While the boat was on the hard over Winter, I took the first steps towards building a new rig, starting from the ground up as it were, with the forestay bridle and bridle u-bolts.
My original bridle was 5/32 1x19 stainless steel wire with 1/4" u-bolts. They were still tight after 10 years, but there was polish and wear on the u-bolts where they take the load from the bridle wires. The plans say that the bridle wires should be 3/16, so they were undersized as built.
There has been much discussion on several internet forums about building reliable bridle anchors. Later Wharram designs were specified with cord lashings instead of u-bolts. However, I was reluctant to drill multiple holes through the stem and wanted to keep the u-bolts. In an internet posting Rory McDougall had recommended 5/16 "or even larger" u-bolts to be on the safe side.
Word to the wise - use forged u-bolts. Wichard makes a range of them and even the 1/4 Wichard u-bolts are rated to 1400 lbs working load and 4,400 lbs breaking load (that is, around a 4X safety factor breaking to working load). Unfortunately, I was on the hard at the boatyard and could not wait for the 2-week delivery time for Wichard bolts. So I bought some West Marine 5/16 316 (non-forged) stainless bolts - big mistake. These were "rated" to "2,200 lbs working load" which is a strange number because the breaking strength of a rolled stainless 5/16 bolt is about 4,000 lbs, and working load is some multiple of breaking strength (2X, 3X, 4X etc.). So 2,200 lbs is not even a 2X safety factor from the breaking strength of the bolt. Further when I was installing these bolts, I torqued them to the rated 11 foot pounds for a 5/16 bolt and they galled (welded in place). This happened to both bolts, so I bought a third bolt thinking that I had over tightened them, and carefully torqued this third bolt, and it to galled before reaching the final torque of 11 foot pounds. Thanks West Marine! Most of the stuff I buy from WM seems good quality, but these u-bolts were so bad that they are dangerous.
So, then I found these stepped Ronstan u-bolts that have a 5/16 thread and a 3/8 U section. This gives me a working load for the threaded bolts of around 1,000 lbs (based on the 4,000 lb breaking load of a stainless 5/16 bolt), and the much larger section of the stepped U to absorb the wear of the bridle. Note that with each u-bolt taking the load on two threaded ends, the true working load is going to be much higher than for a single bolt. Unlike the awful WM bolts, the Ronstan bolts could be torqued and un-torqued to the 11 foot pound rating with no problems whatsoever (that is, good quality steel).
OK, so I had my u-bolts.I then spent considerable time drilling the mounting points oversize, filling them with thickened epoxy and then re-drilling them to the correct clearance size so that the u-bolts could be torqued into place and then removed whenever I want to, rather then being glued into place as the previous ones were. I also made aluminum mounting wedge plates that allowed the u-bolts to mount fair and square from the stems (rather then pointing forward because of the angle of the hulls) and epoxied them in place.
The next decision was which wire to use for the bridle. The plans specify 5/32 6x7 stainless wire for the standing rigging and 3/16 for the bridle. That wire (6x7) is generally no longer available, so I used 7x7 which is ideal for self-made rigging as it is flexible enough to use Nicopress swages. Working load for 5/32 7x7 316 wire is 440 lbs and breaking load is 2,200 lbs (see image below which belongs to Suncor USA). This specification from Wharram is obviously usually fine because there are hundreds of Tiki 21s sailing around the world with standard spec standing rigging. However what are the actual loads that a Tiki 21 rig is exposed to? It turns out that there is no easy formula for this important question and expert sources invariably say that there are too many variables to easily quantify. One source is a small table in Brian Toss's Rigging Guide that suggests that loads on a 20 foot boat are around 500 lbs, which fits with the Wharram's original spec.
