GB power and fuel use.

[Iknowimcrazy]

I'm finding quite a few mid to late 80s Grand Banks 36' Classics for sale. Power options run from single 120hp to twin 210hp and lots of options in between. I'm dubious of trusting gph claims in sales listings. Claims of gph range from 1.2gph to 4.8gph at "cruise." Can anyone chime in with real world knowledge? Thanks

 

[kulas44]

The GB is a semi-planing hull so I would put "cruise speed" well above hull speed. Even at 10 knots it will burn way more than 4.8 gph. Maybe that is "per engine ?". It will never use as low as 1.2 gph at ANY cruise speed with any engine option. Boat brokers are used car salesmen on steroids !!!!

 

[hmason]

I can speak to fuel use on a 36' GB Classic 1984 model as I owned one for 15 years. Satisfaction had a 200 hp British Leyland turbo intercooled engine. By removing the intercooler we essentially detuned the engine to a 165 hp model which British Leyland also produced. This engine is a British Leyland, Not a Lehman. Satisfaction also had a generator, 4kw Onan.

Over the years of ownership which included a trip from CT to FL, we cruised at a speed between 7 and 8 kts with the engine turning about 1750 to 1850 RPM. our average fuel burn was between 2.7 to 3.0 gph depending on generator usage.

The current owner drove the boat from Stuart, FL, to Ontario CD and told me that he calculated 2.4 GPH. I believe he has her for sale now as he wants to move up in size.

Twin engines in a GB 36 is, to my mind overkill as the boat uses about 82 hp at 8 kts. I have a friend with a GB 36 with twins and moving around the engines to service her is a job for a midget acrobat. The single configuration is a piece of cake to service even for a big guy.

We now cruise the ICW in our 46' GB with twin Cat 350s. Our fuel burn is....better not say as the lady captain might read this.

Howard

 

[rwidman]

"Cruise" speed can mean different things to different people. Cruising at the boat's hull speed will be the most economical. You can cruise faster than that on semi-planing hulls but fuel use increases much faster than speed over water.

A twin engined boat will use more fuel for a given time or distance than the same model boat with a single engine.

 

[Nomad Willy]

"GB power and fuel use"

Too much of both unless you have a single engined GB. But the single has a hull optimized for more speed than 120hp can provide. But they are great boats. If you are concerned about fuel burn the single is the obvious choice. Think of the twin as having a 760 cubic inch engine you've got to feed. And the single is probably a better boat out on the water as it's not so heavy.

 

[FF]

>Cruising at the boat's hull speed will be the most economical<

If hull speed is SL x 1.34 it is far cheaper to cruise at SL x .9 to 1.15 depending on how fat and heavy the boat is.

SL is the square root of the actual waterline in feet.

Hull speed is never cheap , its mostly of interest to blow boaters to know how fast they Might go in a 20K reach.

 

[bayview]

Indeed hull speed is the least efficient but fastest speed a hull will go without climbing up .

 

[ranger58sb]

Indeed hull speed is the least efficient but fastest speed a hull will go without climbing up .

Perhaps "the least efficient slow speed..." or maybe "better than slightly higher speeds prior to getting on plane..." some such?

My burn rate can still go up (a lot), and my NMPG can still go down (a lot), at higher RPMs (and speeds) than my "hull speed."



-Chris

 

[kulas44]

Zakly, the faster I goes the more I burns. In my sporty anyway. It doesnt seem to have a sweet spot, unless you call on the pins sweet. It gets proggressively less mpg for each increase in speed. It does particularly bad at 17 mph. and the wake is impressive. Like drounding cows up in the pasture kind of wake.

 

[kolive]

We own a 1974 GB 36 with twin Lehman 2615 E engines. At 1800 rpm's we burned 5.2 Gallons per hour when cruising at an average of 8.6 kts. I measured this after filling up before and after a Gulf Island Cruise. At 1500 rpm's our consumption dropped to 3.6 gph and our speed averaged 7.2 knots. Same measurement method. This was for both engines, not the amount each used.

Single vs Twin, I have owned both and both have pluses and minuses. Found driving either to be something one needs to learn and practice.

