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Old 04-11-2013, 14:17   #1
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Nmpg Per Hour For 35-38' Powerboat With Twin Gas Engines At Displacement Speeds?

Title kind of says it all. I've looked at some 35-38' powerboats with twin gas engines (318s or 350s mostly, don't want to get into the 454's...). Was wondering what kind of mileage I could expect at around 7 knots or so.

Of course I would rather have a more efficient boat...a full displacement trawler with a diesel for example, but can't afford it.

One particular boat I'm looking at, if it helps, is a 38' tolly with twin 318s.
Another I have looked at was a chris-craft catalina with twin 350's.
Both boats were around 8-9 tons dry I think.

is 1.5 or even 2 nmpg/hour unrealistic?

And it seems from everything I've read that running one negine is about the same nmpg/hour as running both engines?

Any input would be greatly appreciated.
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Old 04-11-2013, 15:31   #2
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

I just looked at a nice 36' Carver Aft Cabin with twin 350's. The owner noted his economy setting was 10 knots and 10 gph total, 1 mpg.
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Old 04-11-2013, 16:13   #3
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I've played with computing MPG on a couple of Sport Fisherman types a few years back before GPS would do that for you. What I found that the slower, the better, with the best being at idle. I think you can get 2 kts per gl easy if you can stand going that slow, assuming of course you can maintain steerage
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Old 04-11-2013, 16:23   #4
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

The Tolly is one stout boat, however, the 350 engines (assuming they are GM) are super easy and cheap to get parts for. Plenty of 318's out there, but not so common. Every hot rodder in t he nation is using a 350GM it seems.. The Tolly will likely have Velvet Drives transmissions. Not sure about the CC. Read online regarding these transmissions so you know what to look for in one that is failing, they are expensive, but good. Some tell tale signs that can be determined at the dock as I remember. :>) I wish I could help, but I would think at idle or low rpm you might reach your goal.
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Old 04-11-2013, 19:08   #5
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

I went on the tolly today and he said the PO ran it at 12 knots at 12 gph or so. He brought it up from the pacific coast of WA to port orchard and experimented along the way...said he used about 180 gallons coming up or so and said he usually ran it on one engine going 7-8 knots or so and was getting at least 2, maybe 3 nmpg.

The CC went pending but I think I will be putting an offer down on the Tolly....a hell of a lot of boat for under $10k....and was in pretty good shape overall...the cosmetic work and repairs needed were stuff I could do for pretty cheap. New swim step, replace the shag carpet with new flooring, new paint on the top decks, etc...needed a dinghy but that means I get to pick it....

Will have to find a mechanic nearby to look at the engines. But they were both replaced in '96 so they should probably last a while longer.
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Old 04-11-2013, 21:47   #6
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I have a smaller and lighter GlasPly 28' Long Cabin which probably weighs 14,000 lbs. Twin EFI 1996 Marine Power (350 Chev) engines through V-drives. Several friends have similar hulls with 350 Chev blocks. Most report around 1 -1.15 mpg cruising at about 25 knots.
Without a Flo-scan meter or a carefully monitored run between fuel stops you can only guess and estimate your fuel consumption. A fair assumption is that you will burn lots of fuel, the faster you want to travel. Pay attention to your fuel gauges if you are transiting any distance beyond your fuel tank capacity. Rule of thumb is 1/3 tank consumption to destination leaving 1/3 tank to get back and 1/3 safety margin.
Running out of fuel and having to be towed in is embarrassing, but not as bad as running out in an area where you are all alone.
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Old 05-11-2013, 03:54   #7
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

We used to have a Silverton 31 with twin 360's. It got around 1.5 mpg on plane.

Never ran it long enough at displacement speed to see how much better we could get.

Remember to open her up once in a while if you follow thru. We knew a guy who did like you are suggesting and after a few months started having engine problems. Turns out 4 strokes can foul out the plugs if they never get run up to temperature to burn off the carbon.
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Old 05-11-2013, 04:00   #8
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

I think you'll be surprised at how good the mileage is if you keep it under 7 knots or so.

As a rule of thumb, theoretical hull speed on a monohull is around 1.34 x the square root of the length (at the waterline).

Say your 38' Tolly is 36' at the waterline. Square root of that is 6. 6 x about 1-1/3 is 8. So above 8 knots you're trying to climb your own bow wave. 7 is probably a good compromise, where you get there in reasonable time without wasting too much fuel.

My boat's PO told me how well the boat cruised at 10 knots. And it's true that it rides well at that speed. However, it puts out a mountainous wake, and burns 2-3 more times more fuel to go the same distance, compared to going 7 knots or under.

