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Old 05-22-2018, 09:33 AM   #41
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Former 28' CC Catalina owner. Single 350. We liked to cruise around 2000 rpm. 8 knots. Engine was just loafing along. Planned on 10 gph fuel burn, didn't trust the gauge. Always less than that. If you can get anywhere near 2 mile per gallon with a gas motor your doing exceptional. 1 to 1.5 mpg is realistic.
2 motors shouldn't double the fuel burn, but it will be more..
1 or 2 wide open and the fuel burn will make your knees buckle.
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Old 05-22-2018, 09:36 AM   #42
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Quote:
Originally Posted by TDunn View Post
I burn about 0.25 gph at 5.5 knots and 0.6 gph at 7 knots. That works out to 22 mpg at 5.5 knots, but only about 12 mpg at 7 knots. My boat is 32'8" and powered by a single Volvo-Penta D2-40 diesel. My wife prefers the slower speed to minimize engine noise, although at 7 knots you can still carry on a normal conversation anywhere on the boat.


That is a very efficient boat you have there. That is more efficient than my last two sailboats.
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Old 05-22-2018, 09:42 AM   #43
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This can't be right.....speed and efficiency are inversely proportional...as one goes up, the other goes down. They may be equal at your sweet spot... but I don't think they will be equal above or below that spot.
Some boats when planed out start to lift and that reduces skin drag and wave making drag. Not unusual for there to be a flat section of the burn/speed curve somewhere above 15kts. Depends highly on hullshape. Seen the same on things like the Bertram deep vees.
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Old 05-22-2018, 09:54 AM   #44
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that's a good point....but for the most part the two things go in opposite directions like the graph below:
source: Powercat Trip
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fueluse.gif  
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Old 05-22-2018, 09:57 AM   #45
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My old Phoenix 29 (9000#) with twin 5.7L carb gas engines would get 0.9 NMPG at 6 knots (1400 rpm) and at 16.0 knots (2900 rpm) with trim tabs set perfect. Any other speed was lower to much lower mileage. That was measured with calibrated Flo Scans. The only way to get better was to drop to one engine on the troll and that would give 1.1 NMPG only if flat calm seas.

For comparison, my 21' Sailfish Center Console with a 250 hp Yam 4 stroke gets 3.0 NMPG (3400 rpm) at 21 knots and 3.8 NMPG (1200 rpm) at slow troll.
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Old 05-22-2018, 11:04 AM   #46
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that's a good point....but for the most part the two things go in opposite directions like the graph below:
source: Powercat Trip
That curve looks steep, but look at the choice of scale for nmpg. If zero was included it would be pretty dang flat. About a 30% drop across the range, not too bad at all. Many boats the curve looks that steep even with zero!!

Also real flat from 15.5 to 16.5kts.

And cats are strange. They don't really plane out, and have lots of skin drag as there is lots of skin in the water. But have benefits too as wave making drag from the skinniness of the hulls changes things. I'm no student of cat design though. But sea trialed several and some were slick and some piggy.
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Old 05-22-2018, 11:15 AM   #47
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We were finally able to cruise our Chris Craft Commander twice this month. Our first trip was about 50nm round trip, and the other was around 70-75nm round trip. The first trip we faired pretty well there and back with a tank of gas, but the second trip we coasted into our home dock on fumes on the starboard tank.

After the second trip, my partner decided that we needed to sell it and look into something else a little later on. $500 in fuel this month was just too much to comprehend I guess. So I listed it, and if it sold we've been talking about going the trawler route. (I've always wanted a trawler anyway.)

Thanks!
Questions about your boat?
Length of boat?
Fuel, gasoline or diesel?
Fuel capacity?
Engine size and model?
Weight? (dry)
Speed for each trip?
Were you running the generator? Size of generator?


For example; my AT34 came with one Cummins QSB5.9, advertised 380 hp.
2X200 gal fuel tanks.

