Pod boyancy design

Discussion in 'Boat Design' started by ddrdan, May 11, 2009.

  1. ddrdan
    Joined: Mar 2009
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    Location: Raleigh NC

    ddrdan Junior Member

    If a vessel takes 5" of draft loaded, will making a transom boyancy pod 8" or even 20" deep matter? If the pod will only submerse to the draft level, does it provide any more lift if it's added height is being held above water buy the vessel?
    The pod bottom will be even with the vessel bottom.

    I think it will help to stay a couple of inches above? But, won't the total draft line of the vessel (with the pods) be the max benefit a pod will give. Or does making it higher actually help?

    So when I'm calculating a pods 'actual' boyancy (64#/cf) should the calculation only include 'max draft' as input for the height when calculating the volume?
     
  2. Village_Idiot
    Joined: Oct 2007
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    Location: USA

    Village_Idiot Senior Member

    The pods are to reduce draft due to weight of the engine. On most outboard boats, the stern may be drafting 8 inches while the bow may only draft 3 inches. Addition of pods could bring the stern up to a draft of, say, 6 inches, depending on size of boat, weight of motor and size of pods. Pods are all about buoyancy, and the effect of lengthening the planing hull is a secondary benefit. If all you want to do is gain the lengthening effect, then trim tabs would be a cheaper, easier option.

    By mounting the pods higher, you are gaining a minimal step effect and allowing more bow rise, but losing the main point of the pods, which is static buoyancy, as well as some lengthening effect.
     
  3. ddrdan
    Joined: Mar 2009
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    Location: Raleigh NC

    ddrdan Junior Member

    The bottom of the pod is even at the bottom of the boat.

    What I meant was, if i make the pod 12"L x 12"W x 5"H that would be it's max effect for boyancy, correct? If the the boat, loaded, now has a 5" draft?

    If the pods (2) have 1 SF and I add 60#'s, the boat still has a 5" draft when at rest? Would that be correct?

    Making the pod 12"L x 12"W x 12"H would not add anything to boyancy would it? Because the pod is not going any deeper than the entire boat.
     
  4. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    Don't mix up bouyancy with reserve bouyancy. At rest, in still water, 5" has the same effect as if the boat was longer, if the boat has 5" of draft.
    Problems never occur with the boat at rest. The safety factor has to do with a plunging action. Jonny goes aft to find his tangled lure and you follow him to help and suddenly the boat is 12" deeper in the water at the transom.
    Granted, you also want a reasonable at rest draft too.
    12 x 12 x 12 is going to be worth about 50 lbs more in terms of bouyancy (at rest) if two are used if they are submerged 5" to begin with. But stand with Jonny in the stern and they will be worth 120 lbs more at 12" deep.
     
  5. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    What Alan said.

    True, your draft will remain at five inches if you add approx. 60 pounds to the rear of the boat. However, the reserve buoyancy of a deeper pod is nice, especially when carrying extra cargo/weight in the back half of the boat, towing another boat, adding a heavier engine in the future, etc.
     

  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Another benefit of pods is that they reduce turbulence at below planing speeds. It lowers fuel consumption and makes steering easier.
     
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