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Old 07-23-2016, 11:27 PM   #1
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How does weight impact fuel consumption?

Suppose I can carry 300 gallons of diesel and 300 gallons of water. The diesel weighs 2250 pounds and the water weighs 2500 pounds. If I burn 2.0 gallons per hour cruising at 8 knots on full tanks, would I burn 1.8 gallons per hour at 8 knots on half full tanks because my boat weighs 2375 pounds less?

Obviously my numbers are all hypothetical, but on a Loop voyage where fuel and water are both regularly available, I'd like to know how much less fuel I could burn by running my tanks half full.

Or maybe I'm just geeking out over nothing but math nerdism?
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Old 07-24-2016, 12:38 AM   #2
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Yes ..power required to move the boat (& therefore fuel required to produce the power) is directly proportional to displacement...so less weight will result in less fuel burnt.

If you can measure the draught difference on your boat between the tank conditions ylou want to consider you will get an idea of the order of saving possible
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Old 07-24-2016, 01:21 AM   #3
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Originally Posted by HeadedToTexas View Post
Suppose I can carry 300 gallons of diesel and 300 gallons of water. The diesel weighs 2250 pounds and the water weighs 2500 pounds. If I burn 2.0 gallons per hour cruising at 8 knots on full tanks, would I burn 1.8 gallons per hour at 8 knots on half full tanks because my boat weighs 2375 pounds less?

Obviously my numbers are all hypothetical, but on a Loop voyage where fuel and water are both regularly available, I'd like to know how much less fuel I could burn by running my tanks half full.

Or maybe I'm just geeking out over nothing but math nerdism?
You're not going to carry 300 gallons of diesel and 300 of water on a 2375 pound boat. Your proportion is way off. So all your hypothetical example is so unrealistic that no answer can be given. The impact of weight very much depends on the boat.
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Old 07-24-2016, 03:19 AM   #4
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The difference will likely be negligible, but yes, every pound has a price.
If you really want to save fuel, suggest that you slow down a tad.
8 knots seems like a commonly accepted trawler speed, but the real sweet spot is more often found between 6.5-7.5K.
Of course, all boats are different, (no clue as to what you are running) and it takes some sophisticated fuel measurement devices, EGT and manifold pressure monitoring to really get it sorted out.
Then there's your propeller dimensions that are likely designed around the boat's top speed, which may not be optimal at trawl speed.
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Old 07-24-2016, 05:25 AM   #5
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Your proportion is way off.
Nope...better read it again

To be a fair comparison you'll have to find out (on average) how many extra miles you'd have to travel to visit twice as many fuel docks, combined with how many more times you'd have to pay for moorage because of time delays getting fuel.

I'd just fill the tanks.
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Old 07-24-2016, 05:26 AM   #6
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To get a rough idea of savings at different speeds & loads..
Run light and record speed vs rpm stall few normal cruising speeds.
Then load upon water & fuel and do the same under the same conditions.
The decrease in speed at a given rpm will give you an idea if\ how much difference it makes.
To find a sweet spot divide the rpm by the speed and plot the resulting rpm\ mph

For example...it takes me roughly 500 rpm for each mph up to about 8 mph and I can see a significant drop in mph\rpm above that... I know the engine is working harder and burning more fuel to go a little faster.
See f different loads produce different results... I'll bet you can hardly tell the difference and your sweet spot.. more likely you might see a different speed at wot.
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Old 07-24-2016, 06:56 AM   #7
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Or maybe I'm just geeking out over nothing but math nerdism?

To a certain extent, yes.

The concept is generally correct: less weight needs lower horsepower burns less fuel.

That said, that's about all the math you need for practical application. When you're where fuel docks are common, keep the boat at half tanks or whatever, run as much as you like... and then when fuel docks are less common, fill up for the long legs.

A difference of .2 gallons per hour probably won't make much of a blip in your expenses spreadsheet, relative to all your other costs....

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Old 07-24-2016, 07:11 AM   #8
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I dropped my fuel capacity from 400 gallons to 112 gallons.

