There's been lots of discussion on prior threads of the virtues of a soft rig vs a stiff rig (ie, see recent thread: "Lighter = Less Forgiving?"). One of the reasons discussed is the benefit of automatic gust response reducing the amount of work needed to control the boat and keep it powered up in windy/gusty conditions. I'm a believer in this benefit, having seen flexible masts work their magic on monohull dinghies and having experienced the challenges of a stiff mast on my own dinghy. I was thinking that if this is the case, why is it that so few cats (and no US made cats to my knowledge) use them?

I suspect it's considerably harder to design a properly functioning soft rig than a stiff rig since you need to take into account the "stiffness distribution" along the full length of the spar and match it with the sail cut to be sure the overall combination strikes a balance between being able to develop sufficient power, keep an appropriate sail shape in different degrees of mast bend from downhaul etc, and properly auto depower at the top. The windsurfer masts/sails are fairly eveloved in this regard.

Either most cat manufaccturers don't want to go to the trouble or maybe they feel that any comprimise in stiffness will be too much of a comprimise in power. This raises an interesting issue which I'd like to bring up for discussion: Maybe heavy cat manufacturers CAN'T use softer rigs for this reason. My instinct is that a self depowering rig might only work well on a lighter boat. I suspect that what lets a softer rig work properly with a lighter boat is that the lower mass allows it to accelerate and this in turn allows the rig to power up again sooner, offseting the effects of depowering in the first place. Specifically, as the lighter boat accelerates, I would think the resulting change in apparent wind would allow its mast to staighten sooner and therefor power up the rig more quickly than it could have otherwise (ie, even before the gust has fully passed). On a heavier boat, the softer mast might spill air in a gust, but due to the boat's inertia, the rig won't be able to powerup again until fully after the gust has passed, effectively reducing overall power during this period compared with the ligher boat scenario.

Another area that might be interesting to discuss in this regard: tapered masts vs less stiff non-tapered masts impact on bend geometry and automatic gust response vs rig power. I would think this is a complex issue because bend geometry and dynamics must be related to material stiffness, thickness, and cross sectional profile including tapering if any. My assumption is that with a uniform wall thickness, simply tapering the mast allows it to become more flexible so just varying the rate of taper would allow one to program the bend geometry. Windsurfers use tapered masts as part of tuning the mast stiffness profile and I know some larger high tech sailboats do as well. The Stealth has one. The Taipan uses a straight mast but it's rig is reproted to be quite refined and responsive none the less. I would think that a straight mast would bend mostly near its center, whereas with a tapered mast one should be able to program the position and degree of bend. Maybe more central bend on a straight mast can be compensated for by the right sail cut but I would think that the best automatic gust response would be achieved with a softer mast higher up than lower down. This would let you keep the lower sail powered up all the time maintaining maximum drive in the rig while reducing heeling moment from higher up.

So, in addition to the above issues, I'm curious if sailors, designers, and sailmakers have input on:

1) whether there are real advantages to tuning the bend geometry and dynamics using a tapered mast (I suspect there is)
2) if this does in fact offer even better sailing characteristics such as gust response than a flexible straight mast (I suspect it should)
3) if it might even allow sailmakers more flexibility to cut the sail in a way that offers additional benefits over what can be done for sail shapes on flexible non-tapered masts (such as allowing an even more powerful sail due to larger draft and better control of the sail draft as the mast bends due to downhaul adjustment).

If there are benefits to the above, I assume the reason they're not done currently is due to cost of R&D as well as manufacturing costs. However, it might be in part lack of demand from the buying public due to lack of education on what they should look for in a better boat. Sometimes a specific technology is more expensive only becuase it's done in lower volume; if enough buyers insist on it, prices come down automatically as market pressure forces it's adoption by manufacturers. So I propose by discussing these issues, even if they a slightly esoteric to some, that ultimately we could all benefit.

Lots to discuss!

Jerry

theres no way I'm reading through all that, but really, since you've signed on to this forum, its like a boat design encyclopedia... truely fascinating.

you into amateur boat design/engineering?

Hi Jerry,

This time you`ve answered all your own questions ! Good way to keep the thread short.

