Is there a reason why Balsa is not

Discussion in 'Fiberglass and Composite Boat Building' started by Skua, May 16, 2013.

  1. Skua
    Joined: Apr 2013
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    Skua Senior Member

    used in transoms??? Say for stern drives?
     
  2. Ike
    Joined: Apr 2006
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    Ike Senior Member

    Balsa absorbs water easily. One hole or crack in the glass and waterlogged transom. Balsa is very easily crushed, so bolting an outboard to the transom will crush the balsa. Balsa makes a good composite structure for cabin tops and decks but anywhere below the waterline it is not a good choice.
     
  3. Skua
    Joined: Apr 2013
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    Skua Senior Member

    I knew about the water absorption, and knew it has been used everywhere else, including below the waterline in cored hulls, (Searay and others) with less than great results, I just wondered why they hadn't gone there with it too.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    As noted by Ike it is easily crushed when bolted, as its compression strength when compared to other woods, is poor.

    If you look at say typical marine plywood (Douglas Fir) its compression in the is 50,000kPa, whereas balsa wood is just 14,900kPa. A difference of 3.4 times. More than enough as a simple safety factor!

    D-Fir (marine ply) wood properties.jpg Balsa Wood properties.jpg
     
  5. PAR
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    PAR Yacht Designer/Builder

    Most loaded areas, within a cored laminate, will be devoid of a core, in favor of solid laminate or other high load tolerant product, such as plywood, G-10, metals or other denser material. Chain plates, engine beds and other highly loaded areas, will all be treated this way, so to answer your question, yes, you can use a balsa core in a transom, on a outdrive equipped boat, but the transom ring area will need localized reinforcement, to absorb the loads. This would also be true of the engine beds, which is often the case, as they land on the transom. This said, an outdrive doesn't place nearly as much loading on the transom, as an outboard or bracket mounted outboard would, so the transom doesn't have to be as tough, but it still will need the local reinforcements mentioned.
     
  6. Steve W
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    Steve W Senior Member

    In a perfect world highly loaded areas would be solid as you say Par but unfortunately it is rarely so, on most of the recoring work we do hardware is bolted straight through the balsa including the factory installed parts. There would be no reason not to use balsa in a transom as long as you do it like Par laid out.

    Steve.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I see this too Steve, but it's difficult to call many productions boats, well or properly built structures. Doing it right, is just that and anyone (qualified) that's worked up laminate schedules knows what's right. The question is, will the manufacture follow the schedule.
     
  8. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    All mounting holes drilled through a wood core should be made with an epoxy bushing to prevent water intrusion and rot. Unfortunately this is seldom seen on either factory production boats or those with engines mounted by dealers. The reason is cost, mostly in time, labor and training. There is no reason that an end grain balsa cored transom would not be adequate for a transom if done correctly but the devil is in the details. I have owned balsa cored decks and fully balsa cored hulls. All gave trouble due to less than adequate design and/or construction.

    Plywood or other solid wood cored transoms suffer from the same problems to a lesser degree than balsa. Easily prevented but seldom done.
     
  9. tunnels

    tunnels Previous Member

    I ask why would any one even think of bolting through a balsa core ??
    Would you bolt through a foam core ??
    balsa is one of the lightest woods around if you making models its what you would be using !!
    What about cedar ??
    would you bolt through that ??
    Bit of a silly question really !!
    But in saying that yes you can use Balsa or cedar or foams in a transom as well but not in the part where you carving a hole and through bolting sterndrives or mounting outboard motors !!
    2/3 of the transom is doing very little just the centre 1/3 is carrying a high percentage of the loads but its how you go about making the transom with the different materials that's the key to it being strong and water resistant !!And as for saying water gets in !! its only gets in because you not making a big enough effort to seal all and every hole you drill and fix things to the boat :)
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    You can bolt through anything you like, so long as you can account for the materials deficiencies. You can bolt through Switz cheese if you want, so long as you know in advance the cheese's properties and the load you're applying.

