Designing For Dux Rope Rigging—A Paradigm Shift—Part 2

Hi Damon.

I recommend reading the article John links to, but i can’t resist to comment.

Stability in boats has mainly two reasons. Form stability, like we stand more stable if we move our feet further apart, and low weigth centre, like a pendulum. Inertia, on the other han, is NOT stability, just resistance to change.

A monohull is hoisting a weight up the mast while exposed to waves from abeam: As the weigth gets higher, the centre of gravity goes higher too, reducing stability, of course, but the higher inertia makes the boat slower at reacting to the wave, which is often nice but when it has gotten into motion, it’s also slower at stopping the motion, which is mostly not nice. This means it will roll slower, but move through a greater angle. (Lower frequency and higher amplitude.) This is increased by the reduced stability from a higher centre of weight, that will resist the pendulum motion less.

Conclusion:
No surprises, a boat with a heavier rig will roll slower but more, so it will be more unpleasant to be aboard. It will also sail noticeably slower. Partly because it will lift its bow higher and slam deeper at each meeting wave, and partly because it has less stability and thus less ability to hold its sails. It is NEVER a good idea to make the rig heavier than it must be.

So, am i saying that there are a lot of people out there making false claims? Well, let me think… Hm. Yes, that’s what i do say! 🙂 There’s much complete bullshit out there on this topic, even from “serious” people. I can’t prove by words I’m better. You’ll have to use your own logic to decide what you think is true….

Hi guys. Thanks for responding. I’m a mechanical engineer and understand the forces. But like John’s link says, the physics is complicated and it’s simplistic to talk about specific forces or conclusions. I’m not suggesting to make the rig heavier. I’m asking if there is any risk in making it lighter than the type that has hundreds of thousands of successful ocean miles (and where experience suggests removing the rig drastically reduces capsize resistance). My guess is that the difference in weight we are talking about doesn’t really matter to either the static or dynamic stability, but I wanted to know if anyone had run the numbers. These answers say: “nah, we don’t need to [we already know the answer despite the complicated physics].” Some of John’s link is about moving weight from several feet above the center of gravity to a foot above it and that is unquestionably good. My question was about weight much further away, and even if the effect is overstated, I would think it is worth understanding to best balance different components of stability.

Hi Damon.

I didnt try to be a wise ass. Sorry if it seemed that way. Since my mother tongue isn’t English i probably mess up some, and even worse, being Norwegian means i have no manners. We just came out of the cave. 🙂

I won’t try to crunch numbers. I probably could figure it out about right, strughling for some hours and consulting Professor C A Marchajs books, 🙂 but I’m more into actual experience.

More than long distance cruising, I come from decades of racing, including some of the most extreme stuff on the planet, which has given me some very clear demonstrations of what a reduction in weight aloft does to the boat. On very light boats, small changes are easier to notice, but it works the same way.

The report after the 1979 Fastnet Race really needed to have some “good answers”, for political reasons, and was made by among others, even at that time out dated designers, engineers, etc. It has a seemingly good wiew on much of the topics but quite fragmented on others. The claim that more weight aloft gives a more seaworthy sailboat is correct in such a irrelevant way it must be deemed outrageously flawed thinking.

I’ve raced many boats where the rig has been changed during or between seasons. Monohulls and multihulls. Always lighter rig, if course. Most of them we sailed in rough weather. The difference was always a joy. The boat behaves much better. It handles better, is more comfortable, faster, the works. Disadvantages? None.

As John mentioned in the linked article, a boat with more stability and that handles as goid as possible, is better able to avoid a situation where more inertia would be beneficial. That is, lying with the waves beam on, when most boats, including those with heavy rigs, would be flipped anyway.

I can’t offer specific numbers either, but have spent some time pondering the issue. My best source for this type of questions is “Aero- Hydrodynamics of Sailing” by Prof. C A Marchaj. An old book now, but still quite a nerd bible. He does go into detail on almost everything thinkable, including drag on wires.

That drag is apparently a bigger issue than most sailors realize. The biggest problem seems to be having to many , though. Stays, shrouds halyards, etc exposed to undisturbed flow. The thickness matters too, but apparently less, if not taken to extremes. Changing from 6 to 9 mm is adding 50% to the projected area, and a round profile has close to max possible drag. The drag increase should be close to the square of the area increase.

This does matter for windward ability, mainly in how high the boat can point, as the efficiency of the rig drops, but still will probably only be a clearly noticeable issue for serious racers. For cruisers, i assume it will only matter when forced to fight upwind in a gale. In those cases, the sail area is tiny, while the rig is just as big as normal. The lift to drag ratio becomes very poor, making it hard to make progress to windward. Thinner shrouds etc won’t make a world of difference, but sometime those few degrees was what was needed to get away.

Thanks Andy, S A and Barry for the thoughts. It would be good to someday understand the real effects of windage. Obviously, people often add huge amounts of windage to their boats in terms of cockpit enclosures and similar things but it would be good to eventually have a rough idea of the contribution of standing rigging for those people who try to focus on sailing performance. I have been in the situation where a boat really can’t make progress to weather due to not being able to carry enough sail compared to all of the losses like the area of the hull and it is a scary and weird situation to be in.

Our boat has a pretty tall rig for its size which I generally like but it means that it is not as good when the wind is really up. I have noticed that when there are no waves, starting at about 25 knots steady, our pointing ability starts dropping off dramatically as the wind increases and by the time we have hit 40 knots, we have lost probably 15 degrees of pointing ability. Based simply on how much the boat heels when motoring at the same angle in the same wind, it becomes clear that you can’t fly a ton of sail then. Given that going to weather in this much wind is rarely a good idea as there are usually big waves, it isn’t generally a big problem but you would hate to find that out.

Eric