Calculating fuel consumption at various speeds

Discussion in 'Boat Design' started by Carr, Mar 3, 2009.

  1. Carr
    Joined: Mar 2009
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Veracruz Mexico

    Carr Junior Member

    I am new to the forum. From browsing here and doing several searches I ve been unable to find the information that I need.

    Im currently considering the purchase of a 60' steel pilot boat. Intended use is for a fulltime liveaboard and coastal cruising. I m trying to calculate theoretical fuel consumption curves at various speeds. If anyone can assist me by explaining how to do this it would be deeply appreciated.

    Vessel description is as follows
    LOA 60'6"
    LWL 56.4'
    Beam 17'6"
    Draft 6'
    Displacement app. 125,685 Lbs

    Fuel Capacity 720Gal
    Potable Water capacity 220 Gal

    Propulsion is by a single 250 HP Gardner 8L3B 8 cyl 4 stroke diesel.

    I have a couple of different numbers for published consumption.One figure is 6.6 GPH @ 10 knots. Another figure I have is from her operating logs which seem to indicate that over the last 2500 hours of operation, she averaged 4 GPH at varying speeds.

    Given that she is a double ender, full hull and with with her beam carried fairly far out to the ends, what I am trying to do is determine what type of fuel efficiency could be expected close to square root of LWL which would be around 7.5 kn. I do not have any offsets or lines drawings with which to figure wetted surface.

    Additionally, from the 2500Hr/4GPH it s impossible to know at what speeds the 4GPH average encompasses. For example what part of this time is spent idling at the dock?? Considering her usage it might be assumed that she spends most of her time at 9-10 Kn, delivering pilots or bringing them back from an offing.

    Any assistance with this or a pointer on where to look would be deeply appreciated. From her published specs and from my budget, 6.6 GPH at 10 kn is a lot. Thats why I am attempting to determine whether she might fit for my needs if run closer to square root of LWL and what those figures might be.

    Warmest regards to all and thanks for the great forum.
    Carr
     
  2. Tad
    Joined: Mar 2002
    Posts: 2,309
    Likes: 192, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Carr....

    Sounds like a very cool vessel...fine engine. The good news is that this engine has one of the lowest specific (fuel/hp/hr) fuel consumptions available. Smart man Mr. Gardner....

    If you do buy the boat do your own fuel consumption tests, you have no idea how the figures you have were arrived at. They might be accurate, or they might be wild guesses pulled out of thin air.

    How accurate is that weight figure? What load condition does it represent? Is that full load or empty...we don't know and it does make a difference. There should be a stability booklet for the vessel with true figures for various load conditions.

    4 gph is roughly 82 HP, that should (at the weight you supplied) push her at about 7.5 knots...maybe. The 6.6 gph is about 135 HP which should get you around 8.5 knots.....again maybe. The full 250 HP (Max RPM) will burn 12+ gph and give you 9.5+ knots...perhaps.

    Full trials of your own is the only way to find out for sure. To do this you need a measured mile course and an accurate way to meter fuel consumption.

    Good luck....
     
  3. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 149, Points: 63, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    Hi Carr,

    Welcome aboard tyhe forum. Hopefully you'll stick around and keep us updated on the project :)

    I'm in agreement with Tad on this. Being a former pilot boat I'd hope the logs were reasonably accurate, but as you said you don't know how much fuel was burned at what speed, etc. I think you'd have a hard time coming up with theoretical values that are any more accurate than the measured values in the logs, at least not without some fancy CFD work. No, the way to get fuel consumption numbers is to put a flow meter in the fuel line, mark off a measured mile, and run it both ways at a few different speeds.
     

  4. Carr
    Joined: Mar 2009
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Veracruz Mexico

    Carr Junior Member

    Tad, Marshmat

    Many thanks to both of you for the kind replies. I understand completely the necessity for measured mile trials and have assisted with them in the past...... I remember once using an approximately 5 gallon container that we put the burn and return lines in as a way of keeping an eye on the burn as accurately as possible. I also think (not sure on this one) that VDO makes an instrument that measures this as well, panel mount thing. I only assume that it has flow sensors in both the burn and return lines. Also that its probably pretty costly.