A while ago I asked what a "comptip" was, not being in the US & having not seen or heard of one before. I assumed it was a performance modification which would do exactly what you`re talking about, turns out to be a "safety" feature for lightning protection. I thought, wow, similar to the 49er that has a carbon/fibreglass upper section to the mast for increased flexibility & gust response. Turns out the responses I got to my question were mostly negative, seems the Hobie guys don`t think it is much good for performance. I think they would have to redesign the mainsail to get any benefit from this modification, squaretop main & flatter cut sail might help, but of course, class rules say no. Pity, I reckon if Hobie made their boats a few kilo`s lighter, modified the mainsail to suit the new mast & promoted their spinnaker kit more, they`d be able to upgrade their class to stay with the pack. I found a pic of a rig I think they were testing out for the youth boat selections on their web-site, see attachment. This modification looks suspiciously like they designed the rig to fit into the F16 class, which could only be a good idea in the long term.

Here`s one with the spi.

Broken mast.

MauganH17,

Yeah, the post ended up longer than I originally intended, but I hadn't seen some of those issues discussed on this forum before and thought it would be fun to hear other people's ideas on the subject. Also, it's still too cold to go sailing here, so all I can do at this point is read/talk about it!

I do happen to find boat design issues fascinating, not only because the technology and physics of sailing is interesting, but because I think understanding these issues can be useful in being a better sailor, picking your next great boat vs good boat, and understanding how to sail differently designed boats each to it's own potential.

Jerry

Hey Steve,

There's a difference between speculating and answering your own question!

Thanks for the pictures.

Jerry

In general most sloop rigged cats (w/o kite) do not have a backstay. Cats need a stiff mast to keep their headstay straight.

49ers do not have a rotating mast. The more you derotate the mast, reduce draft, the more the mast bends on the minor axis. Bending on the minor axis increases gust response. When adjusting 12 to 1 downhauls though not automatic work quite well by compressing the mast resulting in flatter main (opens slot) and twists the leech for the gusts. Modern cats shift gears for each wind velocity (light, med, high) quite well, thought not automatic. So if you tapper the mast for one wind range, it will lack speed in another. ie. a tappered mast for heavy air, will not be stiff enough for med air in waves. One could find a compromise here.
As I recall it is a real art to tappering an aluminum mast, and adds a lot of dineros to the cost.

The Tornado class tappers it's mast at the top, but I think they are going to a stiffer, with the advent of dbl trap, more SA, & kite.? Tornado mast design changes all the time. I would really be interested in reading about it's evolution.

I would imagine the A Class and 18HT class would benefit the most from a tappered mast at the top for gust response, because it is no longer concerned about headstay sag.

One other thing that you should consider if you are going to try to make the perfect mast, is the shape of the mast. One of the things that is always referred to as extremely important is the leading edge of a foil shape. Any blemishes on the leading edge of a foil will produce drag and easier seperation of flow of air (or water) over the foil. In my opinion, it should be the same with the leading edge of the sail. In his book on high performance sailing, Frank Bethwaite talked about a long squareback wingmast that he pointed straight into the wind as being better than the ovalized shape that we are currently using. I do not know if that is right or not, but it is interesting to consider, but too expensive to try. Other stuff I have read referred to the perfect mast shape to be about 4 times the width, which is about what you get if you take the front percentage of any reasonably efficient wing shape (like the NACA 64020 or 63020) of 10% camber and mirror that shape for the other side of the mast.

On the soft mast issue, if the mast is too soft, it bends in the middle. I have a carbon A-Cat mast that I have to add layers to to get it stiff enough to get power in anything above about 5 knots of wind. Currently,I have to be real careful to not sheet in too hard as this pulls the sail down the mast, bends the mast more, and removes all my power. It is a pretty fine line between having too stiff a mast and too soft a mast. It is probably easier to tailor your own weight to the mast to get it right than it is to change a mast that is not quite right. Far more painful though.

Another thing is with the predominance of the new square top sails on any modern design, the mast top has to be stiffer to get the leetch to stand up when the wind comes up. I was out on my 200 lb A-cat sailing last fall in around 8 to 10 knots of wind beside a Tornado. The T was powered up on a close reach with a person on the trap and the hull up. I was still sitting on the hull because when I would sheet in hard, the sail would pull down, the mast would bend, the top would twist off and there was no power. The mast is actually supposed to be pretty close to optimum for my weight by the bend numbers. In my opinion, I have to stiffen it up a bit. If I stffen it, and then lose 20 lbs, will it be too stiff? Maybe!

If you like all this stuff, you would be real interestin in a Marchaj book titled "Sail Performance". He gets into all kinds of different stuff including the leading edge planform, and other wacky sail shapes that are more efficient than the current bermudan rig. He also tells why it is more efficient, or powerful. By the way, I made my own A-Cat and am considering trying to make a new mast, too. That is a pretty iffy project though.

