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Author Topic: Balsa chine construction hulls, how far can I push the laws of physics?  (Read 5244 times)

tigertiger

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I am going to build a scale sail model of about 1m hull length, but I am also thinking of doing one of 1.5m+  length.
Chine construction.

I am thinking of using balsa, I would have real problems getting anything else in China.
Unless someone knows a source.

I was thinking of thinning epoxy and impregnation the hull inside and outside, then tissue on the outside.
Would this be strong enough on 1m hull.
Maybe the 1.5.

I could use glass mat (not tissue) inside in addition to this, if I can find mat here (otherwise I will import).
Do you think this will give enough strength.

I am open to suggestions (polite ones only please Dicky D ;))
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kendalboatsman

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Hi Tiger Tiger,

That should work well because you are creating a composite material and using the balsa as the filler. I cannot quote any technical reasons and I am sure someone  probably could. Once built the strength should come from the fibreglass not the balsa. I am only working at 18" length and using thin balsa with Glass Fibre Tissue, but my hull has taken on a much more rigid structure now it is covered.

I also built a one metre hull using 2mm thick planking and epoxied it inside and that became very tough and rigid once set. I didn't thin the epoxy I just warmed up both halves of the mix seperately until much more liquid then mixed and poured it in to the hull. I then brushed it in.

Clive
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boatmadman

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I agree with Kendal boat man.

On my current build, a 48" drifter built of 2 mm cedar (accepted cedar is much stronger than balsa), I epoxied inside with 2 coats and cloth, outside with one coat. It made a huge improvement to the strength of the hull to the extent that bulkheads arent really needed.

I have put some in but purely for internal fitting support and deck support.

So, In my humble opinion, I think what you are suggesting will work very well. I wouldnt thin the epoxy, just make sure its all nice and warm as Clive said, it make a huge difference. For the difficulties I had with the epoxy see my thread on steam drifter build. Warm epoxy solved all my problems.

You probably know this, forgive me if you do, but the hull must be completely grease free prior to epoxy, otherwise the cellulite effect may appear. That includes not touching the areas to be epoxied after cleaning.

Ian
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BobF

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Hi boatmadman,

They do use a balsa sandwhich construction on full size sailing and motor boats.

Bob
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tigertiger

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Would I get away with only using tissue on the inside?
Or only using epoxy on the inside?

For 1m?

and

for 1.5m?
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John W E

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 regitregit or tigertiger just the way I feel today my mate - all back to front  ;D ;D ;D

Hi there, building a large hull with balsa wood - couple of thoughts when I read your posting.    The only weakness, as you may know, with balsa wood - its prone to be a soft timber - dents/breaks easily.  By impregnating the balsa wood with epoxy/polyester resin, it does toughen the outer skin.  This allows you to work the material sanding and so forth quite easily.  By adding matting it does strengthen it - but when you plank the hull, if you double diagonal plank with balsa or even triple plank it, you are producing a form of plywood.   Grains going in opposite direction which increases the strength of the balsa wood immensely.   This would be a preferred method for me if I was making your project.    I would carry on the steps you are going to, fibre glass tissue on the outside and matting on the inside.

There are alternatives to tissue and matting which would aid the strength as has been mentioned on this thread - nappy liners as one.   Another material for adding in is the old net curtain or even silk. Admittedly, it doesn't give the same strength but, it does hold similar properties.

To answer BobF's query, yes, they do use balsa in racing yacht/speedboat hulls at 1:1 scale.  However, balsa  is cut into 1 inch square blocks and then laminated end grain on.  This allows the resin to penetrate the balsa wood blocks and form an immensely strong bond.   The only disadvantage is, when the hull is damaged, its extremely difficult to repair.

Hope this is of some help.

aye
john e
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tonyH

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Two thoughts.

Ronseal make a wood repair resin that is almost like water. Absorbtion is tremendous into really rotten timber and the result is rock-like.

The displacement of the big hull should be 3.75 (If my sums are correct) times that of the smaller hull, there should be plenty of scope for reinforcement by adding extra ribs etc.

Tony
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tigertiger

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Two thoughts.

Ronseal make a wood repair resin that is almost like water. Absorbtion is tremendous into really rotten timber and the result is rock-like.

The displacement of the big hull should be 3.75 (If my sums are correct) times that of the smaller hull, there should be plenty of scope for reinforcement by adding extra ribs etc.

Tony

Extra ribs. I like the sound of that.
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DickyD

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Ah so. But do they sell Ronseal in China ? ;D

Have to agree with you Tony the timber hardener is great, I have used gallons of it in the past. Its about the only rotten wood repair [along with the two part wood filler]
that most councils down here approve.

Richard ;)
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Shipmate60

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The old Dumas kit of USS Crocket was over 1m and made of balsa planks.
Mine was painted with epoxy resin inside and out and was fine.
As long as the keel and frames are man enough to take the weight balsa will be fine.
In fact my old Battle Class at 2m was built this way and she was fine too, so you should be ok.

Bob
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tigertiger

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Re: Balsa chine construction hulls, how far can I push the laws of physics?
« Reply #10 on: April 23, 2007, 03:48:08 am »

The old Dumas kit of USS Crocket was over 1m and made of balsa planks.
Mine was painted with epoxy resin inside and out and was fine.
Bob

Hi Shipmate
What thickness were the planks?
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Shipmate60

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Re: Balsa chine construction hulls, how far can I push the laws of physics?
« Reply #11 on: April 23, 2007, 07:33:57 am »

If my memory serves me right about 3mm I think.
When the epoxy soaks in from both sides the planks become extremely rigid.
The main strength was in the keel and frames.
The keel was 3 lengyhs laminated together and abutted like brickwork for strength. It was almost 1/2 wide and the frames were 2 x 2mm epoxied together.
This produced a very rigid hull with no flexing at all.

Bob
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MikeK

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Re: Balsa chine construction hulls, how far can I push the laws of physics?
« Reply #12 on: April 23, 2007, 08:31:17 am »

Hi Tiger, I have built 3  1m yachts using balsa - round chine admittedly but don't think it makes any difference. On all of them I cut narrow (about 1/8 ") strips of .8 mm ply and laminated them over the shadows 3 thick (the more the stronger, min 3) then planked over them. Ended up as a very strong and light canoe type hull. The first one I just gave umpteen coats of varnish and it worked fine until one day it got clouted broadside by another yacht - big hole - down she went  :o :o The others I used glass tissue outside and painted on resin inside and as far as I know they are still going strong. Decks were liteply. Providing the design is sound I wouldn't like to guess the limits of using balsa.
Cheers

MikeK
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