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Old 10-19-2015, 08:44 AM   #161
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Rule of thumb #'s are a broad way of saying "guess".

The nmpg #'s I've come up with since late 1950's when dad and I calced nmpg together... up to this very point in life... have always been attained by using miles traveled as compared to gallons of fuel usage from fill-up to fill-up. Pretty simple math.

I have a pair of brand new (still in orig package - purchased at an estate sale for very reasonable price) Flow Scan meters that I may decide to install. Would be interesting to watch them, but not really necessary IMO. Also, due to fact that our Tolly is a gasser there is a simple "rule of thump" I do like to adhere to, and that is; the less fuel line junctions the better! So, although I purchased "new" Flow Scans at such a great cost my sense of "gas line safety" may preclude ever installing them. May end up selling em at good price to others!

I've gotta ask:

In following quote from your post above... are you showing that diesel fuel doubles the btu power per gallon as compared to gasoline... or am I mixed up as to the meaning of the following?

"About 20hp/gallon diesel engine ( or slightly less )
About 10hp/gallon gasoline engine ( .. .. .. .. )"
Yep, gas engines only produce half the power output/gal than diesels, but only weigh half as much.Of course they develop their power much higher up the rev band (3500-4000revs), and that leads to higher inefficiencies .

Look at the big ob's at the bottom:

http://www.tohatsu.com/tech_info/fuel_consumpt.html
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Old 10-19-2015, 08:50 AM   #162
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Speaking of phobias, I heard about a guy who was so OC about the singles vs twins discussion on TF that he removed two perfectly good engines and put in one
Isn't this the same guy that moved from Louisiana to Texas, and raised the IQ of both states?
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Old 10-19-2015, 10:53 AM   #163
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In one of your previous posts you asserted a twin uses twice the fuel as a single. So what today determines fuel consumption at hull speed - weight, number of engines, hull design or?
This will never be anything but an old wife's tale because there are too many variables .. both real and imagined to swing it either way. The're lots of twin engined boats and there are lots of single engined boats. And all of them have the number of engines the NA seemed to think was best for them. Krogen put twins on a big hull type that usually is a single and considered by most to be a long range boat. Do you honestly think Krogen offered the twin thinking it was inefficient? There are differences of course .. in both directions but a NA dosn't choose single or twin re how much fuel they will burn. The difference is so small that other far more important design elements lead the way.
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Old 10-19-2015, 11:01 AM   #164
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Krogen put twins on a big hull type that usually is a single and considered by most to be a long range boat.
I have seen the fuel burn numbers for comparing single to twin installs for the KK 52, Nordic Tug 52 and Nordhavn 55. Surprisingly close dependent upon speed but my takeaway in all 3 cases was a 10% differential favoring the single.
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Old 10-19-2015, 11:53 AM   #165
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Comparing a well designed single to a well designed twin, both with properly matched and loaded engines- I'd figure the 10% penalty for twins sounds about right. More parasitic losses, more friction, more pumps spinning, more underwater drag.

But it is certainly not twice the burn with twins.

Many twins are designed to be semi-planing, those could easily burn 2x the fuel at hull speed compared to a full displacement with an engine sized for that speed. Yep, can burn 2x, but that is comparing apples and oranges.
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Old 10-19-2015, 01:45 PM   #166
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With the typical trawler having it's single shaft inside the keel there's definitely more drag for the twins. And typically twins have two of the same engines as the single so there's double the power and a lot more weight. That means comparing anything to do w performance is like comparing Kwis to watermelons.

If the truth is really to come out one would need two boats w identical hulls and displacement. Engines would total the same amount of power ect ect.

Comparing any two boats that actually exist would find one comparing boats that have differences large enough to alter the outcome this way or that. One could not have a turbo and the other not ect. But if two boats were found that almost amounted to the perfect comparison I'd sure like to see it.
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Old 10-19-2015, 01:55 PM   #167
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Comparing a well designed single to a well designed twin, both with properly matched and loaded engines- I'd figure the 10% penalty for twins sounds about right. More parasitic losses, more friction, more pumps spinning, more underwater drag.

Ski your guess is spot on vs my experience doing a few trips on twin and single nordhavn 55s. The op should just buy whatever best suits his needs. For a live aboard tough to beat a houseboat.



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Old 10-19-2015, 02:12 PM   #168
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My observations are that the ER in a twin N55 was quite workable with engines one JD size down and get home eliminated. What are your thoughts?
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Old 10-19-2015, 02:15 PM   #169
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Eric-- GB36s were always available with one or two engines. The engines used in each were generally the same type that was being used by American Marine in that model at the time. So initially one or two FL120s and by the end of production one or two Cummins 220 hp turbo (not sure about the turbo part).

I have talked to the owners of early GB36s with a single FL120 and IIRC they said that at about 7 knots they burned about 2.5 gph. At 8 knots they were up around 3 gph. We have two FL120s and to get 8 knots we run at 1650 rpm and burn, by very rough observation, about 5 gph total.

We used to charter a newer GB36 with a single Cummins 210. To get 8 knots we needed to run at 2000 rpm. However I have no idea how the boat was geared or propped nor do I know what this boat's fuel consumption is at that rpm.
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Old 10-19-2015, 02:37 PM   #170
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Eric-- GB36s were always available with one or two engines. The engines used in each were generally the same type that was being used by American Marine in that model at the time. So initially one or two FL120s and by the end of production one or two Cummins 220 hp turbo (not sure about the turbo part).

