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Old 08-27-2015, 05:40 PM   #1
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Diesel Economy

After reading the gas vs diesel thread I thought I would share these figures from a trip to Grafton harbor from our marina a couple of weeks ago, it's about 130 miles round trip. The other boat is a Gibson 47' houseboat with twin 454 GM power, we both topped our tanks before the departure date. My 38' Bayliner has na Hino diesels, we cruised around 10 mph, had a 2 hour wait at lock #25 and on the way back back had to run about 4 miles at 2800 rpm making 15.5 mph. The Gibson fueled at Grafton and took 64 gallons and took on 138 gallons when we returned to our home marina. I made a 8 mile trip helping some people to scatter their Mothers ashes was the only difference in our total miles traveled. I took on 85.1 gallons of diesel.
Bayliner 85.1 gallons - 138 miles
Gibson 202 gallons - 130 miles

I do think the Gibson is over propped, his hull is dirty and wot rpm is 3600
Mine has fresh bottom paint and reaches the proper rpm.


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Old 08-27-2015, 08:52 PM   #2
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What speed was he running at?
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Old 08-27-2015, 09:00 PM   #3
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We were both running the same speed 9.5 to 10.3 depending on current. He was running 2000 to 2400 rpm, I ran 1500 to 2000 rpm, his secondaries weren't open except on the 4 mile run to the lock.


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Old 08-27-2015, 09:23 PM   #4
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So you're comparing a 47' boat to a 38' boat and you're surprised at the difference? How do the hull shapes and displacements compare? This is kind of like comparing apples and tomatoes, yea there both red.

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Old 08-27-2015, 09:32 PM   #5
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So you're comparing a 47' boat to a 38' boat and you're surprised at the difference? How do the hull shapes and displacements compare? This is kind of like comparing apples and tomatoes, yea there both red.

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Two completely different type boats.
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Old 08-27-2015, 09:41 PM   #6
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I am guessing the house boat is a planning hull and at 10 or so knots is really sucking the fuel..especially if loaded down.


The Bayliner may be a little to the semi-displacement side of shapes and much more efficient in the 10 knot range.


Had he ran at 8 or 20+ knots the numbers may have been closer..but a badly fouled hull and gear throw everything off.


All a wild guess..but....
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Old 08-28-2015, 04:56 PM   #7
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When we ran wot for a bit I was faster which surprised me, Gibsons have a flat bottom and will plane. I knew I would use less but had no idea he would use over twice as much. We both ran our gens probably a equal amount of time, it has been 3 years since his boat got a fresh bottom job. It's my guess that it has a fur coat now and that's why his speed was low and fuel use so high. I thought his fuel use would've averaged about 1 mpg.


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Old 08-28-2015, 09:53 PM   #8
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Really not a reality as to comparing these two hulls, better that two of the same Bayliners one gas and the other diesel run together. These two hulls are not in any way the same configuration to give anything but "He burnt this amount and I burnt this amount"
Find another described Bayliner above and then let us know the results. As a additional part of that experiment, what would be the cost difference between gas engines and diesel in terms of replacement, annual hourly cost.
It is collected data that the forum can digest and make comparisons in relationship to their perspective boats in response. I believe it would develop into an interesting thread.
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Old 08-30-2015, 04:21 AM   #9
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Al there are 3 Bayliners in the marina a 5788, 3988 and my 3870 all are diesel - Manns & Hino, all other boats of a similar size are house boats. There are 3 gas powered cruisers a couple of Rinkers in the 34' to 40' and a 410 SeaRay, non of them cruise over 10 miles on a weekend just far enough to get to a sandbar. The 47' Gibson is the only one I've cruised any distance with. I didn't start this thread thinking the boats were comparable in economy, I started it because I thought the Gibson used a excessive amount of fuel because he is over propped probably because his hull is dirty. I've since talked to another owner of a 47' Gibson with 496 GMs that went from our marina to Minneapolis and back he got .9 mpg. When we planned our trip I told him I thought he would burn about 140 gallons on the trip, I told him I would use 80 to 90 gallons, I was pretty close on mine but way off on his.


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Old 08-30-2015, 07:34 AM   #10
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Hello River Cruiser,


I do not know much about the 47 Houseboat but I do agree that the comparison yields little information that can help as the boats are so different. We usually cruise at least once a year with between 25 and 35 boats (gas and diesel) on the same route of between 100 and 200 miles and although that allows some comparison of speed and fuel use it seems to have minimal advantages unless one is interested in acquiring one of those boats.
Your Bayliner will see an increase of economy of over 1 mpg just by travelling at about 1 knots less then that 10 mph speed. You could even travel at the planning speed of 15 mph to 'make up' the time lost at the lower speed and still come out way ahead in total fuel burn for the trip. Please make sure you can reach 3,000 plus 3-5% fully loaded on a hot and humid day as a WOT test to ensure your engines are loaded OK at high cruising speeds.
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Old 08-30-2015, 11:21 AM   #11
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surprising to me is how much diesel you used.
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Old 08-30-2015, 11:57 AM   #12
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Boattest.com gives fairly rigorous fuel consumption figures for various boat and engines. Modern common rail diesel engines typically are reported as producing 20 hp per gph of fuel at wotwhich is consistent with manufacturer curves. And if you look at curves for older non common rail engines, these range from 17 to almost 20 hp per gph at wot.


Four stroke, fuel injected gasser inboards are reported by Boattest to produce about 12 hp per gph at wot. The gasser may be penalized a little bit by measuring at wot but that is the only way to know exactly how much horsepower is being produced because gasser manufacturers never publish hp and fuel consumption curves.


But with precise fuel air ratio provided by EFI, I don't think it makes that much difference at wot vs lower power loadings compared to the days of 4 bbl carburetors that dumped fuel like an open hose into the engine at wot.