In the end you have two numbers to consider; how far you can go on a given amount of fuel (e.g; MPG), and how long it takes to get there. GPH numbers are meaningless without knowing the speed.
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Old 05-11-2013, 05:11   #9
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

For petrol engines fuel consumption can be estimated as:
Litres/hr = 0.508 x KWprop
For diesel engines fuel consumption can be estimated as:
Litres/hr = 0.274 x KWprop

Where: KWprop = Kilowatts from propeller power curve



gasoline inboard engines use about 0.6 pound (272g) of fuel per horsepower generated
per hour


diesel engines need about 0.4 pound (181 g) per horsepower generated per
hour

A marine diesel engine consumes about 1 gallon per hour (GPH) for every 18 hp generated

Another way to calculate the amount of diesel fuel used (gallons) in 1 hour is to multiply
the horsepower being used by 0.055.
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Old 05-11-2013, 06:07   #10
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

Quote:
Originally Posted by gloriax View Post
For petrol engines fuel consumption can be estimated as:
Litres/hr = 0.508 x KWprop
For diesel engines fuel consumption can be estimated as:
Litres/hr = 0.274 x KWprop

Where: KWprop = Kilowatts from propeller power curve



gasoline inboard engines use about 0.6 pound (272g) of fuel per horsepower generated
per hour


diesel engines need about 0.4 pound (181 g) per horsepower generated per
hour

A marine diesel engine consumes about 1 gallon per hour (GPH) for every 18 hp generated

Another way to calculate the amount of diesel fuel used (gallons) in 1 hour is to multiply
the horsepower being used by 0.055.
That's all well and good but there is no effective way to determine the HP at the prop for the average cruiser. The numbers you state have nothing to do with the HP the engine is rated at unless you always travel at 100% throttle and the motor is dialed in perfectly to what the hull can take.

Propeller HP curves have a lot of assumptions baked in that rarely match up with a specific boat. Rough numbers based on other cruisers experience are probably more accurate as real world conditions are baked in to those numbers.

On a side note: While diesels are more efficent per HP, I think your numbers may be overstating the benefit.
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Old 05-11-2013, 06:23   #11
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

When in the assistance towing business, we idled around for HOURS at times. Albeit most of the boats were single screw diesels, we found that we rarely saw over 1 NMPG.

I own a 28’ Bertram with twin Yanmar 170’s. That boat seems to get within a very narrow margin about 1.6MPG regardless of going 6kts or 20…. above 20Kt she would get down to 1mpg. Same boat with GM 350 CI (she was gasoline when I bought her)….. 0.666MPG at 18Kts
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Old 05-11-2013, 06:29   #12
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

There are more BTUs in a gallon of diesel then in a gallon of gasoline
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Old 05-11-2013, 06:51   #13
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

gasoline inboard engines use about 0.6 pound (272g) of fuel per horsepower generated
per hour, and diesel engines need about 0.4 pound (181 g) per horsepower generated per
hour.On average, diesel fuel contains about 140,000 Btu per gallon (3.8 L), or 10 percent
more energy than the same volume of gasoline.




my boat (FNM HPEP 190) diesel engine is ratad
Maximum output at crankshaft 140 kW/190 hp at 4000 rpm

Maximum output at propeller 132 kW/185 hp at 4000 rpm

this is calculation for cruising speed 35 kts 160 SHP at 3250 rpm
you can use same calcullation for any (HP) setting. petrol or diesel
The density of No. 2 diesel fuel is (0.85) 850 grams/liter = 1.874 pounds/liter = 7.15 pounds
per US gallon.

Regular gasoline is considerably less dense: 6.0 to 6.3 pounds per gallon.


220 g/kWh x 118kW = 25960 g/h or 25.960 kg/h
0.85 kg/l is a typical figure for diesel
25.960 kg/h : 1 L/0.85 kg = 30.5 L/h (8 us gal)
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Old 05-11-2013, 08:35   #14
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

Quote:
Originally Posted by capngeo View Post
I own a 28 Bertram with twin Yanmar 170s. That boat seems to get within a very narrow margin about 1.6MPG regardless of going 6kts or 20
I saw very similar results with my previous boat, a 28' planing hull with a single gas engine (GM 350 block, 300 HP). Whether going 6 knots, or 24-28 knots on plane, I got 1.4 NMPG (which equals 1.6 statute MPG).

However, above hull speed but below full planing speed, anywhere between 8 and 20 knots, MPG dropped off dramatically.

Since we're talking semi-displacement boats here, the top end won't be anywhere near full planing speed.

Keeping it a little below hull speed is probably the best compromise. Which, on the Tolly in question, probably means around 7 knots.

Back to the original question, I think you'll have to go pretty slow to achieve 1.5 NMPG with a twin gasser of this size. My guess is you're not totally out of the ballpark, maybe just a little optimistic.
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Old 05-11-2013, 10:00   #15
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

For those saying they got similar nmpg at displacement and planing speeds, was that using both engines while at displacement speeds? Did you ever try using just one engine at displacement speeds?

Yes, I was figuring the max hull speed was around 8 knots so 7 knots or so would probably be an efficient hull speed. I'm pretty sure a 38' tolly with twin 318's can't get on a plane...or just barely...but it seems like it should get OK nmpg (relatively speaking) running off one engine going 7 knots or so. Then you also have the option to crank it up to 12-13 knots in rough water and use both engines for docking.

I figure I would travel around 1,000 nautical miles per year in jumps of 20-40 knots...just trying to get a comparison to a sailboat (which is what I'm used to). At 1.5nmpg it would cost about $2-3k more in fuel than a sailboat, at 2nmpg it would cost about $1,500-2,000 per year more than a sailboat, etc. So then I can say "yeah, it costs $2k more a year to operate in fuel but I can have a nice solar array for power and a lot more living space so it's worth it." At .75 or 1nmpg it would blow my budget though.
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