got a guick question.
i know there are alot of ways to make a fibreglass hull, female, male, plugs, vacume bagging, ect all ways in which to put resin into the cloth…
but i was just thinking. after you make a plug. you would lay up a mold. hence the female and male

but i was just thinking. what if you could lay up the mold with a 3/32 of an inch space. between the plug and mold…
you could then resin up the cloth. and then use the plug to force the resin into the cloth. and make an even layer… this could be a cheap way of making the hull even… now i know vacume bagging does the same thing without the plug… but vacume bagging it expensive…
i know i have heard of this method somewhere else… just cant remember it… has anybody heard of this… or think this could work?

I have seen this done with full size boats.

I believe this approach is used in some commercial laminate manufacturing but, apart from producing flat sheets by clamping a layup between two flat plates I don’t think it is viable for model boats.

Firstly, for the sort of mouldings we’re producing 3/32nds sounds a bit to thick?

I think your suggested cheapess is illusory; for each new boat you’ll have to produce a male and female mould (probably by making a male plug, laying up over it to the required thickness, work the layup to a finish then laying up the female mould over that) and you’ll still have the problem of maintining a consistent pressure over the assembly when you mould a ‘final’ hull, probably best solved by vacuum bagging. I find it quicker and get good results by simply laying up and vacuum bagging my hulls over the male plug; you do need to wor a finish onto the moulding but this is partly offset by not having to work the surface of the plug to such a high standard of finish as any little imperfections won’t be visible on the outside of the hull lay-up.

The costs of vacuum bagging are largely one-off, in the shape of the pump, but the results are worth the initial cost. Yes, there are some ongoing costs, such as breather fabric and the occasional replacement bag but these aren’t too expensive. I picked up my vacuum pump on Ebay for around £50 (say $75 US).


I have seen a variation on this theme that might be applicable to something like fins and rudders:

from a male plug, produce female moulds of each side of the fin/rudder, with a system for registering the two halves so that they can be accuratly aligned when re-assembled sans male plug; layup the outer skins of the fin/ridder on the inside surfaces of the female moulds; carefully measure and mix an amount of epoxy foam and apply to one of the previously layed up skins; quickly assemble the two female moulds and clamp tightly before the epoxy foam expands; once expanded and cured the epoxy foam will glue the two skins together and form a strong but light core.


There is also the “german rubber” method. finish a plug and mount it to a wooden board. Layup your glass/epoxy on top of the plug, and stretch a sheet of rubber over the layup. Pull it tight and staple to the board. You can squeegee the excess resin out from under the rubber. Once the epoxy cures, peel off the rubber and you have a nice smooth finish on the inside and outside of the hull. It still will not give you a perfect outer hull like bagging inside a female mold will, but it will be pretty nice and easy to sand to your desired perfection.

Do a search for “german rubber method” for some more details.

Cougar -

there is not much to be gained from making a mold, unless you plan to make multiple hulls or for a production application. You can easily shape styrofoam (extruded - not expanded) house insulation, cover with packaging tape, and lay up a couple layers of glass. Regardless of what you do - you still will wind up having to finish the exterior of the hull whether it is a plug, a “female” mold, or a glass covered “plug”. Just leave the layup on the plug as you do final fairing and sanding - so it maintains desired shape.

Now if you are thinking of making a couple dozen boats - then a “female” mold might be worth the time. Check Harbor Freight (Google) - they have small vacuum pumps that work great and are relatively inexpensive and quiet. You can use a compressor, but noisy. You can also make from old refrigerator compressor. Lot os plans and info on line.

Good to hear from you again.


Just a note on vacuum bagging… You really do not have to go to this expense on the little boats that we build! I have made many hulls from a couple of female molds using both vacuum bagging and not. If you squeegee and blot the excess resin from your layup, you will end up with a hull that is only slightly heavier (meaning several grams) than the more complex vacuum bagging.

For one-off hulls, I would still build from balsa planking and cover with a very light glass inside and out. If I think I will make 3-4 hulls, or more, I will finish the planked hull as a plug, and pull a mold from it.

If you have several guys in a club who are interested, figure a way for them to split the mold costs, and let them use the finished molds to build their own boat. Then you will have built a nice one-design fleet at modest cost!

I agree that you aren’t going to get an enormous weight benefit with vacuum backing on model boat hulls but weight isn’t the whole story; vacuum bagging will ensure that your layup conforms to the shape of the plug/mould much better than a hand layup, especially around tight ‘corners’ and my experience is that you get a more consistent laminate that isn’t porous (i.e. have lots of pin holes).


What about cutting the mounting board to shape of the base of the male mold and wrap shrink plastic film material (used for pallet wrapping) around the wet lay-up and apply heat using BBQ gas …

I used these stuff years ago for another application and was surprised how tightly it worked to shapes, I just have trouble finding where I live, but will keep looking.

Cheers Alan

I think its important to recognise we are taliking about two quite different principles here.

Cougars concept uses pressure to mould which in my view would definately assist in eliminating bridging and produce a reasonably smooth exterior surface but you would likely still be left with smalll pockets of air in the laminate which is not cool.

Vacuum is not the same thing as pressure, sounds obvious but it is amazing how many people have trouble grasping the difference.

Vacuum bagging offers many benefits over pressure molding…primarily it sucks the laminates together, eliminating excess resin while evacuating any air from layup.Acheiving this result with pressure would be very difficult to control.

And since you require two moulds to pressure mold you would be better off building one male mold and spending the left over money on a $50 vacuum pump.


