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Old 08-01-2021, 20:31   #1
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GPH---gas engines

I recently went under contract to buy a 28 ft pilot house cruiser with a 7.4 LTR MERCRUISER 454 V8 gas engine. If I run it at half throttle what can I expect for GPH? Thanks....Captain Tony
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Old 08-01-2021, 20:50   #2
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Re: GPH---gas engines

Figure 1 GPH for every 10 hp it is putting out


Half throttle... figure 120 hp, about 12 GPH. Maybe a little less


Works for most modern gassers
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Old 08-01-2021, 22:05   #3
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Re: GPH---gas engines

What are you trying to figure out?
MPG is highly speed dependent?
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Old 08-01-2021, 22:34   #4
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Re: GPH---gas engines

The formula given by Jammer is a reasonable estimate. I have a 28 foot boat, not including swim grid which is a new Merc 350 L sterndrive. The problem with boats with good speed is that, yes you can go half speed, but it becomes addictive to get to your destination faster.

Another formula and true for my boat is that at roughly 75 - 80 % of WOT is your most efficient speed, other than going displacement speed. At 7 knots you can go forever, don't laugh this speed is what a lot of trawlers do, but once you can bounce along at 22 knots or so, 7 knots is painful. My boat is heavy for its size and with 350 hp I can get up to 30 knots.
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Old 09-01-2021, 08:18   #5
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Re: GPH---gas engines

Assuming that engine is correctly propped for somewhere around 4200 - 4300 rpm at WOT, expect somewhere around 2.5 - 3 gph at 1300 - 1500 rpm, around 15 - 16 gph at 3200 - 3300.
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Old 11-01-2021, 08:24   #6
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Re: GPH---gas engines

Biggest factor is weight. At planning (cruise) speed, use this formula to know fuel consumption : Weight/1000 * 2

Example: 10000lbs/ 1000*2 = 20gph
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Old 11-01-2021, 08:27   #7
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Re: GPH---gas engines

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Originally Posted by Davepf View Post
Biggest factor is weight. At planning (cruise) speed, use this formula to know fuel consumption : Weight/1000 * 2

Example: 10000lbs/ 1000*2 = 20gph

I'd say that's very far from accurate and doesn't take speed, hull design or the engines into account. My boat is about 26,000 lbs loaded with twin gas 454s, so by your logic, it should burn 52 gal/hr. That's almost what my engines would burn at WOT. At a 17 - 18 kt planing cruise, actual burn is in the 30 - 33 gal/hr range.
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Old 11-01-2021, 08:44   #8
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Re: GPH---gas engines

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Originally Posted by rslifkin View Post
I'd say that's very far from accurate and doesn't take speed, hull design or the engines into account. My boat is about 26,000 lbs loaded with twin gas 454s, so by your logic, it should burn 52 gal/hr. That's almost what my engines would burn at WOT. At a 17 - 18 kt planing cruise, actual burn is in the 30 - 33 gal/hr range.
It's just a rule a thumb for cruisers in the 25-30 feet range.
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Old 03-02-2021, 13:01   #9
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GPH---gas Crusader engines

I have recently switched over from a 35 foot motorsailing sailboat to a 38-ft power cruiser. It has two Crusader 454 gas engines and used 150 gallons on a run from Providence Rhode Island to Boston Massachusetts. Is this typical for such cruises? The sailboat had a Perkins 4.236 diesel rated at 85 horsepower with a 22-in bronze prop and 150 gallons would get me 1,500 miles. I ran the power cruiser twin engines at 2200 RPM and figure their burning at least one gallon of gasoline per mile. Would appreciate any comments on this thank you.
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Old 03-02-2021, 13:06   #10
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Re: GPH---gas Crusader engines

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I have recently switched over from a 35 foot motorsailing sailboat to a 38-ft power cruiser. It has two Crusader 454 gas engines and used 150 gallons on a run from Providence Rhode Island to Boston Massachusetts. Is this typical for such cruises? The sailboat had a Perkins 4.236 diesel rated at 85 horsepower with a 22-in bronze prop and 150 gallons would get me 1,500 miles. I ran the power cruiser twin engines at 2200 RPM and figure their burning at least one gallon of gasoline per mile. Would appreciate any comments on this thank you.

What speed were you getting at 2200? There's a good chance that's a horribly inefficient speed to run, putting you above hull speed and plowing a ton of water, but not going fast enough to get up on plane. For my 38 foot planing cruiser with twin 454s, as an example, my fastest slow cruise is around 7 kts at 1400 RPM. Beyond that, it's time to get up and go, which means getting on plane and doing at least 15 kts at 3100 RPM. Normal planing cruise for me is around 17 kts at 3300. Anything in between 7 and 15 kts is only done if conditions require it, as it's inefficient, makes a ton of wake, etc.


