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Old 10-17-2015, 04:58 PM   #141
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Originally Posted by High Wire View Post
Thats incredible! What do you use to measure flow?

With twin 5.7L Crusader gas on a 29' Phoenix and a calibrated Floscan, on one engine, the best I could ever get was 5 gph at 1600 rpm for 5 knots for 1 nmpg.
Per math calcs... using time spent, direction and speed of current running in coordination with gps overland speed: I then divide figured miles traveled by gallons used at fill-up to figure nmpg at any rpm/speed.

Was your 5.7L in good condition? When on one engine was your other 5.7 in free wheel or locked prop? Were props correctly sized? What rpm did your engines reach at WOT?

5 gph at 1600 rpm for only 5 knots seems way, way too much fuel burn.

Heck... we get 1 +/- nmpg at full plane doing 16 to 17 knots using both engines.
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Old 10-17-2015, 05:57 PM   #142
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5kts at 5gph is nuts even for a gasser. Something goofy with those numbers. Maybe 2 or 3gph at 1600 for a 5.7.
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Old 10-17-2015, 06:01 PM   #143
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Originally Posted by Rustybarge View Post
Exactly.

I'm surprised nobody has mentioned that twins use nearly double the fuel at displacement speeds.

Rule of thumb:
32'/6 tons/ single[200hp] 2 gals/hr@ 7 kts: 3-4 mpg.
32'/6-7 tons/twin[2x200hp] 4gals/hr@ 8kts: 2 mpg or less.

That's absolutely not true. Twin engines have nothing to do w how much power the boat has unless it does. And then you're not comparing twins w singles but 400hp boats w 200hp boats. Of course boats w bigger engines are going to burn more fuel. No brainer.
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Old 10-17-2015, 06:06 PM   #144
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I find all this worry about extra hull penetrations amusing. It's really such a none issue. And of course twin engines don't double the total hull penetrations in a vessel compared to a single. They add like what, two?

There are plenty of good reasons to choose a single engine over a twin. But worrying about added hull penetrations isn't really one of them.
Well, there was just my message about hull penetrations, but I don't actually consider it a non-issue. Most vessels sink as a result of an existing hull penetration (at least, that's what I seem to recall) and it stands to reason that doubling the penetrations (and hoses, exhaust components, et cetera) would...double that risk. I'd say it's even more likely because of the inaccessibility of the off-side components, but in the spirit of not being argumentative I'll let that one slide.
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Old 10-17-2015, 06:25 PM   #145
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[QUOTE=Art;380186]

Was your 5.7L in good condition?
Yes. These were carb motors.
When on one engine was your other 5.7 in free wheel or locked prop?
Free wheel
Were props correctly sized? Yes and tuned to Class 1 via PropScan
What rpm did your engines reach at WOT?
4400 and 27 knots.

5 gph at 1600 rpm for only 5 knots seems way, way too much fuel burn.
I know but that is what the floscan read. 2 engines at 5 knots was about 6 gpm @ 1300 rpm. I would troll for bluefish at that speed. Going down to one engine only helped a little.
The floscans were accurate to +/- 2 gal out of 100.

Heck... we get 1 +/- nmpg at full plane doing 16 to 17 knots using both engines.
Cruising was about 20 gph (10 gpm each) at 17.5 knots and 3000 rpm.
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Old 10-17-2015, 06:42 PM   #146
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[QUOTE=High Wire;380212]
Quote:
Originally Posted by Art View Post

Was your 5.7L in good condition?
Yes. These were carb motors.
When on one engine was your other 5.7 in free wheel or locked prop?
Free wheel
Were props correctly sized? Yes and tuned to Class 1 via PropScan
What rpm did your engines reach at WOT?
4400 and 27 knots.

5 gph at 1600 rpm for only 5 knots seems way, way too much fuel burn.
I know but that is what the floscan read. 2 engines at 5 knots was about 6 gpm @ 1300 rpm. I would troll for bluefish at that speed. Going down to one engine only helped a little.
The floscans were accurate to +/- 2 gal out of 100.

Heck... we get 1 +/- nmpg at full plane doing 16 to 17 knots using both engines.
Cruising was about 20 gph (10 gpm each) at 17.5 knots and 3000 rpm.
See Ski's post # 142. You say "The floscans were accurate to +/- 2 gal out of 100." How were you sure of that?

Did you check your flowscan reading a few times as compared to mathematically calced nmpg? Maybe flowscan had a problem??

Also... you mention: "Cruising was about 20 gph (10 gpm each) at 17.5 knots and 3000 rpm." ( I think you meant - (10 gph each) - Seems to me your flow scan may have been reading incorrectly at that speed to. I believe our 29' boat should have been getting at very least 1 nmpg... or better! Did you have/use trim tabs?
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Old 10-17-2015, 07:17 PM   #147
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That's absolutely not true. Twin engines have nothing to do w how much power the boat has unless it does. And then you're not comparing twins w singles but 400hp boats w 200hp boats. Of course boats w bigger engines are going to burn more fuel. No brainer.
Nearly every s/d or planing boat over 32' has twins, with the exception of lobster boats, so it its hard to make a real life comparison . If you looked at the consumption graph of twins or a big single in the same hull i would bet that the consumption would be nearly identical at 2500 revs...