    So, in the case of Balsa v typical marine ply.

    If you had an M16 s/steel bolt, with a normal washer and torqued up the bolt to its max permissible, what would occur?

    Well, the max torque allowed would apply a clamping preload which in turn pushes the washer onto the wood, i.e. creating pressure on the surface. So with a standard washer the pressure would be 62.3MPa, or 62300kPa. This would crush the balsa and the plywood, from the figures above. As it is over their compression strengths.

    If the washers external diameter is increase by 10mm, the pressure reduces down to 28100kPa. So the plywood is fine, but not the balsa. You would have to increase the diameter of the washer to 60mm, then the pressure would be 11,000kPa, which is below the balsa's compression strength.

    Thus you can bolt whatever you like, you just need to know what are the limits of the materials you are using and then what load are you applying...and mitigate accordingly. Whether you actually would, is another question entirely.
     
  11. groper
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    groper Senior Member

    Yes, i would... so do many others...

    I know of many boats that use foam core transoms, and probably many that use balsa cored transom aswell....

    I wouldnt use 40kg/m3 styrofoam, nor 60kg/m3 divinycell... ill let you reasearch what i would use and how to use it, for yourself...
     
  12. Oldsaltoz
    Joined: May 2013
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    Oldsaltoz New Member

    The golden rule with balsa fittings /bolts/screws etc is to first drill the hole at least 20% oversize to start with, then use a tool to hollow out the balsa between the laminates (I use an Allen key in a drill).

    Now tape the underside or opposite side to the one you will be working from, make sure you leave a small hole in the tape to allow air to escape.

    Mix a batch or epoxy resin and hardener and apply a coat over the area to be treated. Add some Micro-Fibres to the remaining resin and mix to about toothpaste consistency and completely fill the hole starting around the edges.

    Allow 24 hours to cure then drill the required size hole for the bolt, adding some sealant to the bolt before inserting, tighten finger tight and allow the sealant to cure before nipping it up tight.

    Note: balsa will need some sort of backing plate in most instances to reduce point loading and to spread the loading over a larger area.

    I avoid the use of timber in any form, Aluminium or 316 Stainless is much better, particularly if you place a sheet a thin plastic (Cling wrap) on first the add a later of epoxy resin and Micro-fibres between the backing plate and surface or the ares being strengthened, you have an easy to remove backing plate that has the loading well distributed over the total area.

    Just make sure you remove the plastic and ensure the edges top and bottom of the backing plate are well rounded.

    Good luck and fair winds. :)
     
  13. T0x1c
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    T0x1c Junior Member

    Whilst the balsa is used bois-debout, for a plywood you would apply the compression perpendicular to grain.
    Balsa 14,900 kPa, Douglas Fir 6,000 kPa. This is the reason why balsa is used, there is no better weight/structural combination.

    Balsa is good for epoxy build bottoms -actually better than most foams.
    But not to be used with porous ester resins. When immersed, a polyester will take in water what it has lost in solvant -about 7% in weight. Plus fatigue/microfracture...
    .
     
  14. tunnels

    tunnels Previous Member

    what !!! this dosent make any sense !!

    A outdrive doesn't place nearly as much loading on the transom, as an outboard or bracket mounted outboard would, so the transom doesn't have to be as tough.

    __________________

    Ok I am interested in this quote !!
    What is the difference between a 200 hp outboard pushing a boat through the water and a 200 hp sterndrive doing the same work on the same kind of boat ???
    They are both mounted onto the transom and they both pushing the same weight of boat !! both are pushing on the transom !!
    It doesn't make much sense to me !!what am I missing ???
    Any one care to enlighten this old fella ?? :confused::?:
     

  15. alan white
    Joined: Mar 2007
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    alan white Senior Member

    The stern drive doesn't exert as much leverage as an outboard mounted higher up.
     
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