I guess you would call this a "sandwich" construction but then I guess all fiberglass boats are sandwich? Anyway this particular sandwich is .750 inch, 5.5# Core Cell foam skinned each side with three layers of 1808 biaxial non-woven fiberglass. For those not familiar with structural foam, it is not like insulation foam which is made from things like urethane and Styrofoam. ATC Chemical ,the company that makes this foam, calls it a linear foam but I don't know what it is. My guess it is some sort of modified PVC. Airex (softer and more elastic) and Divinycell/Klegecel (harder more rigid) are some of the other types of foam available. The CoreCell seams to be in the middle, claiming more compressive strength than the Airex and more ability to elongate than Klegecell. The transom is cored with 1.5", 13# Klegecell.
I tried to make sense of the physical properties of the various cores and each type of foam seemed to have strong points but I had the idea that the ultimate strength of the foam had to be tempered with toughness so I chose the Corecell.
I went with a glass/resin/core system that has been successfully used by custom boat builders in the Northern Gulf Coast area. Mainly, I have relied on the advise of my materials supplier Hugh Dykes, the technical sales rep. of Seemann Fiberglass , who is very familiar with this type of construction. Unless you are an experienced builder and have used a particular building technique, I can't stress enough how important it to deal with qualified technical people who can provide assistance with the
The resin is General Purpose Ortho Polyester. This is low tech resin but it laminates well.
I am used ATC's Poly-Fair/Poly-Fill fairing system and Duratec Vinyl Ester primer on the bottom.
The hull bottom as well as the top sides are sprayed
Follow-up on this process, this has turned out very badly in the long term. About 2-3 years later, blisters began forming, first in the hull sides above the waterline then progressing to generously populate almost every surface that had the spray gel coated (almost the entire boat). Needless to say this was heart breaking due the finishing work that included "d-a" sanding with progressive grits to 800 then compounding and polishing with various grits, well, you get the picture, A LOT OF WORK! All this was ruined by the blisters. What happened? This is my analysis. After you mix the gel coat, the hi-gloss additive and the lacquer thinner so it can be sprayed you have a evaporative medium that will be traveling though the air. The air, especially in coastal areas, contains water vapor. As the material travels though the air it picks up the water vapor and surely condenses some of it to tiny drops of water that are mixed with the sprayed material. After curing all these water molecules try and eventually succeed in joining up eventually forming water filled blisters. These blisters are between the primer and gel coat. I am blaming the sprayed gel coat but the moisture may have been deeper, possibly including all underlying material (core, glass, resin, fairing and primer). I could make a case for this scenario as the gel coat is presumably the most impermeable material and it formed the layer that trapped the moisture. If this were the case, an epoxy primer sprayed and cured before the spray gel coat may have helped. However, my money is still on the gel coat it's self. I say this because this was the most "evaporative" material which would have picked up the most moisture and in some cases the under layers would have had time to flash their moisture thought their permeable surface before being sealed off with the gel coat. You could say that I just picked the wrong day to spray but there were many days covering most of an entire year, all seasons. I would have had to be very unlucky! The boat hasn't sank? yet.
So, finishing is one thing that in hind site I would do over, except of course even building it in the first place. I choose gel coat because I wanted the most durable finish. I wish I had used my second choice, urethane paint.
Why I chose a cored construction
My 1968 SeaBird is a balsa cored hull and it has been bust'n waves for 30+ years and particularly rough use by me in the four years I owned it. It is still in great shape. Its hull is light and stiff so I had good feelings about cored hulls. Secondly, using a core gives you a mold to build the boat on. More on that later.
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