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Author Topic: Balsa instead of ply?  (Read 689 times)

Gazzalene

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Balsa instead of ply?
« on: April 11, 2019, 11:25:19 PM »

I am about to start my first ever build. After several questions on kit choice etc I decided to build from a plan.
Its only a basic "speed boat" but it will be a tester.
The plan shows the formers at 1/8th ply,skins 1/16th ply transom 3/16th ply,deck one piece 1/8th ply.
Now I am having a hard time buying ply other than mail order . I can get a variety of balsa from lhs and the 3/16th stringers.
Can I replace the ply formers with balsa? and perhaps the deck?
It will be easier for me to cut with hand tools and purchase of the material. I realise it will be less strong BUT I have no idea if its too weak.
I have seen builds on youtube with balsa and plastic boards etc.
I could add a few more formers.
The boat is called  "cobra 21" and found it online as free download, just printed the plan up on a4 sheets and cut the templates out all ready to go.
its 28" long and 8.5 broad, very old plan. says for .09 -.21
I will be using electric to suit.
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roycv

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Re: Balsa instead of ply?
« Reply #1 on: April 12, 2019, 06:09:13 AM »

Hi Gazz...The bulkheads and internal supports will be OK in balsa but choose a medium to hard grade.
I would stick to the thin ply for skinning the hull, that is the easiest.  However, you could use 1/8th. balsa for skinning and then treat with epoxy to give some extra strength.  I would plank vertically rather than along the length of the boat.

This looks a medium fast boat and you need to be able to survive some inpact damage.  The strength of the boat will be in the outer skinning of the hull.  The deck can be balsa, here use hard, and with a curved deck cut the balsa into planks and slightly chamfer the edges.

A tip here is if you need a hatch then use plank edges for the shape of the hatch and do not apply glue along the length of the hatch. This gives you a good fit.  You are only left with cutting across the planking to free the hatch.

 If you use thicker balsa for skinning you should deduct the extra thickness from the bulkheads or frames to preserve the shape of the boat.

I would think about 30 - 40 watts power would give a good turn of speed.

Hope this helps,
regards Roy
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Gazzalene

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Re: Balsa instead of ply?
« Reply #2 on: April 12, 2019, 10:34:29 AM »

Roy, thank you very much for the reply with help.
I am going to go the balsa way, I will try to finf suitable hardness of the balsa.
Can I just ask, the plan shows 1/16th ply for sides and bottom of hull.
The deck is totally flat ply,1/8th if I recall.
You mention to plank vertically and curve hull, are you saying that this to been done when using balsa rather than ply? altering the construction?
Thanks
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roycv

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Re: Balsa instead of ply?
« Reply #3 on: April 12, 2019, 11:54:25 AM »

Hi, I was suggesting you plank vertically along the hull sides with 1/8th balsa, it is also very economical.  In place of 1/16th ply.
Do not forget to cut an extra 1/16th off the bulkheads to keep the same outside dimensions.
If the deck is flat no problem.  You could make the whole boat from balsa but use epoxy to strengthen the hull so that you can get a nice finish.  Balsa is very open grain and needs a filler to give a good surface for painting or spraying.
With care a balsa hull will last, I have a boat I built over 50 years ago.
regards Roy

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Arrow5

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Re: Balsa instead of ply?
« Reply #4 on: April 12, 2019, 12:27:32 PM »

Your lhs should stock ply.  Why not stick to the plan ?
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Colin Bishop

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Re: Balsa instead of ply?
« Reply #5 on: April 12, 2019, 12:47:04 PM »

You will be lucky to get anything under 3mm at a hardware or DiY store. 1.5mm is all you need for ply side skins.
Most DiY store ply is of poor quality and not really suitable for modelmaking. It's either warped or full of voids or both.
Colin
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Gazzalene

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Re: Balsa instead of ply?
« Reply #6 on: April 12, 2019, 12:58:01 PM »

LHS does have Plywood BUT only in 12in by 12in.
I would need 36in lengths, they cater for aircraft in their wood stock.
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tobyker

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Re: Balsa instead of ply?
« Reply #7 on: April 16, 2019, 10:26:36 PM »

Double diagonal planking sides and bottom in 3/32 balsa, covered with aircraft tissue and Ezycote. Just my two penn'orth!
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DaveM

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Re: Balsa instead of ply?
« Reply #8 on: April 16, 2019, 11:08:48 PM »

I get all my timber by mail order and have done for years. What's the problem? If it's your first ever build then following the plan would be the sensible thing to do. Deviating from it will only cause problems for you.

