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Old 05-28-2002, 10:34 PM
jdardozzi jdardozzi is offline
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Aftmast rigs???

Can anyone comment on aftmast rigs. I read some of Brian Eiland's stuff and recently saw a motorsailer with a aftmast bipod flying a hugh genoa. I liked it a lot and want to rerig my 48 steel yawl this way if I can. Any help would be appreciated.
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Old 05-29-2002, 12:05 AM
tspeer tspeer is offline
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Before considering an aftmast rig, I'd ask the designer to show you the numerical trade studies he's done concerning the drag of the mast (especially a bipod) and rigging vs the expected gains in sail efficiency. "Show me the numbers."
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Old 05-30-2002, 10:20 PM
jdardozzi jdardozzi is offline
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Now you sound a little skeptical...which is good. I would like to hear more of your opinion if you have more. I am looking for a designer who would be interested in this project. What I've read and seen comes from some fairly respected designers like McGurdy & Rhodes, Phil Bolger, etc.
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Old 05-31-2002, 12:41 AM
tspeer tspeer is offline
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Any study of rig effectivness points out that on a per-area basis, jibs are more effective than mainsails. That leads to the conclusion that rigs would better if they were all jib - hence the mast aft rig. What these studies often fail to point out is the effectiveness of the jib comes from its interaction with the main. On its own, it doesn't perform as well. And theoretically the main suffers in the exchange, making it look worse. But the combination is more than either sail acting in isolation.

The finest article I've ever read on the interaction of multiple lifting surfaces is A.M.O. Smith's 1975 Wright Brother's Lecture, "High Lift Aerodynamics" (AIAA Journal of Aircraft, Vol. 12, No. 6, June 1975, pp. 501-530.). He goes into great detail on the five primary effects of slotted airfoils and how they contribute to high lift. Very illuminating with regard to the main/jib interaction.


Then there's the problem of the mast. It's almost impossible to fair an isolated mast because of the range in apparent wind angles. The apparent wind will meet the mast from 20 - 30 degrees either direction. No section shape for the mast will avoid massive separation under these conditions, causing a lot of drag. The drag of a circular cylinder can be the same as an airfoil ten times its thickness and a hundred times longer - with drag coefficients based greater than one. In other words, the drag of an isolated mast can be almost as much per square foot as the sail produces in lift per square foot! When you add up the frontal area of the mast plus any struts and rigging, you get a lot of windage.

The mainsail acts as a splitter plate behind the mast on a conventional rig, preventing the formation of a Karman vortex street in the wake of mast, and the favorable pressure gradient in the slot allows the flow to reattach to the mainsail, minimizing the separation behind the mast. Finally, the jib channels the airflow so that it hits the mast from a much narrower range of angles, making it feasible to reduce its drag by elongating its cross section. So the drag of the mast in a conventional rig is much less than the same mast standing by itself in a mast-aft rig.

Lastly there're the structural considerations. A mast aft rig, especially one that is inclined forward, makes it very difficult to maintain adequate tension in the forestay, leading to lots of sag. The forestay is longer, and the angle between forestay and mast is larger than the angle between backstay and mast. This means the loads on the backstay are far higher than the loads on the forestay, and the mast compression is much greater to achieve a forestay tension anywhere near that of the conventional rig. Making the mast larger, heavier, and needing additional reinforcement to stiffen the boat. You could mitigate some of these problems by mounting the mast more midships and raking it steeply aft to put the hounds at the same location. This would shift the mechanical advantage of the forestay and backstay.

That said, the mast aft rig may still make sense, especially if there are other considerations than pure aerodynamic performance. I've pointed out the downsides because you've already heard the advantages, but I don't mean to sound completely negative on the subject. You have to analyze the entire rig as a system and look at the tradeoffs in quantitative terms. Don't just look at one factor and ignore the drag penalties of others. And qualitative arguments are fine, but they need to be backed up with quantitative estimates so that you can see the relative importance of conflicting influences.

