Fuel Consumption Question

[smitty477]

I expect he means 80% of max RPM, not actually 80% power. 80% of max RPM is more like 60% power on a typical prop curve, so it's much more reasonable sounding in that case.

Maybe that is what he means - but 5gph at 8.2 knots with that boat not likely.
Perhaps in kph or maybe even mph maybe.

 

[koliver]

I expect he means 80% of max RPM, not actually 80% power. 80% of max RPM is more like 60% power on a typical prop curve, so it's much more reasonable sounding in that case.

The best measure, for me, of engine load is to look back at your wash. At 8 to 8.2 knots, over 20 years, my fuel burn is reliably under 4 (US)gph with my twins. This includes generator and diesel stove use.

My wave is below cresting, and low enough that I don't worry about waking others. With that fuel burn, by the numbers, I am using less than 50% of available 200hp. With my SD hull shape and weighing 44,000# by the travelift scales, I can't get much above 10 knots, no matter how much fuel I use. That would only be possible with much, much more hp.
I have run on one engine for an entire season. Keeping that single at the same load as when running with a pair, my speed dropped to 6.5 knots. Hull speed is between this and my usual speed, so little or no wave.

Buying a larger load of fuel rather than many smaller loads, here in SW BC, gets a "volume discount " on a scale at 500l, 1000l, 2000l etc. a few cent a l. I usually fill 2x a year, and usually buy over 1000l at a time.
3.78l = 1USG, 4.54l = 1 Imperial Gal.

Keeping a log of this stuff also allows comparing price /l over time.

 

[Cartouche]

I am thinking about purchasing a trawler having put my sloop up for sale. (I am "a gentleman of a certain age"). But I have no idea of what fuel consumption is on a trawler. One 36 footer for sale has two Cummins 6BT5.9M engines rated at 210HP. Can anyone educate me as to fuel consumption of such a vessel? Thank you in advance for your input.


Ron

See stats of Cartouche attached Fuel burn greatly affected by speed

 

[backinblue]

Didn't we answer the simple question of the OP by now?

 

[Art]

Didn't we answer the simple question of the OP by now?

Ya Think!! LOL

To another factor re fuel usage:

I did differing speed mileage [fuel use] trials on our Tolly... for a matter of fact in knowing her range, etc... I do the same on every inboard boat I've owned... o/b runabouts don't usually meet the need for mileage charts.

So... once getting clear handle on what my cruiser will do [on average]... I do not bother to keep checking. I simply run at speed desired, keep drive line in good condition, keep bottom clean, make sure props are well tuned, be careful to keep anodes fresh, don't let under water intake grates become clogged - AND... Simply fill up when required!

 

[Caballero II]

Fuel Burn

Talk engine brand and model. Then talk RPMs.
Some boats are more efficient/slippery moving through the water at a given RPM.
Also realize, any builder supplied data is taken on a boat 1/2 fuel load, 1/2 water load and fridge is running.
Then we load up out boats with our precious possessions, tools, clothing and various form of food, snack and 'other' liquids. Ah dont forget full load of fuel and also water, spare parts and more undefined stuff.
Figure 1 gph if you run the generator.
Sooooo your milage will most definitely vary.

My single engine Ford Lehman GB 36 at 1500 - 1650 rpm has burned an average of 1.7 gallons per hour (with the Westerbeke 3 KW running) over a period of about 500 hours over (10) years. That runs me at about 7.5 - 8.0 knots. I'm a typical recreational boater, I'd guess. Maybe a little bit more use than most. Not sure.

 

[Art]

My single engine Ford Lehman GB 36 at 1500 - 1650 rpm has burned an average of 1.7 gallons per hour (with the Westerbeke 3 KW running) over a period of about 500 hours over (10) years. That runs me at about 7.5 - 8.0 knots. I'm a typical recreational boater, I'd guess. Maybe a little bit more use than most. Not sure.

Wow - over 4 nmpg! About the best I've heard of at hull speed for that type boat.

 

[backinblue]

Ya Think!! LOL

To another factor re fuel usage:

I did differing speed mileage [fuel use] trials on our Tolly... for a matter of fact in knowing her range, etc... I do the same on every inboard boat I've owned... o/b runabouts don't usually meet the need for mileage charts.

So... once getting clear handle on what my cruiser will do [on average]... I do not bother to keep checking. I simply run at speed desired, keep drive line in good condition, keep bottom clean, make sure props are well tuned, be careful to keep anodes fresh, don't let under water intake grates become clogged - AND... Simply fill up when required!

