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Buying another SD - Ballast tank size for LA 688i

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  • Slats
    started a topic Buying another SD - Ballast tank size for LA 688i

    Buying another SD - Ballast tank size for LA 688i

    Hi guys
    Whilst I'm very familiar with the 3.5" SD (having deployed no less than 12 of these in kit / or ready-made fashion in mine and client turn key boats), I'm wondering about the 15Oz ballast tank capacity of the 3.5" Skipjack SD for the 1/72 688i - my next project. This hull is substantially longer than anything else I have built over the years. I'm figuring that it should be enough as US nukes tend to sit low in the water. I haven't yet got my 688i hull yet, else I'd weigh it and make some calculations. But was wondering if David et.al had more dope on whether it should be adequate.

    Thanks

    John
    Last edited by Slats; 10-04-2017, 05:28 PM.

  • Subculture
    replied
    Otw hulls aren't heavily laid up in my experience. I'm sure they could be thinner if laid up in epoxy and cloth glass as opposed to polyester and chopped matt, but the price would rocket. Sheerline/Eden subs are very thickly laid, I sometimes think they're designed for ramming. The laminates get to 4-5mm thick sometimes, which comes across as a bit excessive.

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  • Slats
    replied
    Thanks Andy... the heaviest hulls I have are my son's OTW Trafalgar and my OTW Valliant. But I do get what you're saying. Cheers John

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  • Subculture
    replied
    Unless the hulls are laid up like a tank, don't think you'll need much beyond 500-600ml. All GRP hulls are denser than water, how much so depends on the glass content, but my own experience tells me most hulls tend to be around about 1.2 times the gravity of fresh water, so if you use weight you'll probably have around 20% reserve capacity to fiddle with.

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  • SubHuman
    replied
    Sounds good! I'll keep my eyes open.

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  • Slats
    replied
    Thanks, David, appreciate it.

    Bob - might be an order for 2 SDs. I'll get Jim (from MSSA who does the hull to weigh it and the sail) and I'll get back to you. No rush. Cheers John

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  • He Who Shall Not Be Named
    replied
    Yeah, the 688's only had a reserve buoyancy of around 15%, so you won't need much ballast tank. Assuming sail and hull is GRP, simply weighing the parts -- the above designed waterline parts -- will tell the tale (GRP has about the same density of water). Same with the resin pieces -- the upper rudder and flushing tube fairing.

    Above waterline weight = ballast tank water weight. Then fudge it up 10% to play things safe.

    David

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