beautiful skeleton

Yes, back when aircraft were covered with cotton the fabric was shrunk with dope. The stuff was pretty obnoxious to use though and remained highly flammable even when dry. Cotton stopped being used on aircraft (except for some historic types) a long time ago now, and pretty much every fabric covered aircraft uses Dacron (polyester). The fabric is supplied unshrunk (normal polyester used for clothing is pre-shrunk, by passing the fabric through heated rollers before the cloth is made up into garments). Being un-shrunk means that the weave is a little looser, making it slightly easier to drape over compound curves, plus it gives it a lot of shrinkage when tightening it up with an iron - maybe as much as 12%.

Here's a link to the Ceconite web site http://www.ceconite.com/index.htm (Ceconite are one of the biggest suppliers of polyester/Dacron cloth for aircraft). Take a look through the articles on there by Ron Alexander and you'll get a feel for how this stuff is used on aircraft.

Jeremy

 

Platt Monfort's "Geodesic Airolite" boats use aircraft Dacron fabric as the covering material. http://gaboats.com/

 

Yes, I know that Platt Montfort uses the same fabric, but what puzzles me is why he didn't just use the same fabric techniques. They've been proven to work well, last a long time (a fabric covered aircraft skin has a typical life of 15 to 20 years before needing replacement) and be easy to repair.

It seems to me that a lot of SOF builders have selected aircraft fabrics but ignored the attachment and finishing techniques, preferring to stick with the way that native peoples attached skins to their craft, rather than use well-proven adhesive techniques.

I've done some rough calcs for typical skin loadings on a small SOF boat and compared them to those on a light aircraft and they are pretty similar. Small boats need a bit more abrasion resistance, which makes me wonder why reinforcing tapes aren't routinely used. These are easy to apply and not only provide additional protection but also act as sacrificial wear strips - they can easy be replaced if they get a bit abraded, or even just have another tape glued over the top.

I've opted to use the heaviest weight Ceconite 101 at 3.3oz, although I think that it's overkill for a one-man rowing boat. I've a lot of my experience has been with the 1.7oz ultralight fabric and I know first hand just how tough that stuff is when properly finished. The received wisdom seems to be that 8 to 12 oz cloth is the preferred thickness, which seems massively stronger (and heavier) than is needed. It also seems that few people use decent anti-abrasion coatings - in my view these are essential for fabric longevity. The PVC based finishes add a fair bit of abrasion resistance and overall toughness to the fabric. Although they don't increase its tensile strength, they do make penetrating damage less likely, by giving the surface a low coefficient of friction and making it less likely that something will dig in and case a tear.

Jeremy

 

Is the major difference you are refering to gluing the fabric to the frame?

There appear to be a wide variety of methods used by SOF builders. I wouldn't lump them all together.

 

Partly, yes, but the main difference I've noticed is that few builders (well, none, that I've seen) use reinforcing tapes. Gluing fabric to a frame requires that the frame be designed to allow a good bond (primarily adequate bond area and wrap angles), but that's not turned out to be too hard to do in my own build (but then I'm used to designing fabric covered aircraft).

The reinforcing tapes make a significant difference to the overall robustness of the covering of an aircraft, providing added strength, abrasion resistance and fabric stiffness where it's needed, next to solid members, but allowing the fabric between supports to remain light and flexible. The flexibility is key to getting good impact resistance, as it allows the fabric to deflect a long way when it hits something - typically 30% stretch or so - before it reaches breaking point. It actually makes the fabric skin tougher by doing it this way, by only adding stiffness where it's useful.

I think the other point is that using PVC based finishes is, in my experience, a superb way to complement the properties of polyester, yet I seem to see a lot of SOF boats just finished in PU varnish, without even any UV blocking pigments. I saw a well-used aircraft here in the UK where the fabric had lost 70% of its strength after just two years use, purely from UV exposure, so I do wonder why so many SOF boats don't seem to be well UV protected.

Jeremy

 

- I’ve long been retired (I really am entitled to that “ancient” label) but it comes from a poster one of the guys that I used to worked with pasted up in his cubicle which said “it’s hard to soar like an eagle when you work with turkeys” although personally when the cubicles were introduced and I lost my office, I felt more like a battery hen.

I think it was the same deal as with model aircraft, the fabric was shrunk first then sealed using the dope. I’m not sure what those folks do these days . . .

 

distortion

I covered my boat with heavy cotton duck, and found that the shape was considerably altered when I stretched and shrunk the fabric. The bottom and sheer were both flattened out quite a bit. The frame was very robust for a SOF boat. Is that the reason that lat Monfort used the diagonal kevlar tapes?

