Gph

Discussion in 'Diesel Engines' started by girvin, May 28, 2010.

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girvinJunior Member

If a full displacement trawler burns 5.5 GPH @ 8knts is there a formula for it a 7knt or 6? 50ft lwl 14.5 beam DD 8v 71

Thanks!

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LandlubberSenior Member

No, too many variables for any formula to be accurate. The engine manufactures fuel flow chart should give some indication, but you will need to know the rpm at 8 knots to be able to even start interpolation.

3. Guest62110524Previous Member

then for every 50shp approx 2 imp gals per hour

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girvinJunior Member

Thanks guys I will find out the rpm. I am just trying to see if this will be a good boat for a passagemaker conversion.

5. Guest62110524Previous Member

oh heck just saw 8v 71, double that!!
you should look, for an engine , that will get you i nautical mpg,
betterr to repower with something like a John Deere Whilst I was with Cummins we had an 8v71 on the dyno with a 903 cummins, lugged back the 8 v was using twice the fuel

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Easy RiderSenior Member

Hard to believe one could push a 50+ foot boat at 5.5 gph w a DD 8-71. But if your numbers are correct The burn at 7 knots should be about 4.5 and at 6 knots about 3gph. That is IF the boats doing 5.5 at 8 knots. The DD should make about 15hp per gallon so that would indicate a power loading of about 75hp at 8 knots. Put that way it sounds more reasonable. The boat manufacturer should have a graph/curve that should show how much power is required to push the boat in question at a given speed. If you can give me the displacement I could approximate power requirements by comparing to some known boat/engine examples

Easy Rider

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girvinJunior Member

Well it was designed for trolling/light crabbing so its a fairly efficient hull. I will try to find the displacement its not my boat just looking at it. Ocean producer under fishing @ pacificboatbrokers.com Its not my first choice but its local and they might want to trade for my sailboat. I was on a 47 a couple of days ago burning 2.5 gph @ 7nts he had a gardner. I want his boat.

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jonrSenior Member

I take it that measuring it is out of the question?

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girvinJunior Member

if you mean the fuel burn I will if I get to a sea trial. I was just seeing if I could figure it out I know it drops alot with speed.

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FAST FREDSenior Member

A 50 ft lwl should run 7K with ease ,

Depending on the weight and loads most long range cruising is at .9 to 1.15 of the Sq Rt LWL, 7 in your case.

Frequently a 1k reduction will lower the fuel burn by 20% .

Detroit's work best when loaded , so you may find the boat does better with a "cruising prop, where more diameter (or pitch if there is no room for a bigger prop) will allow higher loads at lower RPM.

As low as 1200rpm is OK for long range cruise , nice and quiet with great low fuel flow IF loaded.

FF

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Easy RiderSenior Member

I was hoping you'd weigh in here FF. Do you think grivin's burn number could be correct? Gardner ..YES. There was a 58' boat on a brokers site .. Pacific someth'in or other. Looked like an old freighter.
It looks like (from my references) 40 - 50 continuous hp should be required for the kind of hull I think we're talking about. Three to 5hp per ton.

Easy Rider

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girvinJunior Member

So with a 20% reduction I would be burning 4gph @ 7nts I guess that would not be good enough. Would repowering with a 671 make a big difference? O want to get down to 2.5 -3 gph @ 7knts I have been on boats that do it but maybe they had a more efficient hull shape. I was going to buy a 45 troller with a gardner when my boat sells but these guys might be interested in a trade.

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Ocean Producer on Pacific Boat Brokers has a 325HP Lugger.......The Gardner will have the lowest specific fuel burn (gallons per HP hour) and the Jimmy the highest......

There's a lot more that goes into buying a boat than just the fuel burn....If you can get a boat that's perfect structurally for a low price then changing the engine becomes attractive and possible. If you can get a boat that's less than perfect but already has a Gardner installed then that's more attractive....Personally I think hull shape and structural condition are far more important than the current engine.....

But you haven't said what your use is for this vessel.....if you are living aboard and cruising BC/Alaska then fuel consumption probably doesn't matter as much as initial cost and structural integrity. If you are planing round the world voyages then face the fact that you need to either buy or install a Gardner...

Ocean Producer here.....http://www.pacificboatbrokers.com/details.asp?File_Number=LW379

This boat would be the shape you want for ocean cruising.......http://www.pacificboatbrokers.com/details.asp?File_Number=NW2484

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girvinJunior Member

Thanks tad I got the engine confused on 2 different boats. Yes my wife and I are planning to cruise down to south pac and hopefully circumnavigate one day. They boat will be full time live aboard and a surf exploration vessel I might be working out partial sponsership by taking some pro surfers to different spots here and there. I have just trying to get a idea about this boat bc they are considering a trade for my sailboat. Which has been difulcult to sell. It has been very well maintained and is a local boat everyone seems to think highly of it. I like the halibut schooner but 195k for a beat up work boat thats ridiculous. It gets the same burn as the producer. I know the foward cabin isnot Ideal but I am quite restricted on how we do this.

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