Weight estimation for wood coatings

Hi everyone,

I am currently working on a weight estimate for a sailboat using the plywood stitch-and-glue method.

• LOA: 8 m
• BOA: 2.54 m
• Displacement: ~1600 kg

What I am a bit uncertain about is the subsequent coating of the boat. Currently, I am assuming the following build-up for the outer skin:

• Wood
• 2 layers of epoxy primer
• Glass fabric (what weight?)
• Epoxy for the laminate
• Filler / primer
• 4-5 layers of topcoat

What weight for the fabric would you suggest? As a comparison, Dudley Dix specifies a laminate of 450 g/m² for the Didi 26. This could be used as an initial estimate.

I would now estimate the following weights per square meter:

• Epoxy primer: ~250 g/m² per layer
• Glass fabric: 450 g/m²
• Epoxy for the glass fabric: ~370 g/m² (according to an online laminate calculator)
• Filler: ~100 g/m² (what remains after sanding)
• Paint layers: ~50 g/m² per layer

In total, that’s about 1.67 kg of coating per square meter of surface area. Do you think these are realistic numbers, or do you have different experiences and suggestions?

Thank you for your feedback and best regards, Nicolas

 

Is this a proven design you are building to or a one-off design?

The fabric weight is easy, you just weigh it.
As for the resin, about half of the wet weight (container weight).

Yes, 1.7 kg/m2 is reasonable.

 

What is the two layers of epoxy primer?

 

Hi together .. thanks for your replies sofar.

I am currently designing the boat, so it is a one-off design. The build phase is planned to be started around end of this year.

I guess your statement refers to polyester resin, which outgasses during curing. But epoxy does not outgas, so it should have approximately the same weight when cured. I can't imagine a 50% weight loss, but I've never tested it myself.

I thought it might be useful to apply a first barrier layer and soak the wood with epoxy. There are, for example, very low-viscosity epoxy systems that are designed precisely for creating a waterproof barrier layer.

The question is quite justified.
With the glass fiber coating and the varnish, the two primer layers may not be necessary.

But in general.
How would you build up the coating for such a boat and what are your experiences regarding the weight?


Many thanks and best regards, Nicolas

 

But since the glass fiber coating is mainly intended to make the surface more resistant to external influences and not necessarily to contribute to strength, a laminate with 250 g/m² fabric should be completely sufficient, right?

 

I'm guessing that the boat is quite light displacement and excess weight would be harmful to performance.I would suggest that one sealing coat over the hull before applying the glass would be enough to limit the amount of epoxy that migrates into the wood and that working with a glass to resin ratio of around 1:1 would be realistic.I would have more epoxy available personally and I would apply peelply to make the finishing process easier.I don't see much advantage in having a lot of paint build up on the outer surface as it can be quite brittle and prone to chipping.In any event,I wouldn't expect the total weight of coatings over the sheathing to make much more difference than two extra metres of chain on the anchor warp.

 

The filler is probably the biggest variable. It depends largely on how fair and smooth the hull is.

 

You are asking a bunch of philosophical questions.
Most plywood boats are engineered such that the ply is sufficient to take all the loads. Any glass is cosmetic (prevents checking in the top veneer) or for abrasion. How much abrasion protection you wish is your business.
Applying epoxy to the plywood before lamination is a question of preference. How much epoxy the wood drinks depends on the wood species - epoxy viscosity combination. Keep in mind, even if epoxy would be able to completely penetrate the wood and saturate it, with plywood this only applies to the top veneer, it can't go trough the glue line.
Filling the weave with pure epoxy is usually done on clear finished boats, but some people prefer it on painted ones too. Others hot coat with filler, and others use peelply.

I suggest you take a piece of the chosen ply, 0.5m2 weigh it, then apply the usual 3 layers of your chosen epoxy on one side plus paint/laquer (that's the usual boat inside treatment), the chosen amount of plain or twill weave on the other side with the technique you like, then filler layers (sand it to the desired finish), then the desired paint (according to the manufacturers specifications). You weigh the resulting panel and that's your tailored approximate answer. Approximate because plywood density is specified with a +/- tolerance of 50kg.
Don't forget to add all the fillets and tapes if present.

 

Hi ..

Actually, for this type of boat, a 6 mm anchor chain would suffice, weighing approximately 0.8 kg/m. The total outer surface area, including the hull, deck, cockpit, and cabin, exceeds 50 m². Therefore, even if I were to apply just two layers of paint, each weighing around 0.1 kg/m², that would amount to over 10 kg—equivalent to more than 10 meters of anchor chain. ;-)

Thus, the weight of these coatings and paint should not be overlooked in a thorough weight estimation, even if it represents only about 0.6% of the total displacement. However, I agree that the differences are negligible if a single coat weighs +- 10 g/m² or so.

