Rope Trick?

[Howard]

For those of you who have glued a piece of rope to the leading edge of your CB or rudder blades, what type of rope, exactly, did you use? I've heard of everything from cheap braided polyethylene tow line to braided polyester to braided nylon dock lines to something as elaborate as Sta-Set double braid.

 

I would think the qualities you are after are abrasion resistance, compatibility with epoxy and an open enough weave that it can be fully saturated, leaving no air pockets. I can't imagine you could ever wet out something like Sta-Set, so that leaves the rest.......or maybe there is yet another option I've not heard of?

 

 

 

 

 

 

 

 

[Brent]

Cheap braided poly propylene. Notice the open structure which will easily take up resin

[Howard]

By cheap polypropylene, are we talking the hollow braid stuff, such as a ski or tow rope? They also make a solid, single braid polypropylene as well as a stiffer, double braid poly with a core. I can't see using the latter due to the core.

 

At first glance, the hollow braid stuff would be too similar to the polyethylene (different poly?) plastic we use to keep things from sticking. Stuck or not, maybe being imbedded in a glob of slightly thickened epoxy is enough? Beyond that, the hollow core would be just that........a hollow core filled with thickened epoxy.

 

Of the two, my guess would have been the solid single braid. Assuming you can get it fully saturated, it would seem to offer the best properties for abrasion resistance and abuse.

[PAR]

I use a single braid polyester, not polypropylene, which I question in it's ability to stick well to epoxy. Instead of line, solid bar stock in a groove and pulverized stone dust works well too. Line marked as polyethylene is likely a polyester too.

[Howard]

Paul:

 

So would you be using something like New England Rope's Regatta single braid? Or something from a hardware store? Seems to me that any line that floats and does not get wet is not going to be compatible with epoxy. It water will soak into it and it sinks, I would think epoxy would as well, leading up to full saturation and wet out of the line. Beyond that, I would think that a solid braid line is going to hold its round shape better?

 

The only ground stone dust for epoxy that I'm aware of is WEST's 404 filler:

 

http://www.westsystem.com/ss/404-high-density-filler/

 

I've heard it said that a small bag of ground limestone from the garden center is the same thing for a fraction of the cost. Collodial silica is pretty hard too.

[PeterP]

Howard, don't over engineer this. The line has to take epoxy and has to be synthetic. It has to take epoxy so you can stick it to the board. It has to be synthetic so it does not rot when it cracks under impact. And crack it will no matter what line you use. Sort of like a sand bag. Keeps taking the bullet impact getting worse looking each time and when it gets real ugly you replace it. You have been around sail boats long enough -just look in that box of odds and ends and chances are you already have a piece of line that is too long to throw away yet too short to be practical. Just use it. I would not buy anything special. Cheers peterP

[Howard]

Don't over engineer? I am prone to doing that.  (I've heard it said that the first step in recovery is to admit you have a problem)

 

Actually, I tend to think of this as a way to gain understanding of the issues so when you get to that fork in the road and a choice has to be made, I'm more likely to make the better choice. Best way I know of to gain that level of insight is to ask questions of those who have been down that road.

 

You are right about the lines. I must have 10 different types and sizes laying around here. But at the same time, I figure there is a poor, better, and best situation with these. Hate to find out after the fact I picked the wrong one when the best was on hand and available. Stated another way, I'd like to do it right the first time.

[PAR]

Polypropylene is well known for it's "self lubricating" properties, which also makes it hard for things to stick to it. Both synthetic and natural fibers can be used, as they'll become entombed in goo, once installed. Cotton line will work, as will most of the synthetics. I prefer single braid, because it saturates easier. Lately, I've gone away from the "rope trick" and been using decomposed granite. It's literally like having a stone leading edge and absurdly tough. It's difficult to mold and nearly imposable to sand fair, but it's bullet proof. I make a mold of the finished shape of the leading edge with plaster, then I mill the leading edge off the board (or whatever), usually making a groove (or two) to help "key" the cast leading edge in place. I mix stone dust into an extremely tick paste, maybe 70% stone dust and pack the mold full. Once cured the plaster is washed off and the stone casting bonded and/or mechanically fastened onto the board. You will need masonry bits to drill screw holes with this stuff. Decomposed granite is commonly available at landscape and brick paver supply outfits.

[Hirilonde]

Up in New England it is called "rotten rock" and used for driveways and under stone, brick and tile patios.  In reality it isn't rotten nor is it really decomposed as there is nothing organic about it.  But that is what it seems like.  Interesting idea.

