33 gallons aft of midship creates a lot of stern weight 231LB?

Discussion in 'Boat Design' started by the brain, Jan 28, 2020.

  1. the brain
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    the brain Senior Member

    belly fuel tank.jpg


    Main objective is to reduce stern weight by relocating the majority of the weight before the midship (my red line indicating midship is about 4' behind actually midship )basically adding ballast weight reducing stern weight.


    The 22’ vessels waterline increased w/ the inboard outboard engine swap w/ this 230LBs being relocated off of stern how much waterline level will be increased by?I’m expecting a couple inches.


    My fuel tank situation is still a work in progress, Ideally I’d prefer a long belly tank.


    Vessel originally came w/ 2 saddle tanks 15 gallons each I was able to reuse the port side tank the star had rust inside I would like to try to remove the rust I purchased a rust remover tested it on some other rust which failed miserable.

    Additionally I use the star side fuel tank platform for start battery/oil reservoir& tackle box deck space is a good thing.


    Attach image of my temporary setup of the 3 aft tanks. The star is a under deck type I have inside a wooden box.

    The port saddle tank is original which I cleaned added new pickup tube and vacunm fitting. The 2, 6 gallons are reserve tanks.


    The under deck stringers are 25”s from where I’d like the stout ie: the aft of tank will be about 5”s high the front portion will increase to around 18”s so I’ll be removing white Styrofoam floatation from this area.

    Ideally I’d like to add underdeck floatation where the current fuel tanks are.

    Thanks for advice
     
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    If your boat has a ppi of say 400 pounds, moving 200 pounds from the stern to nowhere will lift the boat about 1/2" at the stern.

    If you consider the boat as a seesaw (it is not), then moving the 200 to the bow would drop the bow about half an inch and raise the stern about a half an inch.

    My expectation is about a half an inch of change.

    In order to really change the boat 2"; you must reduce the weight of the boat; not simply shift weights around.

    I recently did a diet on the stern of my boat. I removed 83 pounds and returned 37. My goal was a little less time in the hole.

    As for foam, wise to save ship n crew, but it adds weight, too.

    Moving that tank won't do as much as you hope unless I am way off on my ppi assumptions,
     
  3. the brain
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    the brain Senior Member

    what is PPI?

    I'll take a 1/2" wherever I can get it. remember I'm adding floatation under the dive platform so I expect more inches there also.

    I'm also interested in the slosh effect of fuel, like when the seasawing happens in high seas the force of moving aft ward fuel must have a negative effect on increasing the stern weight.
    Thanks
     
  4. fallguy
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    fallguy Senior Member

    PPI is a measure of the amount of weight it takes to lower your vessel one inch below the design waterline; at least this is my understanding of it.

    if your boat is low astern; you are overweight

    Adding foam to the dive platform will lower the boat unless the dive platform is underwater now.

    Foam adds weight!!!

    To determine the current ppi; a good approach would be to mark the waterline and then add 5 gallon waterjugs to the middle. Once it drops an inch midships; or half inch; you'd know the ppi.

    I also recommend you make a spreadsheet of all the weights onboard. You might be surprised and you might be able to move more forward. For example, a 15 pound anchor sitting in an aft locker that is rarely used can be moved forward.
     
  5. bhnautika
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    bhnautika Senior Member

    The brain forget PPI its the wrong metric you need Mt1 (moment to trim). This is a trimming problem not a sinkage measurement. A rough guide/ guesstimate of the waterplane area for planning boats would be LWL x Bwl x (.7 to .8 ).
     
  6. fallguy
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    fallguy Senior Member

    he remodeled the boat

    he can't modify lwl or bwl

    he may have overbuilt in the remode and he might need to ditch some 3/4 ply
     
  7. bhnautika
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    bhnautika Senior Member

    Fallguy you need the water plane area for the PPI or the Mt1, the LWL and Bwl are of the existing boat
     
  8. fallguy
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    fallguy Senior Member

    can't he just test it to see how much weight drops the waterline say 1/2" so he can come up with the weight loss plan?
     
  9. bhnautika
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    bhnautika Senior Member

    Fallguy yes he could but are we talking about reducing the weight of the boat over all or moving weights around to change trim, to maintain it or a combination of both ? Just a few measurements and you get insight into weights and distances.
     

  10. the brain
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    the brain Senior Member

    this isn't critical below water level at rest /trim situation until at high seas 4-5'.I actually have to trim up the engine to avoid it getting splashed. the engine is mounted at a normal height actually I've seen more vessels w/ lower engines than my height.
    besides the fuel tanks that's all the weighted stuff in the rear. I wanted to relocate the oil reservoir forward but found out it's not recommended also wanted the relocate start battery inside cabin and was advised against that because of battery fumes.
    here's a water level images.close to heaviest w/ completion of inclosure.notice the chin
    dive platform lowered water level to right at the bottom of the DP lower tube.
    here's some floatation I'm adding approxiamently 150LBs of positive floatation while adding less than 2 LBs dead weight.
     

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