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making plug for fibreglass mould

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ashley needham12/07/2017 08:49:23
7353 forum posts
156 photos

Fellow boaters. The time has come to try and make a fibreglass hull.

The question is....what is the best/easiest way to make a plug for the mould??

​Solid wood, foam, plaster....and after the shape has been made, what do you seal the surface with prior to waxing etc



Bob Abell12/07/2017 08:54:55
9334 forum posts
2985 photos

Hello Ashley

How about this for an option?......

..........Use an existing boat hull as the plug and make a mould from it?

Any good?

What type of hull are we talking about?


Ray Wood 212/07/2017 09:11:25
2505 forum posts
864 photos

Hi Ashley

I guess this is a new boat ? I'd carve the shape in Blue foam, plaster of paris skim, then polyester resin, rubbed down to a fine finish, then 10-15 coats of Bees wax polish, should release ok I did this last year to produce a winged keel for my sailing barge. I did buy a book on GRP moulding from Amazon by Ron Warring but it was very basic and 30 years out of date

Regards Ray

harry smith 112/07/2017 11:40:36
1088 forum posts
1272 photos

Hi Guys

The bees wax is OK but, they have a releasing agent which works well!!!

And Ray a winged keel for a sailing barge !!!!

Looks like Rays having a crack at the Americans Cup !!!

By the way, who invented the winged keel !!!!


ashley needham12/07/2017 13:52:46
7353 forum posts
156 photos

Chaps. Yes a new boat so unable to use an existing hull, and no clues as to type.....

​I like the foam/plaster/resin idea as foam is easy to carve. I shall be doing a symmetrical hull so only one half is needed which makes things a lot easier to get even.


Dave Milbourn12/07/2017 15:17:13
4025 forum posts
282 photos

I shall be doing a symmetrical hull so only one half is needed



Paul T12/07/2017 16:22:18
7335 forum posts
1229 photos
2 articles

I think that as it is a symmetrical hull Ashley intends to make it in two halves and glue them together.

It is true to say that a fully symmetrical hull is a very rare beast and knowing Ashley this project will be interesting and very different.

I'm off to google symmetrical hulls and see if I can guess what it is.


ashley needham12/07/2017 16:51:22
7353 forum posts
156 photos

The underwater shape is same both ends as is the above water shape. AT LEAST that's what it says. Pictures are hard to come by, but the (probably builders) half model I have seen a picture of shows a possible rudder port at the aft end...its a bad photo. In which case the blurb is lying and it is not quite symmetrical. I would still make two halves and join them, then cut a hole at the stern and do a bit of re-shaping as easier than making a two part mould, especially for a one off.

I have a preliminary sketch and it looks as if it will be a bit heavy and I am wondering about the chosen propulsion system. Mind you the original was very slow so it doesn't matter that much.

​I have made enquiries with the (possible) owner of this model, and we shall see...or at least hopefully we WILL see, and all may be revealed. The National Maritime Museum is of course a good source of material, but a copy plan is very expensive and you don't necessarily get exactly what you were hoping for.. By the looks of things, making the plug, mould and two halves is not going to ne that inexpensive either what with resins, gel coats, wax and so on. The propulsion system will be the cheapest part of the build!

Paul, good luck with that search....


John W E12/07/2017 16:53:41
276 forum posts
272 photos

1393038765-0.jpghi there Ashley

couple of golden rules with fibre glass - Get the mix right; i.e. hardener to resin - making sure you have the correct temperature (not too hot - not too cold ) and don't mix batches that are too large. In other words the size batch which you cannot use up within 30 minutes.

With regard to materials from which to make moulds - as has been stated previously; there are 100s of items which can be used. Make sure though that whatever you use doesn't react to the resin. You may even use cling film as a releasing agent; the other thing is watch out for undercuts in your moulding for when you release the moulding from the plug. A lot of folk don't foresee this and they don't realise why the moulding doesn't release properly and they end up breaking it. So you are making a symmetrically based hull? Could this be the Russian circular ship from the 1800s? Have to go and do some Google.


Edited By bluebird on 12/07/2017 17:06:40

Colin Bishop12/07/2017 17:04:48
4961 forum posts
6110 photos
411 articles

A war canoe - presumably manned by a bunch of hard boarders?


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