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[Stu]

im just in the process of skinning the hull of my next project, and I'm trying to decide which grade of cloth to use as this is the first hull I scratch built.

[Crossie]

 My suggestion would be to use 100 gsm  or 200gsm 2.2 twill, with either polyester or epoxy depending on size/weight of hull

[Stu]

Thank you kindly

[inertia]

It depends on what you're using it for. For example, if you're skinning a plywood hull purely as a base for a finish then 100gsm is a bit heavy (I use 1oz/sq metre with Z-Poxy resin), whereas if it's there to add a degree of strength then Crossie is right.
DM

[Mark T]

I've got to agree with Inertia here - If its just for getting a finish I have used 25g cloth with Z-poxy resin with great results although even 25g cloth takes a lot of working to get it right.  Anything heavier is for strength.


Here's a link http://www.bucks-composites.com/products/25gm²-ultra-lightweight-close-weave-glass-cloth-sf-127-metre-wide

[Crossie]

  Yes you're right there, 200gsm would be a heavyweight skin, and here I was making an assumption that the hull base was soft/weak/large and maybe a balsa planked job or even foam, though even at this weight 2.2 twill will drape over any curve . I've used a  lot of 90/100 type cloth for skinning over ply , though for model boat use I would have thought 25 , which is more often used on foamie model aircraft, was a bit light for boat use where there's likely to be a bit of 'pond-rash', though it would give a good substrate for building a surface finish.

[John W E]

hi there Stu

Best really to let us know what type of hull you are going to skin - because - depending on the hull and how well you have made it, sometimes there is no need to add cloth.   Several times I have used Z-Epoxy coating, without any cloth or anything else and that has stood the test of time.

Basically, what are you trying to do:
- Strengthen the hull or
- Create a barrier - to stop water absorption - also to produce paint finish

My latest project - I myself used 18 gram cloth - along with ZEpoxy rubbed down to give a smooth coating to finish on.   Also, on the inside of the hull, I used Eze Kote resin to seal the inside.

   John

[Stu]

My bad on this one I'm currently in the process of building hms ocean in 1:72 scale she comes in at about 9 and a bit feet long I have built the ribs out of ply with pine strip strengtheners then using a mixture of ply for the sides and bottom and balsa for the curved areas. I want to generate some strength and give a nice smooth finish.

[John W E]

Hi Stu my way of doing this would be to reinforce the inside of the hull with say 1 oz chopped strand matting with polyester resin and on the outside I would give it 2-3 rebin or even revin or even resin  rubbed down and filled and then either tissue matt or the 18 gram cloth with polyester resin again - when this is all hardened off, rub down with wet n dry to achieve the finish you want.   I have done this several times and achieved very good finishes,.   Thing is remember - don't be tempted to add things on such as bilge keels, A frames for prop shafts or even sonar domes.   Keep the hull as bare as you can with no cut-outs until you have finished all your exterior work.   Then when you have done that you will obviously give it a coat of paint, which will show up any blemishes and you can rectify them; then move on to do the cut outs and install the running gear and so forth - but obviously after you have re-inforced the inside.

The reason I have suggested polyester resin, is, its a lot cheaper for the size of hull.

John

[Stu]

Thanks for the info john very helpful

[derekwarner]

Evening Stu.......a 1:72 scale HMS Ocean 

[Unless the batteries are flat in my 150 year old abacus.......you will certainly need some structural reinforcement and rigidity]

HMS Ocean  = 21,500 tonnes displacement
Model displacement = actual vessel displacement divided by the cube root of the scale
Cube root of 72 = 4.16
22,500 tonnes divided by 4.16 = about 5 tonnes

Having said this, the term displacement for vessels is a real minefield of variables . So to understand this would require extensive reading about HMS Ocean to see if you could find her net gross weight which may be as low as 1/3 of her actual displacement.......

Unfortunately physics and mathematics and scaling can be a little confusing for model builders 

Derek

[AlexC]

Evening Stu.......a 1:72 scale HMS Ocean 

[Unless the batteries are flat in my 150 year old abacus.......you will certainly need some structural reinforcement and rigidity]

HMS Ocean  = 21,500 tonnes displacement
Model displacement = actual vessel displacement divided by the cube root of the scale
Cube root of 72 = 4.16
22,500 tonnes divided by 4.16 = about 5 tonnes

Having said this, the term displacement for vessels is a real minefield of variables . So to understand this would require extensive reading about HMS Ocean to see if you could find her net gross weight which may be as low as 1/3 of her actual displacement.......