All good info. My point is that the OP (who has not responded) is considering moving from sail to power and in his words, has no idea what to expect for fuel usage. He was likely looking for something like "For the boat you describe, expect approx. 2 mpg at 7knots on average." Nothing wrong with all the in-depth data and detail that has been provided, I just doubt he needs that. Just my opinion. Carry on.

 

[OldDan1943]

Moving from sail to diesel, you leave the cost of maintaining the sails and standing rigging behind. That savings can be spent on fuel and engine maintenance.

 

[keith c]

As the owner of a MS 34/twin 240HP yanmars, lets cut to the chase,get down to the bone, terminate the bull . Sorry, I went off the rails there. With 2 rather large diesels running at a reasonable speed, 7,8 or 9 kts one will get 2 mph or likely less in a 36ft vessel !

 

[Nomad Willy]

Wow - over 4 nmpg! About the best I've heard of at hull speed for that type boat.

I get 6 w Willy at 6 knots and 1gph. FD tho and smaller.
Slow down 1/2 to one knot and probably get considerably better .. 8-10?
I ain’t slow’in down tho.

 

[backinblue]

Moving from sail to diesel, you leave the cost of maintaining the sails and standing rigging behind. That savings can be spent on fuel and engine maintenance.

Absolutely agree, also running rigging and other things as well. My point is the OP probably wasn't looking for in-depth analysis of anyone's boat. Nothing wrong with those, but probably not what he was looking for. He was probably just trying to get a ballpark in $$$ of what to expect for whatever type of cruising he expects to do. e.g. if today he sails 500 miles and uses 20 gal of fuel in a season, what should he expect if he does similar boating in the trawler he described? Without any knowledge, he may be picturing $1000 fill-ups several times a season.

 

[Bryant]

fuel consumption

I am thinking about purchasing a trawler having put my sloop up for sale. (I am "a gentleman of a certain age"). But I have no idea of what fuel consumption is on a trawler. One 36 footer for sale has two Cummins 6BT5.9M engines rated at 210HP. Can anyone educate me as to fuel consumption of such a vessel? Thank you in advance for your input.


Ron

My GB 36 Classic has a single Ford Lehman 120. At 7 kts and running the Isuzu generator I burn about 2.5 gals/hr. I prefer 1 engine for a number of reasons, not the least of which is cost of operating and maintaining.

 

[mvweebles]

If you want to know the fuel burn of a vessel, ask a delivery skipper. They will give you an honest answer.

If you ask an owner, the answer you get will most likely be the answer they wish were true.

To directly answer the OPs question, if you stay at 7 kts, you will burn around 2.5-3.0 gph (around 3 nms per gallon). If you throttle up to 15 kts, you will burn around 20-25 gph.

Over the course of 10 years of ownership, with sail spars, sails, running rigging, standing rigging, etc, costs are not too dissimilar. Really depends on how you want to travel. Exposed in a cockpit, or fuzzy slippers on a pilot house.

Peter

 

[Nomad Willy]

My GB 36 Classic has a single Ford Lehman 120. At 7 kts and running the Isuzu generator I burn about 2.5 gals/hr. I prefer 1 engine for a number of reasons, not the least of which is cost of operating and maintaining.

Bryant,
Have you ever been onboard a 36 w twin engines? Wonder if you can feel the extra weight under your feet? The twin will be sluggish to turn, rise to a sea, stop and perhaps as a blessing sluggish to roll ... depending on the size of the seas.

 

[Bryant]

Fuel consumption

Bryant,
Have you ever been onboard a 36 w twin engines? Wonder if you can feel the extra weight under your feet? The twin will be sluggish to turn, rise to a sea, stop and perhaps as a blessing sluggish to roll ... depending on the size of the seas.

What?

 

[Nomad Willy]

A twin w the extra ton of fuel should feel slugggy underfoot.

Perhaps you’d need to be underway. Because of the extra weight their “sea motion” would be different.
I think the two engines are in the same position (fore and aft) so (for example) plowing through a big choppy sea it would be .. sloppy going. And a ton less weight should reduce the wetness quite a lot.

Anyway the GB boats 32, 63 and 42’ are wonderful boats. Would love to have one but my taste in that aspect ratio would brobably be a NT 32.

