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Author Topic: Sheet Balsa Model Construction  (Read 454 times)

GG

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Sheet Balsa Model Construction
« on: November 03, 2019, 01:10:19 PM »

Having come into this hobby via model aircraft, it was only natural that my first models drew heavily on aeromodelling experience and used sheets of balsa to make the hulls, After all, they did look similar to aircraft fuselages!


The first few models worked OK but the need to build watertight hulls and still maintain good access inside them was not easy.  This lead to developing methods which solved these problems.  The result was incremental changes and new ideas which were tested and, if needed, "debugged" through some 40 years worth of models.


No claim to being perfect but with a little cunning you can create good shapes.  Sometimes I've even had to open up my models to prove to disbelieving people that they are just built from sheets of wood.


The two methods I use will be described in the following notes. These will take some time to post on this forum so it might be best not to reply before they are complete.
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GG

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Re: Sheet Balsa Model Construction -Part 1
« Reply #1 on: November 03, 2019, 01:24:21 PM »



Side Sheet Method
This method suits relatively slim hulls such as those used in warships and other fast vessels. The basic hull form is shown in Fig 1.


Design starts with the side view of the vessel on which the model is to be based, Fig 2.  The hull sides are cut to match the shape of the vessel but the their depth is reduced by the thickness of the hull bottom sheet (see Fig 1)
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GG

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Re: Sheet Balsa Model Construction Part2
« Reply #2 on: November 03, 2019, 02:07:24 PM »

Using a plan view of the vessel, a frame of balsa strips is glued to the balsa sheet which is to form the hull bottom, Fig 3.  Note the width of this frame is the width of the vessel at this point less twice the thickness of the hull sides.  The longitudinal strips match the length of the hull sides along the bottom edge from the bows to the up-sweep of the sides.


The transverse strips are positioned to match the bulkheads. The bulkheads are the height of the hull sides at their positions less the thickness of the strips running along the inside of the hull sides along the top of each bulkhead and the thickness of the deck sheet, see Fig 1.   The bulkheads then being glued to strips and hull bottom sheets ensuring they remain perpendicular to the bottom sheets, Fig 4.
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GG

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Re: Sheet Balsa Model Construction Part 3
« Reply #3 on: November 03, 2019, 02:14:37 PM »

The hull sides are then glued to the bulkheads, bottom and balsa strips.  The mid part of the hull is often parallel sided and I usually start there, then pull inwards and glue the sides together to create the bows.  The stern is formed by pulling the sides inwards and gluing the transom between the sides, Fig 5 and 6.
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GG

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Re: Sheet Balsa Model Construction Part 4
« Reply #4 on: November 03, 2019, 02:20:20 PM »

The stern is completed by gluing balsa sheet to the rear edge of hull bottom, edges of the sides and transom.  After which the internal junctions between the sides and rear bottom are reinforced with balsa strips, Fig 7.


The hull section at this stage looks like that shown in Fig 8.  Overall it might not look much like a boat hull but the next stage changes that.
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GG

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Re: Sheet Balsa Model Construction Part 5
« Reply #5 on: November 03, 2019, 02:27:00 PM »

The excess balsa is removed from the hull bottom sheets to produce something more boat like, Fig 9.  The corners of the hull are rounded off, I use a razor plane to remove the bulk, but not so much as to weaken the hull, then sand smooth.  The bows need some reinforcement and are sanded flat before gluing a suitable strip of harder wood in place.  This bow strip is then carved and sanded to blend into the hull shape, Figs 10, 11, 12.
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GG

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Re: Sheet Balsa Model Construction Part6
« Reply #6 on: November 03, 2019, 02:35:26 PM »

So far only a simple flush deck type of hull has been described but this method can accommodate other types as shown in Fig 13.  It is important that any changes in the top edge of the hull side sheets occur at bulkhead positions.


Different bow shapes can also be made by either cutting the bow edge appropriately of inserting suitable pieces of balsa, Fig 14.  A bulbous bow can also be created by this method, Figs 15, 16. 
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GG

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Re: Sheet Balsa Model Construction Part 7
« Reply #7 on: November 03, 2019, 02:49:43 PM »

One handy feature of balsa sheets is that they can be bent into shapes you desire.  Admittedly not extreme shapes but enough for the effect you desire and flare in the bow areas can be created, Fig 17.