Like I said, ask your designer to walk you through the numbers. I'd take the existence or absence of those trade studies as a pretty good indicator of reliability of the designer's claims.
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Old 06-01-2002, 09:25 PM
jdardozzi jdardozzi is offline
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thanks for the education. Assuming you could resolve the engineering issues with the mast and forestay sag, wouldn't a cutter-style rig give you the same results with regards to the slot-effect as a headsail/mailsail?
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Old 06-01-2002, 10:31 PM
tspeer tspeer is offline
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I suppose so, but I've not tried to analyze it. And the cutter rig gives you the option of sailing on the staysail alone, which should present a fairly balanced sailplan in heavy conditions.

The question still remains, what's the drag of the mast(s)? And do you get a net gain?
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Old 06-23-2002, 12:16 AM
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Aftmast rigs

SUBJECT: Mast-Aft Sailing Rig (single-mast ketch)


Contrary to what one might suppose, I actually enjoy receiving some of the criticisms I get on my mast-aft sailing rig concept……particularly when they come from a knowledgeable, intellectual source such as Mr. Tom Speer. He forces me to work harder at justifying the viability of this concept, as obviously his grasp of the theoretical and mathematical aspects of the aerodynamics exceeds my capabilities. I was never a professional in aerodynamics, but I’ve done some studying.

I think Tom Speer & I are both in agreement as to the superiority of the genoa sail from an aerodynamic viewpoint. And we are in agreement that much of its superiority is gained as a result of its interaction with the mainsail. (We had previous discussions of this interactivity under the subject headings, “The Slot Effect” and “How Sails Work”. Where we probably come to differ a bit is the extent of this superiority of the headsail. I give it greater creditability than he does……and particularly as I have it designed.

We know that the restriction presented by the ‘slot’ tends to divert more air around the two sides of the slot, i.e. the windward side of the main and the leeward side of the genoa. This higher flow rate on the lee side of the headsail increases its effectiveness. Now if we also overlap the mainsail with the trailing edge of the headsail, we further increase the effectiveness of the headsail, as it is able to carry this increased flow rate much further aft along its span than if it was to have to dump its flow at free stream velocities up at the leading edge of the mainsail. This overlap is important.

Now imagine looking down on the sailing rigs from directly overhead and evaluating the cross-sections at various vertical levels ( I probably should draw a diagram of this view and post it on my website). You would discover that my two parallel headstays present uniformity in both the slot between the two sails, as well as the overlap of these two sails. And this uniformity is consistent from the foot of the sails up to the hounds.

Significantly this is no-where near the case with the traditional Bermudan rig, either fractional or masthead configured. In both of these cases the throat of the slot is decreasing in size as we move vertically up the mast, while the wind velocity is increasing with this height……sort of a reverse of what we might desire. And the overlap of the necessarily hollow-leached headsail is at best really only effective at the bottom 1/3 of the sailplan. Seems there are many more questions of the compatible interactivity of the main and the headsail of the Bermudan configuration than with my twin headsail arrangement. In this comparison I think my rig configuration will prove significantly superior.

Note also that this nice uniform genoa overlap is attained while utilizing only what amounts to a 110-120% genoa rather than a radical 150-180% sail (Bermuda rig designations). And the foot areas are fully compatible, unlike the raised boom region of the Bermuda rig. Above the hounds the natural twist in apparent wind should help to make this upper portion of my genoa be a more productive sail area, certainly more productive than the thin strip of Bermudan mainsail hidden behind a mast structure on a fractional rig vessel. Just possibly the slot formed between my bare mast and the genoa sail in this upper region may create an upwash that could assist this productive task.

One might equate the twin headsail portion of my rig to the old ‘cutter’ arrangements. In fact the old cutter rigs many times demonstrated a superiority to the standard sloop when a reasonable open slot was designed between the two sails (too close a slot was an Achilles heel of the cutter arrangement). I also find it interesting that the latest ‘state-of-the-art’ Volvo 60’s appear to be evolving from their fractional rig plans to a masthead arrangement with their new flat cambered Code Zero sails (large genoas?) for close reaching work. I think this speaks to the superiority of the masthead verses fractional rig. Just when we thought evolution was favoring a smaller jib/larger main, things re-adjust. Evolution doesn’t always follow mother nature’s preferred path, it can get skewed off- -course a bit following rules put in place by handicappers.

