Fuel consumption at trawler speed.

shaw520

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I know this discusion has been beat to death,..but without a fuel consumption guage im just guessing.
Searay 280 w twin 5.0's,.. Running on plane at 22-24 mph can get costly, im usually never in a rush because my fun starts as soon as i board the boat,..are trip are typically 50 - 150 miles each. ..so ive been running 8-10mph at about 1800 rpms.and saving a ton on fuel. Of course it takes 3 times longer to get where going but we prefer to enjoy the journey...(good practice for when we retire on a trawler) So my question is could i save even more fuel by running one engine and alternating engines to keep the hrs equal. At that speed 8mph, it seems there might be some savings.
 

Scott Danforth

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your two best fuel points are at idle in gear (about 6-7 mph) or just above plane at about 18-20mph

the worst is while the boat is still in the hole.

what is your minimum planing speed with your hull?

running on one or two engines wont change fuel consumption much if at all at idle, as running on one will require more throttle on the one motor than running both at idle

the typical marine boat motor requires 0.40#/HP per hour of fuel.

to dial in your boat best, get a fuel flow meter for the motors
 

tpenfield

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You should be able to find a fuel efficient (if you can call it that) cruising speed. I would think that in the 8-10 mph range would be terribly inefficient as the boat is between idling speeds and planing speed, which tends to be the least efficient.

A boat like that should be getting 1.5-1.8 mpg at its most efficient cruising speed. I don't think a single engine (5.0) will be able to run that boat with any efficiency and probably use more gas than if you ran both engines at a lower speed.

At the slowest of speeds, boats typically get about 2X the mileage of their efficient cruising speed.

Gas is typically only a fraction of the overall cost of boat ownership. So, it is really worth trying to minimize fuel expenditures :noidea:
 

shaw520

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Thanks for your replys guys.... I can stay on plane at about 18-20 mph,..below that she starts to fall off. My most effecient cruise speed seems to be around 22-24mph. But i also notice i get ALOT less fuel consumption at 8mph...fully displaced without any bow rise.
 

b.gagnon

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Running on one engine is not ideal..... If fuel consumption is that much of a concern you need a new hobby! Have you ever considered sailing? A 30' sailboat can cruise @ 5-7knots for 100 miles on very little fuel, and the wind is free! I drove my brothers 32' sailboat aprox 100 miles while under power to transport it to it's new home and I think we only burned about 12 gallons of diesel!
 

shaw520

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Twin Lehman diesel's are most likely in my future,..but not sails.
My 5.0's both draw from one tank so can i get away with one fuel consumption guage?... Ideally youd want to know what each engine is consuming, but the consumption guages ive seen are pricey...any info on the guages is appreciated.
 

Silvertip

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You could use just one gauge but you need two transducers (one for each engine). You then switch between the two transducers to see what each engine is doing.
 

Scott Danforth

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and many of todays sonar/radar units (garmin for instance) of today have screens to show the fuel rate, you may only need the transducers.
 

Ned L

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Or if you don't want to go to the effort of installing a fuel consumption meter you can hook a vacuum gauge to the intake manifold. Learn to run her by that. Up to hull speed takes little HP(intake manifold vacuum will be nice and high), as soon as the the hull starts to move past it's comfortable hull speed the engine will start to work more and the vacuum will start to fall ( also meaning fuel consumption goes up).

8-10 sounds a bit high for hull speed of your boat.
 

shaw520

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All good information to absorb,.. 8-10mph may be slightly above hull speed but still burning a fraction of on plane. The 90's searays are long and sleek,..gliding at hull speed seems effortless.
I have a lead on a.new in the box floscan 6532-CAT-20.l. Are these the older turbine type? I can grab it for 150.
 