FWIW four stroke outboards produce 10-12 hp per gph according to Boattest and of course some of them are auto engine derivatives so that isn't surprising. Even the 2 stroke Evinrude E-Tec engines get 10 hp per gph.


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Old 08-30-2015, 12:10 PM   #13
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Dave- Using the WOT numbers for gassers skews things worse than they actually are. Many if not most gas engines enrich the mixture at full power to protect pistons, valves, etc from heat. Go to cruise and mixture leans and hp/gph improves a little. How much? Who the heck knows, they don't publish any curves!!!

Annoying.

Even CR diesels efficiency drops some at full power, but for different reasons. And not by much.

Gives me the urge to buy a decent dyno and figure this stuff out for myself!!! Probably not my best idea of the week...
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Old 08-30-2015, 01:00 PM   #14
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Quote:
Originally Posted by djmarchand View Post
Boattest.com gives fairly rigorous fuel consumption figures for various boat and engines. Modern common rail diesel engines typically are reported as producing 20 hp per gph of fuel at wotwhich is consistent with manufacturer curves. And if you look at curves for older non common rail engines, these range from 17 to almost 20 hp per gph at wot.


Four stroke, fuel injected gasser inboards are reported by Boattest to produce about 12 hp per gph at wot. The gasser may be penalized a little bit by measuring at wot but that is the only way to know exactly how much horsepower is being produced because gasser manufacturers never publish hp and fuel consumption curves.


But with precise fuel air ratio provided by EFI, I don't think it makes that much difference at wot vs lower power loadings compared to the days of 4 bbl carburetors that dumped fuel like an open hose into the engine at wot.


FWIW four stroke outboards produce 10-12 hp per gph according to Boattest and of course some of them are auto engine derivatives so that isn't surprising. Even the 2 stroke Evinrude E-Tec engines get 10 hp per gph.


David
Tohatsu are the only people who publish fuel consumption figures: Tohatsu Outboards - Fuel Consumption

Your estimates seem to be right on the money.
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Old 08-30-2015, 01:16 PM   #15
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That Tohatsu stuff is nice, but it is just for full power. We want rpm/hp/gph data for part load.
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Old 08-30-2015, 01:47 PM   #16
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That Tohatsu data is interesting. I am surprised that their 2 cycle engines do as well as they do compared to the same hp 4 cycle engine. I thought that only the Evinrude E-Tec comes close for a 2 cycle.


But as Ski notes, the data are only at wot so who knows how good are bad they are at part throttle. I am sure that even EFI gasser engines run rich to protect valves, etc. at wot. The EPA doesn't measure emissions at wot so who cares if they run rich, right. OTOH thermodynamic efficiency is worse under part throttle conditions for a gasser unlike a diesel.


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Old 08-30-2015, 03:24 PM   #17
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Well here is one comparison on Sea Rays. Comparing 2x380 Mercruisers in a Sundancer 370 to 2x380 Cummins in a Sundancer 450. I'll keep looking for something in identical boats.

Test Power: 2 x 380-hp MerCruiser 8.2 MAG DTS with Axius
Range
RPM MPH Knots GPH MPG NMPG Stat. Naut
660 4.1 3.6 2.5 1.6 1.4 334 290
1000 5.9 5.1 3.7 1.6 1.4 326 283
1500 8.2 7.1 5.6 1.3 1.1 255 222
2000 10.0 8.7 10.7 0.9 0.8 189 164
2500 12.0 10.4 17.2 0.7 0.6 142 123
3000 18.0 15.6 24.2 0.7 0.7 151 131
3500 27.3 23.7 27.8 1.0 0.8 200 174
4000 34.1 30.5 40.0 0.9 0.8 178 155
4500 40.7 35.4 51.0 0.8 0.7 162 141
4800 43.8 38.0 64.8 0.7 0.6 137 120

Test Power: 2 x 380-hp Cummins QSB5.9
Range
RPM MPH Knots GPH MPG NMPG Stat. NM
600 5.0 4.3 1.2 4.2 3.6 1095 952
1000 8.6 7.5 3.2 2.7 2.3 706 614
1500 10.5 9.1 9.4 1.1 1.0 292 254
2000 16.3 14.2 21.5 0.8 0.7 199 173
2300 21.7 18.9 26.8 0.8 0.7 213 185
2500 25.9 22.5 27.8 0.9 0.8 244 212
2800 32.5 28.2 35.5 0.9 0.8 240 209
3000 35.9 31.2 39.4 0.9 0.8 239 208
3040 36.8 32.0 40.3 0.9 0.8 240 209
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Old 09-01-2015, 05:50 PM   #18
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You can estimate how much horsepower you are using
from your flo-scan liters per hour:

Diesel 5 hp/Lph (19 hp/gph)
4 stroke gas 4 hp/Lph (15 hp/gph)
2 stroke gas 3 hp/Lph (11 hp/gph)

eg a 454 gas engine burning 50 Lph (for example at 3000-3200 rpm) will be producing an estimated 200 hp

I think this '5/4/3' rule of thumb works best in the normal operating range of the engine, ie excluding idle and WOT
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Old 09-01-2015, 10:00 PM   #19
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Nissan has direct injected 2 strokes also.
Probably Tohatsu powerheads though.
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Old 09-01-2015, 11:31 PM   #20
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surprising to me is how much diesel you used.

I normally run around 5.5 to 7.5 mph depending on the current and would of used 70 to 75 gallons, with gen time, we ran our generator 14 to 15 hours to keep the grandkids comfortable and able to watch tv on this trip. Over 4 years running 90 to 120 hours each year I've averaged 1.8 mpg including gen time, I/2 with the mis river current and 1/2 against.


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