But what about peak loads? I had a Harken High Load (wire) Bullet Block deform and almost break on the mainsheet bridle (which takes the full rig load). I assume that this was with the rig powered up going to windward in the usual 20 knot summer breezes in San Francisco Bay. That block is rated at 500 lbs working load and 2,000 lbs breaking load. This unscientific study leads me to believe that rig shock loads could be as high as 2,000 lbs, that is, approaching the breaking strain of the standard spec 5/32 wire - or, at the very least, that the rig is exposed to loads way beyond the working load safety factor. So what to do? For my new standing rigging I am going up one size to 3/16 wire which rates at 620 lbs working load, and a hefty 3,100 lbs breaking load - well beyond the shock load that nearly broke my Harken block.
All well an good, but what about the loads on my new bridle? The reason why the standard spec for the bridle wires is larger than for the rest of the standing rigging, is that the angle from the forestay to the bridle wires subjects them to a higher load. The graphic below (from LifTechniques.com) shows that lifting slings need to be rated to match the loads of the angle of the sling. The Tiki 21 bridle angle is about 60 degrees from the vertical, and the chart below shows that this angle would double the load on a single sling. Going on the working load for 3/16 7x7 wire (620 lbs), this means that the full working load applied to the forestay would apply double the load to the bridle (1240 lbs). However, that actual load to the bridle is divided by two, as there are two bridle wires. This means that a bridle made of the same 3/16 wire as the forestay would meet the working load requirements.
I misinterpreted the sling loads and forgot to divide the bridle loads by two. This error lead me to make the bridle wires out of 1/4 7x7 wire (working load 1,040 lbs and breaking load of 5,200 lbs. This means that my bridle is overly robust and heavy. I may change them again to 3/16 wires during the next refit.
That just left the fitting to join the bridle to the forestay. The Wharram spec is an oblong quick link fitting, but it always bothered me that the oblong shape left the bridle wires at an odd-looking angle. So I sourced the Maillon Rapide 3/8 316 Stainless Steel triangle delta link that you can see in the above picture. It took me a while to find and order this part, and I went to the trouble because it was the only brand that was available in 316 and that had a test rating.
The bridle is now finished and installed, and is now just waiting for me to finish the new mast and the rest of the rigging - more on that later.
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| Bridle and u-bolt positions |
My original bridle was 5/32 1x19 stainless steel wire with 1/4" u-bolts. They were still tight after 10 years, but there was polish and wear on the u-bolts where they take the load from the bridle wires. The plans say that the bridle wires should be 3/16, so they were undersized as built.
There has been much discussion on several internet forums about building reliable bridle anchors. Later Wharram designs were specified with cord lashings instead of u-bolts. However, I was reluctant to drill multiple holes through the stem and wanted to keep the u-bolts. In an internet posting Rory McDougall had recommended 5/16 "or even larger" u-bolts to be on the safe side.
 |
| 3/8 u-bolt bearing surface and 1/4 stainless wire |
Word to the wise - use forged u-bolts. Wichard makes a range of them and even the 1/4 Wichard u-bolts are rated to 1400 lbs working load and 4,400 lbs breaking load (that is, around a 4X safety factor breaking to working load). Unfortunately, I was on the hard at the boatyard and could not wait for the 2-week delivery time for Wichard bolts. So I bought some West Marine 5/16 316 (non-forged) stainless bolts - big mistake. These were "rated" to "2,200 lbs working load" which is a strange number because the breaking strength of a rolled stainless 5/16 bolt is about 4,000 lbs, and working load is some multiple of breaking strength (2X, 3X, 4X etc.). So 2,200 lbs is not even a 2X safety factor from the breaking strength of the bolt. Further when I was installing these bolts, I torqued them to the rated 11 foot pounds for a 5/16 bolt and they galled (welded in place). This happened to both bolts, so I bought a third bolt thinking that I had over tightened them, and carefully torqued this third bolt, and it to galled before reaching the final torque of 11 foot pounds. Thanks West Marine! Most of the stuff I buy from WM seems good quality, but these u-bolts were so bad that they are dangerous.