The use of a large G-clamp might seem excessive in Fig 18,  but it was used in a controlled fashion to force the sides sheets to maintain contact with bulkhead and balsa strips whilst the glue set.


Some vessels, noticeably warships, feature a "knuckle" in the bows to deflect water.  This can be added with triangular balsa strips glued to the hull sides.  Careful application of a sanding block can blend them fore and aft into the hull sides.  I usually use some filler to blend the lower edge into the hull, Fig 19.
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GG

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Re: Sheet Balsa Model Construction Part 8
« Reply #8 on: November 03, 2019, 03:18:18 PM »

Making a water tight deck opening which still allowed quick and easy access inside the hull was an early problem.  In the end a compromise was made and a "water resistant" method was devised that holds the removable deck sections securely yet still allows their quick removal.  The idea is shown in Fig 20 where a balsa strip framework on the underside of removable deck has to fit snugly in the strips glued around the inner edges of the side sheets and across bulkheads and transoms.  The edges of the deck being trimmed to fit inside the side sheets so the deck is flush with the top of the hull sides.


This method is not hard but needs care!  First the balsa frame or "plug" has to be made to be a good fit into the hull.  The only way to do this is to build it inside the strips into which it must fit, Fig 21, without getting glue in the wrong place!


Before removing it a useful tip is to mark the upper surfaces of this frame.  Likely as not it will only fit properly one way into the hull, Fig 22.  If it will not side smoothly into place then light sanding can be applied but on no account should it be a loose fit.


The frame is then glued to the underside of the deck sheet, remembering to apply glue to its upper surfaces, Fig 23.  Then deck piece is placed on the hull and the frame will partially slide into place and stop when the excess deck sheet contacts the top edge of the side sheets.  Careful trimming to remove this excess can produce a neat line when the deck fits flush, Fig 24.


If you do remove too much from the deck edge then just glue a suitable strip of balsa along the edge and try again.
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GG

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Re: Sheet Balsa Model Construction
« Reply #9 on: November 03, 2019, 03:31:16 PM »

Well that covers the basics of the Side Sheet Method.  It's worth pointing out that whilst I've referred to using balsa sheets, the basic ideas can be used when working with thin sheets of plywood or liteply.


If anyone doubts the durability of models built this way, I'll close with a photo 4 model warships.  This is a series of Roy Navy warships started many years ago.  As for the success of this method of building?  Well, their ages range from 10 to 47 years and all still sound and in working order!
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GG

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Re: Sheet Balsa Model Construction
« Reply #10 on: November 03, 2019, 03:54:41 PM »

Now onto the other way to use balsa sheets,
the  Plan Sheet Method Part 1


As the name suggest, a plan view of the vessel is used to build the model.  This is often the best way for more "beamy" vessels where long side sheets might be reluctant to accommodate the bending required.


The basic hull construction is shown in Fig 26.  The difference being that side sheets will be glued directly to the edges of the hull bottom and deck.  I usually find it convenient to add the side sheets with the grain running vertically, that is from deck to hull bottom.  This will take up hull curves better and as a bonus it can make use of all those odd scraps of wood I'm loath to discard!


Using the plan view, the size and shapes of the hull bottom and deck pieces are found remembering to reduce them to allow for the side sheeting.  The bulkheads width is also taken from these pieces, their height coming from the matching side view and remembering to allow for the thickness of the hull bottom and deck, Fig 27.





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GG

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Plan Sheet Method Part 2
« Reply #11 on: November 03, 2019, 04:05:52 PM »

The bottom might need cutting to create the stern shape.  The following assumes this is the case.


The stempiece and bulkheads are glued to the hull bottom, ensuring they are upright, Fig 28.  A handy tip is to place the last bulkhead so that it slightly overhangs the edge of the hull bottom.  This creates a ledge against which the stern bottom piece can fit when it is glued in place.


When the glue has set, the deck is added and glued into place.  After which the stern bottom piece, any more bulkheads and possibly a stern piece can be added, Fig 29.
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GG

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Plan Sheet Method Part 3
« Reply #12 on: November 03, 2019, 04:20:32 PM »

If the vessel does not have a flush deck, then the deck piece can be made as before and then cut transversely to match the desired profile, Fig 30.  Some of the decks will now need strips gluing across bulkheads to support them when assembling the hull.


