Anyone have a link explaining the physics behind rig tuning? I get the basics, what does what, but would love educate myself on the why and how.
Rig tuning physics
- Thread starter SailingLoto
- Start date
Having followed the settings outlined in guides for my various boats, my understanding is a synthesis of observations. In my view there are two outcomes from a properly tuned mast. First is the control and management of the shape of the mast where we want to be able to shape it fore and aft to manage sail shape while still keeping it in column athwartships. The second outcome is the correct forestay tension to suit the conditions where sometimes you want some amount of sag to gain power and pointing and sometimes in higher winds you need less sag.
The tension levels of the stays are correct for only a narrow band of wind velocities and racers generally adjust them for the anticipated conditions. Those less concerned with 10ths of a knot relative to competitors will find a middle setting and leave it there.
The tension levels of the stays are correct for only a narrow band of wind velocities and racers generally adjust them for the anticipated conditions. Those less concerned with 10ths of a knot relative to competitors will find a middle setting and leave it there.
There is a book "The Physics of Sailing" which touches on some of these issues. I don't remember the author. Here is a search for it:
To add to post # 2 there are additional goals of rig tuning. For instance, to set the center of effort of the sail plan to create the proper helm balance. In fractional rigs mast bend is instrumental in de-powering a sail plan. So keeping the mast in column is less of a priority than for a masthead rig.
And there is the issue of rig tension. While you can crank up the tension all you like but at some point the boat bends and you're not adding any tension. So it is an engineering problem to get the right tension so that the boat isn't bending (Or only a little) and holding the mast in the correct position. And you have lower stays, both forward and aft to manage the shape of the mast as wind is applied to the sails. In all it's an interesting subject.
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And there is the issue of rig tension. While you can crank up the tension all you like but at some point the boat bends and you're not adding any tension. So it is an engineering problem to get the right tension so that the boat isn't bending (Or only a little) and holding the mast in the correct position. And you have lower stays, both forward and aft to manage the shape of the mast as wind is applied to the sails. In all it's an interesting subject.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 23,037
Great ask. Buzz Ballenger at Ballenger Spars in Watsonville CA has made Spars his career. He has several guides on his site. Here is the link to his Light Version of the physics involved in rig tuning.
Buzz provided the aluminum replacement spreaders for my mast refit. First class workmanship. Came across Buzz while exploring the refit of Sequoia, a Cal40 that Fred Cook of Schaefer Marine did in 2015. Ballenger did the mast for Sequoia.
A Mast Tuning Guide - The Light Version | Ballenger Spar Systems
Mast tuning is simple if you remember a couple of basic principles. If you understand these principles, you can tune just about any mast.
ballengerspars.com
Buzz provided the aluminum replacement spreaders for my mast refit. First class workmanship. Came across Buzz while exploring the refit of Sequoia, a Cal40 that Fred Cook of Schaefer Marine did in 2015. Ballenger did the mast for Sequoia.
There's a good article on the how's and why's of mast tuning in www.cncphotoalbum.com. which is the C&C yacht owner's home website. The various spar builders will always have similar offerings. They never mention tuning gauges and such things btw. You'll understand why when you read the articles.
if you like knowing the why and how of this sport, read this.
you wana tune a rig, well, for what weather, what seas, ........
learn the art of sailing from a master.
Why - A properly tuned rig minimizes uneven stress and wear on hardware and optimizes sailing performance. The aim is to ensure a straight mast athwartships, control sail shape, achieve proper helm balance in a variety of conditions and spread loads appropriately on spars, rigging and boat.
How - do an internet search on "sailboat rig tuning" to answer this question and find guidance like these: rigging.pdf (usspars.com), What You Need to Know About Rig Tune & Performance - Articles - Quantum Sail, How to Inspect and Tune a Sailboat Rig | Cruising World and How to Tune a Sailboat Mast – The Rigging Company
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In my ongoing attempt to understand rig tuning better, I made the attached free body diagram (FBD) of a mast. A few opening remarks.
One thing that jumps out right away is how enormous the compressive loads on the mast are, relative to athwartship loads. It's easy to see why masts typically fail by buckling, and why beam strength and stiffness of the deck is so important for a deck-stepped boat.
It's also clear that the mast will bow leeward. It looks to me like, at least in the case of a deck-stepped mast, the extent of this bow is determined by lower stay tension relative to upper stay tension. Would I be correct to focus on upper stay tension to determine the overall tension level and lower stay tension (relative to upper stay tension) to tune athwartship bowing of the mast?
Exactly what is the purpose of tuning the mast bow (in the athwartship direction)? Intuitively, it seems like more bow would make the sail more full and pull the center of effort forward, while less bow would make the sail less full and move the center of effort aft. Is that correct, or am I not visualizing it correctly?
Any other observations that come to mind when you look at the FBD? I'm particularly interested in how this picture changes with heel angle, as that's something I haven't even begun to try visualizing, yet.
- Only athwartship loads are considered. I do plan to look at fore and aft loads, but not here. (Incidentally, is there an antonym for athwart, besides "fore and aft?")
