GPH efficiency?

[dhays]

I have a slow boat. I make 7 knots at 2 gph. I make 9 knots at 6 gph.

So, say I am going to go 50nm. If I go at 7 knots, it will take me a bit over 7 hours and I’ll use 14.25 gal of fuel for 3.5 nm/gal.

If I am in a bit of a hurry and run at 9 knots, it will take me about 5 1/2 hours but I’ll burn 33 1/3 gal fuel for 1.5 nm/gal.

So in this example of one, speed makes a huge difference in the total amount of fuel burned.

[C lectric]

My 32' boat uses ~ 11LPH [2.9USG p/h] at 7 knots.
It uses ~15LPH [3.9USG p/h] at 8 knots.
So ~ 1/3 more fuel for 1 knot so I usually operate at the 7.

Have you tried running the Chris at 7 knots? Maybe the boat can do better if you slow down. Maybe you can drop into an acceptable range of fuel use/cost. I know several people who did that and it worked for them.

You need to find out before making any decisions as any sale/purchase is also going to cost.

If you try it you will find out if you can be OK with 7 knots. Do it for more than one trip though. If the 7K is ok then consider the change.

[Simi 60]

Originally Posted by dhays

I have a slow boat. I make 7 knots at 2 gph.

That's 1 litre a nautical mile.
Almost a mythical beasty.

[toocoys]

Originally Posted by BandB

Don't even think about another boat with that partner unless you've done some serious budgeting together. If $500 in fuel is going to lead someone to sell a boat, I can't imagine what a serious expense will do. Forget fuel data until you resolve that.

Boating is Expensive or Very Expensive. Not a low cost endeavor.

Well, it's a combination of the cost of fuel, and the cost of restoring her.

While she was in running condition when we got her, there were a few things that I did NOT notice when we looked at her. So all winter it was $200 here, and $200 there for this part or that part. And then you put $500 in fuel over two trips and it was just the proverbial straw on the camels back.

However, as I research fuel efficiency and get input from fellow boaters as I'm doing now, I'm learning that I need to slow down.

Our next day cruise is June 2nd, so I'm going to try the 1500rpm thing since its a short trip and see how slowing down helps things.

[toocoys]

Originally Posted by C lectric

You need to find out before making any decisions as any sale/purchase is also going to cost.

If you try it you will find out if you can be OK with 7 knots. Do it for more than one trip though. If the 7K is ok then consider the change.

I have purposefully priced her on the high side, so I'm doubtful she'll sell. (which I'm perfectly ok with.) It's not like they're still making 1967 Chris Crafts anymore, and I'm very partial to the boat. Even in her run down state, she's still a head turner.

My reasons for wanting a trawler are:

Single engine
Interior helm
Helm forward design
Higher freeboard
More room

But, until we can get on the same page, I think our best bet is to throttle down and keep the Commander and finish our commitment to restoring her.

[BandB]

Originally Posted by toocoys

Well, it's a combination of the cost of fuel, and the cost of restoring her.

While she was in running condition when we got her, there were a few things that I did NOT notice when we looked at her. So all winter it was $200 here, and $200 there for this part or that part. And then you put $500 in fuel over two trips and it was just the proverbial straw on the camels back.

However, as I research fuel efficiency and get input from fellow boaters as I'm doing now, I'm learning that I need to slow down.

Our next day cruise is June 2nd, so I'm going to try the 1500rpm thing since its a short trip and see how slowing down helps things.

Now you've shared more of the story and the very reason I made the comment on the $500. $200 here, $500 here, $1000 here happens along the way in boat ownership. You can reduce fuel consumption but you can't escape that there will be $500 expenses at various times.

[toocoys]

Originally Posted by BandB

Now you've shared more of the story and the very reason I made the comment on the $500. $200 here, $500 here, $1000 here happens along the way in boat ownership. You can reduce fuel consumption but you can't escape that there will be $500 expenses at various times.

