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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #11  
Old 09-11-2009
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As others have mentioned I don't think the formula holds true in the real world. Our 62hp Volvo (non turbo) has averaged 1.0 gph consistently over the years (1988 model). At 6.5 knots this equates to - ta dah! -6.5 mpg. A guy in our marina just had a new 32' aluminum catamaran power boat built, he burns app. 25 gph (two 370hp Volvo diesels, turbo charged+intercooled+electronic) and cruises at 30 knots so he is getting 1.2 mpg (and spending $75/hour),ouch!
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There is 112,000 btu's in a gallon of gas. There is 130,000 btu's in a gallon of diesel. Given the same efficiently of engines the diesel would use less fuel to do the same work. However, diesel engines are also inherently more efficient than a gas engine as they burn more of the available fuel and less goes out of the exhaust.
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Guys, the fuel burn per hour seems very rasonable, considering the btu's diesel provide and also the average efficiency on engines nowaday. But seems mostly of you are taking your engine MAX RATED HP in consideration and not the real HP you're using. In other words, my boat has a 54HP Yanmar, and when cruising in calm waters at 6knots, I use barely 1600 rpm, this mean roughly 27HP out of my engine. At this HP, it burns almost what the previous table say. So, if you're measuring you consumption, please note the rpm and go to your engine graph to show you how much HP it puts at such rpm.
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Old 09-11-2009
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Quote:
Originally Posted by Stillraining View Post
According to the calculations then..my 64 hp Perkins should be burning 3.56 gph...It is not as far as I can tell ...but I will have to have more opportunity to check it out over time to be sure ..so far I have only ran it about 2 hours...I cant ignore this feeling in my gut telling me it will come in closer to under 1/2 of that..I will be quite surprised if it does not...as using various other engine consumptions ratios I have around here to compare it with says that calculation is all wet.
If your 64 HP Perkins was actually generating 64 HP I expect it would eat up about 3 GPH. You'll notice that during rough weather in a head wind your fuel consumption will increase even though you're running the same RPM as you always do.

In calm weather with no wind it just doesn't take all that much muscle to move a sailboat around.
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Quote:
Originally Posted by jrd22 View Post
As others have mentioned I don't think the formula holds true in the real world. Our 62hp Volvo (non turbo) has averaged 1.0 gph consistently over the years (1988 model). At 6.5 knots this equates to - ta dah! -6.5 mpg. A guy in our marina just had a new 32' aluminum catamaran power boat built, he burns app. 25 gph (two 370hp Volvo diesels, turbo charged+intercooled+electronic) and cruises at 30 knots so he is getting 1.2 mpg (and spending $75/hour),ouch!
Thats great to hear John...Im counting on mine being no more then 1.5 and I will be estacic if it is as low as yours.....PS: Thats better then my Dump truck gets..I get about 4.5 MPG....but thats 444 hp...ya baby!!

Dully noted Selkirk...I suppose you are right about that..I never run my Heavy Equipment more then 75% throttle and most of the time less then that...per the calculations my Dump Truck would only get 2.5 mpg
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Last edited by Stillraining; 09-11-2009 at 11:03 PM.
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My 120 hp lehman burns 2.3gph no matter the weather as long as I keep it at 1650rpm
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Old 09-12-2009
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One thing to keep in mind: the power doesn't vary linearly with speed (and RPM), it varies with about the cube! So at 3 knots, you need about 1/8 the power you do at 6 knots. Reduce the speed by only 20% and the power is only about half of what it was. That explains the differences between the calculated fuel consumptiong based on rated engine power and actual consumption at cruise speeds.
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JimsCAL, very good point to remember. Also, many new diesels does provide an average chart for rpm/HP (like you say, clearly not linear), so you can beter estimate fuel consumption if you balance against speed.
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Negrini- I realize that I am not using all 62 hp when at 2500 rpm on an engine that will do 3500, but do the math, it still does not equate. Even if I were only using 40 hp to push Laurie Anne (40' - 24,000 disp.) by that formula I would be burning 2.2 gph which is over twice what in fact it uses. By this formula I should only be using 18 hp at cruise speed since I only use 1 gph. In fact, using that formula it is wrong for any of the scores of diesel engines I have owned and have paid for the fuel bills on, so I know how much they use. My current crab boat, for just one more example, has 200 hp, 190 at the prop, and I run it at 3400 rpm. WOT is 3800 and I average betweeen 6 -7 gph at that speed(I log every gallon that goes into it). According to this formula it should be burning close to 9 gph. The Volvo fuel consumption graph is close to my actual burn rate, a little higher. All I am saying is that this does not seem to be a reliable formula to use to estimate anticipated fuel consumption, and therefore required tankage.
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... gallons, littlers, miles, kilometers ..... I just messed up as I don't think imperial. John, redoing the math to my engine, you're correct. Shame on me.
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