Polyester laminate without csm

Hi all.
The question is regards the use of WR laminate without csm.
In the work I have done have never seen a laminate with WR or biax without being sandwiched in between CSM but that concerns boat building(6-15m long) exclusively.
Now I have a opportunity of building and installing standalone pontoons(or jettys) and floating terraces and for that I have to make basically some boxes that will be attached with wooden beams and then a wooden or WPC plates to walk on.
Where I live most of these floaters are made of HDPE but since my business stands in polyester/fibreglass field I would like to have them made from this material.
In order to be economically viable I have to keep the cost down and that means that having them built using gelcoat-csm600-wr800-csm600-topcoat won't work as the csm takes soooo much resin...
I cannot give up on using topcoat on the inside as here and there will be some hatches where you will be able to store thingies and neither gel as first layer since they'll be in water all the time.
The tests I've done during the years revealed that if not under stress or...being pulled using tools or brute force 2 layers of WR with polyester won't delaminate.
And since the floaters will be stand alone I was thinking of going with only 2 layers at once of WR 500(with maybe a bit of csm where some plywood reinforcements will be to have it stiff enough).
As I said I have done during the years some tests but on the long run I don't know if this will work in real life.
The floaters will have a size of 120x60x35 cm and as I previously said my intended layup schedule will be gel then 2x500gsqm WR and then topcoat.
Sorry for the long post.
All opinions are welcomed and thank you in advance.

 

The resin rich area between the two layers of roving is the weak link, it won't fail just sitting there, but when stressed can rather easily. The other problem is that laminates that are only a layer or two of roving tend not to be watertight unless you pay a great deal of attention to detail and get lucky. Roving doesn't hold resin that well, it tends to drain out and/or move leaving spaces that water can find its way through.

 

Except small debris from time to time it shouldn't be stressed or impacted, I'm talking normal usage on calm lakes and European inland canals, If it gets hit by a boat at speed it's not my concern so to speak just as car manufacturers won't bulk a hot hatch in a way to be tougher than a pick up truck.
Regarding waterproofness I agree but I thought the problem will be solved to some extent by having the laminate gel coated inside and out and still having 2 layers of WR not one. Maybe going down with the weight of WR like 2x300 with a 225 mat in between to fill the gaps? I'll do some tests and see since except for the stiffness(provided by beams and some plywood)a floater might as well be the thickness of a PET bottle as long as it is waterproof.

 

Since you will need to take the time to accurately cut and locate darts in the WR to follow the shape of the part,there may not be any time saving.As an alternative,have you considered getting hold of a chopper gun and saving a lot of time?Yes it can be messy but the speed should increase considerably and you can fine tune resin levels with experience.

 

Chopper guns are the fastest, but use more material. However, labor is usually more expensive so there should be savings.

 

Strength reasons aside there's no other reason what-so-ever to use CSM. WR or Biax are easy to make watertight with thickened (w/ silica) resin.

 

Gelcoat is more expensive than resin, and it looks like you will be using a lot of it in lieu of resin/mat filler?
Like Gonzo mentions above, chopper gun layup is still pretty much the go to method for production fiberglass parts.
If I were shopping for dock floats, I would definitely choose HDPE over FRP due to durability, impact resistance, UV resistance and such, so make sure you have a market!

 

I chopped and threw away a unfinished canoe that stood 4-5 years in the sun, rain, snow and down to -5 to -8C and it seemed pretty durable and uv resistant not having a topcoat because was not upsidedown and always full with something
On people's waterway won't be any kind of hard impact as I said but even so with all the "risks" some just want to own nicer things than others and if they afford so be it.
HDPE is for pipes and septic tanks
As for the thickened resin it will add weight, labour and might crack so I'd rather use some light mat to hold it

 

You are quite free to make whatever product you like,just as the customers are free to purchase whatever product they like.The problem comes when no customers come forward to buy the product you would like to make.Do you have contingency plans for such a scenario?I do understand that investment in tooling for HDPE would be considerable,but once in place it can be used to create utilitarian floats quickly and inexpensively.With the added bonus that HDPE can be recycled,and this can be a consideration for some buyers.

 

A decade ago my son and i attended a 3 day workshop with Derek Kelsall to gain hands on experience with resin infusion. The workshop was actually to promote his Kelsall Swiftbuild technique for building cored composite catamaran hulls but we were there for the infusion. Now Derek had been building foam cored multihulls since the 1960's and had always used polyester resin and had never seen any reason to change. So the 42 ft catamaran hull we built was infused as 2 x full length sides from the keel line to the sheer on a flat table using only unidirectional glass laid in pre-determined orientations with polyester resin. To be clear, no csm between layers, no stitched fabrics, just uni. Derek is credited with designing and building the first foam core multihull in the form of Toria, a 40' trimaran which he went on to win the around Britain race in 1966 or there abouts and went on to design and build some of the largest racing trimarans in the world in the 1970's so he had a bit of experience to back up his belief in polyester, foam core and no csm. CSM was a necessity back in the days when we used heavy woven roving where there would be very little surface area to bond if you just put roving on roving. These days, since the early 1980's, we have had low profile stitched fabrics that really don't need the csm.

 

CSM in between WR layers is a Class Rule. It does help in strengthening the bondline or "glue line". Resin is the weakest link, followed by CSM. WR has the intermidiate strength with UDR as the highest.

It helps to transition the strength in a more gradual way instead of introducing shear stress.

As a compromise however, Epoxy is used in between WR and foam. Poly is weak. Veil cloth or surfacing mat can be used instead of CSM for lightweight. Or surfacing mat in between Gelcoat and next layer which can be A CSM or WR.

In general, Resin is a good water barrier. Add some colored solids and you get a gel coat, not as good as pure resin but gives you a colored surface. Where color is not important and good water barrier is more important, Pure resin, then surfacing mat, then CSM, then WR.

 

Your plan sounds reasonable for standalone pontoons and floating terraces, especially since they won't be under heavy stress. Using 2 layers of 500g WR without full CSM is viable for this application, especially if your past tests showed no delamination. Including CSM only at reinforcement points (like around plywood) is a smart way to save resin while maintaining strength where needed. Keeping gelcoat outside and topcoat inside is the right choice for waterproofing and durability. Just ensure good bonding between layers and proper curing. Overall, it seems like a cost-effective and workable solution for your use case.