While on this flow subject, I wish to bring up one other matter. Almost no one including the textbooks addresses the triangular nature of the slot (throat of the slot) in the traditional Bermudan rig, and how this might redirect (divert) some portions of the airflow up or down vertically (3 dimensions)? Everything is treated in a 2 dimensional manner, in a plane parallel to the water’s surface. Wouldn’t this more restrictive slot at the upper regions combined with an increased flow velocity, tend to deflect some airflow in a vertical manner? I’ve certainly seen it with my telltails…..and if my Bermudan mainsail is diverting wind upwards it must be pushing back downward on my mainsail (for every action there is an opposite and equal reaction). Or how about that ‘lift’ I seem to experience with some headsails. Where is the theoretical data to suggest and/or collaborate these observations? Things in nature don’t always follow our assumed calculated rules.

As Tom noted “then there’s the problem of the mast”. Believe me I have always been concerned about the drag of my ‘bare’ mast. I’m acutely aware of those dramatic statistics that implicate round cylindrical sections (mast or wire rigging) with creating a drag force that is somewhat larger than would even be suggested by just their cross-sectional areas. For this reason alone I would not even consider using a round-sectioned mast. Rather my mast is a foil-section for both drag purposes and strength purposes. The significant fore-to-aft forces imposed on my rig (particularly at the hounds), in combination with the necessity to keep the mast in column under significant compression loads, dictates an asymmetrical (elongated) mast section. The ideal sectional shape would be determined in the tests that I hope to conduct prior to building this rig, and may be different yet from that of a conventional rig. The aero configuration of this section will be of prime consideration, both its own drag and its affect on the mizzen sail. In any case I don’t feel it will have anything near to a ‘ten times’ affect hinted at in Tom’s posting. In fact if my sailing rig is really doing its job properly, that of smoothly redirecting the air flow so that my vessel benefits from the reactionary push, then my twin headsails will be directing that airflow over the mast located in their wakes…..and in this case my mast may actually experience smaller angles of apparent wind incidence than a mast sitting up front exposed to the much more variable incoming airflow. This coupled with the airfoil shape should help minimize some of the separation behind the mast. Then there is always the possibility of adding a small flexible piece to the rear of the mast to act as a flow stabilizer (splitter plate, per Tom’s reference).

Per Tom’s comments, “when you add up the frontal area of the mast plus any struts and all the rigging, you get a lot of windage”. How true! I once added up all the frontal area of a sailing rig on a traditional 50’ boat and came up with something like the equivalency of almost a 6’x 8’ sheet of plywood. Wow! Imagine sailing to windward holding up a sheet of drag producing board like this. Sure would be nice to have some wire covers of some sort, that could swivel around to the airflow and fair out these trailing edges. Actually some of the newer wire rigging substitutes (kevlar, zylon, PBO, etc) are of such a strength that their required diameters are much smaller.

Lets compare the frontal area drag of my rig with a traditional rigid-masted Bermuda rig and a modern 3 point stayed, rotating mast rig. In the case of a twin spreader, rigid mast, sloop rig, the required spreaders, diamonds, shrouds, forestays, backstays are just about the same rigging as required for my rig, except I have one extra rear jumper strut at the hounds, one wishbone boom on the mizzen, and a ‘bare’ mast…. However, I also have a rig that’s about 25% shorter in height for the same sail area. Net result, I probably have a slight bit more drag area, but I’m betting tests will show I have more drive per same sail area…..so my drive to drag ratio is similar or better. Close call.

I’ll concede that my rig has considerable more drag than a 3 point stayed, rotating mast rig…...and for that reason it very well may not be a candidate for a ‘RACE’ boat. Believe me, I’ve surely experienced the dramatic improvements in sail propulsion offered by rotating wing-mast rigs compared to rigid rigs. But wait a minute, what if the boat lengths increase for the next ‘RACE’…..and surely they will! I wrote of this subject in a letter/proposal to Paul Cayard (copy available). If the boat lengths increase (*note), then surely the sail driving areas must also, and yet, can it be done with a sloop rig?? Mr. Ollier has carried out EXTENSIVE tests and determined that the size of the mainsails carried on the boats of this past ‘RACE’ were the maximum size that could be handled by man-power alone (unassisted by power). If this is so, then what can be done to increase the sail areas….a multiple mast rig…..a ketch, schooner, etc?? Now, I just might be tempted to challenge a double-masted rig with my ‘single-masted ketch’. I think I might have less frontal area in this case.

