Direct female molding

I think with 3mm pvc board, you can have a little fun fairing the thing afterwards, but give it a go!

 

yes, the mold is not perfectly fair, its pretty close but not perfect like a normal production mold has to be... im not going to bother trying to get it perfect either... the laminate is epoxy and will be sanded afterwards prior to antifoul application so i figured a little fairing then.

i chose to do it this way not because i wanted a nice finish, but more so for practical reasons. This way i can get both sides of the sandwich done in one shot and also means it will come out stiff instead of floppy, easier to handle and no need to realign everything before glassing the other skin- i can put them aside and continue building the rest of the boat.... It also means more resin and glass gets done in each shot, so i get more done with each infusion for the setup time and consumables cost associated with each one.

Hopefully ill only need to apply a small amount of bog, give them a sand, then they should be ready for antifoul - its all below the waterline anyway and noone will ever see it etc...

 

My major input here is to slow down a moment and tune that mold to the best of your ability. It seems faster now to just bog and fair out later, but that will add weeks to your project.

I had a sloppy mold tune on my first hull and it took a lot of painstaking, back breaking sanding to fix. Weeks. Spent a lot more time (one day?) tuning the mold on the second hull and it only took me a little under a week to fair it (before the infusion). Take your time in setup. It pays off in spades later in the project.

 

Like i have mentioned before no one really knows How much vacuum you really need to hold the floam in place till the resin has gone hard !!.
LOOK AT THE AMOUNT of pressure that exerted on a sheet if you even get 90% of protentual full suck !!
On a 8x4 sheet its 46,000 psi !!! why do you need 14 lbs per square inch
I used vac to make panels and all kinds a things for 4 years making some really big panels with 40 mm thick hard cores and 6 layers of glass each side !! ,Went right down to 10mm foam for small special things ,we never had to use any more than 20% of max vac and on some large panels just 5% was completely ok !!, they came out beautiful with not the slightest blemish any where !!
Everything we were making exceeded DNV specs for quality and consistancy !!. The DNV inspectors never came to our department to check as they knew we were exceeding there specs all the time !! This used to really get up the noses of the other contractors on site as it showed up there incompetance and poor workmanship , specially if the had to grind off and redo some of there shody work !!.

So how much vac do you need ?? thats the question , Provided you have a good big bag and lots a spare and no tight areas its only pulling the core against the glass under the core !! Do a sample dry and see what happens !!, get a couple gauges so you can keep a eye on what happens where and when and at what pressure !!
You might be plesantly surprised at how little vacuum you actually will need .
I was luck and we had the ultimate system with huge capacity with 2 pumps running all day long . One hi capacity and the other hi pressure so could suck many jobs all day long , all at the same time and never run out of suck !
Learning never stops !

 

but the mold is flexible... its 3mm foamed PVC sheeting... i dont even know how much the mold will move when vacuum is applied??? so, is it worth spending the time on it?

i thought of applying some layers of glass inside, then doing some more fairing on top of that before painting it - which would seal it, stiffen it a little and get it a little more fair perhaps, but fairing the inside of this mold is difficult as the surfaces are all concave - i dont think id ever be able to get it perfect even if i tried...

On top of all this, its below the waterline anyway, so im not going to spend weeks fairing it to a perfect finish afterwards as noone will ever see it!

Its not to bad at the moment tho, its actually reasonably fair - just not perfect...

 

Ok i painted the mold this morning... now you can see things more clearly, its not as fair as i first thought

so once again im not sure which way to proceed

i had one idea, to put the skin coat of glass on then release the skin coat off the mold but leave it in there. Then do the infusion with the skin coat as the mold side vacuum bag...

Another idea, is to lay a vacuum bag inside the mold first, then layup the stack and another bag over the top - so a full envelope bag - with the mold just to hold the shape... it could be held down inside with sand bags etc...

Anyone else have any ideas?

Also, im not sure i will be able to get the corecell to fit well in the bow area as the radius up there is very tight - maybe only 50mm or so, any ideas on how to get the core to take such a tight radius?

