SilPoly

That looks like some interesting fabric. Why do you assume less water absorption? Given the hydrostatic heads listed on the website, it seems like SilPoly could be less waterproof than their 20D silnylon. It also merely says "less stretch" than their silnylon.

Edit: how does the strength and abrasion resistance of polyester ripstop compare to ripstop nylon?

Ceteris paribus, nylon will be stronger and more abrasion resistant than polyester, and a bit lighter. Polyester is substantially more UV resistant, and it's inherently hydrophobic, so it absorbs a lot less water than nylon. Nylon also loses a lot of strength when its waterlogged. Polyester is much cheaper than nylon, but lots of factors go into retail pricing, so it might not always be.

Of course, things are rarely equal, so things like thread count and fiber size and coating quality matter a whole lot.

"Given the hydrostatic heads listed on the website, it seems like SilPoly could be less waterproof than their 20D silnylon"

That's the part that still remains to be seen. RBTR has put out good products thus far, so I assume this stuff will be waterproof enough for general use in tarps. Time will tell.

Ryan

I am just about finished with another pyramid tarp made from RBTR's 20D silnylon. I'm impressed with the material so far. Waterproofness seems excellent from my bathtub test, and it seems quite strong too.

Maybe I will get some SilPoly in the hopes it would be good for a bathtub floor. It seems many tent manufacturers are using low denier PU coated materials for floors now.

There is a silpoly thread on hammock forums that has some decent info.

https://www.hammockforums.net/forum/showthread.php/101360-RBTR-SilPoly-Initial-impressions

To test waterproofness, just filling it with water isn't sufficient, although that's a good test.

In the rain, rain drops drop through the air and hit the tent with velocity. Pressure is applied to the fabric. The fabric can hold a gallon of water without leaking, but leak in the rain.

You can set it up in the rain and see if it leaks.

You can shoot water at it with a hose but it's hard to get the same pressure as if it was raining.

You can do the hydrostatic head test where you have water at some pressure, for example a column of water in a pipe, with the fabric attached over the end, increase pressure until beads of water start leaking through.

Well I didn't just fill it up with water :) I soak a towel, put a layer of fabric on top, then a cotton t-shirt on top of the fabric, and stand or kneel on top of the shirt. Any moisture forced through shows up on the shirt.

With the 20D, there were no damp spots, even when standing on the toes of one foot! Most 30D silnylons I have tried show at least some dampness with this test.

Also, I think water coming out of a hose is at a higher pressure than rain, particularly if you have an adapter on the end to get a sharp jet. Water droplets should reach their terminal velocity pretty quickly.

Interesting that these new fabrics are all driven by hammock people.

Looks interesting Ryan, thank you for the heads up.

If it lacks waterproofness, one could always boost it, but it would add some weight.

On a theoretical note, it's too bad they don't make a very tightly woven, microfiber or near microfiber, and UV treated polypropylene fabric.

It could be more burly for less weight than polyester, with less stretch than nylon, but a little more than polyester with less moisture absorption than either. Problem would be waterproofing it. I would just combine (by sewing and/or specialized tape) it with light weight polycryo, or theoretically you could melt (by calendaring?) a thin layer of non woven PP film into the top, filling in the interstices of the woven PP fabric below. This would make a pretty good, light weight, and very waterproof tarp for cheap. PP is closer to polyester in expense than to nylon.

standing on it will apply pressure so closer to what would happen in rain.

it'll be interesting to get a HH measurement or someone using it in the rain.

good test though and interesting fabric

terminal velocity of a rain drop isn't very high, and is reached after a drop of less than 10 meters. If you have decent water pressure, it's possible to spray water that far in the air. If you've got a handy three story building (or tree), you could just use a bucket…

Thanks for the heads up on this. RBTR only gives the unfinished weights of fabrics.
1.1 oz, 30 denier uncoated nylon with a DWR (durable water repellent) finish has been readily available on the DIY sites for a long time, along with 30D silcoated nylon that weighs in the 1.3-1.4 osy range after coating.

Along comes RBTR with 1.1 oz fabric that weighs 1.2 osy after the silcoat. Thank you RR for the link to the post where the polyester fabric was weighed finished. But it can't be much of a silcoat at that weight (less than .1 osy).

Granted, as DS states, the quality of the coating is just as important as the quantity. This site abounds with tales of leaky silnylon with thicker coatings.
But less than .1 osy is a bit much. There was a similar discussion here recently with Argon silnylon with a light silicon coating until RN tested it for HH and found it wanting:
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=89500&disable_pagination=1&id=bENuhKgK:184.153.187.236
Richard's posts are dated 4/7&14/2014.

The improvised tests described here may be useful to detect silnylon that has virtually nil waterproofing; but are not helpful in distinguishing fabric that is going to leak under lengthy and heavy rain from fabric that is not. An HH tester, especially when applied to swatches from different samples and used both before and after an accredited 'aging' process, provides a much better guide, and that is why the better tentmakers, including cottage ones like MLD, HH test their fabric.

For several years, I sent swatches to BPL equipment editor, Roger Caffin, for HH testing that he graciously provided, and the difference in results for materials that easily withstood hose and other improvised tests was amazing. Very little of the stuff was reliably waterproof. Silnylon that is reliably so, currently comes from Asia in places where less restrictive environmental restrictions apply. But alas, because quality widely varies, even on the same roll, you've got to either HH test it yourself or rely on someone you trust to do it if you want to be reasonably sure.

It takes me a long time and a lot of effort to design and construct a tent, or even just a modded tent, so I want to be reasonably sure. Camping can be a joy with a reliable shelter, but not at all with a leaky one. Been there and done that.

So does the HH test correlate with waterproofnes from an extended rainstorm?

Of course, Richard Nisely does the most HH testing. Doesn't he do this as a professional as opposed to us amateurs : )