 

[markpierce]

Hull speed is never cheap , its mostly of interest to blow boaters to know how fast they Might go in a 20K reach.

Fuel consumption more than doubles moving from one knot below hull speed to hull speed on my boat.

 

[hollywood8118]

We own a 1974 GB 36 with twin Lehman 2615 E engines. At 1800 rpm's we burned 5.2 Gallons per hour when cruising at an average of 8.6 kts. I measured this after filling up before and after a Gulf Island Cruise. At 1500 rpm's our consumption dropped to 3.6 gph and our speed averaged 7.2 knots. Same measurement method. This was for both engines, not the amount each used.

Single vs Twin, I have owned both and both have pluses and minuses. Found driving either to be something one needs to learn and practice.

Another way to look at it...
8.6kts /5.2gph = 1.66 nmg
7.2kts /3.6gph= 2.0nmg

not a lot of difference on a 50 mile trip

8.6 kts= 30.25 gal/fuel
7.2 kts= 25.00 gal/fuel

5.25 @ $ 4.00 = $ 21.00

Depending on time available I can earn way more per hour than $ 21.00.. if your retired slowing down Earns you $ 21.00. The Admiral would be really pissed if the weather was turning to crap and I didn't spend $ 20.00 to get her out of the slop an hour earlier.. better yet I would be really pissed at my self.. I would pa $ 50.00 to not have to suffer " The Wrath"

HOLLYWOOD

 

[Bob Cofer]

"Can anyone chime in with real world knowledge? Thanks"

One question. One answer. Eleven opinions, seems like the normal ratio of TF thread creep.

Single Lehman, 7 knots cruising speed, 1.6 gph.

 

[Nomad Willy]

Bob,
Yup ...... about 30 hp.

Variations in how fuel consumption is measured can be significant. Many only consider fuel put in and engine time. That is most often considerably different that cruise burn minute by minute. People usually don't realize how much time is spent below cruising rpm.

 

[rwidman]

1) Most boaters cannot accurately determine fuel burn except over hundreds of hours and on the condition that they top off their fuel tanks at the beginning of the measuring period and again at the end of the measuring period. This will provide an average fuel burn but unless you travel at a constant speed (over water, not GPS) you will have just that, an average fuel burn for your boat the way you operate it.

It would take an accurate fuel flow computer to determine fuel consumption at a particular speed or RPM.

 

[rwidman]

Another way to look at it...
8.6kts /5.2gph = 1.66 nmg
7.2kts /3.6gph= 2.0nmg

not a lot of difference on a 50 mile trip

8.6 kts= 30.25 gal/fuel
7.2 kts= 25.00 gal/fuel

5.25 @ $ 4.00 = $ 21.00

Depending on time available I can earn way more per hour than $ 21.00.. if your retired slowing down Earns you $ 21.00. The Admiral would be really pissed if the weather was turning to crap and I didn't spend $ 20.00 to get her out of the slop an hour earlier.. better yet I would be really pissed at my self.. I would pa $ 50.00 to not have to suffer " The Wrath"

True perhaps but on a one hundred day cruise at 50 miles per day the difference would be $2,100.

I don't think any of the folks here want to spend more on fuel than we have to. We each pick our own sweet spot between idle speed and WOT based on time available, comfort and fuel cost.

 

[Marin]

I'm finding quite a few mid to late 80s Grand Banks 36' Classics for sale. Power options run from single 120hp to twin 210hp and lots of options in between. I'm dubious of trusting gph claims in sales listings. Claims of gph range from 1.2gph to 4.8gph at "cruise." Can anyone chime in with real world knowledge? Thanks

We have a GB36 with twin FL120s. With our props we cruise 8 knots at 1650 rpm and a total fuel burn of a tad over 5gph, or just over 2.5gph per engine.

 

[FF]

Depending on time available I can earn way more per hour than $ 21.00.

Probably need to gross about $65 to put that $21 in your wallet

 

[Capt.Bill11]

With my old GB 42' MY with twin 135 Lehman's I figured 5 gal per hour total with 8kW genset at 9 knots.