GPH efficiency?

We were finally able to cruise our Chris Craft Commander twice this month. Our first trip was about 50nm round trip, and the other was around 70-75nm round trip. The first trip we faired pretty well there and back with a tank of gas, but the second trip we coasted into our home dock on fumes on the starboard tank.

After the second trip, my partner decided that we needed to sell it and look into something else a little later on. $500 in fuel this month was just too much to comprehend I guess. So I listed it, and if it sold we've been talking about going the trawler route. (I've always wanted a trawler anyway.)

However, after starting a few fuel efficiency conversations in various places and forums, the general consensus is that fuel efficiency ends up working out the same either way. Whether you're burning 2gph and taking a 12 hour trip, or you're burning 11gph and that same trip is only 2 hours.

So what I'd like to know is what kind of boat do you have, what engine(s) are in it, and what your fuel efficiency is at whatever your cruise speed is?

Thanks!

The ratios don't work.
My boat--single engine-we cruise at about 7 kts at 2.5-3 gph.


You won't get a similar sized boat to go 21 kts at 7.5 to 9 gph

Twin Lugger LP668T - 174hp.

Manufacturers figures:

1600 RPM ..................................... US gph (lph) 2.8 (10.5)
1800 RPM ..................................... US gph (lph) 3.7 (14.1)
2000 RPM ..................................... US gph (lph) 5.0 (19.0)
2200 RPM ..................................... US gph (lph) 6.6 (25)
2400 RPM ..................................... US gph (lph) 8.7 (33)

I get ~ 9 knots at 1850, ~7 at 1500. My numbers are close to that though not identical, in fact slightly better.

we cruise out President 41 with twin SP225 Lehmans at 10 knots and burn about 7 to 8 gallons per hour. We could slow down to 8 knots and burn less but we like the little higher speed.

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

Agree with your numbers don't work. My boat at ~7 knots = 3 nautical MPG. Same boat powered with an engine big enough to plane around 12 knots gets maybe 1.5 NMPG. Bottom line going faster takes more horsepower which is exponentially more fuel per mile.
Monk 36
Perkins 6.354M 130 hp
Normal cruise 6.8 knots, 1.8 gph

around 4-5 mpg on my Mainship 34 single diesel @ hull speed(6-7knots)..

Same boat can run 20 knots with ~300hp burning about 20gph..

4-5x more efficient at hull speed.

My 31’ Mainship with twin 454’s gets roughly 1 MPG at 7 kts. If you were to run her at 20 kts you’d be looking at at fuel flow similar to a DC3.

My 45' boat came with a 450 HP Cummins. It would do 14 knots at 21 GPH (1.5 gallons per mile). I re powered with a 135 HP John Deere. It will cruise 6 knots at 1.2 GPH (5 MPG), 7 knots at 2 GPH (3.5 MPG), 8 knots at 3.7 GPH (2.2 MPG). I cruise at 7 knots. Around 8 years ago, diesel spiked to $5 per gallon. Part of my rational for 7 knots 2GPH was that I can't control the cost and don't want the price, whether it's $2.50 or $10 a gallon, to prevent me from cruising.

Ted

menzies said:

Twin Lugger LP668T - 174hp.

Manufacturers figures:

1600 RPM ..................................... US gph (lph) 2.8 (10.5)
1800 RPM ..................................... US gph (lph) 3.7 (14.1)
2000 RPM ..................................... US gph (lph) 5.0 (19.0)
2200 RPM ..................................... US gph (lph) 6.6 (25)
2400 RPM ..................................... US gph (lph) 8.7 (33)

I get ~ 9 knots at 1850, ~7 at 1500. My numbers are close to that though not identical, in fact slightly better.

Click to expand...

Per engine?

I burn 3 litres (0.8 US gallons) per hour @ 1800 rpm averaging 6 knots
That works out to 0.13 gallons per mile in imperial measurements. or 7.5 mpg

If I want to save fuel I can raise the sails and reduce the rpm.