 

[Caballero II]

fuel burn

My GB 36 Classic has a single Ford Lehman 120. At 7 kts and running the Isuzu generator I burn about 2.5 gals/hr. I prefer 1 engine for a number of reasons, not the least of which is cost of operating and maintaining.

Bryant, unlike Weebles (my curmudgeon trawler brother), I don't think an owner is going to try to lead you astray out of some misplaced pride. I am interested in why you're reporting a 2.5 gph fuel burn while I calculated a 1.7 gph after a trip to Mexico and back using about 300 gallons. I doubt your Isuzu, as opposed to my 3 KW Westerbeke generator would burn that much more to make that kind of difference, so I'm wondering about which of us screwed up on our arithmetic. The number of gallons is on the receipt, so that is not in question. It would be a lot of work to go back thru the hours to see if I duplicated, yet, hours from fuel load to refuel would also seem to be pretty straightforward. Hmmn? What do you think? Could you have it wrong?

BTW, I currently use 2.5 gph anyway for range planning based on initial estimates, just to be safe and have reserves. Nonetheless, I'd love to get to the bottom of this to be sure I don't have a false premise.

 

[Bryant]

Fuel consumption

Bryant, unlike Weebles (my curmudgeon trawler brother), I don't think an owner is going to try to lead you astray out of some misplaced pride. I am interested in why you're reporting a 2.5 gph fuel burn while I calculated a 1.7 gph after a trip to Mexico and back using about 300 gallons. I doubt your Isuzu, as opposed to my 3 KW Westerbeke generator would burn that much more to make that kind of difference, so I'm wondering about which of us screwed up on our arithmetic. The number of gallons is on the receipt, so that is not in question. It would be a lot of work to go back thru the hours to see if I duplicated, yet, hours from fuel load to refuel would also seem to be pretty straightforward. Hmmn? What do you think? Could you have it wrong?

BTW, I currently use 2.5 gph anyway for range planning based on initial estimates, just to be safe and have reserves. Nonetheless, I'd love to get to the bottom of this to be sure I don't have a false premise.

I’m basing my assumption upon the amount of fuel I take on between refuelings taking into account the hours the main engine has run but not taking into account the hours that the generator has run while on the hook while the main engine is not running. It’s an average fuel burn figure that is much better for planning than 1.7/hr in my opinion. If you plan for and provide for 2.5 and actually get 1.7 you will never run out of fuel. If you plan for 1.7 and achieve 2.5 you will be seriously screwed. BTW, my gen is a 9kw. I don’t know if that makes a difference but it might.

 

[Bryant]

Fuel consumption

A twin w the extra ton of fuel should feel slugggy underfoot.

Perhaps you’d need to be underway. Because of the extra weight their “sea motion” would be different.
I think the two engines are in the same position (fore and aft) so (for example) plowing through a big choppy sea it would be .. sloppy going. And a ton less weight should reduce the wetness quite a lot.

Anyway the GB boats 32, 63 and 42’ are wonderful boats. Would love to have one but my taste in that aspect ratio would brobably be a NT 32.

My apologies but I have no idea what point you are trying to make. GB boats with twin engines are side by side, not fore and aft. Another reason I don’t want two engines. I can walk around my engine 360 degrees and access every part on it. You can barely get into the engine room of a GB 36 with two engines.

 

[backinblue]

If you want to know the fuel burn of a vessel, ask a delivery skipper. They will give you an honest answer.

If you ask an owner, the answer you get will most likely be the answer they wish were true.

To directly answer the OPs question, if you stay at 7 kts, you will burn around 2.5-3.0 gph (around 3 nms per gallon). If you throttle up to 15 kts, you will burn around 20-25 gph.

Over the course of 10 years of ownership, with sail spars, sails, running rigging, standing rigging, etc, costs are not too dissimilar. Really depends on how you want to travel. Exposed in a cockpit, or fuzzy slippers on a pilot house.

Peter

Respectfully, I don't think the OP was asking about whether sail vs. power is better. In my experience neither is as you describe, but we can save that for another thread.

 

[mvweebles]

Respectfully, I don't think the OP was asking about whether sail vs. power is better. In my experience neither is as you describe, but we can save that for another thread.

I understand your point, but I think the OP's question went unanswered except for my post. He is looking at a semi-displacement 36-foot boat with twin Cummins' 210 hp engines capable of cruising at 15+ kts. He did not receive a single response on what the fuel burn would be at cruise. Only prideful responses about what is economical at displacement speeds.