But at low revs its a different story. A big diesel at idle will not burn the fuel cleanly, and consume more diesel.

So most twins are run at the lowest revs that will give clean combustion where the engine is on the torgue curve: 1200-1500 on medium speed engines.

Most singles are run at 1500-1800 revs to give about 7kts cruise.

Its a fact that twin driven props lose 20% through drag, plus the higher operating fuel consumption, plus double the engine heast loss, plus the higher weight of two engines.

In other words doubling all the inefficiencies and losses plus 20% drag; that adds up pretty quick.
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Old 10-17-2015, 07:17 PM   #148
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You know...that emoticom really hits home doesn't it?
Especially if you are a horse.
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Old 10-17-2015, 07:47 PM   #149
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Originally Posted by Rustybarge View Post
Nearly every s/d or planing boat over 32' has twins, with the exception of lobster boats, so it its hard to make a real life comparison . If you looked at the consumption graph of twins or a big single in the same hull i would bet that the consumption would be nearly identical at 2500 revs...


But at low revs its a different story. A big diesel at idle will not burn the fuel cleanly, and consume more diesel.

So most twins are run at the lowest revs that will give clean combustion where the engine is on the torgue curve: 1200-1500 on medium speed engines.

Most singles are run at 1500-1800 revs to give about 7kts cruise.

Its a fact that twin driven props lose 20% through drag, plus the higher operating fuel consumption, plus double the engine heast loss, plus the higher weight of two engines.

In other words doubling all the inefficiencies and losses plus 20% drag; that adds up pretty quick.
Free wheel – VS Fixed wheel… prop drag!

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Old 10-18-2015, 12:53 PM   #150
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[QUOTE=Art;
See Ski's post # 142. You say "The floscans were accurate to +/- 2 gal out of 100." How were you sure of that?

Before a trolling trip I would fill the main tanks full. After burning through 60 to 100 gallons as read on the Flowscan Totalizer, I would again fill the tanks full. The fuel pump gallons would be within 2 gallons of what the totalizer read. It took 3 years and many fillups to tweak it to be right on the money. I am absolutely sure of its accuracy.

Did you check your flowscan reading a few times as compared to mathematically calced nmpg? Maybe flowscan had a problem??

The flow scan was right on proven many times.

Also... you mention: "Cruising was about 20 gph (10 gpm each) at 17.5 knots and 3000 rpm." ( I think you meant - (10 gp[B]h[/B] each) -

Yes, 10 gallons per hour per engine at 3000 rpm equals 20 gallons per hour for both engine running synchronized.

Seems to me your flow scan may have been reading incorrectly at that speed to.

Based on what facts?

Those were co I believe our 29' boat should have been getting at very least 1 nmpg... or better!

Did you have/use trim tabs?

Yes, Trim tabs were always used about halfway down for cruise best speed and lowest GPH values. The only times the tabs were up were for trolling and full speed test runs where tabs hurt performance on that boat.
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Old 10-18-2015, 01:36 PM   #151
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[QUOTE=High Wire;380433]
Quote:
Originally Posted by Art;
See Ski's post # 142. You say "The floscans were accurate to +/- 2 gal out of 100." How were you sure of that?

Before a trolling trip I would fill the main tanks full. After burning through 60 to 100 gallons as read on the Flowscan Totalizer, I would again fill the tanks full. The fuel pump gallons would be within 2 gallons of what the totalizer read. It took 3 years and many fillups to tweak it to be right on the money. I am absolutely sure of its accuracy.

Did you check your flowscan reading a few times as compared to mathematically calced nmpg? Maybe flowscan had a problem??

The flow scan was right on proven many times.

Also... you mention: "Cruising was about 20 gph (10 gpm each) at 17.5 knots and 3000 rpm." ( I think you meant - (10 gp[B
h[/B] each) -

Yes, 10 gallons per hour per engine at 3000 rpm equals 20 gallons per hour for both engine running synchronized.

Seems to me your flow scan may have been reading incorrectly at that speed to.

Based on what facts?

Those were co I believe our 29' boat should have been getting at very least 1 nmpg... or better!

Did you have/use trim tabs?

Yes, Trim tabs were always used about halfway down for cruise best speed and lowest GPH values. The only times the tabs were up were for trolling and full speed test runs where tabs hurt performance on that boat.
You sure seem to have everything in sync. I simply cannot understand what appears to me an overabundance of fuel per mile usage per engine. Don't you feel the 350 cid engines used too much fuel per mile in the 29 Phoenix... or do you feel their fuel usage was as to be expected?

Maybe Ski in NC can offer clear reasoning.