Frontal area drag is not really as prime a concern, induced drag is far more critical. I expect much less off the bottom of my rig than the traditional low jib/high boom configuration of most Bermudan rigs. But at the top, I’m unsure as to all the factors, particularly with as many as 3 generating sources (3 sail tips), and the constructive or destructive interference with those portions of the rig behind the leading elements. Induced drag can easily be as high as 75% of the total air drag of a sailboat. Shaping the sails for minimum induced drag is most important for good performance…..and many a sailmaker has anticipated great results from a shape developed in theory, only to find it didn’t pan out that way in real life. I’ve not seen a lot of theory that predicts with any certainty, or qualitatively, the induced drags of a sailing rig. My rig would be interesting to view in a wind tunnel smoke test.

Most of the mast-aft rig experiments I can find thus far, have all moved the mast to the very stern of the vessel. Thus they are unable to maintain adequate headstay tension. Without this tension our headstays sag with increasing winds, our sails become fuller rather than flatter, which is exactly what we do not want, and thus the poor performance of previous experiments. With sag they all had to continually re-adjust their sails, and thus they were all seen as problematic. I think I have solved this problem. The key to proper forestay tension is proper backstay tension! First look at my masthead. The single backstay at this location is actually pulling aft at a more favorable angle than on most all other sloop rigs. This might suggest that we could decrease the tension load of this stay, but wait, lets keep it tight in order to maintain the mizzen’s leading edge shape as well. Hard to see on the website drawings, but the bottom end of this backstay is attached to the frame member of the vessel that supports the mast…the backstay’s tension upward is countered by the mast’s compression load downward…….less strength required from the frame structure, and it cancels out the dependency of this backstay’s tension on the stiffness of the aft sections of the hull structure. (The frame structure itself is also a major bulkhead of the vessel). At the hounds is where we really load things up. We have the inner forestay tension we wish to maintain, and we have the significant forward push of the aft jumper as well. Lets employ two quasi-conventional pieces of rigging: 1) the two forward facing ‘baby’ jumper struts are rigged in such a manner that both jumper stays are always sharing the load rather than allowing one to go slack, 2) the lower backstay from this hound location is split into two legs in a geometry that promotes both legs are always sharing the load rather than the normal situation with a slack leeward stay. All the rigging is working full time, which imposes less load to any single member and less load to their attachment points. Note I have 3 backstays working full time. The x-wide staying base of a multihull allows for x-wide spreaders (note no overlapping genoa) thus lower compression loads from the shrouds…..and they in turn are anchored to this same cross frame member that supports the mast. Patent protection has been sought with respect to some of these rigging details in combination with the mast-aft configuration.

My mast will experience lots of compression loading, particularly in the lower sections, so the key is to keep the mast in column. Heavier wall sections are not necessarily the answer. Keep it in column. This is where subtle changes to the geometry of the sailplan could produce the most optimum form……it might predict factors such as the optimum overall height, spreader placement, jumper strut lengths, attachment points and methods, etc. This is where I believe the computer and a finite element analysis could be very helpful in analyzing all of the rigging loads as a result of modifying any single link and determining the load requirements for all the parts. I am seeking funds for this comprehensive study so we get it right the first time out….and I would love to have someone with Tom’s analytical capabilities involved, including Tom himself.

For a moment look at the profile drawing of my rig and picture it as though the mast was standing straight up vertically with its masthead in the same location as mine now is. Contrary to Tom’s statement, my forestays are really no longer than a conventional sloop rig, in fact they are likely shorter. And his comments about the angles between the backstay & the mast and the forestay & the mast are not quite correct. In reality it’s the horizontal (not vertical) component of the tension forces in these two stays that determines the ability of the rig to resist sag in the forestay……my masthead backstay has a more advantageous (rearward pulling) angle than does my forestay pulling forward. Interestingly, this phenomenon was probably most detrimental to the Prout mast-aft rig. Their short vertical mast at the aft position resulted in a highly sloped forestay that was both too long for the rig’s overall height, and it could over-exert a forward pulling force on the masthead that was indefensible by the shallow-angled backstays…..result, big time sag, sails too full. This rig had other problems as well….not an example of a successful mast-aft rig, but a case study in some things to avoid.