Oh I know that.

Before we had the Chris Craft, we had a 18.5 foot Chaparral bow rider. Loved that little boat! We didn't even have it a month and my partner took it out without me. I told him that it wasn't like normal boats, it sat high out of the water and was hard to control. He took it out with a friend and ended up swinging the stern into the rocks near the boat ramp and tore up the prop and ripped the skeg off the outdrive. That was $1200 worth of fun not even a month after we had it. He refuses to drive the boat now.

[darkside]

My little boat uses 0.75USG/NM at 25kn, both engines.
I don't have any numbers for 7kn as I don't do that speed much.
Going fast doesn't have to mean a big fuel burn.
You just have to be small (36ft) and light and of course, have the correct number of hulls.

[Scratchnsaw]

Personally I would never attempt to compare one boat to another, solely from their fuel burns. Size almost always matters. But the age of the boat when comparing similar sizes can vary. Older boats in similar sizes almost always burns more fuel per hour than the newer hulls simply because the construction materials have changed making boats weigh less, along with the engines, which have seen major improvements in efficiency when just reducing their weights while using improved electronic metering.

Take for instance in my case the four stroke outboards are far superior in fuel use than the older two strokes, which have now also been improved in their remakes. The same applies to the smaller diesel engines like Yanmar versus the older type GMC marine diesels like the 653 or 853 that were used in the earlier models of the heavier production fiberglass hulls in the states.

The older inboard gas engines like the Commanders sucked gas like a rotten leaky wooden planked hull leaked water. There is not enough money and desire to deal with either scenerios in the 21st century, especially if a person wants to use their boats more than just a dock condo.


I will add that with the price of the newer hulls, some of the older hulls looks mighty good. You can buy a lot of fuel for price differences of upgrading to a modern hull. Pay as you go works for some folks on any type of boating budgets.

[Kaz]

Slower is more fuel efficient than faster. Smaller boats are more fuel efficient than larger. Full displacement hulls, especially sailboat hulls, are more fuel efficient than semi displacement. A single prop is more fuel efficient than duel.

The same rules seem always to apply. If both speed and fuel efficiency are the priority, the only people beating the system seem to be the small planing boats and the power cats. Some power cats seem to be a able to produce those 2 to 3 nmpg efficiencies while going 18kn. I don't own one. However, it does make you think.

[bayview]

OC if you had gone slow with that Cummins fuel would have been less too

[Mrwesson]

Did some research for you and it doesn't look very good.

I had to use a modern vortec 350 to get GPH numbers so yours will probably be even more but..

You'll need 21hp to move your boat @ 7 knots.. 11,500lbs @ 31'

Each engine will burn 2.5+GPH @ 1,500 and since your boat is propped to go faster you'll need this RPM for any meaningful speed.

That's at least 5Gph to go 7 knots for a modern twin efi 350.

My guess is that you'll burn closer to 7@7knots with the older 350 getting about 1nmpg..

Based on some research @ the chris craft forums another user is burning 16gph@14knots @ .9 NMPG.

A planing hull wont gain much @ displacement speeds because..

1. They are geared to plane. You only need 21hp to move your boat @ displacement speeds but your boat has a small prop and higher pitch to move the boat fast.

2. You have too much power. Try running 1 engine @ displacement speeds if possible(may damage the other by freewheeling and may cause drag).. Pull the prop if you have to.

3. Gasoline engines need to rev to make power and they do make more of it per liter of displacement but low rpm thrust is what makes a boat efficient(at hull speed) so we run diesels.

The hull isn't the "problem" it's just not set up for what you want to do.

You'll have to do more than just slow down.

[denverd0n]

Originally Posted by toocoys

However, as I research fuel efficiency and get input from fellow boaters as I'm doing now, I'm learning that I need to slow down.

BINGO!

