Force vectors

[dmonn]

The last time I did force vector analysis was in 1971, so I'm a little rusty. I'm actually working on some rigging for my friend's 45 ft sailboat, but figured you guys could check my analysis.

I'm trying to figure out a reasonable length for a bridle. A second line will be attached where it can slide a very small amount along the bridle line. The second line will carry an essentially static load of 600 pounds (the log in the sketch). We'll be using Amsteel lines. We want to make the size of the bridle line as thin as would be safe because of what it will be attached to.

By adjusting the length of the bridle, the angle at point C changes, and that effects the tension on the bridle line. Have I done the calculations correctly? Conclusion: A longer bridle reduces the bridle tension.

Next consideration is the attachment at points A, B and C. Points A and B will be the sides of a stainless steel U-bolt. The diameter of the bolt is not known yet, but looks fairly small in the photo I've seen. Maybe 3/16 or 1/4 inch. If I splice eyes in the end of the bridle, I'm concerned about the bend radius around the bolt. Do I need to put bushings on the bolt to increase the bend radius? If the bolt only gives me a 1:1ratio, there could be a 70 - 80 percent loss of strength for the Amsteel. (Marlow Rope tech info if I read the Dyneema chart correctly).

The attachment at Point C is the next consideration. Movement along the bridle will be minimal. I was thinking of using a spliced eye with a stainless steel thimble, a rigging ring, or ? Whatever is there might bang into the fiberglass hull of the boat, so steel or anodized aluminum would not be the most desirable substances. If either of those are the best or only good choices, we could probably come up with some way to prevent it from banging into the hull.

So to sum up my questions, have I analyzed the forces correctly and what should/could I use at Point C?

 

[ghostice]

Besides a buncha engineering/ cad type packages (learning curve and $$$ warning) there's stuff out there like RigRite and others. Maybe engineering toolbox on the web has something too. Good hunting. Cheers.

 

[Dan Cobb]

I didn't notice any errors in your calculations. However, I'm at a total loss trying to picture the application, especially when the ring (or other hardware) may slap the hull. Don't "standard" solutions exist that achieve the goal of this rigging?

 

[dmonn]

Thanks.

The application is putting in a retractable bowsprit. It is for the line that angles down and aft from the tip of the bowsprit to a small hole in the bow of the boat. The U-bolt is on top of the bowsprit and the bridle would wrap down and partly around the sprit. This prevents the sprit from being ripped upwards off the boat when the Spinnaker fills. I don't think many people put retractable bowsprits on Beneteau Oceanis 45s, so there's no "standard" way of doing this. Tom likes trying to turn his luxury cruiser into a racing boat.

 

[Bart_]

Two points. If you can let your bridle line be as long as possible to keep angle y small because the opposite is making a line sweating force amplifier that tries to tear out your anchors toward each other. The other is the eyelet or ring etc where the lines join at C. The bridle is less than 180 degree wrap but my testing shows a lot of friction likely so the tension ratio could be 1.5 in the bridle legs during self alignment. You could help equalize the load sharing by having at least a self lubicated bushing/pulley that traverses the bridle. Equalizing the loads buys you more headroom in loading vs strength limits. Bushing will buy you more bend radius too. Also comes down to whether the bushing spins or if it skids across the bridle.

On the positive side dyneema is about the slipperiest fiber so the tension ratio will be lowered if you use it for the bridle. In bend life/loading testing I did decades ago dyneema/spectra came out favourably vs kevlar and vectran.

 

[dmonn]

Thanks Bart. Good points.

 

[Reach]

Thanks Bart. Good points.

One concern I would have there is the brittleness of Dyneema. When the spinnaker deploys, it will hit hard, and Dyneema does not like shock loads. It seems to me that something with a hint of elasticity might not be a bad idea, unless your Dyneema is far is stronger than it needs to be.

 

[dmonn]

Thanks Reach. I'll point that out to the owner (Tom).

I like Dyneema for this application, with consideration to what Reach said, is that the rope needs to go through a grommet and into the chain locker. Tom wants to minimize the amount of water getting into the chain locker (which has a small drain). An eye splice in Dyneema is smaller and more "squishable" than a Class I eye splice. The eye needs to be pushed through the grommet and captured with a dogbone. By using extra strong Dyneema, the size of the grommet can be minimized, and therefore limit the leakage when we smack into an 8 foot wave and the bow goes 2-3 feet underwater. I am the bow guy, and love sailing this boat in heavy weather!

 

[Bart_]

Dyneema/spectra strength is way up the chart. So is it's durability. We chose it as our test winner. The principle of load cycle testing is what percentage of ultimate load you operate at vs how many cycles you get, so it's a given that it will operate well below ultimate loading. But it is very inelastic. One could consider whether a "rigging" (!) ring (ironic!) that skids on the bridle could act as a shock damper if the geometry works out. Spectra isn't brittle. It's used for bullet proof vests.

 

[Bart_]

Y'all got the irony that rigging rings originated on sailboats? Back to seriousness, if you're concerned about shock loading the lines then consideration should be given to the anchors especially if in fiberglass or carbon fiber. My understanding is that fatigue failure in those is progressively snapping more fibers each load cycle till you get a fracture or tear out. So the benefit/idea of a rigging ring skidding into position and acting as a load spike damper is real. If you want to try something neat and your amsteel is hollow braid, make a skinny football shape from elastomeric material like polyurethane foam etc and put it inside the hollow braid. Voila, inline shock damper.