Tank Placement

Discussion in 'Boatbuilding' started by fishbuster, May 27, 2011.

  1. fishbuster
    Joined: May 2011
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    Location: Florida

    fishbuster New Member

    Redoing a 20' Dusky with a Yamaha 200 HPDI (I know not a seacraft) With a bracket and fixing to install a 100 gallon fuel tank and was wondering if i can get yalls 2 cents.

    Originally the boat came with a 60 gallon fuel tank but want to increase the fuel tank size to a 100 gallons.

    The new fuel tank is 72" L x 30" W x 10" H.

    The old fuel tank was 37" L x 33" w x 11" H next to the stringers and 16" H in the V.

    Some people was mentioning just to mount it in the original location just have the tank go forward from there. The original location was 48" from the transom.

    I know putting the boat in the water and start putting weight in the boat would be the best option but i would have to mount the bracket but still have to paint the boat. But if thats what it takes thats what i would do.

    But prior to tearing the boat up the boat sat stern heavy but also had a lot of wet foam, stringers, and transom. And a heavy dusky drive.

    I replaced the transom with coosa and replacing the stringers with sandwiched 3/4" plywood. also have a hermco bracket that supposedly has 800 lbs floatation (i know doesnt matter when on plane). The HPDI weighs a tad under 500 lbs.

    Also planning on putting a fish box up front of the boat and that would account for 200-400 lbs. Was thinking about measuring center line of the boat and moving the tanks center alittle under 2 foot beyond that. What would yall do if it were your boat?

    Thanks
    Randy
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The center of gravity of the tank will be different. A problem is that when you get up on plane, the fuel goes back in the tank. Tanks have variable centers of gravity. The new tank, being longer will have a larger variation. It is impossible to tell how that will affect the boat without some major calculations or testing. Can you install two small tanks instead of one big one?
     
  3. fishbuster
    Joined: May 2011
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    Location: Florida

    fishbuster New Member

    Gonzo,
    Ive already bought the new fuel tank so wont be able to exchange them or anything. Wish i would of gotten two 50 gallon tanks but hindsight is 20/20. Ive talked with a couple of ppl and they were saying to mount the tank where the old one ended and then go forward with the new tank. So didnt know if anyone on here that had some insight on this type of boating. thanks
    Randy
     
  4. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    It may be difficult to get up on plane. Also, it may bow steer.
     
  5. fishbuster
    Joined: May 2011
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    Location: Florida

    fishbuster New Member

    I forgot to mention prior to ripping up the boat when i had a 60 gallon tank under the leaning post I mounted an extra 30 gallon tank inside the cabin and didnt have a problem with it getting on plane or bow steer. The only time when i had trouble getting on plane was when i had a full fuel (90 gal) plus 250 lbs of ice, and 16 tanks with full dive gear but i still managed but my motor was mounted low prior.

    With that all in mind i believe if i mounted the tanks back end where the old tank was and go forward i think i should be ok. any input? also i am upgrading the livewell and instead of a 30 gallon livewell i have a 55 gallon on in the leaning post now.
    Randy
     
  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    From your description the weight forward should not be more. If there was no problem before it should be OK>
     
  7. leaky
    Joined: Sep 2008
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    Location: nh

    leaky Senior Member

    If that's a 100 gallon aluminum tank, look into having it cut in half and re-welded. I'm sure you can find someone to do it.

    Trouble with a single big tank is you end up toting around lots of extra fuel (and weight) when you don't need it. At least for me, the only thing I trust as far as knowing how much fuel I have is to see it piss out the vent.

    It sounds like you've got it right, just keep in mind you can overload a boat to the bow. I've got a 26 ft. Seawolf (Watkins) with twin opti 150's on a bracket that's got 180 gallons starting 5 feet forward of the transom. My long term plan is to cut it into two 60 gallon tanks (just don't need 180) and shift it back a few feet. Boat is very sharp in the bow, with a cabin, a hardtop, etc.. Sit's bow heavy at rest or low speed.

    Jon
     
  8. Mark Cat
    Joined: Jun 2011
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    Location: Michigan

    Mark Cat Senior Member

    Basic rule:

    If the tanks width is less than 20% of overall beam and the capacity of the tank is less than 12% of displacement, you can neglect the free surface (slosh) effect on pleasure-craft work. If tanks are wide athwartships, split into two separate tanks. Then look at CG relative to waterline in handling in relation to a low and high level in the tanks.

    For example, 20% and 80% full. Where is the tank CG relative to the old installation, fore and aft of craft CG and the new heavily loaded water line.

    Mark
     

  9. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The difference between 60 and 100 gallons is 2 adult persons sitting 48" from the transom.
    I guess they won't keep your boat from planing.

    Should that prove to be difficult, the next time don't fill it all the way....
     
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