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Old 10-04-2020, 09:38 PM   #1
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Going slow in a fast trawler

Hi Folks. Please don’t pile on the what is a trawler discussion, but I have a 2002 Sabre 34 running two diesel yanmars at 310 hp each and I am trying figure out if we can extend its range to get to the Bahamas from Florida by going slower. It has an odd consumption graph where the faster you go the more miles per gallon with the best going 17 to 23 knots with one gallon per knot at 3000 rpm with a range of 225 nm. Anything less drops the mileage to .7 nm per gallon. The consumption chart from Sabre stops at seven knots at 1400 rpm at 160 nm range. I have researched and my waterline is 30.25 feet so my hull speed is about 7.2 knots. If I am below hull speed, do you think I can extend my range or is this just the downside of a fast trawler?
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Old 10-04-2020, 10:13 PM   #2
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Old 10-04-2020, 10:23 PM   #3
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Rough calculations, 1,166nm range with 250 gallons of fuel at 7kts. 2,500nm range at 5kts.
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Old 10-05-2020, 12:29 AM   #4
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Answer: Yes

Considerations: Operate one engine at a time to keep rpm to healthy temps.
Note: You will not likely realize any fuel savings, rather you will have better engine temps. Diesels are temp sensitive for proper treatment.
Not great to idle a diesel. Diesels have an operation range healthy for continuous duty. Therefore consider the following options.
1. Find out if your transmission and gear can safely freewheel the prop in neutral.
2. If freewheeling is w/o damage, go to next step. If not, secure prop shaft.
3. Run one engine to keep rpm up to healthy level while at slow speed.
4. Running very efficiently is (1.3) under hull speed, or less.
5. Many have charted will Hull speed showing not so efficient.
6. Around 50% of hull speed will be near 50% less fuel consumption.

This will apply to your fast trawler as well as most other twin engine boats.
Others will likely provide example charts to attest to this.
There are several variables that others will bring up, such as off center single engine opts, having to secure a prop shaft if freewheeling is damaging (Coast Guard does it), drag of freewheeling prop, very slight difference in fuel consumption vs. twin engine opts, etc..

So yes, you can save fuel and extend your range operating at below hull speed.
Link below is one charted example someone put here:

https://share.icloud.com/photos/0gZZ...Fmz15ru8uGnveg

.
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Old 10-05-2020, 12:41 AM   #5
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Old 10-05-2020, 07:24 AM   #6
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In general, doing a knot below hull speed will burn much less fuel than running up on plane. Figure your nmpg at 6 kts will be 2 - 3 times what you get at planing cruise.
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Old 10-05-2020, 08:36 AM   #7
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Read your engine operators manual first.

One fellow I know did that. He was trying to save on fuel costs in a 30' planing boat with two big engines going south to Bella Bella. Gummed up his turbos something fierce. Didn't end well.

The operators manual for our Yanmar says that if you have been running at low speed for a certain length of time, you are supposed to run it up to a stated higher rpm, again, for a certain length of time.

You'd have to factor in those increases in rpm (if required) into your calculations.
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Old 10-05-2020, 09:08 AM   #8
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If you have a flow scan, make your own chart.
Usually, when the builder talks about fuel range, he is talking about 1/2 filled fuel tanks, 1/2 filled water tanks and for some reason the fridge it on.
EVERYONE puts stuff in their boats and that will have a great effect too.
If I run with a full water tank, (150 gallons) that amounts to a bit over 1,200 pounds. Then fuel (400 gallons) about 2,800 pounds. Total about 4,000 pounds or about 2 tons of weight you are pushing through the water.
That is ignoring all the good stuff we put onboard for creature comforts.
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Old 10-05-2020, 09:40 AM   #9
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What OldDan said.. Only way to know for sure is to run it. Full load, full tanks. Measure (dip) the tanks to get an accurate measurement. Or fill to the cutoff point every time.
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Old 10-05-2020, 12:05 PM   #10
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Thanks Folks. I was thinking the same direction. I have the PSSA non drips so I think freewheeling will not be an option to keep them cool. The Yanmars are turbos so I will check on that. I have 240 gallons of fuel so the other option is bladders in the cockpit, but I would be fine traveling at 6 knots. We have been life long sailers so 6-7 was our old world.
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Old 10-05-2020, 12:42 PM   #11
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Maybe not relevant to bigger boats.

Many years ago with a smaller planning boats 18-24 this discussion would come up. At the time those with same type boats complained of higher fuel burn when they could not get on plane.
So I did the tests at varying rpm and came to the following conclusion.
As long as there is a related increase in speed with increase in rpm the fuel burned over the measured mile was negligible differences noted. If at any time rpm was added without increase in speed more fuel was burnt.

