Where to foam this foam boat?

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Nov 9, 2022.

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    This is a corecell boat, hand laid on male stations. It will host or is hosting a Nanni 200hp or so diesel.

    I told a friend he ought to do some foam and he asked for guidance on where. Fuel tank is going aft in the big center spot.

    We already did some calcs and the goal is to keep the boat upright aft near the engine in any sinking event. The back alone is a bit insufficient, so the boat might float, but barely, so foam might save occupants and engine. Meditteranean location, for curious.

    2934E0D9-B8F1-48C9-8F31-4E73A8464A63.jpeg
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The foam should be above the bottom. If the center of buoyancy is too low, the boat will capsize when flooded. The sides are the ideal places. A bit on the bottom is OK because the engine will balance it out.
     
    VinTin likes this.
  3. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    As Gonzo said the foam should be high enough to float the engine in an upright attitude. You've already got a lot of flotation built in (the Corecell) So you really only need to support the machinery and persons. The hull should support itself. How big is this boat? What does the engine weigh? here is a link that can help you. Boat Building Regulations | Level Flotation https://newboatbuilders.com/pages/flot2.html (level flotation)
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    The engine is about 650 lbs from memory. I did some calcs and it seemed that the boat would sink pretty far on the aft end and perhaps roll before finally floating.

    26' boat with cuddy cabin
     
  5. Ike
    Joined: Apr 2006
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    Ike Senior Member

    When an inboard boat gets up to 26 feet or more, it takes a lot of flotation to achieve level flotation. It can be done, especially with a boat made of what is essentially foam. That's one of the reasons why inboard boats are not required to have level flotation, just basic flotation (doesn't have to float upright) . Also that is why Federal rules apply only to boats under 20 feet in length. ABYC recommendations go bigger but they still only recommend basic flotation. Just a rough calculation says you need 10 to 11 cubic feet of 2 lb density foam just to float the engine. I didn't take into account that the engine weighs less when submerged which would reduce the amount of flotation needed to float the engine.
     
  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I think 11 cubic feet is about right. But I don't see any reason to go higher up the sides than say 18" or maybe really only 12" up off the sole because the boat is not going to sink much past a foot, so foam higher is just weight higher.

    abyc is irrelevant; this boat is in Greece; I recommended he add enough foam to offset the inboard, but wasn't sure exactly where.
     

  7. mudsailor
    Joined: Mar 2013
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    mudsailor Junior Member

    The reason the rules apply only on boats up to 20’ is that nearly all the accidents happened in boats under 20’……..so that’s how they wrote the rules back when.
     
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