How about the use of a bi-pod or A-frame mast as I have hinted at previously. The slickest example I can reference was onboard Olaf Harken’s innovative “Procyon” (photos avail). But I’ll save this discussion for another time, as I just received a whole packet of new info on this subject.

My apologies Tom for not ‘showing you the numbers’. My observations are still more qualitative than quantitative, but I think I’ve put together enough qualitative material to justify some serious wind tunnel testing time, finite element analysis and/or CFD. And even though I may not go racing with this rig, the cruising potential of this ketch style rig is a very viable option. Chris White certainly appreciates a ketch rig. Check out his Concept 63 ‘Sailplan’, ‘Sailing Report’, and ‘Whitbread Comparison’ on his website
<www.chriswhitedesigns.com/concept63/index.php>



*Note: I’ve just learned that Mr.Ollier has designed a new boat for the next ‘RACE’, and it is longer. No details have yet been released as to its details and in particular its rig.
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Old 06-23-2002, 04:32 PM
tspeer tspeer is offline
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My appologies if I've sounded like I was running down your design, Brian. I didn't mean to do that.

I'm genuinely in the dark as to how this arrangement would improve efficiency of the rig. My comments were intended more to encourage you to come up with some quantitative trade studies, either through prediction or test. I think that if it does turn out to be superior to a conventional rig, it would be very interesting to see why.

It's certainly an innovative variation on mast aft rigs. Past efforts at mast aft and bipod rigs have been disappointing, and I've often suspected it was because the designers didn't treat the whole rig as a system - they tried to make gains in one place at the expense of huge losses elsewhere. I hope you'll be able to avoid this and come up with a balanced design that is a real step forward.

Cheers,
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Old 02-12-2004, 05:51 AM
Tord Tord is offline
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Aftmasted rigs

Seems there is a lot of merit in a rearward rig with a really big genua and small, double-sided, rigid main, in conjunction with an mostly unstayed mast, where the main essentially is a flap (to use an aircraft term) to the genua - double rear stays might be needed to keep the front stay rigid!

This way the drag of the mast will be very low, the rear stays can have an aerodynamic fairing, like double-decker aircraft used to have, or have an oval cross-section. If the mast can rotate with the "main" the design of the main sail can be simplified, and the mast be profiled, to further improve flow.
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Old 02-12-2004, 08:55 AM
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gonzo gonzo is offline
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Since the thread was about adding a sail to a motorsailor, upwind performance is not really important. An advantage to a tripod setup is that it can clear the side decks of rigging clutter. Also, it can be used for other amenities like hanging a tarp and a couple of hammocs under it. Another use could be to eliminate the radar arch an other supports that would be installed anyway. In a race boat it may not pay to have a tripod, but in a motorsailor I think the pros are predominant.
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Old 02-16-2004, 04:38 AM
CT249 CT249 is offline
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Re

"I also find it interesting that the latest ‘state-of-the-art’ Volvo 60’s appear to be evolving from their fractional rig plans to a masthead arrangement with their new flat cambered Code Zero sails (large genoas?) for close reaching work. I think this speaks to the superiority of the masthead verses fractional rig."

Not really, surely. It's just that the VO 60 rules allowed a Code Zero of much larger area than the VO 60's #1 genoa. Not mystery to the fact that a much bigger Code Zero powers a boat up much more than a smaller #1 (and I think only Dolphin & Youth went for large overalap on the #1, so they were small sails).

If the VO 60 "masthead" rigs are so great, why aren't Shockwave, Zana, Xena, Scandia Wild Thing, Pyewacket, Morning Glory, etc masdtheaders? Answer - 'cause the VO 60 "masthead" rig is created by the rule.


re "Just when we thought evolution was favoring a smaller jib/larger main, things re-adjust. Evolution doesn’t always follow mother nature’s preferred path, it can get skewed off- -course a bit following rules put in place by handicappers."

