Calculate GPH

[Shamfarlango]

I've searched and read quite a few posts but have not been able to find an answer.

What is the best technique for calculating your GPH? I know weather conditions have a major influence on it, but how do i get a fairly accurate base understanding?

The second part of my conundrum is I don't know the horse power of my engines or how to calculate that.

I have twin 1985 OMC 3.8L Engines with stringers outdrives on my Thundercraft 280 Magnum.

I'm sure i'm not the only one looking for a way to calculate this.

Note: had to edit post when i realized i put 4.3L engines. They are 3.8L . Not sure how i brainfarted that one

 

[haulnazz15]

Re: Calculate GPH

Well, you'd just need to know the fuel usage for your model of engine at a given rpm. However, that would only be a ballpark figure as each engine will vary slightly depending on tune and overall condition. A fuel flow meter is the best way to be sure. Weather has nothing to do with GPH usage (air temp notwithstanding), but it will affect fuel mileage.

 

[arks]

Re: Calculate GPH

What is the best technique for calculating your GPH?

Think about it
It's really SIMPLE:
- Fill your tank(s)
- Run the boat for one hour at normal cruising speed
- Re-fill your tank(s)
The amount of fuel used is your GPH

The second part of my conundrum is I don't know the horse power of my engines or how to calculate that.

You don't need that info. This procedure works with ANY setup.

 

[JoLin]

Re: Calculate GPH

The second part of my conundrum is I don't know the horse power of my engines or how to calculate that.

I have twin 1985 OMC 4.3L Engines with stringers outdrives on my Thundercraft 280 Magnum.

Try here:

http://www.piranha.com/OMC_Sterndrives.php

Looks like either 175 hp or 205 hp. I assume the difference is in compression ratio and carburetion, with a 4V carb model putting out 205.

As for calculating GPH, the others have answered it. It all depends on your RPM. Mine uses about 15 at 3200-3400 rpm with a normal load (about 9000 lb. total).

 

[Shamfarlango]

Re: Calculate GPH

Thanks Jolin, I guess i have the 205 HP because i have the 4 barrels on mine. thanks for the link.

I don't think i'm going to go through the expense of a fuel flow meter to get my answer. So I will just try the fill and run and fill method suggested by Arks.

Much appreciated guys

 

[QC]

Re: Calculate GPH

What is it ultimately that you want to know? Your GPH will be different at every RPM, and as you said, weather and water condition too. Almost any carbed 205 curve will be close to baseline as prop load curves are basically linear. They differ by boat at the inefficient points just below and through just barely planing, as that happens at different RPM in different boats.

I think to be conservative you could look at this carbed 220 5.0. Newer, but also more hp, so should be close even by RPM. This assumes you are propped right too. Yes I know you have two, double it

 

[Mischief Managed]

Re: Calculate GPH

Gas engines require about .08 gallons per hour to make 1 HP. Your twin 205 HP engines would burn 32.8 gallons per hour in total if both engines were running at full power. Most gas boats cruise at around 50% power, if powered well, so figure 16.4 GPH (plus or minus 15%) under normal conditions at best cruise.

 

[Thalasso]

Re: Calculate GPH

Gas engines require about .08 gallons per hour to make 1 HP. Your twin 205 HP engines would burn 32.8 gallons per hour in total if both engines were running at full power. Most gas boats cruise at around 50% power, if powered well, so figure 16.4 GPH (plus or minus 15%) under normal conditions at best cruise.

I have twin 5.0 270 hp. At 3000rpm's ( planing speed) i am burning 16gph on each engine. Measured by flowmeters.It gets worse after that.

 

[sschefer]

Re: Calculate GPH

I have fuel flow sensors that provide data to both my Lowrance HDS-8 and my SmartCraft gauge. The one that is connected to the Lowrance allows me to setup a very accurate fuel gauge and gives me GPH while the one in the SmartCraft can only tell me Fuel usage and GPH unless I spend more money and convert my tank sending unit. It makes no sense to me why Merc did it that way but after spending a couple of hours trying to input the fuel level I finally called Merc and they told me the bad news. Thats when I bought the second NMEA 2000 flow sensor and used the Lowrane instead.

My analog gas gauge is accuarate when the tank is full and when it's empty, anything in between is just guessing depending on the attitude of the boat. With a fuel flow sensor and proper gauge you will know exactly how much fuel you have used and what you have left. You can then easily and accurately apply the rule of thirds, (one third out, one third back and one third reserve).

Sure, you could use the run it at cruise for an hour method but that's pretty much useless if you're trying guess how far you can go and find your self bucking a head wind or tide.

 

[dingbat]

Re: Calculate GPH

I run fuel flow meters on the boat. My fuel usage can change by a factor of 10 based upon the weather conditions. Unless you can come up with a formula to account for that variable, taking a wild ***** guess would be just as accurate on any given day.

 

[Shamfarlango]

Re: Calculate GPH

Thanks for all the replies guys. A lot of good information here.

Is there a relatively inexpensive fuel flow meter solution? I'm seeing about $300 as the cheapest

 

[dingbat]

Re: Calculate GPH

The cheapest solution is to add the fuel flow function to your existing chartplotter. Otherwise, a Flowscan ~$300 is about your best bet.

 

[Beefer]

Re: Calculate GPH

Well, you'd just need to know the fuel usage for your model of engine at a given rpm. However, that would only be a ballpark figure as each engine will vary slightly depending on tune and overall condition. A fuel flow meter is the best way to be sure. Weather has nothing to do with GPH usage (air temp notwithstanding), but it will affect fuel mileage.

That's a contradiction. Weather does and will play a part in GPH and MPG. You can't change one without affecting the other.