DaveM
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Gazzalene

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Re: Balsa instead of ply?
« Reply #9 on: April 18, 2019, 09:23:02 PM »

Whats "lite" ply?
What would the difference be using same thickness?
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chas

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Re: Balsa instead of ply?
« Reply #10 on: April 18, 2019, 10:58:55 PM »

Hi Gaz, please may I offer some simple advice. As this is your first model, build it as the designer intended, they usually know what they are doing, and specify materials, grain direction etc for a good reason. It's certainly true that experienced modelers will substitute some materials  in a build, but this is usually based on hard learned lessons gained over time, and for specific reasons.
   Go on, bite the bullet and try mail order, they are very good and it ain't that hard.
Chas.

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DaveM

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Re: Balsa instead of ply?
« Reply #11 on: April 18, 2019, 11:13:56 PM »

Whats "lite" ply?
What would the difference be using same thickness?
Very briefly, liteply is laminated from a softish type of wood and uses a water-based adhesive in the process, whereas marine ply is made from hard birch and uses a waterproof resin glue. This makes liteply about half the weight of marine ply and a lot easier to work using hand tools BUT it's not as inherently strong, has a much more pronounced grain pattern and will quickly de-laminate if you don't waterproof it throughly. That said, I've been using it for a long time and never had any problems with it. However, as Chas says, the designer knows more about the way the model should be built than you do if only because he's built at least one more than you have. My advice is worth repeating  - don't make problems for yourself and only substitute materials if you really are unable to obtain the specified ones.

DaveM
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John W E

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Re: Balsa instead of ply?
« Reply #12 on: April 19, 2019, 09:20:34 AM »


hi there


After reading all the posts about you wanting to build a model of the speed boat Cobra; I went ahead and downloaded the plans so I could look myself to see what they were like.  Although they are a fairly simple plan to follow and build; it would be for a person with previous experience in the modelling field.  So, may I make a suggestion - if you look in the Masterclass builds on here there is a build for the Fairy Swordsman for the beginner.  Although the plans aren't free - they cost about 12 + postage from Sarikhobbies.com - there is a step-by-step building guide on this forum.  There are a good few people who have built this model and will guide you through any pitfalls and if you stick to the plans; and don't deviate it will give you a good grounding in building experience.   Obviously, if you go onto Sarik hobbies and look at the plans they hold, you will find there is another gent called Glynn Guest - he has done a lot of easy to build plans with a good deal of information such as write ups in model boats magazines and he has a good selection to pick from.  As far as purchasing materials Cornwall Model Boats do a good supply of timber at a reasonable price along with SLEC manufacturing - I have had lite ply and birch plywood and all manner of timbers from both parties and quite happy with them.   So, as has been stated in previous posts when building from a plan for the first time - stick to what is recommended on the plan as far as materials/processes go.  Thus minimising things that can go wrong.   


When you gain more experience in this particular hobby and building - then - that's the time to start experimenting and altering things.


That anyway is my two and a half pence worth or a tanner in old monies worth :-)


Happy building whatever you feel you want to do.


https://www.modelboatmayhem.co.uk/forum/index.php/board,139.0.html


John
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DaveM

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Re: Balsa instead of ply?
« Reply #13 on: April 19, 2019, 09:38:27 AM »

The original plans for my Swordsman design (as mentioned above by John Elsy) were given away free with the magazine; Sarik Hobbies have only the right to sell hard-copy. If anyone would like an electronic version of the two-sheet plans - in PDF format, to be printed out by your local copy-shop - then just send me a PM with your regular E-Mail address. These would be for your own use ONLY. SLEC will shortly be releasing a full kit for a totally new Swordsman design.
Dave Milbourn
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DaveM

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Re: Balsa instead of ply?
« Reply #14 on: April 19, 2019, 10:17:38 AM »

I forgot to say that although electronic copies of the plans are free I would expect you to make a donation the next time you pass an RNLI collection box.
DM
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Gazzalene

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Re: Balsa instead of ply?
« Reply #15 on: April 19, 2019, 06:19:01 PM »

Hi John, Thanks for taking the time to look at the Cobra plan,i am working with.
As you say its a good plan BUT not much info for the newb.
I went with that because I like the "speed boat" look with no cabin structure.
I will probably go with the suggestion of the huntsman build.
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Arrow5

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Re: Balsa instead of ply?
« Reply #16 on: April 19, 2019, 07:28:04 PM »

gazz, Fairy`s have a racing tradition, including Huntsmen. Put a couple of crew members on your model with orange coloured helmet, open face style, life jackets and some black race numbers on hull (one each side near front, one front deck). They competed in major offshore races with some success.
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