Maybe being something of a pedant and perhaps ignoring common usage langage in favour of actual definitions but of course there is actually no vacuum involved in vacuum bagging. A vacuum is a volume/space within which there is/are no gas/particles. What we do with vacuum bagging is evacuate the gas (air) from the volume (i.e. the bag) but at the same time allow the volume to collapse; if there is no volume there can be no vacuum. The bag is still occupied by the plug/laminate so does not contain a vacuum. So, yes vacuum bagging does remove air from inside the bag that might result in pin holes but no vacuum bagging doesn’t suck the laminates together, it creates a pressure differential that compresses the laminates against the plug.


Not 100% correct - not all of the volume is removed, as the bleeder wouldn’t work…

Thankyou Ray, the differential pressure and internal vacuum is a better defined explanation than mine…what i was trying to explain in laymans terms is by reducing internal bag below atmospheric pressure the laminates are compressed between bag and mould.

Therefore this is not the same principle as applying pressure by squeezing two mould together.

However, as you said maybe a little pedantic, but you are most certainly creating a vacuum when vacuum bagging. Albeit a partial vacuum.Full vacuum can be found in outer space, for our purposes we work with partial vacuums…what we create inside the bag is most certainly partial vacuum and not pressure.

Moreover as Astute points out the breather blanket does leave volume in the bag , with out a breather of some sort vacuum bagging laminates would not be effective.


With vacuum bagging the maximum pressure differential we can produce can be no more than one atmosphere at sea level (and in reality somwhat less than that given our imperfect technology). That pressure diifferential is sufficient for most of the volume of the bleeder cloth to be removed but not enough to compress it completely and there remain sufficient ‘voids’ in the bleeder for it to be able to allow the remaing air to be able to flow towards the pump and for it to absorb resin. So, yes, not all of the volume is removed, nor all of the air for that matter, which reinforces my point that there is no vacuum involved in vacuum bagging, it is just a form of pressure moulding


I think it is just the same principle, both approaches are simply applying a pressure to the laminates in order to consolidate them and remove voids.

No we aren’t, we are simply evacuating the air from the bag so that the pressure differential causes the bag to collapse onto the laminate/mould.

and maybe not even there…

What we create is a pressure acting on the outside of the bag.

It is the presence of the Earth’s atmosphere that creates that pressure, without it vacuum bagging wouldn’t work.


[QUOTE](and in reality somwhat less than that given our imperfect technology)/QUOTE]

Not all use imperfect technology…
Some strive for perfection in every way.

Now, do you agree that a digital gauge reading 1 mbar @ sea level connected to the bag, not the pump…is about as close to a vacuum as you can get? I have attached a diagram to show.

We process laminates at these levels, and better…
It is all about pressure differential causing the bag to compress the laminate, and autoclave technology takes this further yet.

Maybe “Vacuum” should look at its trademark and begin some legal action?!!

[QUOTE=Astute Composites;57765]

(and in reality somwhat less than that given our imperfect technology)/QUOTE]

Not all use imperfect technology…
Some strive for perfection in every way.

Now, do you agree that a digital gauge reading 1 mbar @ sea level connected to the bag, not the pump…is about as close to a vacuum as you can get? I have attached a diagram to show.

We process laminates at these levels, and better…
It is all about pressure differential causing the bag to compress the laminate, and autoclave technology takes this further yet.

Maybe “Vacuum” should look at its trademark and begin some legal action?!!

Perhaps we need to be clear, vacuum is an absolute - it is a volume/space inside which there is nothing. We have technology that can evacuate most of the contents of a volume but not everything so by definition the technology cannot be perfect and probably never will be. Using the term partial vacuum, one could argue that the earth’s atmosphere is a partial vacuum.

Of course part of the human race’s endeavours is about striving to improve the technology we possess but I don’t believe anyone yet uses technology that cannot be improved, ergo it is imperfect.

That said, yes our technology is pretty good at creating a pressure differential that allows us to use pretty much all of what our atmosphere’s pressure has to offer.

Thanks Dick… still been here… but just quiet

the idea was to create a pressure mold. and force the resin ,INTO the glass at a even rate ( ie no high spots or low spots…) you would get an even layer… the 3/32 inch. was just a number i pulled from the air… I know i have seen this down with latex … where they lay up the “hull” then make a mold… then after the mold is set. the lay the clothe in and then inject the resin then replay the master…and then with 2 bars ( across the mold) then push down on the master untill the resin comes out… I have tried something simular… but no success using wieghts…

at my club we are trying to get a class going( just a club class) and i have been approached to make the mold. as i have some drawing experience and some molding experience… ( i am not great at the lay up) this is why i am trying to find a better way…
i would like to try vacume… but they start at $500… and for a club boat… I am not going to foot that bill…
if i could find a way to do it with under $200 then we talking

Cougar. it shouldn’t cost $500 to set up a basic vacuum bagging system. If you’re content to run the pump continuously during the cure you don’t need a reservoir system, just the pump, a gauge, a bag and some tubing/fittings to connect it together. If you need to be able to have variable pressure available then there are simple manual valves that allow you to introduce a ‘leak’ into the system that will effectively reduce the pressure. You should be able to pick up a pump for less than $100 - I got mine off ebay. Check out this site for vacuum bags, consumables, etc.


Check here Cougar:

The one for $79.95 is more than adequate. And if you already have an air compressor that can run continuously, the $13 venturi will work!