Never noticed a difference in any performance numbers over the last 800 hrs, 5000 miles.

Wind and tide have a much larger effect.
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Old 07-24-2016, 08:08 AM   #9
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The simple rule of thumb is 3HP per ton (2240lbs) of boat weight at high std cruse.SL x 1.15.

So reducing the weight by a ton might save 1/5 of a GPH if your diesel is well matched to the vessel.

AS noted going slower will do more for the fuel bill (on a displacement boat) than getting a bit lighter.
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Old 07-24-2016, 09:03 AM   #10
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Nope...better read it again
Best not to read and post at 2:21 AM as I did....sorry. Still need to know what boat.
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Old 07-24-2016, 09:31 AM   #11
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If you had a planing hull and were trying to get up on plane the weight would matter. Additionally, the boat will sit lower in the water so if you are now dragging your keel through the mud it would also matter.
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Old 07-24-2016, 10:33 AM   #12
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Saving 0.2 gph by dropping a ton of boat weight do seem hard to see in the complexity of wind, tide, idle time, and speed. Over the course of a 1500 hour Loop where one averages $3 a gallon for diesel, it might add up to $900. I don't think boat weight impacts my cruising speed; that is a function of my own sense of serenity (or lack thereof). Since leaving my 20s, slower has always been better.
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Old 07-24-2016, 11:15 AM   #13
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When running just under hull speed, I can notice a difference from heavy load, but my boat is light so adding 300gal is adding 15%. If you add 300gal and it adds 3% to your weight, you probably won't notice the difference. Also, my tanks are aft so added weight there is unfavorable to slow speed trim.
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Old 07-24-2016, 11:36 AM   #14
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You're not going to carry 300 gallons of diesel and 300 of water on a 2375 pound boat. Your proportion is way off. So all your hypothetical example is so unrealistic that no answer can be given. The impact of weight very much depends on the boat.
The difference one word makes. He said his boat would weigh 2375 pounds less! He didn't mention how much his boat weighed.
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Old 07-24-2016, 11:39 AM   #15
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The difference one word makes. He said his boat would weigh 2375 pounds less! He didn't mention how much his boat weighed.
Yes, that was called to my attention earlier. The "less" on my tablet carried to the next line and after 2:00 AM I obviously overlooked that.
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Old 07-24-2016, 01:10 PM   #16
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Yes, big difference. That would be quite a boat which can carry 300 gallons of diesel and 300 gallons of water, yet weigh only 2375 pounds.

In this case, the boat weighs 20,000 pounds.
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Old 07-24-2016, 01:38 PM   #17
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I've observed no discernible difference with my heavy-displacement boat.
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Old 07-24-2016, 02:10 PM   #18
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Yes, big difference. That would be quite a boat which can carry 300 gallons of diesel and 300 gallons of water, yet weigh only 2375 pounds.

In this case, the boat weighs 20,000 pounds.
That's still a very light boat to be carrying so much fuel and water. For instance a Mainship 34 is about that weight and only carries 250 gallons of fuel and 70 gallons of water. It would very much be affected by that added weight. A Nordic Tug 34 carries 205 gallons of fuel and 100 of water. I think it would be affected. However, a Helmsman 37 which weighs slightly more carries 300 gallons of fuel and 140 gallons of water and I don't think the added weight would be noticeable for it.

I've found most builders size fuel and water in such a way that the boat is only very moderately impacted by having both full. I have seen planing boats that do lose up to 2 knots of speed at top end having full versus 1/4 full tanks. Even there the efficiency loss may be 32 vs. 30 knots and so the fuel impact is only about 7%.

In doing the loop, I can't imagine you'll ever really notice the fuel savings from running half full of fuel and water. Now, again, I don't know the specific boat you have in mind.
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Old 07-24-2016, 09:33 PM   #19
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That makes sense. Thanks all.
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Old 07-25-2016, 05:26 AM   #20
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If the object is to save money through lower fuel burn, having the capacity to carry larger quantities purchased at a lower price will probably be the second biggest savings. As mentioned, largest savings will come from slowing down a knot or so.


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