0007

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2400# loaded weight starcraft 1974 ss with 19p 3 blade stock prop, 65 degrees F, no wind, no current, 2.0 gear ratio, and at each 500 rpm level....
5500 = 14 gph
5000 = ?
4500 = ?
4000, 3500, 3000...
 

dingbat

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200 HP Ocean Pro

Fuel flow, speed and chart plotter configured to report nautical miles.

Average fuel burn at cruise (~4300 rpm, 27 kts.) under good to decent conditions = 1.9 MPG, or roughly 16.3 GPH

Raising or lowering cruise by 1500 rpm typical results in a plus or minus 0.1 mpg change.

In rough conditions, I've seen fuel consumption more than double for a given rpm
 
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mr 88

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So, the rule of thumb is 10% of hp is WOT gph usage. I have an 83' Mercury 115hp with I'm told is the year they changed to different hp rating (at the prop rather than crank?) making my 115 more like a 140...meaning I "should" see about 14 gph at WOT.
Pretty sure you have that backwards . Your 115 is more like a 100hp figuring a 10% loss of hp between the crank and the prop . Try Googling it .
 

jimmbo

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1982 was the year Mercury started Prop Rating Engines larger than the 50 cubic inches.
There have been rumours for Decades that in 82, Mercury rebranded the 115 as a 90, and the 140 as a 115, the 175 as a 150, the 200 as a 175, and the 225 as a 200. Maybe, maybe not, as the Specified Rpm Ranges remained the same, ie): 90hp 4500-5000; 115 5000-5500; 140 5300-5800.
Looking thru the Parts Lists, shows many part # Changes but no evidence that it was a Decal Swap...
That said, there has been a rule of thumb for Decades that to attain a 3mph increase in speed required an increase of 15 hp, or 5hp for each mph increase.
In 1984 I replaced a 78 90hp(crank rated), with an 84 115hp(prop rated), The boat which did 37-38mph with the 90hp, increased to 48-49mph, which would indicate a HP increase of approx 55hp, which would be very close to the 140 crank rating. That 115 was faster, and had better Hole Shot, and burned less gas than an 82 Evinrude 140hp(crank rated), on that same boat


As for fuel consumption. It is not Linear with Rpm and/or Throttle Lever Travel.
 
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lapl0007

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Pretty sure you have that backwards . Your 115 is more like a 100hp figuring a 10% loss of hp between the crank and the prop . Try Googling it .
If it's a 115hp rated at the prop, then due to loss, the 115hp would go up, meaning a "125hp" crank would lose 10% and land to 115hp prop. So sure not the old 140hp but still thirstier than the "old" 115hp crank rated which is what we normally do the old 10% WOT gph.
 

cyclops222

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If all the motors were in PERFECT CONDITION ? Some real results could be expected.
 

lapl0007

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1982 was the year Mercury started Prop Rating Engines larger than the 50 cubic inches.
There have been rumours for Decades that in 82, Mercury rebranded the 115 as a 90, and the 140 as a 115, the 175 as a 150, the 200 as a 175, and the 225 as a 200. Maybe, maybe not, as the Specified Rpm Ranges remained the same, ie): 90hp 4500-5000; 115 5000-5500; 140 5300-5800.
Looking thru the Parts Lists, shows many part # Changes but no evidence that it was a Decal Swap...
That said, there has been a rule of thumb for Decades that to attain a 3mph increase in speed required an increase of 15 hp, or 5hp for each mph increase.
In 1984 I replaced a 78 90hp(crank rated), with an 84 115hp(prop rated), The boat which did 37-38mph with the 90hp, increased to 48-49mph, which would indicate a HP increase of approx 55hp, which would be very close to the 140 crank rating. That 115 was faster, and had better Hole Shot, and burned less gas than an 82 Evinrude 140hp(crank rated), on that same boat


As for fuel consumption. It is not Linear with Rpm and/or Throttle Lever Travel.
So on that 84' 115 did you see 11.5 gph wot or closer to 14 gph wot?
 

jimmbo

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So on that 84' 115 did you see 11.5 gph wot or closer to 14 gph wot?
It took a little less than a 1/2 hour to burn through the 5 Gal(Imp) Tank. I guess that would be about 13 1/2 gal(US)/hr. It got about 4mph at WOT, it was really good at about 38 mph, a Tank would last over an hour
All the Motors I listed above were Broken in by myself and were in good condition and tune.
 

