T&T: Bimini fabrics

[Robert Phillips]

I have done the primer on thread and will now address some of the  
different fabrics available for bimini's.

Sunbrella, very popular due to the wide selection of colors and great  
marketing by Glenn Raven, is a woven acrylic fabric.  It has good UV  
resistance and the ability to breath, making it ideal for sail  
covers.  Being a woven it is fairly stable in the 0 and 90 degree  
axes, with little stability on the bias, making it susceptible to  
permanent distortion if stretched in any but the 0/90 directions, as  
happens when water is allowed to pool on it.  In the Caribbean,  
Sunbrella has a usable life of about five years when exposed  
horizontally, longer when used vertically.  Stitch holes tend to  
elongate over time resulting in seam leakage.  While water resistant  
initially, that resistance deteriorates rapidly and requires  
"waterproofing" if you wish to stay dry under it.

There are several hybrid acrylic fabrics, Sunbrella Plus, which has a  
coating making it "highly moisture resistant", and Sunbrella Supreme,  
which has a waterproofing layer bonded to it.  Another company,  
Haartz, has a product named Seamark, which has a layer of vinyl  
bonded to the underside of a layer of Sunbrella, both reinforcing and  
waterproofing the fabric.  While these address the issue of moisture  
transmission, they don't prevent distortion from pooling.  If you  
have a woven fabric top, make sure you keep it tight enough that it  
will shed water in even the most severe rain.

There are a number of "marine" vinyl fabrics, typified by the white  
top / blue underside fabric we have seen for years.  These fabrics  
tend to be "laminated", which means the vinyl is applied to a  
reinforcing scrim under pressure, giving a light, medium strength,  
fabric.  The down side is the layer of vinyl is very thin, having  
been applied under pressure, and the plasticizers, compounds that  
keep the vinyl pliable, deteriorate resulting in stiffening and  
cracking.  Here in the Caribbean the life is around five years if  
exposed horizontally, more if laced tight and never removed, less if  
allowed to flex or removed and folded, as is done periodically when  
these nasty 'H" thingies come calling.

Keeping in mind that the marine fabric market is but a single grain  
of sand on the industrial fabric beach, it is no wonder that we  
haven't kept up with the rest of the world, most of which uses  
"coated" vinyl fabrics.  These are fabrics where the layers of vinyl  
are spread over a reinforcing scrim with a "knife", resulting in much  
thicker layers, from which the plasticizers take much longer to  
migrate.  There is more choice of reinforcing scrim with the most  
common being Nylon and polyester, each having slightly different  
characteristics.  These fabrics are often used for soft sided  
containers, towed on the Intrastate for millions of miles at high  
speed and in all weather conditions, a far more severe environment  
than our boats ever experience.  Even the manufacturers have no idea  
of life expectancy as the conditions in which they are used are so  
varied, but it is a long time.

I started using these fabrics eleven years ago and the first top I  
made, 48' x 18' on a 105' trimaran, is still as pliant as when it  
went on the boat.  The white vinyl cover on the T-top on my loft  
chase boat, a 23' Seacraft, is ten years old and still going strong;  
we know we have washed it twice and think there may have been a third  
time, but it looks nice and white and is a great advertisement for  
the fabric's self cleaning abilities.

We use 18 oz. fabrics for our biminis and lace on tops, with Nylon  
reinforcing, slightly stretchy, for a bimini that has to "drape"  
around a traditional bow frame and polyester reinforcing for a lace  
on top.  Both of these exhibit no permanent distortion when water  
pools and return to the original shape when the water evaporates; I  
have seen a 14" droop in a 4' area and it came right back into shape  
when the water was dumped out.

These fabrics weld easily, yielding permanent and waterproof seams.   
Stitch holes don't elongate and the fabric doesn't fray on the edges  
so hems can be single folded.

My favorite fabric is named Shelter-Rite, made by Seaman Corporation,  
18 oz. tarp.  However, as this fabric was designed for trucks the  
only colors we have found to be color fast are white, gray and tan.   
When we need any other color we use an Italian fabric made by  
Ferrari, named Precontraint 502.  Same construction, but designed for  
the commercial awning industry, which is a bit less forgiving of  
color fading that the truckers.  Given who makes it and the decline  
of the dollar, it is about four times as expensive as Shelter-Rite,  
but material cost is usually less than thirty percent of the cost of  
a canvas job so the difference isn't that great.  The 502 comes with  
an eight year warranty; Shelter-Rite one year, although I give a ten  
year warranty on the tops I build with it.

There is a coated vinyl named Stamoid, which is popular for marine  
use, available in two weights, light with a vinyl coating on one side  
and heavy with a coating on both sides., 8.3 oz. and 12.7 oz.  We  
have tried it but have found it didn't hold up well to wind load and  
stitch holes elongated causing leakage.

This is one instance where keeping away from marine will yield  
significantly longer life, especially if you are in an area where  
there is high UV, industrial fallout, or just extreme weather.

Bob Phillips,
Another Asylum, Tortola, BVI