Weight 18,700 Dry

Factory specs;
1400 RPM.... 8 knts .... 2.1 gph
2000 RPM... 10 knts.... 5.1 gph
2400 RPM... 12 knts... 11 gph
2800 RPM...16 knts .... 15 gph

I have a 6KW generator and I suspect my fuel consumption is about 1 gph.

In other words, I can almost go a long way at 8 knts.
If I am in a hurry, I will pay the 'price' in fuel consumption.

Other things that affect the gph, besides SPEED, sea conditions and how much other stuff do you have on board.

Two other questions; do you have sight glasses on your fuel tanks? The second question, why are you running on just one tank? Why not cross connect the tanks? If you run a tank dry, you have the problem of priming the fuel system before switching tanks. Remember to save 1/3 of the total fuel capacity for unplanned 'fuel eating' sea conditions.
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Old 05-22-2018, 05:00 PM   #48
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OC if you had gone slow with that Cummins fuel would have been less too
I put over 1,200 miles on the boat with the 450 HP Cummins at 7 knots. At 1,200 RPM the boat was getting less than 2 MPG. I basically cut my fuel consumption in half for the same speed. Very happy with the repower.

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Old 05-22-2018, 07:55 PM   #49
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The older inboard gas engines like the Commanders sucked gas like a rotten leaky wooden planked hull leaked water. There is not enough money and desire to deal with either scenerios in the 21st century, especially if a person wants to use their boats more than just a dock condo.


I will add that with the price of the newer hulls, some of the older hulls looks mighty good. You can buy a lot of fuel for price differences of upgrading to a modern hull. Pay as you go works for some folks on any type of boating budgets.

Actually, all things considered, my original 51 year old 327F engines only burn about 11gph at 17 knots. Compared to some of the experiences that I've been given, they aren't all THAT bad.

And when you're looking at trade values vs gas prices, I'm miles and miles in front of a new boat since I own her outright.
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Old 05-22-2018, 08:06 PM   #50
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Two other questions; do you have sight glasses on your fuel tanks? The second question, why are you running on just one tank? Why not cross connect the tanks? If you run a tank dry, you have the problem of priming the fuel system before switching tanks. Remember to save 1/3 of the total fuel capacity for unplanned 'fuel eating' sea conditions.

I do not have sight glasses. The original tanks were taken out in 2007 by a previous owner and replaced with two 55g cube shaped tanks, one behind each engine. There is a cross connect but its at the top of the tanks, so I don't understand how it exactly works. I usually run it in the closed position.

I also do not understand why my starboard tank empties faster than the port. After the last cruise that I spoke about, my starboard tank is only about 1/8 full or less, and my port tank still has a little over 1/4 of a tank.

Could be a carb issue though since I have an Edelbrock 1409 on my port side, and a Carter AFB on my starboard side.
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Old 05-22-2018, 09:10 PM   #51
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Ah, you run each engine on its own tank? Two different carbs. Your thoughts might be true.
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Old 05-22-2018, 09:16 PM   #52
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but if both engines are at the same rpm, wouldn't they use the same amount of gas, regardless of the brand of carb ?
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Old 05-22-2018, 09:32 PM   #53
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but if both engines are at the same rpm, wouldn't they use the same amount of gas, regardless of the brand of carb ?

Unfortunately not. Carter AFB's are infamous for being finicky. Oddly, thats the carb I rebuilt though.
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Old 05-23-2018, 12:02 AM   #54
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If you try to run at a slower speed DO NOT pick an rpm or a speed. Let the boat show you where it is more efficient.

Watch the bow wave. If a large mustache is climbing the bow you are pushing to hard.

Watch the stern wave. If a large rolling stern wave comes up behind the boat you are pushing too hard. This can also be seen by another boat pacing you looking at the dip between the bow wave and where the dip comes up at the transom. If the dip appears to rise AFTER the transom you are pushing too hard. The dip should rise just a bit before the transom.