And another problem perhaps, getting this to release out of the mold easily - without destroying the mold? im doing some test peices of glass on a few random places to see if the PVA is going to keep things from sticking too much...

 

Do you have a very tight turn in the bilge along the entire length?
If so one otion is to initialy leave the core out of this just finish the core edges at 45 deg. Infuse the job bringing the two skins together, later bag strips of foam into this and glass over, or alternatively build up the section with more glass and coremat if you like.
Have you considered making the hull bottom solid glass or glass coremat? how different would the weight be for such versus full core?

 

Building on Andrew's suggestion, you could also strip plank that tight radius from bow to stern using the core material.

Very long, thin boards of foam running fore and aft in the tight curve, then simple flat panels on the sides and bottom could be a simple way to do it. Just bog in the cracks.

Strip planks are naturally fair because they act as full length battens from bow to stern, station to station.

 

great advise guys, i had not considered these options...

I like the coremat idea andrew, would simplify things a great deal for only a small weight penalty - the tight radius only occurs over a very small surface area. If i only used coremat along this radius and up in the bow, the increased weight would be virtually unnoticeable.... but save me a lot of work...

 

For you, Groper. An image of how I did my strip planking and the results on one of the half hulls:



In the picture, notice there is a skim coat of bog on the balsa deck, which is the right most part of the hull in the picture. Then, below that big brown splotch is a very tight radius. You can really get an understanding of how tight that radius is if you look way down by the vacuum sitting in the distance. Look at the curve there. That's my sheer radius/curve, leading to the topsides, then finally to the keel, which is at the left most part of the picture.

Notice the strip planks in the tight sheer curve just below the brown bog on the right side of the picture.

They come out like that. You make up long pieces on a table saw, then cut some bevels or angles along the length of the strips, so they fit nicely into the tiny curve.

As a side note: The rest of this half hull was thermoformed Core Cell. I tried the oven, but it was a huge waste of time trying to build an oven instead of a boat. I then just bought a quartz heater and learned how to "cook" the foam just right so that it went soft, but wasn't overcooked. It's a learned art and I do mean art. It's a lot like cooking food well. The advantage of the quartz heater over the oven is that the foam becomes hot right at the spot where you are getting ready to push it down into the mold. It gives you a lot more working time to get it right before it hardens and cools again.

Lastly, the mold tolerance was within 1/8" (a few mm), as you can see here...

 

skin coat is done... will do a vacuum test on it once it cures and see if i need to do another coat to get it sealed... notice the lumps and bumps? wish it were fair...

 

It really doesn't look that bad to me in the pictures. Your fairing skim coat of bog should easily take care of any small bumps we can't see in the 2nd picture.

 

im just sealing the mold here, will probably put another coat of resin only wet on wet to make sure its vacuum tight... then ill put the PVA mold release and begin the infusion layup after that...

but so far, this epoxy doesnt look like its going to go hard... still tacky after 5 hrs, oh dear...

 

ok luckily the epoxy was fine - it just had a very long open time of +9hours...

ive done another coat of resin only to fill the weave left by the cloth...

So now its quite shiny, the lumps and bumps are quite easy to see and my fibreglass supplier forgot to include the perforated film with my order so i cant infuse anything for another week until it arrives so... i might have a crack at fairing this mold a bit more!

I was thinking of sanding down the epoxy to get a mechanical bond and spraying it with some automotive "spray putty" which is basically a thinned polyester bog from what i understand or "some other high build primer"? so building it up a bit to fill the low spots then longboarding this down again to level it a bit more and finishing it off with some dark colour automotive paint? is there a better way?

See its not so much the finish im worried about as im always going to have to fair the hulls afterwards, its being able to get it out of the mold easily without it sticking! Is there a better mold release than PVA for this type of thing? i even considered wiping the whole thing with motor oil - but realised it would contaminate the glass haha!
How do i ensure it wont get stuck?

what do you think?

 

aha! what if i line the whole thing with cling wrap before the infusion to stop a potential stick? any problems with doing this?