Small wood vs big steel

Discussion in 'Boat Design' started by girvin, Feb 7, 2011.

  1. girvin
    Joined: Dec 2009
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    Location: Vancouver Island

    girvin Junior Member

    So my wife and I are still on a hunt for a boat but we did finally sell our wharram and lost our *** on it LOL.

    I have a little question and I might not have enough info but we will try. I have looked at 2 boats for cruising the pacific and se asia and maybe going around some day.

    First is a 54 ft carvel planked ex seiner with a 6ft draft and 14ft beam. 1500g fuel tanks and 250 water. Burns 3gph at 7.5knts. Its reported to be very seaworthy and the hull is in excelent shape. The hold has already been converted.

    the other is a 75 ft steel buy back shrimper with 20ft beam and 8ft draft. it burns 12 gph at 9knts but I think it can be detuned and regaered for cruising I was hoping 9gph at 9knts. It does have a 14,000 g fuel tank and 2500g of water. It has enough room for four extra bedrooms for surf chartering at a couple of remote areas we will be going to. I have my own clientele that care more about surfing than being on a yacht. Yes I know you can't charter a boat in other countries legally.

    So they are the same price. The smaller wood boat is easier to haul out and has better burn rate. We will have to refeul in most countries bc of the small tanks. But the maintenence might be similer between the two?

    The steel boat has the room we want and the ability to do small cargo work at my home until we leave. We always anchor when we cruised on our sail boat. So slip costs don't worry me much. I like the fact that I could fill up once every two years and fill up in Venezuela or SE asia where there is cheap diesel.

    I know there is a huge difference in Size but I wonder over everything maintenence, Fuel and ability to bring more parts, ect if they would be about the same cost to cruise on. We are going to remote surfing atolls and reefs for scouting waves. We don't really do the normal cruiser thing. In the long run we want to start a surf boat charter like the ones so popular in Indo but in the SP.

    Sorry if this seems like a rambling post but two very different directions for us and all of our work and money. Sorry About the spelling and grammer....
     
  2. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    You can take steel to places overseas that wont allow a wooden vessel - eg Australia
     
  3. girvin
    Joined: Dec 2009
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    Location: Vancouver Island

    girvin Junior Member

    Wow I never knew that thanks. I am a deliverys captain and am used to these sizes I just and trying to figure out what price difference will be. I am leaning more toward steel if I could just get the burn rate lower some how.
     
  4. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    What is the engine, what gear ratio, and what is the propellor (you also want to know the injector size on the engine and the duty cycle).....then waterline length, beam, draft, and displacement if you can get it (travellift weight as a rough estimate) Draft and displacement will be subject to load condition........

    Only with this info can one start to make guesses about fuel consumption vs speed........also do not believe any consumption data given by word of mouth by owner's or brokers......only believe a sea-trial data sheet prepared and signed by the installation engineer....otherwise do your own tests.....
     
  5. girvin
    Joined: Dec 2009
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    Location: Vancouver Island

    girvin Junior Member

    Thanks Tad you always have helpful info I will try and get all that data together.
     
  6. girvin
    Joined: Dec 2009
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    Location: Vancouver Island

    girvin Junior Member

    prop is 60 gear is 6:1 cat 343 with 400 hp and might have n55 injectors. I think the waterline length is 70ft by 20ft beam and draws 8ft. This is all the info I have got so far.

    How much would it cost to pull out the engine if the burn rate is too high and drop in a cummins or a 671, 871 detroit? I hate how much cats are to work on but they seem quite nice. I have only driven one boat with a cat.
     
  7. girvin
    Joined: Dec 2009
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    Location: Vancouver Island

    girvin Junior Member

    anybody have the formula so I can try and get a idea of how low I could get the burn rate?
     
  8. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    Next to length and speed, displacement (the weight you have to push through the water) is the most important factor in fuel consumption. So we need to guess weight but haven't a clue because we can't see under water.........:D

    We do have some dimensions though....waterline length and beam, 70' by 20', draft is 8' but we don't know the load condition and that's partially just a plate keel (adding little to displacement). So we'll go with a canoe body (ex-keel) draft of 5'0" just for fun. A box 70 * 20 * 5 = 7000 cubic feet. Multiply that by an approximate Block Coefficient (Cb) of .35 (hard chine hulls) gives us an underwater volume of 2450 cubic feet multiplied by 64 for salt water equals 156800 pounds displacement....that might be in the ballpark though will be incorrect with 14,000 gallons of fuel aboard.......

    So a 70' waterline boat of 160k displacement will require around 90HP (at the propeller) at 8 knots (smooth bottom in flat water) or 4.5 gallons per hour at the very best specific consumption (not a 2cyl. jimmy). 9 knots requires 152 HP or 7.6 gph, and 10 knots 208 HP or 34gph........

    Without knowing the canoe body draft accurately we could be off by 50% or more........so this guess is worth about what you paid for it..........
     

  9. girvin
    Joined: Dec 2009
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    Location: Vancouver Island

    girvin Junior Member

    Hahaha Thanks Tad thats awsome, PM sent
     
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