Our trawlers appear to operate the same way burning proportionally more fuel getting an extra knot of speed. Yes, less than hull speed is most efficient.
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Old 10-05-2020, 12:49 PM   #12
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Small gas powered boats can be an unusual exception to the rule. A smaller, lighter boat pays a smaller penalty for running up on plane vs slow (and slow means really slow with a short waterline). And gas engines lose a ton of efficiency at light load. So with a small boat and a big gas engine, you can end up getting the same or better nmpg on plane vs at low speed.

With a 34 footer, that's not going to happen. My 38 footer gets double or a little better the nmpg at 6.5 kts compared to 17 - 18 kts. If I had diesels and didn't have as bad a drop in engine efficiency at light load, the difference would be triple rather than double.
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Old 10-05-2020, 06:58 PM   #13
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When we do our next local trip 50-70 nm, I will give six or seven knots a try with full tanks. Do you think I should try six or should I see enough extra fuel mileage by just going the hull speed. I guess this is one example where analog engines can’t give you instant answers like their digital new ones. Not that I am complaining
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Old 10-05-2020, 07:02 PM   #14
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One other follow up question on the idling comments..you are right that six knots is idle with both engines. Given I have the no drip, would you suggest I tie one shaft so it doesn’t freewheel. Is is rubber bungee enough? Seems that would break if I accidentally fired the engine up.
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Old 10-05-2020, 07:28 PM   #15
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The answer re: running on 1 engine lies in the manual for your transmissions. Some (Like my ZF) freewheel happily according to the manual.
The PSS seals will stay cool (maybe10-20 deg hotter) at 6-7 knots even without crossover plumbing, while running on the other engine. You can verify with IR gun easily. At planing speed, u will need crossover plumbing (but then u wouldn’t be conserving fuel, duh).
Agree, approx. 1knt below theoretical hull speed with either 1 or 2 engines will give best fuel economy. Open her up for a few minutes just before teaching final destination. A few long trips at fast idle in an engine’s lifetime isn’t going to make or break the durability.
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Old 10-06-2020, 05:24 AM   #16
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"Do you think I should try six or should I see enough extra fuel mileage by just going the hull speed."

Theoretical hull speed (SL x 1.3) is never an efficient speed perhaps 2x the burn of the boats efficient cruise speed.

Cruising speed (depending on the boat) is usually considered to be from .9 to 1.1 times SL
(SL is the sq rt of the LWL while underway.)

For inshore sailors the good news is SL speed is figured in nautical speed ,Knots,so when using ICW charts in Statute miles roughly 5 K = 6 MPH or 6K is almost 7 statute . Helps planning long daylight fall runs.
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Old 10-11-2020, 03:03 AM   #17
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Don't worry about keeping engines heated up.
1000 to 1200 rpm generates enough load to keep engines happy.
When you put engines under load the increased heat and exhaust velocity will clear the soot and carbon.
Yanmar has modern high pressure pop-it injection, maybe yours are electronic injection that virtually eliminates liquid fuel from combustion process.
No diesel should be left idling for long periods or running cool. Keeping engines heated up is related to old direct injection diesels, especially 2 cycle Detroit's and to much lesser degree Cummins with PT injectors. These were very poor at no load idle and suffered cylinder wash... Cylinder wash occurs when engine does not reach operating temperature and 300-400F exhaust temp. Again this is greatly reduced by pop-it injection systems.
Pop-it injectors (Cat, Cummins B, C series, Perkins, Lehman, Mack, Westerbeke, Hino) etc, use injectors with pre loaded needle seats that do not open until 3000 - 5000 psi giving excellent atomization spray pattern. Little is left to coat cylinders after engine heated up and under small load.
Turbo charger does not care what load is on engine. When you increase fuel rate heat increases and spins turbo faster and the increased heat burns soot and carbon out. Turbo coking is another problem related diesels with poor no load and idle injection systems, passing liquid diesel fuel through exhaust, mixing with soot and carbon to make glaze like substance.. Modern diesels do not have this problem unless run completely out of normal parameters.
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Old 10-11-2020, 07:13 AM   #18
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Thanks for the response. We are on the water this weekend and for me to run at hull speed 7.25, the engines are running at 1400 rpm. Idle pushes the boat at about 5 knots.
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Old 10-11-2020, 01:30 PM   #19
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Rough calculations, 1,166nm range with 250 gallons of fuel at 7kts. 2,500nm range at 5kts.
2500 nm at 5 knots would mean 500 hours and it won't run 500 hours in neutral and idle. This is where theory loses to reality. Until you're actually tested in real conditions don't fall for numbers like this.

Can you extend range? Sure, but don't bet on the results without running tests.

My solution to your range problem is to find a closer point of the Bahamas. If going to northern Bahamas go to West End and then from there. If going to Nassau or Southern then make your entrance stop in Bimini.
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Old 10-11-2020, 01:55 PM   #20
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Miami to Bahamas is like 60 miles with 200 plus gal. How could you not make it?
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