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Old 11-04-2013, 05:47 PM   #21
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I apologize if I brought this up in the wrong area. Obviously new to site. Give a guy a break.
Hey, I wasn't chastising per se....but with forums, in general, you want to put topics where they belong so people know where to look for them. It also allows more related searches and traffic to provide you the information you are looking for....IOW, people do not come to the "Welcome Mat" to find out about fuel saved by running a twin on one engine!!! If you put that in the powerplant section, then more people are likely to find it and reply to it....make sense?....
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Old 11-04-2013, 05:58 PM   #22
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A forum to discuss the maintenance and repair of Chris Craft Constellation 500 and related motoryachts
You'll notice you are currently in the Chris Craft section

It's a little thin content wise right now so invite your friends and enjoy
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Old 11-04-2013, 06:29 PM   #23
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Nice pic CPseudonym.
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Old 11-04-2013, 08:31 PM   #24
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Hi All,
Pat: Glad to see that you managed to find our new forum on this site and post a message. Unfortunately the Boat US forum is getting less and less reliable. I think we need to transfer our posts to this forum. Sorry to hear that your latest nmpg figures were less than expected but suspect it had something to do with the way the fuel consumption was measured. Iíll try to explain the nmpg you should get.
A Connie running at 1400 rpm on both engines gets 10.5 knots and burns 14 gph. That comes from running at that speed for several years plus the Detroit Diesel engine performance curves. So the specific fuel consumption is 10.5/14.0 = 0.76 nmpg. When running on one engine the speed drops to 9.2 knots and the fuel consumption drops to half of the two engine fuel consumption or 7.0 gph. This makes the specific fuel consumption 9.2/7.0 = 1.31 nmpg. Thatís a (1.31-0.76) = 0.55 nmpg saving or a 42% saving over the twin engine fuel consumption.
A few posts ago you thought you were getting 1.85 nmpg so the 1.125. nmpg you think youíre getting now must be a great disappointment.
Your latest data gives a specific fuel consumption of (180.0/1.15)/160.0 = (156.5/160.0) = 0.978 nmpg at 9.0 knots which is only 0.978/1.31 = 74.7% of the nmpg you would expect. But I assume that you estimated the fuel consumption based on the fuel gauge readings. My experience is that you can only read them to an accuracy of about 5 gallons and the saddle tanks are tapered top-to-bottom which throws the reading off a bit more. So I donít think you can really trust the gauge readings to better than 10 gallons. So you might have only burned only 150 gallons which would make the specific fuel consumption (156.5/150.0) = 1.04 nmpg.
Thatís still only about 79.4% of what you should expect but it is (1.04/1.125) = 92.4% of what you think you actually got. Either way itís not what you should expect so there still must be a factor we havenít accounted for. But an 8% error in this type of measurement isnít bad. Weíll have to wait for further data to really nail this number down. Expect the answer to be somewhere near 1.31 nmpg.
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Old 11-04-2013, 08:41 PM   #25
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Hi Baker & Ranger2C

Yes, I was surprised that this thread was put in the "Welcome Mat". I suggested that it be in the Trawler Forum.

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Old 11-04-2013, 08:50 PM   #26
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Hello Pete, glad we caught up. I started with full saddle tanks and then topped them off. Definitely room for error. Plus the factor of fuel returning to a different tank. Non the less, we leave Demopolis Al on early Wednesday morning and will run on both to make 10 knots so we can reach a decent anchorage. It will be nice to let "The Ladies" run.
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Old 11-04-2013, 08:55 PM   #27
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Lets not forget the buoyancy of salt water compared to mud.
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Old 11-04-2013, 08:56 PM   #28
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Hi CPSeudonyn,

I have an address list of about 200 Connie owners. This forum running under the Boat US Manfacturing Forums ran for seven years, had 2000 posts and nearly a half million reads. We also have a 2.5 gigabyte Connie Owners Library on DVD which documents what we have learned about Connie in the past seven years. When I have notified the Connie owners about this new forum by mail this site should get a lot thicker.

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Old 11-04-2013, 09:10 PM   #29
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I would sure like to get my hands on one of those dvd's.
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Old 11-04-2013, 09:26 PM   #30
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You'll notice you are currently in the Chris Craft section

It's a little thin content wise right now so invite your friends and enjoy
WOOOOHOOOOOO!!!!!! Great job Craig!!!!
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Old 11-04-2013, 09:27 PM   #31
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I would sure like to get my hands on one of those dvd's.
What would be even cooler is if we could find somebody to host that data and then we could put it at the top of this forum as a "sticky". We do not offer that service but it would surely be cool if somebody had some space on their server!!!
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Old 11-04-2013, 09:36 PM   #32
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Wow! This forum should raise huge numbers of inquiry and reaction to the goal.

