Hull mold layup schedule questions

I'm in the very early stages of building a plug for a 22' bay/mild offshore hull. Once that is finished a mold will be made then on to layup. Currently everything i've worked out says that this layup schedule is going to work out fine for laying the part when everything else is done.

BOTTOM
-gel coat 20-22mils
-1 1/2oz. mat
-1708
-core mat (4mm)
-1 1/2oz. mat
-1708
-1 1/2oz mat
-1708
(this yields about a 3/8" stock with out gel coat)
(the transom outside will be 2 times the above listed layup +/-1/2")


Sides
-gel coat 20-22 mils
-1 1/2oz. mat
-1708
-1708
-1708
(this yields about a 3/16" stock with out gel coat


i'm planing on using vinylester resin


just noticed i spelt schedule wrong in the title

 

MuddyWater, welcome to the forum. I can see some improvements to make in your hull mold layup.

First, the hull mold for this size boat should all be the same layup, bottom, side and transom, so that over time, the whole mold will expand and contract evenly with changing temperature. It will hold its overall shape better.

Second, do not use a bidirectional fabric so close to the gelcoat. Even though you are using vinylester resin, you will get some shrinkage over time (not as bad as polyester) and that fabric pattern will print through and be reflected in every part. You'll never get rid of it. It is far better to use something like the following lay-up:

gelcoat
3/4 oz mat
1-1/2 oz mat
1-1/2 oz mat
Coremat
1708 Double Bias (DB)
1808 BiDirectional (BD)
1708 DB
1808 BD
1708 DB
1808 BD

You can substitute equivalent thickness of fewer plies of triaxial if you don't want so many plies of the 1708 and 1808. You do not have to make the transom heavier than the rest of the hull. This is a mold--it does not have to carry any normal boat loads, it only has to hold the hull when the hull is being laid up. The mold should be generously reinforced and attached to a cradle, preferably an articulating cradle that swings or adjusts side to side like a baby's cradle, for ease of lay-up.

The reason for doing the layup this way is to make sure that you have a very stable mold surface that won't print through the gelcoat and onto the part. When I worked in a boat factory (TPI--Chief Engineer) we did not use Coremat in our molds, and so we had something like the above lay-up to start, with more mat replacing the Coremat. Then we had a structural layer or two, then we had balsa core, and then finished with an equal number of structural layers as on the other side of the core. These molds lasted a long time.

Good luck, I hope that helps.

Eric

 

Eric


Thanks for the reply very good info. i guess i should of explained myself a little better. the schedules i posted were for the actual hull coming off the mold. my mold schedule is very similar to yours stated above with the addition of a layer of veil between the gelcoat and the first layer of 1.5 oz. thanks again for the reply and if you have any further suggestions on my lay up please feel free to let me know. knowledge is invaluable and im never against learning something new.

P.S. i have already built the cradle for the mold spins 360 with a hand crank gear and chain.


tim

 

Hi Tim,

Similar philosophy applies to the actual part. I would remove the 1708 between the gelcoat and the Coremat to avoid print-through of structural fabrics in the part, and actually, I would go with a thinner Coremat, say 2mm. Coremat is sometimes used as a structural core, but a very poor one. All it does is soak up resin (and weight) so it's like having lots of layers of mat in there. Coremat builds up thickness quickly, so it's easier to use in many cases rather than building up lots of layers of mat or chop. The most common use of Coremat is to build up thickness between the gelcoat and structural layers to build imperviousness against water ingress into the laminate. Actually, you already have pretty good imperviousness with the vinylester resin.

Finally, I would drop out the layers of 1-1/2 oz. mat between the layers of 1708 because the -08 is already a bit of mat which is good enough for interlaminar adhesion.

So, to summarize, for the actual hull, I would recommend:

Bottom (about 3/8" thick, with gelcoat)
gelcoat
3/4 oz mat
1-1/2 oz mat
Coremat--2mm
1808 BiDirectional (BD)
1708 Double Bias (DB)
1808 BD
1708 DB
1808 BD

For the sides, you can drop out the last 1808 and 1708 which gives a little more than 1/4", and if you want it just a bit thinner, drop the next 1808 This keeps the Coremat in the sides as well. For the transom, you can build up more layers of 1708 and 1808 as you had originally suggested. If the transom is going to support an outboard engine, then you might want some hard core material to build up a lot of thickness and rigidity to support the motor.

Thats my take, I hope that helps.

