What is Your Fuel Burn?

[eyschulman]

I AM INTERESTED IN COMPARING MY #S WITH OTHER BOATS OF SIMILAR SIZE. LOA IS 48FT AND WL IS 46 WEIGHT 34,000LB. MY BOAT IS TWIN BUT I WOULD LIKE TO COMPARE TO SINGLE AND TWIN BOATS. AT LOW END 8K/4GAL-AT 9K/6GAL AT MID RANGE 10K/8.1GAL 13K/14GAL AT HIGH END 15K/17.5GAL AT 18K24GAL ALL ABOVE ARE TYPICAL CRUISE SPEEDS I USE. THE HULL IS DOWN EAST FAST SEMI DISPLACEMENT AND I WISH TO COMPARE TO ALL TYPES.

[djmarchand]

Most boat owners will report x gph at y kts, and they usually say that that was from fill to fill fuel measurements. But that is always misleading. You spend a significant amount of time idling and getting in and out of marinas, dropping the anchor, etc that never gets accounted for.

So I have concluded that the only realistic way, unless someone reports Flowscan, real time data, is to look at it somewhat theoretically. There are two approaches to do this.

One is to use a pretty good rule of thumb that it takes 15 hp per 10,000 lbs for a true, round bottom displacement boat, and 20-25 hp per 10,000 lbs for a semi-displacement hull like downeaster to push the boat to displacement speed- defined as 1.34*sqrt(lwl).

That would mean that your boat would take 68-85 hp. High output engines at lower rpms typically make 15 hp per gph of diesel burn. So your boat would take 4.5-5.7 gph to go 9.1 kts. That is on the low end of what you reported.

The other way is to plug your boat's parameters into boatdiesel's power required calculator and then use 18 hp per gph for the higher hp loadings. If I plug your data into the calculator, I get 350 hp at 15 kts or 19.3 gph, within 10% of what you reported.

So, collect all of the data that respondents give you, but take it with a big grain of salt.

BTW, downeaster hulls are pretty inefficient, but handle heavy weather better than almost all other hull types. I have a 34' downeaster and it burns a lot of fuel at 15 kts.

David

[Nicholson58]

36 tons; 16 beam; 58 feet LOA; single screw; adjustable pitch Hundested 3-blade 24-inch. 115 HP 6-cyl Westerbeke; 4 kn-miles/gallon under power @ 7 - 9 knots. We ship 380 gallons in 5 tanks for a range of 1500 miles. Nothing "semi" about our displacement.

[Cheechako]

44 ft cutter, 38 ft waterline, 28000 lb., 51 hp Perkins 4-108. Usually motored about 8+ knots in small chop/flat water. Fuel burn average for the 4 years of my ownership .65 gallons per hour.... about 12 miles per gallon.

[eyschulman]

Please understand that this post is under power boats. Being a sailor I understand the economy of sail boats powered or under sail. What I am looking for is similar boats to mine power 30,0000lb the hull type as long as a power boat is not so important. If the boats speed stops at 8 or 9k it is hard to make a comparison. Best comparison would be boats capable of mid teens or higher speeds. I know my boat sets no economy records not why I posted just want a feel for what other similar boats are getting.

[River Cruiser]

38' 18,500 lbs, 13'6" beam, twin 175 hp na diesel. 3.1 gph over 3 years includes gen time. Total hours by hour meter 323 on mains, the gen hours I don't recall & the records are on the boat & I'am back home in Kansas. This is a average burn rate for 3 years of use for everything from idle to wot. This years numbers aren't included because I spent half the year helping to repair a boat in Minnesota that a friend bought & I only put 30 hours on mine.

[Seachase]

Beebe's Voyaging Under Power has some graphs for calculating fuel requirements for diesel displacement power vessels. Probably not applicable to your vessel except at displacement speeds.

[donradcliffe]

Grand Banks 42 with twin 200hp Leylands--4 gph at 8k, 3 gph at 7.5k, 2.5 gph at 7k.

[captpeter]

Hi I have a 43feet motoryacht Sessa42, with twin D6 volvos. I worked out the burn by filling the tank then cruising at various steady speeds (8kts,16,20,25,32) and after each run I topped the tank again and worked out how many litres used at that speed. This enable me to calculate the fuel required for long trips and was accurate. It was about 3.6L per nm.The overall burn appears to be about 50litres per hour average for both engines together (25L each). Hope this helps.