Yep, testing the coatings on a sample piece would be ideal. However, I currently don’t have all the materials on hand, so I’m reaching out to see if anyone with experience can provide insights.

Please don't get me wrong, but, to be honest, this answer is not less philosophical.
The boat I’m designing is a standard recreational sailing vessel, not a fishing or workboat that would require additional abrasion protection. If you have experience in this area, I would appreciate your thoughts on the necessary level of abrasion protection in terms of fiberglass fabric weight, or any recommendations you might have.

So, if anyone has built a plywood boat, whether sailing or motor, and has documented the materials applied to the wood surface or conducted a similar test as suggested by Rumars, please share your insights.
Thank you very much, and best regards.

 

Don't get any supposedly special epoxy designed to penetrate better, all they do is add a solvent. Epoxies don't get along well with solvents, it significantly reduces the physical properties and you pay extra for cheap solvent.

Just apply a layer of your normal epoxy and let it soak in, sometimes the wood is thirsty and needs a bit more. If you let it at least start to get hard it won't suck any epoxy out of the glass, and any air in the wood wont create bubbles under the laminate. Then do whatever laminate you decide on.

 

On my ply dinghy half the size of yours I did 2 epoxy layers on the wet side, with sanding and minimal filler between, with glass tape on the chines and edges only, not fully glassed; after over 2 years regular use it is holding up well against normal use and wear. Inside I used cheap indoor varnish to seal the ply, with a UV resistant water based paint over it, and it also is still in good condition. Unless you are beach launching, or beaching on gravel, you shouldn't need any more skin protection except at chines and centreline. Glass is a sponge for resin. You can always glass the hull after a few years of use, if you find you need to.

 

I'll cite the Gougeon book:
Epoxy coating of wood, one layer: 0.14kg/m2, minimum of two layers needed, three for the outside.
Laminating 330g/m2 fiberglass: 0.5kg/m2

For a painted plywood "standard recreational vessel" I consider a single layer of woven fiberglass (plain or twill) 150- 390g/m2 to be sufficient. Plywood made from harder wood species (sipo, meranti) doesn't need any outside glass, but many people do it nevertheless. Softwood ply (spruce, pine, fir) usually gets glassed to prevent the veneer checking and destroying the finish. Soft hardwoods (okoume, poplar, khaya) are a matter of preference, most people do glass to improve the surface hardness.
Of course others might disagree and I'm fine with that. You can find examples that work for every combination and weight of ply/fiberglass/other fibers (dynel, xynole, kevlar, etc.)/no fibers at all/no epoxy at all.

If you use stiched fabrics they should be always covered with a layer of woven if mechanical damage is expected.

 

Hi ..

these are the information I was looking for, thanks.

This is a good hint, thanks. I'll do so.

Good suggestion, but before I potentially spend this effort, I'll glass the hull right from the beginning.

Thanks for sharing your insights and the source. These are good information with which I will further proceed in the weight estimation. Lets see what turns out at the end. When we do all the coatings, I'll take a sample piece and check all the weights of the individual layers. I guess this would be well spent.

With stiched fabrics you mean, e.g. biax layerd fabrics? Can you please explain the reasons for that in more detail and why these fabrics should be covered with woven ones? That would be great.

Thanks

 

Stiched fabrics are those where the fibers are held parallel by a separate thread that isn't structural (straight fibers have better strenght), so unidirectional fabrics, biax, triax, etc. Because of this construction in some damage scenarios the individual fibers can be lifted out of the resin matrix for a longer lenght, resulting in a much greater damaged area then with woven fabrics, where the threads running in different directions are mechanically locked to each other and arrest the movement. Best practice is therefore to cover all stiched fabrics with a woven final layer, it acts as a ripstop on the surface. With heavier unidirectionals the woven layer also improves surface finish because it's threads are finer.

In my previous post I forgot to mention that the figures from the Gougeon's book are for mixed epoxy consumption (resin+hardener) only. So for the fiberglass lamination you have a total weight of 330g glass + 500g resin = 880g. The 500g of resin consumption figure is high, but if you substract the 140g the wood is supposed to absorb you land at 360g, wich is 30g more then the fabric weight. Still more then the modern literature suggests, but approximately in line with the rule of thumb for amateur hand laminators of same weight of fabric and resin.

 

The hull thickness, fiberglass coating, framing, etc. are driven by the overall design. What works for one may not for another. Maybe you could get a book with simple formulas for designing boats, like Gerr's.