Personally, in calm water, I get about 0.55 nmpg at 17 kts (3300 rpm) and about 1.3 nmpg at 6.5 - 6.7 kts (1300 rpm).
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Old 03-02-2021, 13:24   #11
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Re: GPH---gas engines

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Originally Posted by Davepf View Post
It's just a rule a thumb for cruisers in the 25-30 feet range.
I have a 38 foot cruiser rated at 12 tons. I made a run from Providence Rhode Island to Boston Harbor and burned 150 gallons. These are Crusader 454/350 gas engines and the RPM was 2200. I figure I have to plan at least a gallon per mile. I recently switched from a 35-ft motor sailor with a Perkins 4.236 diesel engine and 22-in bronze prop, to now this big twin engine cruiser. With the sailboat 150 gallons would get me 1,500 miles. That's a run equivalent from Boston down the coast to Florida. I recently made a switch from having the motorsailer, to now running this twin engine cruiser. Would you consider this kind of fuel consumption normal? Also it would be assumed that you don't run your boat during the winter up there in Canada? I only run during the winter months if the water temperature is 40° F or higher and the engine room about the same or at least 50° F for the fear of tearing up the seawater pumps.
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Old 03-02-2021, 14:11   #12
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Re: GPH---gas Crusader engines

I figure about 10 knots. But this varies and also take note that I am new to running a cruiser. I used the ebb flow from Providence, down to Newport. Then Buzzards Bay northbound it does not matter what the current is because it flows east to west and vice versa. Coming through the canal at slack water. I literally zipped through hitting 15 knots or better. Then up Boston's South shore with the ebb flow drawing 150°. There is a lot of resistance, coupled with tide and wind. In Buzzards Bay with a 15 knot wind the hull was slapping (some people call this heavy chop) I didn't like the noise and throttled back and the boat traveled much smoother and to the best of my knowledge making 10 knots. Considering there are four barrel carburetors, three quarters to one gallon of gas per mile, appears to be normal consumption for these engines. About coming on plane I'm not experienced with that either, plus it was discovered the trim tabs don't work. Because I run up and down the East Coast my concern is not speed but just getting the best days travel for the cost. From Boston to Savannah it is 1300 miles and the cost of gasoline is currently $2.50 to $3.75 a gallon. It's going to be a very expensive trip to say the least. My email is quad computers@yahoo.com. I'm going off here but would appreciate the knowledge you have on these concerns with gas engines. With the Perkins 4.236 diesel, 10 miles per gallon @ 12-15 knots was very typical. With or without the sails.
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Old 03-02-2021, 20:38   #13
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Re: GPH---gas engines

A 454 in a pickup gets 5-6 mpg. Not exactly an economic engine and you're running 2.
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Old 05-02-2021, 05:51   #14
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Re: GPH---gas engines

ONE GALLON A MILE IS NORMAL
I have TWIN 4.3 Volvo Penta ,
6 cylinder Chevy Vortec engines
If you plan on long voyages on gas engines , Fill ups you will CRY

Over the 6 years of owing my boat , No matter what experiment I do I still get same gas mileage , 1 engine hours - 2 engines slow - 10 knots , 20 knots . ONE GALLON A MILE
I seldom go fast ,
Sail boats pass me
Past summer I went on a 50 mile trip , Topping off when leaving
Get too destination , topping off fuel , checking oil discovered raw water pump failure ,Water circulating never effecting temp but salt did damage too alternator , start coil pak , weeks later starter ,
Following day returning too port one engine - around 8 knots
took around 8 hours . get too home port , topped off
Burned same amount of gas .
You would think going slow just pushing over water , basically little faster then idle Still burned 50 gallons

I have been considering a different styled boat a motor yacht , since were retired , want too cruise weeks , I am so afraid on a wrong purchase
Lots of Silertons , Carvers , show beautiful , many within my budget ( willing too spend on a boat ) all had gas engines.
Trawlers burn 2 gallons a hour , but not crazy about looks ,
Was considering sail , but recent health issues concerns me ,
Then if I get something too big will I take out often or it will sit as a dock queen with a 20 ft cobia tied too transom , fishing , fun

Presently Gas at marina is $ 3.19 gallon down from $ 4.39 ( last year )

During recent weeks Gas has gone up $ 50 cents a gallon on the Street ,
I expect marina gas too go back too were it was possibly reaching $ 5.00 bucks a gallon . ( talk at gas dock )
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Old 05-02-2021, 06:09   #15
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Re: GPH---gas engines

Quote:
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ONE GALLON A MILE IS NORMAL
...
X2

For that size of a boat, yes 100% normal !
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