30 foot boat; full displacement hull, 7 tons
Vetus (Mitsubishi) 42 hp diesel

Simi 60 said:

Per engine?

Click to expand...

Yes. Per engine.

Going slowly between the islands in the Abacos at 1500 I was seeing (rough guess) 1.6 - 1.7 MPG. No current and minimal wind effect. So each engine was burning ~ 2 GPH.

2GPH X 2 = 4GPH. ~ 7 knots = ~1.75MPG.

I don't have any fuel scanners nor am I going to install any. I just don't want to be concerned about a gallon here or a mile there. I want to enjoy the experience.

menzies said:

Yes. Per engine.

Click to expand...

Phew, had me worried there for a minute.

We worked our economy out by marking the sight glass/tube at 500 litre intervals during fill so not highly accurate but close enough.
A mate has a longer, beamier and heavier boat with the same engine AND he has floscan or similar fitted.
His numbers aren't far from ours at the same rpm.

We get around 2 US mpg doing 8. knots @ 1250rpm and a bit better again at 7. knots @ 1150 rpm

That's on 60ft and 75 US ton of boat with a single 14 litre Cummins nta855m 325hp no electronics engine.

Your numbers seem pretty close to ours.

I've got a planing hull boat, so I can choose to go slow and save fuel or go fast and burn it up. It has a lot of miles on it and I keep careful fuel dipstick readings and the numbers are very consistent:

7.7kts at 950rpm, 1.9gph, 4nm/gal

19kts at 1900rpm, 11gph, 1.7nm/gal

Takes about 2.5 times the fuel to cover the same distance fast. On other boats I have set up or tested, that factor is pretty typical.

38' cold molded, about 14k lb wet, single Cummins 450C.

Fuel economy

My Lien Hwa 47 with twin Lehmans 375s got 1 gal per nm at 10 knots total. My brother's Offshore 48 with twin Cat 3208s got 1 gal per nm at 10 -12 knots.

The best fuel economy I have ever seen on a planning hull was 2 nmpg on a 24’ searay. Sailboats can easily average 10 nmpg with out sails. Sub 30’ displacement/semi displacement hulls often get 6 nmpg at 5-6 kts. From 40-50’ fuel consumption drops to 4 nmpg but speed is 6+ kts. Over 50’ fuel consumption drops to 2 nmpg and speed moves up to 7 kts.

This is an avg for avg boats. Some semi displacement hulls can’t make these numbers and some full displacement hulls are optimized to beat these numbers.

I find most people running at 10kts are burning 1 gpnm. I think this is a cost/time comfort zone. I run at 1800rpms with a pair of cat 3208’s at 10kts and burn 10.5 gph. Out in big swell & chop my economy has dropped to 1.5 gpnm. I can drop to 1200 rpms and get 2 nmpg but my trip will take almost twice as long and Cat does not recommend running the engines below 1500 rpms.

toocoys said:

After the second trip, my partner decided that we needed to sell it and look into something else a little later on. $500 in fuel this month was just too much to comprehend I guess. So I listed it, and if it sold we've been talking about going the trawler route. (I've always wanted a trawler anyway.)

!

Click to expand...

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.

BandB said:

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.

Click to expand...

Well said

Here's my numbers. 31'er with a single 200 diesel and displaces about 12,500 lbs (approx).

BandB said:

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.

Click to expand...

If it cost me $500 a month in fuel then I'd be making some changes, as well.

Boating does not have to be expensive.
Boating can be whatever budget you want it to be, as long as you are realistic about it. Just don't buy a mega-cruiser when you have a row boat budget.

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.

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.

dhays said:

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

Click to expand...

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

BandB said:

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.

Click to expand...

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.

C lectric said:

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.

Click to expand...

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.

toocoys said:

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.

Click to expand...

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.

BandB said:

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.

Click to expand...

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.

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.

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.

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.