[PeterP]

Howard, do this experiment and you will understand the beauty and simplicity of the rope trick. Soak a piece of line in epoxy and let it cure for a couple of days. Mix up a cup of your rock/epoxy stuff. If you want- do a third sample of steel wool and epoxy. When it is all well cured lay it on a concrete floor and whack the samples with a hammer. What you'll see is that your rock stuff will shatter in zillion pieces because there is nothing to hold it together. The rope on the other hand - although fractured - will hang together. Ditto for steel wool. The difference is the fibre matrix. Obviously, the steel once cracked-even though stronger to begin with -will rust out in no time at all. The synthetic line wins hands down. I always thought this one of Tom's real clever ideas. Simple, cheap, effective. Hard to improve on that. PeterP

[PAR]

Actually the rock mixture (if it's dust) will not shatter into a zillion pieces, though some localized fracturing will take place. It'll look more like a road stone bruise on a car's windshield - strongly fragmented at the impact point, with radiating and decreasing fracturing, the farther away from the impact site you inspect. Because of it's density, it takes a strong impact to do damage of any significance. In fact, if you compare apples to apples, I'll bet it takes twice the force on a pulverized stone mix, to get the same damage as you get with most other materials. It also adds a little bit of ballast along the leading edge, which can be taken to another level if desired, with a much bigger pulverized stone area. This mixture will be heavier than concrete, though no where near lead or steel. I'll bet a 160 pound per cubit foot mix could be made.

[Howard]

OK.....on to the second part, which is how to actually build the thing. As I recall the original concept, after the line was soaked in epoxy, it was stretched down the leading edge of the foil. I also seem to recall the board was shaped first, then trimmed to allow for the line. Once the line was stretched out down the leading edge, plastic sheets were pulled tightly over it, pinning it down. The force on the plastic was enough to compress it slightly against the edge, which as I recall, more or less forced it into the leading edge shape of the foil? It was then faired in and glassed over.

 

I'm going to try a slightly different approach. First off, it seems to me that most of the B&B foils are designed more or less to a NACA shape of 0010, meaning the width of the board is 10% of the chord length (fore and aft dimension). The leading edge of a NACA foil is round in shape. As I understand it, the leading edge is a 90 degree quadrant of a circle. For a NACA foil, the radius of the circle is 1.1% of the chord length. This is important as this more or less defines the diameter of the line you would use. A board with a chord of 12 inches would use 1/4" line:

 

(12" x .011) x 2 = .264"

 

The Princess 26 CB has a top chord of 17.5 inches and since the CB is 1.75 inches thick, this appears to be a NACA 0010 foil. Thus the line diameter to use would be:

 

(17.5 x .011) x 2 = .385 (3/8 = .375) (7/16 = .4375)

 

On the plans, the diameter for the leading edge of the foil is listed as 7/16 inch. Glassed over, a 3/8" line will probably bulk up to reflect a diameter of 7/16". Beyond that, the board tapers down to the tip. Closer to the tip, the chord becomes 16.5", but since the board does not taper in thickness, the % of chord (NACA foil shape) increases slightly. Not much, but some. But anyway, the size of line I should be using is 3/8".

 

So not only do you have to decide on a type of line, but the one you select needs to be available in the right diameter.

 

Having done a few CB's and rudders, I've come to the conclusion that getting a good shape to them is important for performance, but the shape is probably not nearly as  important as getting good symmetry. Whatever the shape, it is important that both sides are shaped the same, else you may get more lift on one side than the other. This is really noticed with rudders with the tiller wanting to pull or drift to one side. Bad symmetry might also show up as vibration or a hum at speed. You don't want that.

 

Getting good symmetry starts off with accurately marking the centerline of the leading and trailing edges and shaping to those. I can do that with pencils, but somehow, during the shaping process, those lines always seem to smudge and wander.

 

Current plan is mark the centerline of both the leading and trailing edges. On the leading edge, I intend to use a half round router bit and router in a groove down the leading edge. I'll stretch the line soaked in slightly thickened epoxy down that, where it will become the perfectly shaped leading edge for my board. On the trailing edge, that needs to be a sharp edge and to be durable, needs to also be made from thickened epoxy. I'll use a 1/4" flute bit and router in a square channel down the centerline and fill it with epoxy. That then becomes my trailing edge and a fixed and identifiable reference to shape to.

 

I did a mock up on a piece of scrap and it seemed to work well. With a couple plywood templates trimmed to scale, I should be able to build some pretty good foils this way. BTW, that is 1/2" single braid polypropylene line (individual strands get all fuzzy when cut) soaked in slightly thickened epoxy. I whacked it hard enough with a hammer I knocked it out of my hand, and I didn't even dent it. I can see that having such a line out front is certainly the way to go