If the masthead rig is so great, and fractional rigs only survive because of handicapping, why are so many classes OUTSIDE of the handicapping rules fractional? Just about every high-performance "open" class from the cats for The Race, to 18's skiffs via sportsboats are fractionals (apart perhaps from some use of light-air Code 0s)?
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Old 02-17-2004, 09:10 AM
Aleksander B C Aleksander B C is offline
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president

I design afterrig 20- 25 years ago. I name It thie "polish rig" and disuss it with
John Letcher. Ask him to calululait mast. He cud'nt. (not convensional). I find very simple solution for it. Bay the way this baoat is ander costruction!, (also not cnvensional) . Are you interesting? Aleksander Bronislaw Ciechanski
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Old 06-19-2004, 11:00 AM
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brian eiland brian eiland is offline
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Mast-Aft Sailing Rig (single-masted ketch)

(I reposted this posting as I had made it as a 'guest' rather than a registered member, and was having trouble finding it for use as a reference)


Contrary to what one might suppose, I actually enjoy receiving some of the criticisms I get on my mast-aft sailing rig concept……particularly when they come from a knowledgeable, intellectual source such as Mr. Tom Speer. He forces me to work harder at justifying the viability of this concept, as obviously his grasp of the theoretical and mathematical aspects of the aerodynamics exceeds my capabilities. I was never a professional in aerodynamics, but I’ve done some studying.

I think Tom Speer & I are both in agreement as to the superiority of the genoa sail from an aerodynamic viewpoint. And we are in agreement that much of its superiority is gained as a result of its interaction with the mainsail. (We had previous discussions of this interactivity under the subject headings, “The Slot Effect” and “How Sails Work”. Where we probably come to differ a bit is the extent of this superiority of the headsail. I give it greater creditability than he does……and particularly as I have it designed.

We know that the restriction presented by the ‘slot’ tends to divert more air around the two sides of the slot, i.e. the windward side of the main and the leeward side of the genoa. This higher flow rate on the lee side of the headsail increases its effectiveness. Now if we also overlap the mainsail with the trailing edge of the headsail, we further increase the effectiveness of the headsail, as it is able to carry this increased flow rate much further aft along its span than if it was to have to dump its flow at free stream velocities up at the leading edge of the mainsail. This overlap is important.

Now imagine looking down on the sailing rigs from directly overhead and evaluating the cross-sections at various vertical levels ( I probably should draw a diagram of this view and post it on my website). You would discover that my two parallel headstays present uniformity in both the slot between the two sails, as well as the overlap of these two sails. And this uniformity is consistent from the foot of the sails up to the hounds.

Significantly this is no-where near the case with the traditional Bermudan rig, either fractional or masthead configured. In both of these cases the throat of the slot is decreasing in size as we move vertically up the mast, while the wind velocity is increasing with this height……sort of a reverse of what we might desire. And the overlap of the necessarily hollow-leached headsail is at best really only effective at the bottom 1/3 of the sailplan. Seems there are many more questions of the compatible interactivity of the main and the headsail of the Bermudan configuration than with my twin headsail arrangement. In this comparison I think my rig configuration will prove significantly superior.

Note also that this nice uniform genoa overlap is attained while utilizing only what amounts to a 110-120% genoa rather than a radical 150-180% sail (Bermuda rig designations). And the foot areas are fully compatible, unlike the raised boom region of the Bermuda rig. Above the hounds the natural twist in apparent wind should help to make this upper portion of my genoa be a more productive sail area, certainly more productive than the thin strip of Bermudan mainsail hidden behind a mast structure on a fractional rig vessel. Just possibly the slot formed between my bare mast and the genoa sail in this upper region may create an upwash that could assist this productive task.

One might equate the twin headsail portion of my rig to the old ‘cutter’ arrangements. In fact the old cutter rigs many times demonstrated a superiority to the standard sloop when a reasonable open slot was designed between the two sails (too close a slot was an Achilles heel of the cutter arrangement). I also find it interesting that the latest ‘state-of-the-art’ Volvo 60’s appear to be evolving from their fractional rig plans to a masthead arrangement with their new flat cambered Code Zero sails (large genoas?) for close reaching work. I think this speaks to the superiority of the masthead verses fractional rig. Just when we thought evolution was favoring a smaller jib/larger main, things re-adjust. Evolution doesn’t always follow mother nature’s preferred path, it can get skewed off- -course a bit following rules put in place by handicappers.