Steve,

I don't think the hobie comptip was for lightening. It was a response, to a few frivolous lawsuits, b/c people hit their masts on Powerlines, and a few died.

I say frivolous, b/c, it is the operators responsibility not to hit powerlines with you mast. My Nacra, has an aluminum mast. If I hit a powerline with it, it's not PC's fault.

Hi Sail7Seas,

You raise some interesting points about the potential for headstay sag and I'd like to hear other's experience and opinion on this matter. Some sag may not be a bad thing in gusts if it opened the jib leech slightly depowering it as well in gusts. It does seem clear that there are very successful and admired cats out there such as the Taipan which do sail sloop with a less stiff mast and don't seem to have a problem with this. Since it is the top of the mast that deflects the most and the headstay attachment point is well below this, it's not clear to me that the forestay would sag much on a rig with a properly designed flexible mast. Also, even if it could sag, I would think that headstay sag would be controlled adequately on a flexible mast via diamond wires tension and spreader rake while still retaining gust response.

Jerry

ConradQ, thanks for your comments.

You said:
On the soft mast issue, if the mast is too soft, it bends in the middle. I have a carbon A-Cat mast that I have to add layers to to get it stiff enough to get power in anything above about 5 knots of wind. Currently,I have to be real careful to not sheet in too hard as this pulls the sail down the mast, bends the mast more, and removes all my power. It is a pretty fine line between having too stiff a mast and too soft a mast. ... when I would sheet in hard, the sail would pull down, the mast would bend, the top would twist off and there was no power

I would have to agree that it is possible to make a mast too soft or not match it properly with sail cut and that it is a fine line between too soft and too stiff but it seems that if some care is taken during the design process this could certainly be done properly. It's probably just easier for some manufacturers to go with a stiff mast and not worry about getting it right, but this does mean the crew has to work harder as a result. Also, with a softer mast, it is possible to depower it out of it's groove and this would be a problem if the designer made the mast too soft in it's middle. That's why I brought up the topic of tapered masts - seems like it makes it easier to keep the mid mast stiffer and put the softness where you want it at the top. I would think that if the sail and mast stiffness are matched right, the top of the sail would twist off with downhaul or gusts but the rig wouldn't be depowered by tensioning the mainsheet as you describe.

This brings up a related issue to my original post on softer vs stiffer masts related to tuning spreader rake and diamond wires on stiff vs softer masts. Once a decision is made to go with an overall stiff mast instead of a softer mast by a designer/manufacturer, I would think that this decision limits the ability of the crew to tune the mast.

With a softer mast you can adjust stiffness and prebend somewhat independently. For example, you could tune mast stiffness for crew weight but not prebend athe mast allowing you to depower interactively with downhaul and still have good overall rig power and automatic gust response.

With a stif mast, you can't make it less stiff with spreader rake and diamond wire tension, you could only make it stiffer (what's the point) or set prebend. Once you prebend you've limited your ability to power up the rig. I would think this approach would be limiting when conditions change, ie, most of the time.

So it would seem to me that a stiff mast has limitiations for how a cat can be tuned and sailed by the crew: you can't tune mast stiffness, you need to work harder playing the mainsheet in gusts, and you don't have much of a range of downhaul to interactively depower overall rig power and instead must live with whatever prebend you guessed would be the best comprimise. Finally, I would also guess that sails might last longer on softer masts than stiffer masts since stiffer masts would put more strain on the sails due to their inelasticity, particularly when the crew tries to make the downhaul do something by really cranking on it.

Jerry

Though headstay sag opens the slot, it also makes the sail fuller, and will backwind the main (it is more obvious on a monohull without battens). You could put negative luff curve in the sail to counter this, but it may be to flat for other conditions.
Comparing two boats with all things basically equal the boat with the straighter headstay will point higher.

Again, a historian from the Tornado class could share their wisdom on this one as they have worked it for years.

The advantage to taper is in the upper unstayed sections. Without the taper (most production beachcats) sheet tension results in more bend occurs in the section close to the hounds than does at the tip. It is easier to design a sail shape to a uniform bend than a bend that isn't. Taper isn't the only way to get a more uniform bend.
Some mfg's taper the layup rather than the section to achieve it. That's not going to happen in alum. though, they always taper the section.

There is no easy answer I can think of to the main point of your question. Sails can be cut to most any mast stiffness within reason. But at both extremes of the spectrum, you become unable to adjust to conditions, and more specialized.