 

[QC]

Re: Calculate GPH

Let's try to keep this open without too much fighting, but technically I agree with haulnazz, beefer and dingbat to varying degrees. Yes, if a headwind puts additional load on the bow she'll make less speed for the same RPM (increased slip BTW), and she will have a higher GPH for the same RPM (slight lug). However, I don't believe a factor of 10 ever. Sorry, and dingbat knows his stuff, but I wouldn't believe a factor of 2 and would have a hard time with 1.5 regarding GPH by RPM variation . . . Wheeled vehicle? Yes. Fixed pitch propeller? No.

beefer is right too as it affects all three, GPH, MPH and the resulting MPG. haulnazz gets that propeller lug curves are pretty darn linear so variation by RPM is minimal unless climbing a steep swell or planing with different loads. Once at cruise type speeds the swing is minimal.

I guess the swing could be significant between some specific examples. Let's say you are climbing a steep swell while trying to plane vs. surfing the backside already on plane at the same RPM. Hard to do but the GPH swing would be large in that case.

 

[haulnazz15]

Re: Calculate GPH

That's a contradiction. Weather does and will play a part in GPH and MPG. You can't change one without affecting the other.

The effect of weather on GPH is minimal unless you are finding a way to differ the load a significant amount. An engine uses a specific amount of fuel per hour at a constant rpm, not a whole lot is going to change that. Now MPG is completely different, and has a ton of variables that contribute to it, weather being one of the many. Don't confuse GPH and MPG, as they indicate different circumstance.

Short example: You run @ 3000 rpms for an hour in no wind/no current and use 10GPH and travel 10 miles, so 1 MPG. You go out the next day into a large headwind, run @ 3000 rpms for an hour and only go 8 miles. You are still going to burn close 10GPH on the second trip (sime run time, same rpm), but your MPG is now 0.8. See the difference? While there are a few extreme situations which can drive GPH to increase appreciably (which QC touched on), you cannot generally associate weather with GPH calculations.

 

[QC]

Re: Calculate GPH

This may help illustrate why I struggle with 10x GPH swings. I've used the same to support the torque discussions, but don't run screaming I've separated out just the fuel curve(s):

This is s diesel, but the concepts are the same. Look at the lower curve at say 3000 RPM The prop's typical load is right at 4 GPH while the engines max is right around 6. That's a 1.5 swing between typical and a full lug, and I don't think you can create a full lug at that RPM once on plane . . .

 

[dingbat]

Re: Calculate GPH

I guess the swing could be significant between some specific examples. Let's say you are climbing a steep swell while trying to plane vs. surfing the backside already on plane at the same RPM. Hard to do but the GPH swing would be large in that case.

Tides and waves can have a very pronounced affect on fuel consumption. Those of use who deal with them day in and day out have a healthy appreciation for mother nature.

A typical day find us trolling 800-900 RPM with the tide and 1000-1100 RPM trolling against the tide to maintain 2.5 kts. Throw in a 10-15 Kt. north or south wind blowing opposite the tide and you could easily be pushing 1500-1800 RPM to maintain the same speed. The difference in fuel consumption over an 8-10 hour day under those conditions is substantial. Throw a following sea into the mix on the way home and you?ll think you have a hole in the bottom of your fuel tank.

I don?t know how many times we?ve run 30-35 miles out in the morning at 4200 RPM while only using 16-20 gallons of fuel only to find ourselves later that afternoon, slugging our way back home( off plane) at 1800-2500 RPM while praying we make it back to the dock with a 150 gallon fuel capacity.
The last example may seem extreme to some but others know exactly where I?m coming from. It?s certainly not hard or uncommon to use twice the expected fuel on any given day.

 

[haulnazz15]

Re: Calculate GPH

Tides and waves can have a very pronounced affect on fuel consumption. Those of use who deal with them day in and day out have a healthy appreciation for mother nature.

A typical day find us trolling 800-900 RPM with the tide and 1000-1100 RPM trolling against the tide to maintain 2.5 kts. Throw in a 10-15 Kt. north or south wind blowing opposite the tide and you could easily be pushing 1500-1800 RPM to maintain the same speed. The difference in fuel consumption over an 8-10 hour day under those conditions is substantial. Throw a following sea into the mix on the way home and you?ll think you have a hole in the bottom of your fuel tank.

I don?t know how many times we?ve run 30-35 miles out in the morning at 4200 RPM while only using 16-20 gallons of fuel only to find ourselves later that afternoon, slugging our way back home( off plane) at 1800-2500 RPM while praying we make it back to the dock with a 150 gallon fuel capacity.
The last example may seem extreme to some but others know exactly where I?m coming from. It?s certainly not hard or uncommon to use twice the expected fuel on any given day.

But keep in mind, you are getting worse fuel mileage due to increased rpms to maintain your 2.5kts, not because GPH was affected by the waves. If you held the rpms constant (of course your speed would drop) when compared to going with the waves, you GPH would be much closer. Again, it's mainly a disparity between MPG's, not GPH. The only reason you burn more fuel in worse weather is because: 1) you increased rpms to maintain a calm-day speed, or 2) you had to run the same calm-day rpm's for a longer period of time in order to reach the same distance.

 

[dingbat]

Re: Calculate GPH

But keep in mind, you are getting worse fuel mileage due to increased rpms to maintain your 2.5kts, not because GPH was affected by the waves.

I needed more RPM to stay at the same speed becouse of the force of the waves yet the increase in GPH wasn't caused by waves?

Here is one for you. I'm going from Cheaspeake Beach to Ocean City and back next week. It's 120 miles each way. I get 2.17 miles per gallon or 14 gallons per hour at cruise. How much fuel am I going to burn round trip?

 

[QC]

Re: Calculate GPH

He means GPH as compared to RPM . . . RPM went up, GPH goes up, speed stays flat, MPG goes down