While on this flow subject, I wish to bring up one other matter. Almost no one including the textbooks addresses the triangular nature of the slot (throat of the slot) in the traditional Bermudan rig, and how this might redirect (divert) some portions of the airflow up or down vertically (3 dimensions)? Everything is treated in a 2 dimensional manner, in a plane parallel to the water’s surface. Wouldn’t this more restrictive slot at the upper regions combined with an increased flow velocity, tend to deflect some airflow in a vertical manner? I’ve certainly seen it with my telltails…..and if my Bermudan mainsail is diverting wind upwards it must be pushing back downward on my mainsail (for every action there is an opposite and equal reaction). Or how about that ‘lift’ I seem to experience with some headsails. Where is the theoretical data to suggest and/or collaborate these observations? Things in nature don’t always follow our assumed calculated rules.

As Tom noted “then there’s the problem of the mast”. Believe me I have always been concerned about the drag of my ‘bare’ mast. I’m acutely aware of those dramatic statistics that implicate round cylindrical sections (mast or wire rigging) with creating a drag force that is somewhat larger than would even be suggested by just their cross-sectional areas. For this reason alone I would not even consider using a round-sectioned mast. Rather my mast is a foil-section for both drag purposes and strength purposes. The significant fore-to-aft forces imposed on my rig (particularly at the hounds), in combination with the necessity to keep the mast in column under significant compression loads, dictates an asymmetrical (elongated) mast section. The ideal sectional shape would be determined in the tests that I hope to conduct prior to building this rig, and may be different yet from that of a conventional rig. The aero configuration of this section will be of prime consideration, both its own drag and its affect on the mizzen sail. In any case I don’t feel it will have anything near to a ‘ten times’ affect hinted at in Tom’s posting. In fact if my sailing rig is really doing its job properly, that of smoothly redirecting the air flow so that my vessel benefits from the reactionary push, then my twin headsails will be directing that airflow over the mast located in their wakes…..and in this case my mast may actually experience smaller angles of apparent wind incidence than a mast sitting up front exposed to the much more variable incoming airflow. This coupled with the airfoil shape should help minimize some of the separation behind the mast. Then there is always the possibility of adding a small flexible piece to the rear of the mast to act as a flow stabilizer (splitter plate, per Tom’s reference).

Per Tom’s comments, “when you add up the frontal area of the mast plus any struts and all the rigging, you get a lot of windage”. How true! I once added up all the frontal area of a sailing rig on a traditional 50’ boat and came up with something like the equivalency of almost a 6’x 8’ sheet of plywood. Wow! Imagine sailing to windward holding up a sheet of drag producing board like this. Sure would be nice to have some wire covers of some sort, that could swivel around to the airflow and fair out these trailing edges. Actually some of the newer wire rigging substitutes (kevlar, zylon, PBO, etc) are of such a strength that their required diameters are much smaller.

Lets compare the frontal area drag of my rig with a traditional rigid-masted Bermuda rig and a modern 3 point stayed, rotating mast rig. In the case of a twin spreader, rigid mast, sloop rig, the required spreaders, diamonds, shrouds, forestays, backstays are just about the same rigging as required for my rig, except I have one extra rear jumper strut at the hounds, one wishbone boom on the mizzen, and a ‘bare’ mast…. However, I also have a rig that’s about 25% shorter in height for the same sail area. Net result, I probably have a slight bit more drag area, but I’m betting tests will show I have more drive per same sail area…..so my drive to drag ratio is similar or better. Close call.

I’ll concede that my rig has considerable more drag than a 3 point stayed, rotating mast rig…...and for that reason it very well may not be a candidate for a ‘RACE’ boat. Believe me, I’ve surely experienced the dramatic improvements in sail propulsion offered by rotating wing-mast rigs compared to rigid rigs. But wait a minute, what if the boat lengths increase for the next ‘RACE’…..and surely they will! I wrote of this subject in a letter/proposal to Paul Cayard (copy available). If the boat lengths increase (*note), then surely the sail driving areas must also, and yet, can it be done with a sloop rig?? Mr. Ollier has carried out EXTENSIVE tests and determined that the size of the mainsails carried on the boats of this past ‘RACE’ were the maximum size that could be handled by man-power alone (unassisted by power). If this is so, then what can be done to increase the sail areas….a multiple mast rig…..a ketch, schooner, etc?? Now, I just might be tempted to challenge a double-masted rig with my ‘single-masted ketch’. I think I might have less frontal area in this case.