Here in Ketchikan I recently had the occasion to discuss one back yard marine engineer type who had removed two low block 671's from his boat, installed a newer high block 671. (I may have reversed hi/lo on these if so sorry). He install the new engine between the inboard engine stringers. Then he mounted a hydraulic pump that distributed fluid to two additional counter rotating hydraulic pumps. (Just turned one around as I recall) then he placed the hydraulic controls on each of those and ended up with a single 671 powering two shafts. He obtained the 8 knots that he desired at 1400 RPM.
This is an approximately 50 foot steel trawler. Will take a photo of the boat tomorrow when I am working our boat nearby.

I have to repeat that this forum will have an impact on discussion over twin/single particularly if the reported results are accurate and who is to dispute them unless more individual test are under taken.
Thanks!!


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Old 11-05-2013, 07:33 AM   #33
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It certainly is!! But not in the "Welcome Mat" part of the forum.

Oh, yeah, THAT!

I typically use the "New Posts" option and so I usually forget to notice what particular section a post might appear in

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Old 11-05-2013, 07:50 AM   #34
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BTW... relative to the freewheeling issue...

I recently had to replace the rudder reference for our autopilot, after only 4 years. We routinely did our Spring trolling on only one engine and using trolling valves to step down speed to approx. 2.5 kts... and there's lots of autopilot use in there... usually just using the "go straight" (maintain course) function. In some sea states and/or with some wind directions, the A/P was not always able to compensate under those conditions.

I have come to wonder (very recently) if autopiloting (?) on only one engine put a higher degree of strain on the rudder reference, causing its premature failure.

No way to tell, of course... although I'll probably usually troll on both engines from now on. At idle RPMs and in this scenario, the extra fuel burn is inconsequential...

Anyway, I throw that out there just in case it has any bearing...

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Old 11-05-2013, 09:07 AM   #35
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Hi Timjet,

The speed with one engine will always be more than half of that with two engines. You haven't taken the specific resistance curve into account. Unfortunately I haven't yet learned how to post graphs yet. When I do, I'll post Figure 74 from Basic Naval Architecture by Kenneth Barnaby.

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Old 11-05-2013, 09:32 AM   #36
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Hi Pseudonym,

I misspoke in a previous post. This forum (when part of the Boat US Manufacturer's Forums) had 5473 posts in the last seven years (not 2000). That's about 781 per year or 2.1 per day. Have patience we're just getting started. It'll thicken up very quickly.

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Old 11-05-2013, 09:48 AM   #37
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Hi Miss Donna,

I usually send out to get the DVDs made. About 25 at a time. But with the chaos on the Boat US site and the transition to a new site I'm way behind. I have to make up new address lists and mail out about 200 letters. It's a lot of work and I don't want to do it until I'm sure this site is going to work out. It seems pretty good so far but I haven't tested picture upload and download yet or a lot of the other features. It takes time to get used to a new forum.

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Old 11-05-2013, 09:58 AM   #38
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Hi All,

I'm working on getting the address lists for the Connie forum updated. When finished I want to send them to a firm that can make up a batch of DVDs, apply the address labels and send them out. Does anyone know of a firm that does that sort of stuff?

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Old 11-06-2013, 05:18 PM   #39
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Well, I promised to upload Figure 74 from Barnably and finally figured out how to do it. But I haven't figured out how to resize the image yet.

Anyway the figure shows the specific resistance of a trawler in lbs. per ton on the vertical axis and the speed length ratio on the horizontal axis. Fuel consumption is proportional to specific resistance and speed of the boat.

Note that the number of engines is not included because it has nothing to do with the fuel consumption. Fuel consumption rate is proportional to the specific resistance times the speed length ratio regardless of whether one engine or ten engines are used.

The only way to reduce the fuel consumption is to reduce the weight and/or the speed of the boat. This comes from Basic Naval Architecture by Kenneth Barnaby. Trawlers are efficient because they go slow.

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Old 11-06-2013, 06:18 PM   #40
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Hi Timjet,

The speed with one engine will always be more than half of that with two engines. You haven't taken the specific resistance curve into account. Unfortunately I haven't yet learned how to post graphs yet. When I do, I'll post Figure 74 from Basic Naval Architecture by Kenneth Barnaby.

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Pete,
I don't understand what the specific resistance curve is showing other than resistance/drag increases as speed and or displacement increases and how does this relate to single vs twin fuel economy.

From a purely fuel burn consideration if the speed on one engine will always be greater than half the speed on two then one engine operation is always more economical UNLESS the fuel burn on the engine running after it's mate is shutdown increases even though it's rpm remains the same. Fuel flow indicators would be necessary to determine this.

In my case when I shut one engine down I do not need to increase throttle position on the remaining engine to maintain rpm.

So my question is; even though I do not increase throttle position to maintain engine rpm, because of the greater load on the remaining engine due to the rudder deflection and free wheeling prop, engine fuel consumption has increased even though rpm has remained the same. Correct?
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