- This rig configuration and geometry doesn't depict any specific boat, but it is loosely based on the rig from (formerly) my Mac 26S, only because that's the rig I'm most familiar with. I presume that it's conceptually representative of many boats of similar size to the Mac.
- The mast and stays are shown in grey. Force vectors at the stay, spreader, and mast foot attach points are shown in red. For convenience, I've made the lower stay and the spreader attach point the same point. I assume that's a reasonable approximation of reality, in most cases. Sail load (speculative) is shown in blue. There was no science behind the sail load distribution shown, it's only meant to be eyeball-representative of what's typical. For simplicity, there's no heel angle and no sail load component parallel to the mast. There's also no athwartship boom load separate from the distributed sail load, which is obviously not accurate but I'm trying to keep it simple, for now.
- Lower stay tension is equal to upper stay tension in this example, only for ease of constructing the FBD with proportional vector lengths. Note that this equality of upper and lower stay tension results in a combined lower-stay+spreader load that's net to weather, so I assume that, in a properly tuned rig, lower stay tension would be less than upper stay tension, resulting in net force to leeward. Essentially, adjusting the lower stay tension rotates this force vector at the stay-spreader attach point.
- Also for simplicity, the sail loads are balanced by the stay+spreader loads such that there is only compression loading at the mast foot (vertical force vector). I'm aware this is not necessarily the case, but you can easily visualize that increasing the stay tension (especially the upper stay) will rotate the force vector at the mast foot.
- I've assumed that this (deck-stepped) mast has negligible rotational constraint at the foot. That was certainly the case for my Mac, and I assume that it's normal for deck-stepped masts.
- I get that not everyone appreciates, or even approves of, a theoretical approach such as this. You're more than welcome to make your case for that, but I'll ignore it. I'm also aware that the stay, spreader, and sail loads are interdependent such that changing any one affects the others. It's actually the purpose of a FBD (in part) to help visualize those inter-relationships. Feel free to point out that this diagram only depicts one such loading case and other loading cases will be different but, again, I'll ignore it. Generally speaking, I'll ignore pedantry.
One thing that jumps out right away is how enormous the compressive loads on the mast are, relative to athwartship loads. It's easy to see why masts typically fail by buckling, and why beam strength and stiffness of the deck is so important for a deck-stepped boat.
It's also clear that the mast will bow leeward. It looks to me like, at least in the case of a deck-stepped mast, the extent of this bow is determined by lower stay tension relative to upper stay tension. Would I be correct to focus on upper stay tension to determine the overall tension level and lower stay tension (relative to upper stay tension) to tune athwartship bowing of the mast?
Exactly what is the purpose of tuning the mast bow (in the athwartship direction)? Intuitively, it seems like more bow would make the sail more full and pull the center of effort forward, while less bow would make the sail less full and move the center of effort aft. Is that correct, or am I not visualizing it correctly?
Any other observations that come to mind when you look at the FBD? I'm particularly interested in how this picture changes with heel angle, as that's something I haven't even begun to try visualizing, yet.
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Why does your force vector for the lower shroud not align with the shroud? In my experience the mast doesn’t seem to bow much of at all - when sailing upwind the upper shroud will tend to get slack at least as much as the lower. My assumption is that the lower shroud tension does compensate for both the side force from the sail and the compression from the spreader.
Good to know. Thanks!lower stay tension will always be more than upper stay tension.
not assumption, but fact.
Humm, I've got a forestay and backstay. If I tension one the other one always follows, tension wise, since the lengths are close. I don't have any lower stays.lower stay tension will always be more than upper stay tension.
not assumption, but fact.
Or maybe we should all be speaking the same lingo. I've always like shrouds and stays to separate fore and aft vs athwart ship.I wish Frank Bethwaite would have written about the flying he did into thunderstorms and seeding clouds, in a glider, none the less.
A great question @SailingLoto.
And this is because you say so ? ? ? ?not assumption, but fact.
"NO" I used to tell my students, it's a fact when you prove it to me or you quote a source that's a hell of a lot smarter that you are.
fair enough, shrouds. they are all stays i think. shrouds are the side stays and then there are fore and aft stays, and so on.
ralph, lay you mast on it's side. you pick it up from the area around the spreaders while wifey picks it up at the head of the mast. who is carrying all the weight?
and ralph, most everyone is smarter than i am, but i did do my homework.
and ralph, most everyone is smarter than i am, but i did do my homework.
For what it’s worth the Selden guide at https://support.seldenmast.com/files/595-540-E.pdf says the following:
• While sailing, check that the mast is straight laterally. Adjust if necessary using the lower shrouds.
• While sailing, check that the mast is straight laterally. Adjust if necessary using the lower shrouds.
That's interesting, thanks. I had been under the impression that some degree of lateral bowing could be used to tune the main sail, but the Selden guide only talks about tuning to ensure that the mast is straight, laterally, for all rigging types.
Fore and aft bowing can definitely be used to tune the main (with more bend flattening and depowering the sail), but I’ve never heard of lateral bend for that purpose.
I found out it is pretty easy to get the mast S shaped laterally and also pretty easy to fix by asymmetrical adjustment of the shrouds.