For the life of me, when I read threads on fuel consumption I really wonder why y'all even bother with anything other than a displacement boat. An interesting thread would be what percentage of SD running time is at 1.3 S/WL or faster. If burning double-digit gph of fuel bugs people, why not just get a boat that is purpose built with a correctly sized rudder, etc.?

Peter

 

[rslifkin]

For the life of me, when I read threads on fuel consumption I really wonder why y'all even bother with anything other than a displacement boat. An interesting thread would be what percentage of SD running time is at 1.3 S/WL or faster. If burning double-digit gph of fuel bugs people, why not just get a boat that is purpose built with a correctly sized rudder, etc.?

For some, I think it's just that the boat they liked best happens to be SD. And some live in places with bar crossings where they may want the extra power / speed even if they don't use it often. Plus, plenty of SDs handle well enough at low speed that it's not really a big downside.

 

[OldDan1943]

My N46 carried 1000 gal of fuel. The ER was a nightmare to work in. It had a 20kw gen.
Over the years since I own it, i have Ben told, the Lugger was rebuilt, the 20kw was replaced with a 12kw, the 8kw gen was removed from the aft lazzarett and 2 of the 4 fuel tanks were removed. Now the ER has more space to work at the expense of reduced range. That was the owner’s decision and perhaps a wise one if there was no intention to cross the ocean.
I have no doubt the Gph increased significantly due to the reduction in weight.

 

[Xsbank]

How much horsepower does it take to reach your desired speed? There is a direct relationship between fuel required to make that horsepower and it is a very tight ratio that varies very little between makes and configuration.

The annual cost of fuel is about 1/3 of my insurance cost (no claims); 1/5 of my moorage;
equal to annual haulouts; 1/15 of annual total costs. Fuel is important but nowhere near the highest expense and therefore the wrong area to be concerned about.

The cost of capital and depreciation are the biggest but my knowledgeable boating friend says "boating is the best time you can buy."

A pair of heat exchangers will cost as much as your average annual fuel bill if you are an average user.

 

[Nomad Willy]

My apologies but I have no idea what point you are trying to make. GB boats with twin engines are side by side, not fore and aft. Another reason I don’t want two engines. I can walk around my engine 360 degrees and access every part on it. You can barely get into the engine room of a GB 36 with two engines.

Same position “fore and aft” = each engine is the same distance from the bow and or same distance from the stern. Wether it’s a twin or single the engine weight is in the same place “fore and aft”.

Because of the house/cabin design the engines are more fwd than on most boats. Ideally engines should be aft of center because the bow’s ability to support weight is much less due to the fact that that end of the boat is the pointy end.

Yes but engine access has nothing to do w how a boat handles at sea.

 

[backinblue]

1 engine equals 1/2 the maintenance, at least in theory....and does make for a roomier engine room. Also nice if your single prop is protected by a keel or skeg. Definitely. saved my butt last summer.

 

[Nomad Willy]

B&B,
Yes as long as it’s a trawler whereas both engines are the same and the singles utilize the same old engine.
That means any comparison will be comparing a 120hp boat to a 240hp boat.

In some ways they can be compared. However if you’re going to compare the two boats need to have the same amount of power. Total power.

 

[koliver]

Bryant,
Have you ever been onboard a 36 w twin engines? Wonder if you can feel the extra weight under your feet? The twin will be sluggish to turn, rise to a sea, stop and perhaps as a blessing sluggish to roll ... depending on the size of the seas.

Are you suggesting the WL on a single is not the same as on a twin? That the overall weight of the whole boat is increased with twin engines? That the designer didn't take the weight of the power plant(s) into account when specifying ballast or no ballast, when locating batteries, etc, etc?
What is your take on a pair of 80 hp v a single 160 hp, approx same total weight?

 

[rslifkin]

Are you suggesting the WL on a single is not the same as on a twin? That the overall weight of the whole boat is increased with twin engines? That the designer didn't take the weight of the power plant(s) into account when specifying ballast or no ballast, when locating batteries, etc, etc?
What is your take on a pair of 80 hp v a single 160 hp, approx same total weight?

Depending on the boat, there can be a noticeable weight difference. It's not necessarily bad, but it's there. In your example of small twins vs big single, things will often be more comparable, as the engines are likely better chosen (rather than 1 good configuration and 1 sub optimal one).