building a female mold from scratch (without a male plug)

I found this method of construction, basically, they use a computer generated stations to build a female mold directly in MDF. They plank the inside of the mold, rather than the outside, and use it directly as a female mold. the surface is then treated and faired (not sure if they glass it or just gel-coat it) and prepared for molding.




I'm tempted to do the same method for a 6.5m yacht. What are your thoughts on getting the mold fair? the computer model would need to be spot on and completely fair, and I imagine it would not be as easy to fair as a male plug. But all in all, I think it would be a cheap way to make a mold for say 2 or 3 hulls. (which is all i want it for).

http://www.sayerdesign.com/sayerdesign construction ryujin.htm

 

This technique is becoming somewhat more common- I've read about it in four or five places in the last year or so. I haven't tried it yet. Fairing should not be appreciably harder than for a male plug, provided that your frames are accurate and are set up within acceptable tolerances. (This is much easier to do now, with CNC cutting and laser alignment, than it was a few decades ago!) You do need a smooth, resin-tight and air-tight surface finish with enough bulk that it can be sanded during the final fairing and polishing steps (I have had very good results with CASS Adtech ES-219 on previous projects; their ES-218 should work just as well if you aren't heat-curing it).

 

It works well for to pull 5-10 products from the mould. The mould kit should be fully worked out in 3D CAD and cut with an NC router. There are special techniques for making the various pieces fit together properly and then for sealing it so you get a good surface finish.

Also ideally the surfaces of your hull should not have too much double curvature otherwise it is very difficult to make a mould like this. Developable surfaces are of course ideal.

I know somebody who can deliver a mould kit flat packed for you boat if you require. Send me a private message if this is interesting...

 

Also depends on the shapes your trying to achieve.

A nice gradual curvature like that shown above, is great for this technique. If the surfaces are completely developable, then there is no need to make a mold like this - it can be built from flat panels bent into shape. However if the curvature is much tighter - like a very narrow catamaran hull or tri float etc, then it becomes very difficult to fair the highly concave surface in terms of physically sanding it- which is where a male plug becomes much easier to get it fair.

 

Have found its best to get you frames cut on a cnc for cosistancy and accuracy . the setting up and how you sheath the inside and what you use is really important as to the final shapes you will end up with !!
if the shapes are flat you could simply use glass panels that will become the boat its self so even easyer just have a set of frames evenly and closely spaced .
we have finished moulding and assembling a 90 foot lake boat where the mould frames are wood and then plywood and covered with a commercial grade of formica stuck down with contact adhesive . waxed gelcoated and then the boat was built same as usual . the whole outside is resonably fair but needs 100% sanding and long boarding all over undercoat and painting .
I have posted pictures from the early stage up till quite recent . its due in the water 5th of november !!

 

Thanks for the comments,

Marshmat and Scottorf, good to know its been done before. great to know how long the mold might last and what surface treatment to use.

Groper, the hull as its currently designed is definitely not developable, but I am considering a hybrid design that's got compound curvature below the waterline, and developable topsides. That way I can make a smaller mold of below waterline, and then attach developable topsides. This would save money and time compared to a completely molded shape, but still get the benefit of a more streamlined hull. Its one of those situations i like to call a double win. I have been following your build, its awesome work! Is it difficult to bend the panels once they are infused? I noticed for the tight bends you lay a cut down them, but for the longer bends, as in the side of a hull, is much force required to get them into line?

Tunnels, I looked through your photos, very impressive! Am I right in thinking that the majority of her hull was developable (except perhaps for the bow section)? I want to build a small yacht hull, and i think planking the areas with high curvature would be more difficult than a 90' hull where its bigger, and the curvature is less. Did you need to build the surface up much when fairing it, or was it quite good from the CNC frames?

 

Hi glad you liked it !
Most of the problems we had was ripply caused by the formica !! there was quite a long delay between the waxing and the actual lay up and weather affected the mould so some areas where there was tension were loose hence ripples . was better on the deck that was a while later and during the hotter part of our summer . wooden moulds move a lot!!, like it or not so unless you seal the outside of the wooden mould as well as the inside to help stabilize the whole thing ! or if its small have a 100% controlled enviroment and maintain a complete stable humidity and have just a small rise an fall with temprature you will get lots a movement .
n South Korea 2007/8 we had a whole project of plugs (hull and deck )inside a tent once we got to the stage of paniting and sanding etc it was a 24 hour a day 7 days a week control !! worked 18 hours a day 7 days a week till it was to the stage of the hull and deck plugs forthe boats were 3/4 made then slackedned off a little by that stage not much was going to change Humidity wasn the problem just the temprature at minus-15c outside and 600mm of snow for weeks on end and we maintained 25c inside the tent and when finished work let it drop to 22 c slowing then back up to 25 when we arrived at work next morning .
The designer came from nz and everyone was really on edge and holding there breath , he walked round the plug and rubbed his hands along both sides and the transom and had a great big smile when he came back to where were were all standing and said it was the best fairest plug he'd seem !! yeah all our countless hours of multi colored coats of hi-build and hours of long boarding all paid off . The boats when they came out of the mould were really good , had 8 boats to be made so there was no time to sitaround and congratulate ourselves . in under 9 months plugs moulds and 8 fully completed boats were made and on the water .
Any way a yacht hull could build a female mould and strip diagonal plank the whole boat as long as the planks have a tougue and groove system they will mate easy and hold there shape . we used one router with a grooving bit and another that made a tongue so one cut to a shape ran each one then assembles once you get the system going its a piece of cake and quick and easy when all is set inplace them a roller coat of epoxy resin will tie everything inside together completely then with the add of hi build coats you could make a pretty good job and last coat duratech and then buff and semi polish . Most places manufacture a samding fairing gelcoat that coule be you first step for the boat then a couple of csm layers that in the worst case situation could be sacrificual !! some where to get the nice finish you either spend time on your mould and make it shiny or on you boat !! My chice is the boat its quicker and easyer to fair outside that inside . been and done both and theres no contest !!. just remember make it quick dont take months and months the longer it take the more the mould moves and twists out of shape . build the mould ontop of a big sheet of plastic so if you intend to tent one of 6 sides is semi sealled already . man theres a lot of moisture comes out of concrete floors and walls , its inbelieveable !!even new concrete with plastic under the concrete makes vertually no differance .
Its best to not stick the planks to the frames just screw and or nail so its possible for them to move fractionally if they want !!.