QBhoy

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Hi
Interesting topic...I have a real interest in these kind of posts...mainly because of fuel costs in the UK and because I'm Scottish ! Haha.
Really surprised to hear you get a better economy figure when running at 1800 rpm, than you do up on the plane at the lowest permitted rpms. I would have thought running at 1800rpm would be about the worst possible situation for fuel consumption.
Traditionally I used to get up on the plane, cut back to the lowest possible rpms to keep it there and thought that would be the most economical. But...according to the smart craft gear, it isn't. It's actually a little further up the rev range...feels unnaturally and I don't always abide by it when cruising, but worth a thought anyway.
An example of my set up;
She will sit on the plane at around 22mph at about 2150rpm
According to smart craft, it's most fuel efficient sitting at 2600 rpm and around 26-28 mph gps.
 

shaw520

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I should have mentioned im running 4 blade props.
Anything below 3k rpm (18mph) and im falling off plane...between 2-3k rpm im pushing alot of water and it feels very ineffecient.
I will note that this is the first boat ive ever owned that theres vitually no transition point,..once on plane if i backvoff on the throttles ill fall off plane...as opposed to all other boats ive owned where once on plane i could back off on the throttles. This may be a trait of the 4 blade props.
 

QBhoy

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Hi
That's fascinating. What about sitting above 3000rpm...say 3200-3500...you would think that would still be better than sitting at 1800 rpm ? Obviously you would be doing around 2.5-3 times the speed for less than double the rpms ?
 

boatman37

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i have a 1986 250 sundancer with a merc 260 (5.7). i can run about 7 to maybe 8 mph max before the bow starts raising. no idea what my best RPM is for efficiciency but feels smoothest at about 3400RPM and about 26 MPH. i have a 3 blade 14.5x17
 

bruceb58

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I have a fuel flow meter that is tied into my GPS so I can get actual MPG numbers and I can tell you that at 8 MPH, I am not using less fuel than on plane. I am pushing a lot of water out of the way.

If someone tells you what their most efficient speed is and they don't have a flow meter, they are pulling their results out of thin air. I always thought my most efficient speed was at 3000 RPM for years until I installed a flow meter. My most efficient speed is closer to 3800 RPM. Every boat/engine is different so don't rely on one persons results to match what yours would be.

Get a flow meter.
 
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shaw520

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Im guessing the 4 blade props are achieving more 'bite' at lower rpms,..but probably creating more drag on plane. I may change over to 3 blade. According to merc chart I should be considering 15x17p or 16x16p. I may just have to try the 15x17p (opinions welcome)
My last trip was on plane the entire way,... 30 miles (aprox 1.5hrs) and I estimated 1.2 GPM.
 

bruceb58

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You are the perfect candidate for a flow meter. It would pay for itself fast.

The main reason I bought a flow meter is for boating on a lake like Lake Powell where gas docks are far away from each other and you need to get as many miles out of every tank. Not uncommon to go on 50 mile runs.
 

shaw520

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You are the perfect candidate for a flow meter. It would pay for itself fast.

The main reason I bought a flow meter is for boating on a lake like Lake Powell where gas docks are far away from each other and you need to get as many miles out of every tank. Not uncommon to go on 50 mile runs.
Yes Bruce,... we take weekend trips sometimes 100 miles,... theres a favorite beach we go to almost every weekend that is 45miles,..(90 miles round trip) and like you say, finding the optimum orientation for gas consumption can easily pay for itself in just a couple months.
So Bruceb58, is this a decent enough unit (floscan 6532-CAT-20),.... I can get it for $150.
 

shaw520

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ooops,...scratch that,... that one is designed for diesel
 

Silvertip

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There is another use for a flow meter besides MPG or GPH. How about having established a bench mark for the way you normally run the boat and what the average load is. Whatever that flow rate is is becomes your bench mark. Let's say one day you depart the dock with average load and you glance at the flow meter and see a very abnormal reading. Would you keep going, stop and investigate, or head back. What you do depends on how much variation there is in from the bench mark. A sudden change indicates a problem -- perhaps a fuel line leak and the bilge is being filled with fuel. Or perhaps a carb/injection issue, fuel pressure regulator gone bad. My point is that simple flow meter can alert you to a problem very quickly. But then you do need to look at it periodically.
 
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