At some point there may be a relatively sharp rise at the bow. There will be a small and gentle rise as the boat gains speed but where the rate of rise become more distinct you are pushing too hard. When that happens the boat is attempting to climb over the bow wave readying to plane. What is going on is the transom is falling into the dip described above.

When you see one or more of those conditions back off a bit or you may well defeat the idea. When one or more of those conditions occurs the HP required to carry on that way will rise sharply.
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Old 05-23-2018, 03:04 AM   #55
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50 foot 30 ton I have a Gardner 6LX and at 1050 RPM get 8 knots at 8 liters per hour.
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Old 05-23-2018, 04:39 AM   #56
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Originally Posted by toocoys View Post
I do not have sight glasses. The original tanks were taken out in 2007 by a previous owner and replaced with two 55g cube shaped tanks, one behind each engine. There is a cross connect but its at the top of the tanks, so I don't understand how it exactly works. I usually run it in the closed position.

I also do not understand why my starboard tank empties faster than the port. After the last cruise that I spoke about, my starboard tank is only about 1/8 full or less, and my port tank still has a little over 1/4 of a tank.

Could be a carb issue though since I have an Edelbrock 1409 on my port side, and a Carter AFB on my starboard side.
"I also do not understand why my starboard tank empties faster than the port."

Could it be...
- the genset is drawing off one tank
- one engine is well out of tune
- the props are not balance
- your transmission ratios are different
- your fuel draws or returns share a semi open line at some point

All of these are fairly easy to check
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Old 05-23-2018, 06:25 AM   #57
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To the OP- Take boat for a run at your desired cruise speed and run for at least ten minutes. Come back to the dock and shutdown with minimal idling. Or drop the anchor. Let engines cool a bit and take a couple of spark plugs out of each engine and post photos of the firing tips. Insulators should be whitish tan. If black, that carb is running rich and may explain why that engine is burning more fuel. If both engines plugs are whitish, then for some reason the load is different on engines. You want to check with minimal idling as idling can darken plugs if idle mix is rich, and rich idle mix is not going to make a big change in cruise burn rate.

Are the actual cranks counter rotating from port to stbd? If so, gear ratios likely exact.

Tachs could be off so when needles are in sync, actual rpm might be a little different. That will change burn more than one would think.
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Old 05-23-2018, 09:02 AM   #58
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Tachs could be off so when needles are in sync, actual rpm might be a little different. That will change burn more than one would think.

Yes, tachs can be a problem as Ski points out. Do not assume they are correct. Helped a friend a few years ago. Twin engines. The tachs side to side were off about 200 rpm. On each side they were incorrect for that engine.

Using a phototach we worked through a range of revs of the actual vs the tach readings. Once we actually made the table and he started running by it he found:
--picked up ~ 3/4knot
--steered straighter without cranking wheel over . Wasn't much but noticeable. The steering was mechanical so the wheel returned to the same spot, unlike a hydraulic system.
--fuel use actually dropped.
--the noise level dropped.
--with the table he could get close enough revs each to then be able to finish the synch by ear.

I figure the imbalance overloaded one engine, the rudder goof meant the boat was off tracking somewhat, the revs were so far off the usual rr,rr,rr rotation of sound was so far off it was not detectable.

Check your tachs. Decent, accurate units can be had from Ebay for ~ $20.00 or less. Far more accurate than the typical dash tach. They are just not robust enough to be roughly handled but, for we amateurs, are more than good. Laser pointer digital phototachometer.
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Old 05-23-2018, 09:18 AM   #59
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I run enough twins engines to never expect them to be the same. If you run 24 hrs and one burns a gallon more per hr it starts to add up. It does not hurt to double check all the settings but would not spend a lot of money chasing it. It is one of the reason similar boat can vary in mpg.
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Old 05-23-2018, 02:57 PM   #60
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How many people shut a main down while under way? How does it effect mileage and handling?
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