Weight distribution/balance for project houseboat

Discussion in 'Boat Design' started by rsimon, Feb 4, 2013.

  1. rsimon
    Joined: Jun 2011
    Posts: 36
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Land locked Florida

    rsimon Junior Member

    The boat is a Marinette "Rivercruiser" houseboat: 34'LOA, 12' beam, weight=7,ooolbs, but I've also heard 7,400lbs (maybe 7k is "dry weight." ) Draft 28" with a cruiser-type hull. With my project I removed the old Chrysler v8 and paragon v-drive and heavy rudder and shafts etc. I fabricated an outboard motor bracket on which a modest outboard will hang. I also built 2 shelves or platforms overhanging over stern. One shelf has a vented enclosure in which a generator will be (approx. 170lbs with fuel.) The other shelf will have a dedicated propane locker (with 2 propane tanks.) I also fabricated a support structure like a cage that supports a davit/crane lift which is located down inside the engine compartment just below where the davit lift will be. I also want to locate the main panels and batteries at the forward part of the engine room (between engine rm and interior. Water tanks will be located about mid-way along the boat (where utilities will generally be located.) Is it possible to get even a close approximation of how bad the stern will dip down with the added weight (or if it will dip at all?) I removed a lot of weight with the motor and 80 gal. factory gas tank, but added some after doing all that. That weight I added in the form of support brackets, gussets and braces as well as the shelving was obviously not part of this boats original design, but I am REALLY WONDERING about this. Is there a way to figure out how or even a computer program that will calculate how bad the boat will dip down (in the rear) with these design changes? What would I need from Marinette? The fulcrum point? (so called dadum point?) Would I need the area of the hull? the project can be viewed to better describe these things on the site:

    Thanks so much, Roger from Florida
  2. Submarine Tom

    Submarine Tom Previous Member


    Google "weight and balance"

    If you've already done the work, then what's the point?

    I only got through 55 of your 165 photos.

    That's a real fixer-upper!
  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    What you need to determine is the PPI ( pounds per inch of immersion) and MT-1" (moment to trim 1").

    To calculate PPI, use the water plane area (sq. ft.) and multiply by 5.34, which will give pounds per inch (10.25 for kg/cm).

    To calculate the MT-1", take the square root of the water plane area, multiply by .35 (or .68 for sq. meters), then divide by the waterline beam. This will give you a figure that is the ft. lbs., per inch (or kgm/cm).

    To figure the water plane, multiply the LWL by the it's waterline beam, then multiply by .8 (in your boat's situation). This will be the area within the water plane in square feet or meters, depending on what you employed.

    With these figures, you can estimate how she'll trim (rotate about her CB) and how much it takes to sink her down a specific amount. It would be nice to have an idea of her CB (center of buoyancy), which on your boat will be roughly about 55% aft of the forward end of the LWL. The distance forward or aft of this point is what will be the multiplier for trimming moment leverage. For example, if you had a perfectly balanced boat, teeter tottering on a piece of round bar stock and placed a 100 pound weight directly over the CB, the boat will weigh 100 pounds more and will sink down (trim) uniformly too. On the other hand, if this 100 pound weight was 36" aft of the CB, you now have 300 pounds of leverage over the CB, not 100 pounds, because it's 3' aft. This also will affect your trimming moment.

    Frosty would be pissed with this (my) post . . .
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