Unfortunately physics and mathematics and scaling can be a little confusing for model builders 

Derek

LOL Derek... have you been at the amber nectar again!!!

Model displacement = full size displacement x the scale ^3.

1/72^3 = 2.6791838^-6

To get model displacement in lbs: -

(21,500 x 2240) x (1/72^3)= 48,160,000 x 2.6791838^-6 = 129.0295lb.

Just a good shade lower than 5tons I believe.

Time to turn the batteries round in your old abacus me old mate

Keep happy.

Sandy.

[Crossie]

 Derek, my arithmetic tells me that 5 tonnes would roughly equal a displacement of 176 cu.ft of freshwater so would a model (aircraft carrier?) 9 foot long displace that much? That would surely be a very wide and deep hull.  Since my knowledge of Royal Navy ships is the square root of  zilch, I would have thought that perhaps 9 foot long model would displace a bit less than 5 tonnes ?

[Colin Bishop]

Surely, weight must be equal to displacement - or has Archimedes been wrong all these years?

Anyway, your result is out by a couple of decimal places. Result should be 0.06 tons or around 135lb. Calculation is 22,500 /72 /72 /72, about right for a 9ft model

Warships can have light, standard or heavy displacement depending whether they are fully loaded or not and this used to be quoted in reference books.

Colin

[John W E]

Stu

You are going to have to add a lot of lead acquired from somewhere  for Derek's calculations - build the model first, worry about the weight later - theory is one thing; making it work in the modelling field is another 

[inertia]

As Crossie implies, a classic example of following the maths and not thinking about the result. Is it reasonable that a model boat which is substantially smaller in volume than my car (which weighs about 2 tons) should weigh/displace 5 tons?
Nope.
It's like those Chinese speed controllers which claim to be capable of carrying 320A continuously, but use connecting cable which would melt at much above 50A.
Nope³
DM

[John W E]

I am going to get wrong here - Dave your car weighs approx. 2 ton - is that with you sitting in it after a steak dinner and 6 bottles of wine 

[inertia]

I lied.
It's 1440kg (1.4 tons).
With me in stated condition add 130kg (=1.54 tons).
So it's not 2 tons;
But it's certainly not 5 tons.

BGFTBGGR

[Colin Bishop]

Does your car displace half a garage Dave?

Colin

[Stu]

Wow that grew legs quick.  On the subject of weight it will mainly be batteries to bring her down but I read an article about how a gentleman had used old wine bags and a 12 volt pump to ballast a carrier which interests me quite a bit.

[AlexC]

Wow that grew legs quick.  On the subject of weight it will mainly be batteries to bring her down but I read an article about how a gentleman had used old wine bags and a 12 volt pump to ballast a carrier which interests me quite a bit.

Hi Stu,

Sounds like a good ploy, however, make sure you use Australian water as it is 86times heavier than ours
(at least it is by Derek's calculations)  which will save on space and number of wine bags required.

Keep Happy.

Sandy.

[inertia]

Does your car displace half a garage Dave?
Colin

Colin
Coincidentally it does, while Liz's car displaces the other half. Handy that, isn't it?
Bloobs
In the true spirit of experiment and pushing the boundaries of knowledge blah blah et cetera I'm going out this evening to have a steak meal and six bottles of wine. Do you know of a public weighbridge with an NG postcode which might be open on Sunday evenings? It must also be totally unknown to the rozzers...
DM

Is Australian heavy water H₂O_z? 

[AlexC]

Colin
Coincidentally it does, while Liz's car displaces the other half. Handy that, isn't it?
Bloobs
In the true spirit of experiment and pushing the boundaries of knowledge blah blah et cetera I'm going out this evening to have a steak meal and six bottles of wine. Do you know of a public weighbridge with an NG postcode which might be open on Sunday evenings? It must also be totally unknown to the rozzers...
DM

Is Australian heavy water H₂O_z?

Hi Dave,

I suspect that is about right... just be sure you don't drink Oz wine for your experiment since this will contain said H₂O_z, in which case it may overload the weighbridge.

Keep Happy.

Sandy.

[ballastanksian]

This is a fun topic and one that is also helping me with planning for glass coating a model. (Though I am tempted to try Ron Ress' tights and resin/varnish technique.

[inertia]

Sad to report I chickened out and had fish (if both are possible in the same sentence) and a pint of Abbott Ale. We may never know, John... 
DM