If you keep it at or just under hull speed then your fuel economy will be about as good as it's going to get. The only way to get more miles per gallon is to buy a sailboat, or buy a significantly smaller/lighter boat.

Slapping the name "trawler" on a boat does not magically give it better fuel economy. Trawlers tend to get better fuel economy mostly because the people who buy them are happy traveling slower -- at or just under hull speed.

[TDunn]

I burn about 0.25 gph at 5.5 knots and 0.6 gph at 7 knots. That works out to 22 mpg at 5.5 knots, but only about 12 mpg at 7 knots. My boat is 32'8" and powered by a single Volvo-Penta D2-40 diesel. My wife prefers the slower speed to minimize engine noise, although at 7 knots you can still carry on a normal conversation anywhere on the boat.

[Gmarr]

I have a hunch that the reason for the 50 and 70 mile trips was that’s where you both wanted to go. So to save on fuel- it’s going to take longer to get there. How will that work?

[smitty477]

Our last boat is a good example of how speed affects economy - these numbers are rounded just a bit for ease of use:

6 knots = 3.5 nmpg
8 knots = 2 nmpg
10 knots = 1 nmpg
17 knots = 0.95 nmpg

Its worth a look to see what happens when we travel into a 3 knot head sea as well.....
All speeds SOG in 3 knot head sea
3 knots = 1.75 nmpg
5 knots = 1.25 nmpg
7 knots = 0.7 nmpg
14 knots = 0.78 nmpg

As they say YMMV but the speed will vary the mileage way more than you think.

[FF]

"I think our best bet is to throttle down and keep the Commander and finish our commitment to restoring her."

If you can be comfortable with the slower trawler like speeds , there are fairly cheap modifications that will reduce your fuel burn.

Operating on one engine might be OK for your tranny.

Simply installing a vacuum gauge and watching to operate below where the secondaries start to open.

If you can get under 3 GPH at say 7 MPH (not knots) would that work?

[Benthic2]

Originally Posted by SOJOURN4

SWIFT TRAWLER 44
For the most part - MPH = MPG

Sweet spot is between 17 and 20 MPH. Burn Rate 17 GPH and 20 GPH respectfully

This can't be right.....speed and efficiency are inversely proportional...as one goes up, the other goes down. They may be equal at your sweet spot... but I don't think they will be equal above or below that spot.

[BandB]

Originally Posted by Benthic2

This can't be right.....speed and efficiency are inversely proportional...as one goes up, the other goes down. They may be equal at your sweet spot... but I don't think they will be equal above or below that spot.

That's his cruising speed. However, at 8 knots he gets 2.2 nmpg, at 6.7 knots, 3.7 nmpg, at 5.7 knots, 5.2 nmpg. 9 knots is 1.8 nmpg, 10 knots is 1.3 nmpg.

RPM MPH Knots GPH MPG NMPG Stat. Mile NM dBa
700 4.2 3.6 0.6 6.9 6.0 2297 1997 62
1000 6.6 5.7 1.1 6.0 5.2 1993 1733 62
1250 7.8 6.7 1.8 4.3 3.7 1430 1243 65
1500 9.2 8.0 3.7 2.5 2.2 826 718 64
1750 10.3 9.0 5.1 2.0 1.8 677 589 67
2000 11.5 10.0 7.9 1.5 1.3 481 419 68
2250 13.4 11.7 10.0 1.3 1.2 445 387 72
2750 19.5 16.9 16.0 1.2 1.1 404 351 73
3000 22.6 19.6 19.0 1.2 1.0 394 343 76
3250 26.0 22.6 22.5 1.2 1.0 384 334 76
3500 28.6 24.8 26.0 1.1 1.0 365 317 78
3600 29.4 25.6 28.0 1.1 0.9 349 303 79

[Benthic2]

I assumed as much. But to state that "MPG=MPH" is saying that you're miles per gallon will be double if your speed is doubled, and your most efficient speed would be full throttle.