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Old 10-18-2015, 01:58 PM   #152
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Sure seems like an outlier. I can see burning 5gph at 1600, but that should give well over 5kts. If 1600 only gives 5kts, then prop load is pretty light on the engine and it should burn less. Double the rpm and get 3.5 times the speed, so that makes prop slip off the charts??? Guess that can be from little props and running on only one.

Sounds like the gph is pretty well confirmed by multiple tank fills and comparison to flowscans. Maybe boat is hard to push and maybe engine is running rich, which many marine engines do.

Who knows.

Might as well run 17.5kts, dang near same NM economy.
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Old 10-18-2015, 03:13 PM   #153
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FWIW, I dug out the actual performance curves from 15 years ago. The numbers posted in previous posts were from my foggy memory.
This is both engines running in sync and tabs set for best performance at rpm. As you can see, they are nowhere near 2 nmpg at any speed.
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Old 10-18-2015, 04:27 PM   #154
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Without reading this entire thread---
We had a rough ride across Pamlico Sound to the Neuse River today. Heard a fellow call the CG because he lost his tranny in his single engine boat. He didn't seem happy. BTW, we lost our steering a few years back and made it home steering with our twin engines.
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Old 10-18-2015, 06:55 PM   #155
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FWIW, I dug out the actual performance curves from 15 years ago. The numbers posted in previous posts were from my foggy memory.
This is both engines running in sync and tabs set for best performance at rpm. As you can see, they are nowhere near 2 nmpg at any speed.
Then... it seems that an item or a compounded of items in Ski's post # 152 and some items in my and other previous posts might be the culprit[s] regard what surely sounds overage fuel burn in your 29' Phoenix. Cause - I simply cannot understand that much fuel burn. I've had or have twin 350 cids in a 31' 1973 Uniflite sedan sport fisher and 1977 Tollycraft 34' tri cabin. Both could get better mileage per gallon than your 29'er.
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Old 10-19-2015, 12:05 AM   #156
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Well, there was just my message about hull penetrations, but I don't actually consider it a non-issue. Most vessels sink as a result of an existing hull penetration (at least, that's what I seem to recall) and it stands to reason that doubling the penetrations (and hoses, exhaust components, et cetera) would...double that risk. I'd say it's even more likely because of the inaccessibility of the off-side components, but in the spirit of not being argumentative I'll let that one slide.

If you are not doing proper basic maintence the possibility of sinking is going to be an issue on a twin or single engine boat.

But barring that, having a couple of extra hull fittings because you have twin engines is hardly a reason to loose any sleep at night.
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Old 10-19-2015, 06:08 AM   #157
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Then... it seems that an item or a compounded of items in Ski's post # 152 and some items in my and other previous posts might be the culprit[s] regard what surely sounds overage fuel burn in your 29' Phoenix. Cause - I simply cannot understand that much fuel burn. I've had or have twin 350 cids in a 31' 1973 Uniflite sedan sport fisher and 1977 Tollycraft 34' tri cabin. Both could get better mileage per gallon than your 29'er.
There are so many variables in boat performance that boat builders get away with downright lies about fuel burn. A few years ago Fleming had an ad on their website claiming that their 55' trawler ' did about 1 gal/nm at 10 Kts...!!!!' [ aha, perpetual motion drive maybe]

So there's a strong argument for rule of thumb ball park figures for that very reason : probably more accurate than manufacturers numbers.

About 20hp/gallon diesel engine ( or slightly less )
About 10hp/gallon gasoline engine ( .. .. .. .. )

5 HP/ton to push most hulls at just below hull speed.

The beauty of this formula is that gass engined boats are lighter than diesel, and this is taken into account.

How much did your Tolly weight last time you had it lifted with all your ' stuff' onboard ?

Do the rule of thumb no's work for your boat?
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Old 10-19-2015, 07:52 AM   #158
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There are so many variables in boat performance that boat builders get away with downright lies about fuel burn. A few years ago Fleming had an ad on their website claiming that their 55' trawler ' did about 1 gal/nm at 10 Kts...!!!!' [ aha, perpetual motion drive maybe]

So there's a strong argument for rule of thumb ball park figures for that very reason : probably more accurate than manufacturers numbers.

About 20hp/gallon diesel engine ( or slightly less )
About 10hp/gallon gasoline engine ( .. .. .. .. )

5 HP/ton to push most hulls at just below hull speed.

The beauty of this formula is that gass engined boats are lighter than diesel, and this is taken into account.

How much did your Tolly weight last time you had it lifted with all your ' stuff' onboard ?

Do the rule of thumb no's work for your boat?
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 ( .. .. .. .. )"
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Old 10-19-2015, 08:06 AM   #159
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About 20hp/gallon diesel engine ( or slightly less )


5 HP/ton to push most hulls at just below hull speed.

?
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?
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Old 10-19-2015, 08:40 AM   #160
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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?
A combination of all those factors!

I think Richard in his Kk 42 managed about 3mpg/6kts overall across the Atlantic on a 120hp ford Lehman.

The challenge: can anyone on this forum beat 2mpg on a 32' or larger twin engined trawler ?

The evidence is in the actual fuel numbers.
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