 |
| 5/16 threads and nylock nuts on shaped base |
So, then I found these stepped Ronstan u-bolts that have a 5/16 thread and a 3/8 U section. This gives me a working load for the threaded bolts of around 1,000 lbs (based on the 4,000 lb breaking load of a stainless 5/16 bolt), and the much larger section of the stepped U to absorb the wear of the bridle. Note that with each u-bolt taking the load on two threaded ends, the true working load is going to be much higher than for a single bolt. Unlike the awful WM bolts, the Ronstan bolts could be torqued and un-torqued to the 11 foot pound rating with no problems whatsoever (that is, good quality steel).
OK, so I had my u-bolts.I then spent considerable time drilling the mounting points oversize, filling them with thickened epoxy and then re-drilling them to the correct clearance size so that the u-bolts could be torqued into place and then removed whenever I want to, rather then being glued into place as the previous ones were. I also made aluminum mounting wedge plates that allowed the u-bolts to mount fair and square from the stems (rather then pointing forward because of the angle of the hulls) and epoxied them in place.
 |
| Rated triangle link ready to go |
The next decision was which wire to use for the bridle. The plans specify 5/32 6x7 stainless wire for the standing rigging and 3/16 for the bridle. That wire (6x7) is generally no longer available, so I used 7x7 which is ideal for self-made rigging as it is flexible enough to use Nicopress swages. Working load for 5/32 7x7 316 wire is 440 lbs and breaking load is 2,200 lbs (see image below which belongs to Suncor USA). This specification from Wharram is obviously usually fine because there are hundreds of Tiki 21s sailing around the world with standard spec standing rigging. However what are the actual loads that a Tiki 21 rig is exposed to? It turns out that there is no easy formula for this important question and expert sources invariably say that there are too many variables to easily quantify. One source is a small table in Brian Toss's Rigging Guide that suggests that loads on a 20 foot boat are around 500 lbs, which fits with the Wharram's original spec.
But what about peak loads? I had a Harken High Load (wire) Bullet Block deform and almost break on the mainsheet bridle (which takes the full rig load). I assume that this was with the rig powered up going to windward in the usual 20 knot summer breezes in San Francisco Bay. That block is rated at 500 lbs working load and 2,000 lbs breaking load. This unscientific study leads me to believe that rig shock loads could be as high as 2,000 lbs, that is, approaching the breaking strain of the standard spec 5/32 wire - or, at the very least, that the rig is exposed to loads way beyond the working load safety factor. So what to do? For my new standing rigging I am going up one size to 3/16 wire which rates at 620 lbs working load, and a hefty 3,100 lbs breaking load - well beyond the shock load that nearly broke my Harken block.
All well an good, but what about the loads on my new bridle? The reason why the standard spec for the bridle wires is larger than for the rest of the standing rigging, is that the angle from the forestay to the bridle wires subjects them to a higher load. The graphic below (from LifTechniques.com) shows that lifting slings need to be rated to match the loads of the angle of the sling. The Tiki 21 bridle angle is about 60 degrees from the vertical, and the chart below shows that this angle would double the load on a single sling. Going on the working load for 3/16 7x7 wire (620 lbs), this means that the full working load applied to the forestay would apply double the load to the bridle (1240 lbs). However, that actual load to the bridle is divided by two, as there are two bridle wires. This means that a bridle made of the same 3/16 wire as the forestay would meet the working load requirements.
That just left the fitting to join the bridle to the forestay. The Wharram spec is an oblong quick link fitting, but it always bothered me that the oblong shape left the bridle wires at an odd-looking angle. So I sourced the Maillon Rapide 3/8 316 Stainless Steel triangle delta link that you can see in the above picture. It took me a while to find and order this part, and I went to the trouble because it was the only brand that was available in 316 and that had a test rating.
The bridle is now finished and installed, and is now just waiting for me to finish the new mast and the rest of the rigging - more on that later.
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