How does weight impact fuel consumption?

[HeadedToTexas]

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?

 

[stone beach]

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

 

[BandB]

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.

 

[kapnd]

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.

 

[MurrayM]

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.

 

[Bacchus]

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.

 

[ranger58sb]

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....

-Chris

 

[psneeld]

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.

 

[FF]

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.

 

[BandB]

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.

 

[alormaria]

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.

 

[HeadedToTexas]

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.

 

[Ski in NC]

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.

 

[Recb]

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.

 

[BandB]

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.

 

[HeadedToTexas]

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.

 

[markpierce]

I've observed no discernible difference with my heavy-displacement boat.

 

[BandB]

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.

 

[HeadedToTexas]

That makes sense. Thanks all.

 

[O C Diver]

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.


Ted

 

[psneeld]

What Ted said.....

Fuel planning can be done down to a gnats butt....however in reality as it's been said. Wind and tide will have the biggest effect unless you don't plan ahead and buy your fuel in the best spots.

I saw fuel vary last year from a $1.75 to over $3.00 along my limited 300 mile range. So, even though I do look ahead and try to plan around some predictable extreme tidal flows... it is not always is just that simple...but where I buy fuel is always under my control.

 

[BandB]

.

I saw fuel vary last year from a $1.75 to over $3.00 along my limited 300 mile range. .

I just pulled some current Florida numbers. Let's say you're sitting in Fort Lauderdale and heading north. In Fort Lauderdale, you can plan and pay as little as $1.95 for a fuel truck delivery or wait until you're in North Palm and pay $3.08 at Old Port Cove. If you're low on fuel and have to purchase at Port Canaveral then it's $2.77 at Ocean Club, but wait for Jacksonville and pay $2.30 at Lambs. Or if you plan it right then wait for Brunswick, GA and pay $1.84 at Ocean Petroleum. So, by filling in Fort Lauderdale and next in Brunswick, I could average $1.90 per gallon or by filling at a marina in Fort Lauderdale or Palm and then again in Canaveral I could have averaged $2.90 per gallon. By planning my fuel purchases I could save 34% on that trip or probably about $400 by selecting the least expensive fuel options. That is without changing my cruising habits at all.

We consume a lot of fuel and we aren't going to slow down to displacement speeds but we definitely are aware of fuel pricing.

Also, if you're filling, buying larger quantities, you open up the possibility of many discounts. Last, don't hesitate to ask. If we're planning on docking at Ess-Kay in Brewerton and Winter Harbor, across the canal is 10 cents a gallon cheaper, we won't hesitate to ask Ess-Kay to match. We could just as easily the next morning on our way out fuel at Winter Harbor. Oh, and we're not about to pay the price at Brewerton Boat Yard which is 70 cents higher. Now, typically for 10 cents a gallon, we're not going to inconvenience ourselves, just ask for a courtesy match, but for 70 cents a gallon we sure will.

Our biggest disparity is cruising regularly to the Bahamas. Right now the going rate there is around $4 versus the $1.95 above in Fort Lauderdale. In our ideal world we leave home full and arrive back home with 20% of a tank.

 

[HeadedToTexas]

My thinking was limited to just what the boat used, but using range to take advantage of price variations makes a ton of sense. Thanks guys.

 

[FF]

Cheap fuel is nice , CLEAN fuel much better.

Stop where the fish boats (real trawlers) fill, but be advised they may have BIG nozzles and big pumps.

But if its not busy they will deal best for CA$H.

 

[BandB]

Cheap fuel is nice , CLEAN fuel much better.

Stop where the fish boats (real trawlers) fill, but be advised they may have BIG nozzles and big pumps.

But if its not busy they will deal best for CA$H.

It can be both. We do find commercial marinas to offer both, but also fuel trucks do, and many marinas are high volume and very clean.

 

[HeadedToTexas]

And it's water that makes diesel un-clean?

 

[tadhana]

Our Helmsman 38 displaces about 26,000. We carry 400 fuel and 150 water. We have a fully electronic engine so we can, and do record our fuel burn and graph it. The fuel tanks are amidships, and they make little difference in trim. We burn 2.5 gph at 7 kt. cruise fully loaded and 2.4 at light load. the fuel tanks are amidships and as the fuel load decreases the boat comes up almost level. On the other hand, the water tanks are quite far aft. When fully loaded they do cause us to burn more, particularly if our fuel load is light. When fuel is light and water is full we cannot make rated RPM only get to 2550. Full fuel and half water we hit 2560. So at the end of the day yes you might see a few percentage points savings but at the end of the year the net effect on the cost of boat ownership is decimal dust.