 

Great point tunnels, I didn't even consider the effects of heat on the mold. I thought about humidity, but more the direct effects of it getting wet rather than just the humidity. I live in near tropical conditions. Summer is hot wet and humid, winter is warm and less humid. Could be a bit of a problem. I can't really control the temperature without spending too much on airconditioning, but getting a de-humidifier might be worth it for construction. Hopefully the effects on a smaller hull will be less than on a bigger mold

 

Points to remember with dehumidifyling and the like its a ongoing problem ! once the conditions are right they need to stay like that as long as possible like 24/7for the length of the build no good turning it on and off or getting conditions righ then lifting and making a opening and all the dry air escapes and its start over again !! same with materials they to need to be conditioned to the enviroment they will spend there life in .
like i said the frame and shell do not want to be solidly attached as they will move diffrently from each other!! not all materials behave the same !!
Size is another factor to consider as well . big volumn takes longer to get to where you want it !! small volumn is easyer to control and maintain
!! good luck !!

 

Good point about keeping the dehumidification constant. I usually get about 6 weeks off over christmas, half cos its a quiet time at work, and usually take some leave too. Too late to do it for this year, but thought i'll aim to do it at the end of next year. That way i have plenty of time to prepare, get all the materials sorted and then just go crazy building!

For a 6.5 m hull, 3m beam, what frame spacing and thickness do you think i should go with on the MDF for the mold (the hull is a compound shape and no surfaces are developable, but some come close):

I was thinking:

Sections: 16 or 20 mm MDF, spacing at 500mm, total 14 frames

Planking: 10 or 12mm, roughly 75 mm width but narrower where required.

I want to use the minimum structure for it to last for probably 3 (maximum) hull's.

As it's a relatively small hull, i thought about welding up a reinforced steel frame on wheels, so I can move it round to get better access, although it will be better to keep her in the one spot during the build.

Also not sure what's best in terms of building the hull mold. Hull in one piece (+ deck) or two pieces (as shown in the photo in the first post). Two pieces would be easier to build and store, but i am concerned that the two halves might not mate perfectly. One piece would make a better hull, but i think access around the bow might get difficult. I could always do another smaller mold just for this area, but it seems like that could get tricky.

 

The least number of moulds the better thers always problems with joining and alignment

 

Thanks Tunnels, Good to get some help from someone who has done it before.

 

Does anyone have any tips for fairing concave surfaces? Types of tools or tricks of the trade to get it fair with the least amount of blood sweat and tears?

 

Yeah dont put on loads of fillers !! A one metre long steel ruler is one of the best fairing tools i ever used ,inside or outside !!
Get the grinder and just touch off the very sharp corners each end .
The other trick is to use a sloppy filler and then the steel ruler its finds it own bend and gently but quickly scrap it across, it has a sharp edges so scrapes well .
Longest one i used was 1.200mm long the ruler is wide and fractionally thicker THAN THE 1 METRE RULE .
Sanding board , 3mm 3 plywood cut off so 2 grains go across and one grain in the middle runs lengthwise !!, gets a nice bend ,cut the other way with 2 layers of veneer running length ways its quite stiff and not so much bend. Find a thin layer of polystyrene and glue it to one side right to the edges and 2 mm over hang . Its so the ply never touchs the surface your sanding and leave marks in the filler ! The board can be up to a meter long or 800 or so is cool and place 2 easy to hold handes across the board about 300 apart close to the centre and glue annd screw through into from the other side befor thhe polystyrene gets glued on !!!
Staple the sand paper on ,make sure the staples dont come all way through the board and stick out through the papaer .
The board bends and you can sand a slight curve in the polystyrene real easy so when you press down have a curve both directions the rest is up to you . sanding the out side curve we use 4 mm plywood and have 2 guys and 4 handles !! all good fun !!!

 

for easy to sand coats i always add 1 paper cup of qcells to one spray gun bowl of hi build 2 pot undercoat to make it easyer to spray add a little fast drying thinners so it evaporates really quickly . i use acetone if its compatable with the paint !!
It dries quickly and your able to get sanding as soon as its to the stage where it sand and dosent pull off the surface ,3 to 4 hours usually !!.