Eric

 

Thanks again, so the small amount of CSM on the back of the biax is sufficient for adhesion ive always wondered if the manufactures use enough.

 

Also yes the transom will be supporting a bracket and outboard. that will be cored with coosa, airtex, or other dense foam core material. leaning towards coosa just because I've done quite a bit with it and never have had any problems.

 

i guess i could remove the coremat entirely to save weight. any suggestions on that

 

some points to consider !!

Points to think about when making moulds with shiny surfaces are porosity in the gel coat!! so the catalyst and your gel time plus the viscosity linked to temperature AND humidity !!! all these things will have a affect on the actual surface finish , a second coat of gel coat to build thickness behind the first layer of gel coat is necessary!!!
A resin rich layer of glass !! . Now we used fine glass tissue onto the gel goat and let it go harden over night , we also used Vinyl-ester resin in that layer and the first layer if chopped strand P matt!!
Reason is its a much harder resin and acts like a barrier shell !!.
Use at lease 4 layers of chopped strand matt and well cured before any heavier weight glasses of any kind are added to the lay up !!
Print through is always a problem from the glass weave if it to close to the moulds gelcoat !!,also core matt I feel need to be kept away from the surface layers as it holds resin and resin shrinks !! , yes its only 2 mm core matt but is just like adding a layer of blotting paper !! but it does get used as a glass fibre print blocker !! to me its not doing much !!
I like to use 5 mm gore matt in and under the last layers of heavy glass near the end !! WHY ?? well I have seen first hand the affects of heavy handed workers with rubber hammers whacking the mould really hard and causing really fine star cracking the gel coat so the thick core matt on the outside helps to protect the mould from that kind of damage as well as stiffening the surface !!.
Just my thoughts of what to take care of !!

 

Hi Muddy,

I'll second the incorporation of a tissue layer behind the gelcoat, also the 3/4oz-225gram chop next- we used to use what was described as a split strand chop where the fibre bundles were thinner, dunno if you can still get it.
Also plan the attachment of framing very carefully to avoid direct attachment of transverse framing members to the mold skin.
Jeff.

 

I quite like some coremat in the mix, as previously stated get some bulk in the skin beforehand, remember it's quite resin hungry & easy to overheat in the thicker varieties, proper installation is very important, best to bed into wet choppy, back wet the coremat before laying into the chop, fully wet out coremat & sandwich with another choppy layer- check your resin consumption as you go as dry coremat is useless.
Jeff.

 

Here in china its what we used on all the boats we made as well ! I layer of tissue resin rich and left to go hard and then a 300 gram P matt that has powder binder and that was put on also resin rich and hardened then 2 layers of 450 csm P matt all hardened between each layers, never saw any fibre print through any where ever in the last 3 years !!
Another thing is curing the mould for 12 hours before you attempt to take it off the plug !! that makes a difference to the stiffness !!

 

You have the completely wrong impression of core matt !! its not resin hungry at all its the complete opposite and its using quite a bit less resin for a given thickness than it would if you used solid glass !! Plus it has the advantage of becoming a core and stiffening a surface !!
Core matt used the way its intended is a good material just it gets used in ways it was never intended to be used !! and the soaking up resin thing is people not thinking about it properly !!

 

There you go, two valid opinions on the pros and cons of Coremat. Try it using it and see how it works. So much of fiberglass boatbuilding is finding out what materials and processes work for you.

Eric

 

core matt in the construction of a boat hull is like a bomb waiting for the right moment to go off !! because of its very nature and way its made it shears like crazy like rips completely apart from end to end my first and last experience was watching the guys use 4 mm core matt in the bottom of a boat and up the top sides as well !! Lucky they didn't go past the chine with the bottom core matt
Was a few weeks later the same boat was back in the workshop with the whole outside skin missing on one side of the bottom and the other side flapping like a wet blanket !! The core matt has sheared completely right through is middle ! When I opened the skin that was still holding on 2 mms of core on the outer panel and 2mm on the inner skin !!
If a panel has any kind of movement DO NOT USE COREMATT IN IT !!!!

 

thanks again to all of you for the info. research before construction saves a lot of headaches in the long run. ive done some more reading and also talked to a rep from Lantor. He said for a cold molded process like im going to be doing core mat either xi or xm will not have much structural benefit if any at all due to the amount of deflection in the hull during use. the stuff seams to be more for rigid panels with little or no deflection. so with that im thinking an extra layer or two of biax will do the trick.