    The information contained in fuel burn figures is I think pretty fascinating, I would guess it says a lot about an individual boat, engine health, slickness of a hull, how easily she slides through the water and so forth. In my last boat, a 48' aluminum ketch, I had a 50HP Perkins (4-108) and a Hundested variable pitch prop. I bought her in Bay City Michigan, sailed her from there through the Macinaw and down to Chicago. In Chicago we unstepped the masts and did the rest of the trip to Mobile Al. under power. Habit from my old merchant marine days, I kept accurate logs. At the end of the trip (which took the better part of a month of hard going, underway time) we averaged .8 gallons per hour. Eight tenths. Calculated from engine run hours vs. fuel burned. I was amazed. Granted we had current help on part of the trip but several areas most notably the Ohio River was upstream as I recall. Still pretty amazing....that engine would push that hull between 5 1/2 and 6 knots in calm water, that was about it...if you tried to go any faster the exhaust started to dirty up and cooling water temp started to rise so you had to be careful....

    Tad, I very much appreciate the figures on theoretical fuel consumption provided here.As for the questions on the published displacement figures I dont have any idea of where they came from nor whether at those figures, the ship is wet or dry as far as tankage goes. Im trying to be very careful with this, moreso than usual. Reason being that this boat is being auctioned off by the pilot authority. Instead of the offer/sea trial and survey/accept or reject procedure thats normal in a boat purchase, you do your diligence, make an offer and if you are the high bidder then its yours.No seatrial and I dont think they will make any provisions for a survey either. So no chance to do any measured mile stuff as part of a trial..... There is a recent audioguage (from November of last year) as part of her continued in class inspection necessary every year, and reportedly they drew the tailshaft to check for clearances as part of this along with painting and so forth. The pilot association reportedly keeps very thorough records of everything as well for what thats worth.....

    Additionally all of this is a half a world away from where I am, in the British Isles. I ve talked pretty extensively to the broker over there as well as a marine surveyor who is not associated with any of this. I was amazed at how much slippage (haulout) cost on this size boat...just for a trip out on a railway and back in in 2 days time its between $2000 and $3000. For a crane capable of lifting this its even more. there are drying out stands there where one can take advantage of the tides if one doesnt mind 12 hour shifts....but either way it seems a lot more costly than what Im used to- even more surprisingly in the entire British Isles there do not seem to be any travel lifts that could pick this boat up.....

    On the back side of all of this there is the possibility depending on how the bidding goes, that one can buy a lot of boat for the money. She has a huge amount of room below decks and several of the spaces would need conversion, since she isnt designed with berthing spaces. Additionally if one can afford to operate her then one could within reason go pretty much anywhere one wanted, unlike some of the glass sided gin palaces that pass for production power boats...she is also a lot more trawler than most of the trawler yachts available on the market today with 5 W/T compartments. Of course the North Sea /Bristol Channel pilots have to routinely come and go in weather that would be considered heavy to extreme in many parts of the world so one expects her to be well hung together.....I ve pretty much done the drawings on what I would do with her different compartments to change her to suit my purposes. The negative factor is her fuel burn coupled with the whole economic climate in the world right now. Im not so sure that back to a sailing vessel might not be the wise thing for my next boat, but the wind is not free by any means...... I had a 54' aluminum cutter for a number of years and when I bought her in San Diego in 1994, she needed re rigging. By the time I had the mast painted (the only part of the job I did not provide labor for), new wire and line aloft, a bat car system and a fully battened main, I had the better part of $30 K in everything above deck.One is looking at replacing standing rigging on an offshore vessel at a maximum of every 10 years so figure the rig, provided you take care of the sails is going to cost you $3000 a year, to sail. So thats according to current fuel prices in the UK, around 300 hours of operation in this pilot vesseland that does not count the cost of engine rebuilds and so forth, doesnt include haulouts (on either the pilot vessel or the aluminum cutter) or any other cost of maintenance. Just getting the boat from place to place.... enough of my rambling though maybe this thread will be of benefit to someone else as well.... Tad can you recommend either a website or books with reference materials on things like resistance curves for various types of hulls, where fuel burn figures (either lb/ bhp/hr or lb/shp/hr) are published for different engines, and approximate formulas for figuring out things like resistance??The information you ve provided here is invaluable and it looks like particularly now since we seem to be in an atmosphere of perpetually rising fuel costs,invaluable to any boat owner, either power or sail.....

    Regards and thanks again to Tad, Marshmat and everyone else here for a fine forum. Ill keep you updated on what I decide to do on this one, whether I bid and how it goes....

    Carr
     
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