lapl0007

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It took a little less than a 1/2 hour to burn through the 5 Gal(Imp) Tank. I guess that would be about 13 1/2 gal(US)/hr. It got about 4mph at WOT, it was really good at about 38 mph, a Tank would last over an hour
All the Motors I listed above were Broken in by myself and were in good condition and tune.
Thank you for some actual numbers. It's refreshing to have an answer to a question rather than old rule of thumbs repeated back to me
Thanks again.
 

jlh3rd

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Looking for actual numbers if anyone has some.
ok...here. This is a 2021 Merc 115 hp CT pushing a twin tube (25") pontoon.
5800 - 10.6 gph.......2.35 mpg
5000 - 7.0. ". ......3.1. "
4000 - 4.1. ". .......4.3. "
3000 - 2.15. ". ......5.5. "
2000 - 1.2. " ......5.4. "
These #'s ,+/- 0.2, are from 3 of my props I used. 2 3blades and one 4blade.

6000 - 10.0 gph.........2.2 mpg
5000 - 6.0. ". .........3.2 "
4000 - 3.8. ". .........4.2. "
3000 - 2.1. ". .........4.7. "
2000 - 1.2. ". ..........5.4. "
these are from a 3-16x12 merc. prop

yep, my preferred cruising rpm is around 3500.
 

Scott Danforth

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A 2021 mercury isn't a 1983 TOP running with a BSFC of nearly 0.6. the 2021 motor will burn about 80% the fuel of the 1983 motor
 

Chris1956

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It is very hard to give accurate numbers for your boat. It depends on hull design, trim position, load in the boat and other factors.

i.e I had a Merc 1500 IL6 on my speedboat, for years. Once I had the boat loaded with camping gear, more than I ever loaded it up before or since. I ran flat out (~55MPH) on a full 6 gal tank, and it lasted 25 min or thereabouts. So it was about 23 miles on 6 Gal, or 14.4 Gal/hr.

However, the rest of the time, with a boat load averaging 3.5 people, cooler etc., she was more fuel efficient, maybe twice as good, especially since I rarely ran flat out.

Most outboard motors are designed such that the ign timing advances to maximum, and the carbs are between 3/4 and 7/8 open. Opening the carbs that last bit increases the speed, but the fuel consumption increases dramatically. Fuel useage is not linear as speed increases.

I now have a 135HPV6 (122 cu in) on that same speedboat. The fuel use seems to be much more than the IL6, which was 99 Cu in.

IMO I always thought the IL 6 motors from Merc were much more fuel efficient, than my V6. Not sure if this helps, but it is one more data point.
 

bajaman123

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Many of the boating mags have reports that show gph fuel use at various rpm when doing boat tests. GENERALLY speaking there's always a 'sweet spot' that takes place when you are planed out nicely and running ~50% of WOT, and again generally you'll see fuel consumption around 35% of WOT. Note that this is often not your most efficient place to run; that would be where your fuel use and speed marry perfectly.
 

lapl0007

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Aug 15, 2024
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ok...here. This is a 2021 Merc 115 hp CT pushing a twin tube (25") pontoon.
5800 - 10.6 gph.......2.35 mpg
5000 - 7.0. ". ......3.1. "
4000 - 4.1. ". .......4.3. "
3000 - 2.15. ". ......5.5. "
2000 - 1.2. " ......5.4. "
These #'s ,+/- 0.2, are from 3 of my props I used. 2 3blades and one 4blade.

6000 - 10.0 gph.........2.2 mpg
5000 - 6.0. ". .........3.2 "
4000 - 3.8. ". .........4.2. "
3000 - 2.1. ". .........4.7. "
2000 - 1.2. ". ..........5.4. "
these are from a 3-16x12 merc. prop

yep, my preferred cruising rpm is around 3500.
That is exactly what I'm looking for. Thank you! If other have this sort of data I would greatly appreciate it!
Thank you again
 

jimmbo

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13,515
Here is some of the Mercury Documentation of the Prototype Black Max compared to the I6 1972 1400. The 72 1400 had better Hole Shot than the I6 1500 that followed in 73
V6 report 4 001.jpg
 
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