 

My guess is that Monfort used the kevlar tapes to keep the boat from bending up at the bow and stern under use. If you don't you need to make the frame members heavier so that they individually resist the bending caused by the persons weight being supported by the water spread over the length of the boat. The tapes should make the boat act more like a solid skinned boat by providing a diagonal shear load path.

Did you have many ribs on your boat to support the tension caused by the shrinking fabric?

 

The diagonal tapes would resist distortion during skinning but if you can get the skin in place without changing the hull shape then the skin does the same job as the tapes, which become redundant.

 

Terry,

Whats the stiffness of Kevlar? Pretty big in tension I believe, in compression it would not count since a tape would quickly buckle.
Whats the stiffness of Polyester. Pretty small, of course the entire skin would help. Wish I had some real numbers but Polyester is so small that no one cares in my business. I know a guy I would consider an expert about skinning an aircraft - I'll try to ask.

I really expect the stiffness increase with polyester is really small even with the large area compared to kevlar with its small tape area.

I'm day dreaming about making a catamaran like hull with SOF. I am concerned that the frame would have to be so heavy to get the vertical bending under control that there might not be weight savings like I had assumed. Kevlar tapes might allow me to keep the frame small (the section of each piece) and still not have it too bendy. The hull would be 18" x 6" wide with 9" depth. I could easily build it strip planked an it would be significantly over sized as far as being stiff vertically.

 

I don't really know but I read on George Dyson's web page that polyester is much less elastic than nylon FWIIW. I have found it so in other applications. I haven't used Kevlar but it has a rep . . .

 

Terry,

Can you give me the link to Dyson. Must be something wrong with me, again, but I can't find it.

Whats "FWIIW" ?

Marc

 

If the structure is properly designed to take the fabric tensile loads then the skin will add a lot of additional strength and stiffness. It's not unusual for the fabric on an aircraft wing, for example, to increase the strength by around 10 - 20% over the uncovered wing (it's for this reason that structural testing should be done without the fabric, as fabric strength degrades with age and UV exposure).

The heaviest gauge polyester aircraft covering, the stuff used on high speed aerobatic aircraft and Dakota control surfaces, has a breaking strength of 125lbsf/in and a bursting strength of 275psi, yet is only 3.5 oz/yd².

When heat shrunk and coated it retains a degree of stretch that will reduce the strength and stiffness contribution it makes to the structure (due to off loading to other stiffer components in the frame) but nevertheless it will still make a significant contribution by adding a tensile member all around the outside of the frame.

The Kevlar cords used on the GA boats are, in my view, potentially problematic. The impact resistance of polyester fabric skins comes from allowing local deflection to absorb the impact before reaching the fabric bursting strength at the impact point. Anything that restricts deflection under point loads will tend to make the fabric more likely to fail from a point impact. Anyone who's worked on fabric covered aircraft will know first hand that the areas that get holed tend to be those where the fabric is closely supported; areas with wider spaced supports tend to be less subject to damage. Kevlar has a very low stretch before failure, only a couple of % I believe, whereas the polyester will stretch a great deal before it fails.

A key point is that the frame has to be designed to take the fabric tensile loads, both to avoid distorting the desired shape of the boat and to ensure that the skin remains tight when the boat is loaded and in the water. In an open boat the skin will tend to pull the gunwales down towards the keel and flatten the sheer. Simple measures, like fitting the gunwales to the top of the frames, rather than the outer edge of them, will help, by directing a greater proportion of the fabric tensile loads into the frames in compression, rather than bending. Adding thwarts to keep the frames in shape will also help to resist distortion, as will bracing the keel to prevent it tending to hog under the loads.

Kayaks are inherently simpler to design in this respect, as the deck skin compensates for the tension of the hull skin.

Jeremy

 

scantlngs

My boat has a full 1" X2" mahogany keelson, 3/4" X 3" gunwales and 5/8" X 3/4" laminated ribs on 10" centers. I should have braced the frame side to side and longitudinally as I stretched the skin. It is a lot different shape skinned than as a bare frame, but now it is very rigid with thwarts and floor boards installed. Live and learn, I guess.

 

Me neither. For What It Is Worth I think it has disappeared from the Web. There was a lot more technical information and advice out there a few years ago but very little now apart from the links in post #15: I am not sure if this is a method by his publishers to promote sales of his book, or what . . . he still operates a business selling the tubing, cloth and other stuff.