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[Sandy]

Hi,
Newbie to forum here, but a not entire newbie to model boating.

I have been reading many threads on here about building GRO hulls and the construction of olds or plugs and I am slightly confused. Most of the plugs seem to very elaborately done. Is this because they hope is that they can be used again and again?

Back in the late sixties, my father built a five foot six inch hull for a Queen Mary II from fibreglass using purlboard as the basis for the mould (but I guess the correct term is plug). He glued together six or seven 1 inch sheets and formed the shape of the hull then laid on the fibreglass and then simply hollowed out the purlboard and threw it away once the glass had cured (after hours of sanding down the outer glass). Is this no longer an accepted way of doing it, based on the sheer elaborateness of some of the threads on here.

My plan, for a Vietnam era riverine monitor, was to form the plug out of this:-

http://www.allscotltd.co.uk/product.asp?strParents=&CAT_ID=&P_ID=434

lay on the glass then hollow out the foam. The foam should be easy to work with and shape, if it is affected by the resin then it doesn't hugely matter as that is inside the hull. Am I missing something here?

Any advice greatly appreciated.

All the best
Sandy

P.S. If this is an acceptable method of doing it, just how much smaller should the plug be to allow for fibreglass thickness to return the hull to the 'correct' size?

[Drkomen86]

I think the main reason for making a plug and then a mould is that you can get an almost flawless finish over and over again.
Also you don't need to spend hours sanding down the horible fibreglass or adding lots of weight with filler to level off the surface.

Hope this helps,

Chris

[nick_75au]

Hi,
 I work in the fibreglass boat industry and have watched the evolution of drawing to plug to production model. Plug is used to make a female mould so many products can be pulled. It is then discarded.

The reason for am "elaborate" plug is so the mould formed off the plug is perfect, any imperfections in the plug will transfer to the mould and be replicated in every hull produced. the better the plug the better the end product.

There is nothing wrong with the "Male Mould" method for ONE OFF production of a hull, but you will spend as much time smoothing the hull as if you were building a plug as the rough surface becomes the eternal skin of the hull.

To save the effort of scraping out the foam, cover the foam (wood etc) in clear or brown packing tape, the resin wont stick to it then.

A reasonable lay-up for a model will be around 1-3 mm thick depending on the size so your plug should be reduced by this.

The polyurethane foam you linked to is not dissolved by polyester resins, be careful with shaping as it is unhealthy to work with. If you don't tape it then I believe that petrol will  dissolve the foam leaving you with a hull.(and a whole lot of sticky petrol to dispose of)

 Polystyrene foam is dissolved by polyester (but not Epoxy resins) The foam will dissolve before the resin has set leaving you with a sticky mess.

Cheers
Nick

[johno 52-11]

Hi Sandy,

What you have described it a perfectly acceptable way of making a one off hull but you will need to do a lot of rubbing down of the fibreglass and then add a coat or two of gel-coat and then more rubbing down. One way to stop the resin attacking the foam is once you have the shape you require give it a couple of coats of emulation paint.

The other way is to produce a plug or patten that is as detailed as possible and polished to a high gloss. you can then use this to make a mould in fibreglass. Once the mould is queried  it can be used to make multiple copies of the hull. A couple of things to note moulds sometimes need to be made in two or more parts to make it possible to get the hull out. also the mould should be made a lot stronger and thicker then the finished hull so if you have a problem getting a moulding out it will be the moulding that brakes first not the mould.

Hope this helps

John

[Sandy]

Thanks for the replies.

My terminology may be over the place as I assumed a mould was a hollow female boat shaped thing into which you put the fibre glass, hence I can see that it needs to be perfect and smooth so the outer skin of the hull is perfect, but I guess that you make the female mould by building a perfect male plug..

I appreciate what you are saying about the exterior finish of the fibreglass if using a male plug. I do recall my father spending hours/days in the cellar with his power sander getting the hull sanded down correctly.

John, when you say gel coat, are you meaning lay up the matt to make the hull, let it cure then gel coat it to get it smooth and finished?

The Vietnam era monitors are quite simple hulled things (glorified Mike 6 landing craft). The model 1:24 scale so is about 30 inches long. I could use plywood, but reckoned glass would be more fun   Maybe not if the sanding process is going to be tiresome.

Assuming I use tape to seal the polyurethane, is it a case of apply resin, apply mat, soak mat, resin some more, roll out to get rid of excess and wait. Would one skin of mat do?

I will go and see Allscot ( it is years since I dealt with them) and get some advice from them also.

Thanks again.

All the best
Sandy

[Sandy]

Also, I was thinking of fitting individual bar armour/buoyant sections to the monitor.

This means that hull is not a simple U-shaped but an inverted T-shape which makes the destroyable male mould more acceptable as I can leave some polyurethane in the under water bars of the T, if you see what I mean?

[gingyer]

Hi Sandy,
I was reading the thread where abouts are you
If you are going to allscot it is safe to presume you are near glasgow

We have a member who is strangely attracted to vietnam riverine craft he is building
the ATC(H) in 1/16th scale

You are more than welcome to come along to the club and the guys will help you if you are interested
and knowing Shug will be more than interested in your model

Colin

[Sandy]

Hi Colin,

That sounds good.

Whereabouts in Glasgow is the club? I am in Darnley.

All the best
Sandy

[gingyer]

the club is at Richmond park opposite shawfield stadium

here is the website http://glasgowrichmondmbc.co.uk/

[Umi_Ryuzuki]

Stryrene foam will melt when contacted by polyester resin, but is safe if using epoxy resins.

Polyurethane foam will be inert to polyester resin, and will work well with epoxy resin also.
However depending on how porous and the density of the foam you use, an intial coating of
paint or tape to prevent the foam from absorbing too much resin.

Here's a thread where I created a styrene foam hull using epoxy resin.
http://www.rcgroups.com/forums/showthread.php?t=551258

 

[nick_75au]

John, when you say gel coat, are you meaning lay up the matt to make the hull, let it cure then gel coat it to get it smooth and finished?

The Vietnam era monitors are quite simple hulled things (glorified Mike 6 landing craft). The model 1:24 scale so is about 30 inches long. I could use plywood, but reckoned glass would be more fun   Maybe not if the sanding process is going to be tiresome.

Assuming I use tape to seal the polyurethane, is it a case of apply resin, apply mat, soak mat, resin some more, roll out to get rid of excess and wait. Would one skin of mat do?

Also, I was thinking of fitting individual bar armour/buoyant sections to the monitor.

This means that hull is not a simple U-shaped but an inverted T-shape which makes the destroyable male mould more acceptable as I can leave some polyurethane in the under water bars of the T, if you see what I mean?

I think that one layer of 600 Gsm mat would be sufficient as long as you keep the power sander away, knock off the tops of the high spots only and use filler to bring the low spots up, you want to avoid exposing the glass fibres. Additively make sure you get resin with wax added, laminating resin is air inhibited(wont cure i air.

Gel coat is a devil to work with when applied to a hull, additives are needed as gel goat is also air inhibited. Paint after its smoothed is fine.

On your additional post, that is fine, I would glass the inside of the exposed foam after you removed the centre part

Nick