toggle strength
 

I use toggles from time to time on my boat, and wanting to use them more (cheaper and lighter than shackles), I began to wonder how much force they're subjected to when loaded. Has anyone calculated this? What percentage of the load on the halyard, say, translates into a bending force on the (getting technical here!) sticky-out bits that the loop rests on? If anyone cares to do some destruction testing, I'll gladly spare a few toggles (I turn them myself out of hardwood on a lathe).
Ben

Benz,

I have pretty much removed them, and shackles from my boat. For about $3 you can make dyneema soft shackles that have a working strength of 4,000lbs, and weigh nothing.

Not always suitable
 

Dyneema shackles are not always suitable. If you put a thin dyneema cord through a eye spliced in a fatter dacron cord, the thinner line can chafe and cut at the dacron, which is softer. But speaking of dyneema soft shackes, why not splice a toggle into the soft shackle and not lose the strength that the knot takes away?

Do you mean toggles like you use on standing rigging, usually eye and fork but sometimes fork and fork? Like with clevis and cotter? They are for a given pin size almost always stronger than shackles and given that they are drilled for pins are more suited for the high loads of rigging. But you'd not want to feed anything flexable - wire or fibre - through the eye. So I'm thinking you mean something other than what I know as a toggle.

Different
 

Hi Ian,

Not that sort of toggle. What I mean is a little hardwood rod which you splice a line around tightly (so it can't slip out). Then you can quickly work the toggle through a loop spliced in another line, and presto! The lines are joined without a knot or a shackle. Harken is making something similar out of aluminum that they call a "Dog bone", but those are expensive and (I think) ugly. if I can find a picture of one I have made up, I'll try and post it later.
Ben

Ah. Of course. I should have thought of that.

Here's a pic--not mine, but a toggle in use. Sorry I couldn't get to load as a picture.
http://t1.gstatic.com/images?q=tbn:A...-3PCSo1_-o3x4w

I have been working on this for several weeks after seeing an incorrect use of one an aluminum dog bone for a halyard on another rigging blog. There is a little trick that puts the pull force right above the head plate that one of my readers came up with. Here is how it looks on an old headboard.


I wanted the strength of aluminum but wanted it secure. I also wanted something I could make myself so that left out the doggie bone. The dog bone I saw (SOAK) was a little nicer than the Harken one but wasn't even available yet and not priced so clearly not an option. What I did was encase an aluminum rod in some tubular webbing and sew the splice to the toggle.

I wrote it all up HERE

Allen
L-36.com

Hi Allen,

Pretty nifty--I wouldn't have thought of covering a piece of rod. I was going to have a little spliced loop that would accommodate the toggle every place that it was necessary. I don't have sails with headboards, but that's a nice way of getting the pull straight. I had considered drilling a small hole in the middle of a metal (bronze, of course) pin, spicing the line over the hole, and then carefully working a cotter pin through both legs of the loop and the hole, thus keeping the pin from slipping out of the splice. It could also be lockstitched that way...

I thought about a hole in the rod but it would be right where the stress was the highest and I didn't want to compromise the strength. I should have thought about it more as I just ran some numbers for this reply and it isn't that bad, especially if aligned correctly.

I noticed the toggles that SOAK are going to offer are 10mm in diameter and neck down to 7mm. My rod was about 8mm. The dogbone is then about 45% heavier and roughly 60% of the strength of the solid rod. On the other hand, drilling a 1/16 inch hole in a 5/16 rod would decrease its strength only about 3% if aligned perpendicular to the line and would be 68% stronger than the dogbone. It does seem to be very sensitive to the alignment of the hole as aligning it parallel to the line decreases the strength by 34%. In any event, keeping the hole or holes more or less perpendicular is how you would want it and it would still be stronger than a dogbone that necks down to 7mm.

I ran these numbers quickly so I hope I didn't get them wrong. The main point is that most of the strength of a beam comes from the top and bottom surfaces (think I-beam) so that a hole that runs perpendicular to the load doesn't change the strength much. On the other hand, the strength is diameter to the 4th power so a very small increase in diameter makes for a much stronger toggle.