Once again, this method of construction is not limited to balsa.  Fig 31 shows a model being built out of "lumber! and gave me the satefying experience of being able to hit it with a hammer whist driving nails in to supplement the glue. Some flare can also be incorporated even when using plywood for the sides, Fig 32.  But, with plywood being stiffer than balsa, it needed lots of encouragement and quite a few nails to achieve!
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GG

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Plan Sheet Method Part 4
« Reply #13 on: November 03, 2019, 04:29:07 PM »

With some models, a simple up-swept shape at the stern just doesn't look right.  An answer is to make the hull bottom piece in two parts and the stern part being fitted above the main one, Fig 33.  After the side sheeting is glued in place, some extra balsa strips and suitable filler can be added to this area to create a less angular shape, Fig 34.



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GG

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Plan Sheet Method Part 6
« Reply #14 on: November 03, 2019, 04:40:25 PM »

So far I've only talked about making displacement hulls but, this method works well for the "Hard Chine" types.  The difference being that the angular hull sections needed are made by moving hull bottom piece inside the hull and making it narrower than the deck piece, then gluing a Keel under the hull, Fig 35, 36.


This adds some more twists to the side and bottom sheeting, but balsa seems to cope with it, Fig 37.
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GG

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Plan Sheet Method Part 7
« Reply #15 on: November 03, 2019, 04:55:25 PM »

Access into the hull cannot be by the same method as used in the Profile Sheet Method since the deck is a structural part of the model.  The simplest method is to used a removable hatch fitted over coamings fitted around openings in the deck, Fig 38.


Cutting these openings out of the decks before starting to construct the model or after construction is your choice.  If the hull structure will be robust enough to allow the deck opening to be made before starting construction, this may be the easiest way.  However, I have built a few models where holes in the deck could have been embarrassing weak pints during building and so cut them out later.  If you go this route then make sure no glue is applied to the top of the bulkheads beneath the hatches!


Another tip is to build detachable superstructures directly around the coamings. This will go a long way to producing secure and water resistant fit, Fig 39 
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GG

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Re: Sheet Balsa Model Construction
« Reply #16 on: November 03, 2019, 05:01:45 PM »

Of course there is always the model that is a bit of a mongrel and uses both methods of construction.  This was the case in a simple canoe type of model, Plan method at the bottom and Profile method at the top, Fig 40.  This gave me a simple rugged hull but with access to all parts.  It worked and "Bert" and "Ernie" seem quite happy paddling about, Fig 41.


Glynn Guest
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terry horton

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Re: Sheet Balsa Model Construction
« Reply #17 on: November 04, 2019, 09:01:42 AM »

An absolute boon to new modellers in balsa.
 I've made a few successful models based on Glynns' Model Boat mag plans  and all were a great success! :-))


Regards
Terry H
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redpmg

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Re: Sheet Balsa Model Construction
« Reply #18 on: November 04, 2019, 09:12:10 AM »

How would you go about a multi chine hull Glynn - the only way I could think of  is make the basic assembly as you suggest , then chop a bit off between the side & bottom sheeting - put in a bit of reinforcement - sand to shape -  then plank the open bit . Don't think it would be an option to put in yet another sub deck as you effectively do with the single chine hull........Thanks for all the ideas and designs over the years- great inspiration.
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GG

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Re: Sheet Balsa Model Construction
« Reply #19 on: November 04, 2019, 10:26:43 AM »

redpmg,
           The sub-decks do not have to be a solid one piece items.  You can cut away all but the outer edges of the sub-decks.  I have done that with fast models, like torpedo boats and similar, where it was essential to keep internal items (battery, motor, etc) as low as possible.
 Hope this helps.
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GG

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Re: Sheet Balsa Model Construction
« Reply #20 on: November 04, 2019, 01:20:37 PM »

redpmg,
             Knew I had a couple of photos somewhere to illustrate this method, this ought to make it clear.
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GG

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Re: Sheet Balsa Model Construction
« Reply #21 on: November 05, 2019, 10:23:32 AM »

Just found a few photos to show that the Plan Sheet Method is not limited to simple "balsa box shapes".  More realistic and maybe pleasing hull shapes can be made.  It's a little more demanding but still quite easy, at least once you have figured it out!


Glynn Guest
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