I have talked to the owners of early GB36s with a single FL120 and IIRC they said that at about 7 knots they burned about 2.5 gph. At 8 knots they were up around 3 gph. We have two FL120s and to get 8 knots we run at 1650 rpm and burn, by very rough observation, about 5 gph total.

We used to charter a newer GB36 with a single Cummins 210. To get 8 knots we needed to run at 2000 rpm. However I have no idea how the boat was geared or propped nor do I know what this boat's fuel consumption is at that rpm.

OK so 3 gal/hr at 8kts(2.6 mpg) on single Vs. 5gal with twins(1.6mpg)....

Nearly twice the fuel burn, give or take: ok about 60% more...
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Old 10-19-2015, 02:44 PM   #171
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Thanks Marin,
I'd like to have a GB32 w two Mitsu engines just like mine. Should be 80hp (total) and 2gph at a 50% load. Just a guess but I'll say about 7knots. Double the fuel burn of Willy but almost a knot faster .. but again just a guess.

Rusty Barge wrote;
"Nearly twice the fuel burn, give or take"
Rusty Barge nearly twice the power, give or take.
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Old 10-19-2015, 04:14 PM   #172
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Eric---- American Marine actually did build a tiny handful of GB32s with twin engines. I read about them years ago on the GB owners forum. There was at least on of them in BC, I think the Vancouver area. They used a pair of FL80s, the four-cylinder version of the Ford Dorset diesel that Lehman marinized into the FL120.
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Old 10-19-2015, 09:49 PM   #173
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Interesting Marin,
Too bad they didn't use Perkins 4-108 engines. Those two 4cyl FL's at an idle slightly out of sync would rattle windows and clear the table.
I think the 120hp GB32 has a top speed of 10 knots. An 80hp version should cruise at 7.5 to perhaps 8 knots. Perfect.
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Old 10-19-2015, 11:32 PM   #174
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I know a guy in the Sausalito (San Francisco Bay) area that has a GB32 with twins. I'll find out the manufacture and report back.
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Old 10-19-2015, 11:49 PM   #175
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The IG32 here came with a variety of engines, incl 2 80hp Lehmans, which vibration apart would work well. Some had a single FL120, in one extraordinary case, twin 200hp Volvos. I`ve seen twin 230 hp Volvos in an IG36, what were they thinking?
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Old 10-19-2015, 11:51 PM   #176
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Originally Posted by Rustybarge View Post
Yep, gas engines only produce half the power output/gal than diesels, but only weigh half as much.Of course they develop their power much higher up the rev band (3500-4000revs), and that leads to higher inefficiencies .

Look at the big ob's at the bottom:

Tohatsu Outboards - Fuel Consumption
Diesel fuel does not contain all that much more energy than gasoline.
http://www.diffen.com/difference/Diesel_vs_Petrol
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“Energy Content of petrol vs diesel
Gasoline contains about 34.6 megajoules per litre (MJ/l)while diesel contains about 38.6 megajoules per litre.”
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Old 10-20-2015, 12:31 AM   #177
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Interesting Marin,

I think the 120hp GB32 has a top speed of 10 knots. An 80hp version should cruise at 7.5 to perhaps 8 knots. Perfect.
I don't know that American Marine ever built a GB32 with one FL80 in it, but I could be wrong.

BTW, the FL80 is the four-cylinder version of the FL120. The FL90 is the four-cylinder version of the FL135.
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Old 10-20-2015, 11:25 PM   #178
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The IG32 here came with a variety of engines, incl 2 80hp Lehmans, which vibration apart would work well. Some had a single FL120, in one extraordinary case, twin 200hp Volvos. I`ve seen twin 230 hp Volvos in an IG36, what were they thinking?
Was that IG with the big Volvos for sale in Anacortes? We looked at one up there this summer with giant turbo Volvos that was an estate sale. It was super clean and well priced, and engines aside was probably the best boat we looked at from Vancouver to San Francisco. I just can't remember exactly where it was.
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Old 10-21-2015, 12:05 AM   #179
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I`ve seen twin 230 hp Volvos in an IG36, what were they thinking?
Probably the same thing American Marine/Grand Banks was thinking when they made two Cummins 220 turbo engines the stock offering in the GB36. The boat could be driven pretty fast which appeals a lot to boaters with time constraints on their cruising who want to get somewhere fairly quickly, then putz around for a week or two, and then get home quickly.

This is a popular operational mode for a lot of Grand Banks boats in our harbor, particularly in the big charter fleet. They bomb on up to Desolation Sound at 14-16 knots in a day or two, a trip that takes us some four days at 8 knots, cruise around at 8 knots or so enjoying the place, and then bomb on home.

There's a big market here for boats that can do that.
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Old 10-21-2015, 12:36 AM   #180
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Was that IG with the big Volvos for sale in Anacortes? We looked at one up there this summer with giant turbo Volvos that was an estate sale. It was super clean and well priced, and engines aside was probably the best boat we looked at from Vancouver to San Francisco. I just can't remember exactly where it was.
The one I saw is at Brooklyn, in Sydney, Australia, I think it is under contract. I`ve seen another advertised with 200hp twin Volvos, and trim tabs. Trim tabs on a trawler?!
GB36s built here by Riviera usually had twin Cummins 5.9L engines,some had Volvo twin 165s.
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