One more point of interest. If aluminum is 2.7 times heavier than oak and 20 times stronger we can increase the diameter of the oak by 2.1x and get the same strength toggle and be only 16% heavier. Not bad.

Hi Allen,

Thanks for running those numbers, and the info on pin-hole alignment. Since my halyards are 7/16", then a hardwood pin that "looks" right ought to be sufficiently strong. Splicing too small a pin on a thick line just looks silly, and puts a tighter radius on the splice. And even my biggest toggle pins (or my wooden belaying pins, for that matter) are still much lighter than a SS shackle. I might try a bronze toggle pin where I have high-load HM lines (running backstay), but for lesser things I think the wood will be all right.

I agree, wood looks like a good choice. I think the numbers I ran are for white oak. Teak would be about the same and may have some advantages.

The thing I have not done is run some numbers on how large a diameter toggle we should use. That would be an interesting exercise but would depend on the size of the hole in the headboard so may be a little hard to generalize.

I am going to show my work as I am unsure of it. Perhaps someone can verify what I have done or correct it.

The question is, how large a toggle do you need to hold the halyard per the technique shown HERE and discussed above. I am going to solve this for 5/32 Amsteel which is 4000 pounds breaking strength and assume that I want to be able to hold that load at breaking strength. Finding halyard load is not easy. The chart in the back of Brion's book (page 372) for a 35 foot boat says 800 pounds. That would be a 5:1 safety factor, which sounds fine so I went with that.

The way the line is led is not exactly a knot but it does offer some stress relief to the toggle. I tested that around a very smooth post which for sure would be worst case compared to the friction of a head board. It took only 4 pounds to hold a 22 pound test weight. Thus the 4000 pound load could be held with 727 pounds on the toggle.

I assume a .75 inch opening in the headboard and 727 pounds gives a bending moment of 51 inch pounds per this formula ( w * l^2 ) / 8 .

Using a pin of radius r the load on the outer most point r from the center of the pin is 51 * r / J where J is the moment of inertia of a circle which is ( pi * r^4 ) / 2

Combining these the maximum stress as 32 / r^3. Aluminum is 35,000 psi which says r minimum is .097. That is a diameter of .194.

Oak is 7440psi so r is .16 and diameter is .325 or just over 5/16. Being wood, perhaps an additional safety factor would be appropriate and a 3/8 to 1/2 inch toggle would give that. Wood has the additional issue that the surface might crush and I did not analyze that.

I would love for someone to verify these numbers and I would encourage nobody to use them or rely on them until that happens.

Allen rocks!
 

I've always loved smart people. I asked a physicist once to work out a the measurements of a gaff sail of 300 sq ft following the ratios in the Gaff Rig Handbook. I expected him to take a week or so......he did it right there on a torn-off sheet of notebook paper using equations he had memorized. I kept the paper for a souvenir, I was so impressed. Sounds like Allen could of done it in his head.
Anyhow, my 7/16" regatta braid halyards are nowhere near strong enough, it seems, to break a 1/2" wooden toggle, and the toggles I'm turning are a little fatter than that. Also they're Rosewood and Cocobolo, so they're lots denser and harder than oak or teak. I'll probably go with some 5/8" bronze rod pieces I have left from another project for really heavy-duty loads, but it seems that for my halyards wood'll be fine. Easier to undo a toggle on an icy dark line with gloves on than most shackles.
Love to hear more about the math of it and testing.
Ben

Just be sure to remember that a lot of the strength comes from the way the line is arranged, the semi-knot. Not sure of your setup but I know that I could not get 7/16 line to pull the loop through the hole in a headboard. It is even difficult with my 5/32 Amsteel because I did both a brummel and a lock stitch and the lock stitch is too far away from the toggle and tends to interfere with the loop. I will probably cut it all off and move the lock stitch to the inch or so closest to the eye.