Frontal area drag is not really as prime a concern, induced drag is far more critical. I expect much less off the bottom of my rig than the traditional low jib/high boom configuration of most Bermudan rigs. But at the top, I’m unsure as to all the factors, particularly with as many as 3 generating sources (3 sail tips), and the constructive or destructive interference with those portions of the rig behind the leading elements. Induced drag can easily be as high as 75% of the total air drag of a sailboat. Shaping the sails for minimum induced drag is most important for good performance…..and many a sailmaker has anticipated great results from a shape developed in theory, only to find it didn’t pan out that way in real life. I’ve not seen a lot of theory that predicts with any certainty, or qualitatively, the induced drags of a sailing rig. My rig would be interesting to view in a wind tunnel smoke test.

Most of the mast-aft rig experiments I can find thus far, have all moved the mast to the very stern of the vessel. Thus they are unable to maintain adequate headstay tension. Without this tension our headstays sag with increasing winds, our sails become fuller rather than flatter, which is exactly what we do not want, and thus the poor performance of previous experiments. With sag they all had to continually re-adjust their sails, and thus they were all seen as problematic. I think I have solved this problem. The key to proper forestay tension is proper backstay tension! First look at my masthead. The single backstay at this location is actually pulling aft at a more favorable angle than on most all other sloop rigs. This might suggest that we could decrease the tension load of this stay, but wait, lets keep it tight in order to maintain the mizzen’s leading edge shape as well. Hard to see on the website drawings, but the bottom end of this backstay is attached to the frame member of the vessel that supports the mast…the backstay’s tension upward is countered by the mast’s compression load downward…….less strength required from the frame structure, and it cancels out the dependency of this backstay’s tension on the stiffness of the aft sections of the hull structure. (The frame structure itself is also a major bulkhead of the vessel). At the hounds is where we really load things up. We have the inner forestay tension we wish to maintain, and we have the significant forward push of the aft jumper as well. Lets employ two quasi-conventional pieces of rigging: 1) the two forward facing ‘baby’ jumper struts are rigged in such a manner that both jumper stays are always sharing the load rather than allowing one to go slack, 2) the lower backstay from this hound location is split into two legs in a geometry that promotes both legs are always sharing the load rather than the normal situation with a slack leeward stay. All the rigging is working full time, which imposes less load to any single member and less load to their attachment points. Note I have 3 backstays working full time. The x-wide staying base of a multihull allows for x-wide spreaders (note no overlapping genoa) thus lower compression loads from the shrouds…..and they in turn are anchored to this same cross frame member that supports the mast. Patent protection has been sought with respect to some of these rigging details in combination with the mast-aft configuration.

My mast will experience lots of compression loading, particularly in the lower sections, so the key is to keep the mast in column. Heavier wall sections are not necessarily the answer. Keep it in column. This is where subtle changes to the geometry of the sailplan could produce the most optimum form……it might predict factors such as the optimum overall height, spreader placement, jumper strut lengths, attachment points and methods, etc. This is where I believe the computer and a finite element analysis could be very helpful in analyzing all of the rigging loads as a result of modifying any single link and determining the load requirements for all the parts. I am seeking funds for this comprehensive study so we get it right the first time out….and I would love to have someone with Tom’s analytical capabilities involved, including Tom himself.

For a moment look at the profile drawing of my rig and picture it as though the mast was standing straight up vertically with its masthead in the same location as mine now is. Contrary to Tom’s statement, my forestays are really no longer than a conventional sloop rig, in fact they are likely shorter. And his comments about the angles between the backstay & the mast and the forestay & the mast are not quite correct. In reality it’s the horizontal (not vertical) component of the tension forces in these two stays that determines the ability of the rig to resist sag in the forestay……my masthead backstay has a more advantageous (rearward pulling) angle than does my forestay pulling forward. Interestingly, this phenomenon was probably most detrimental to the Prout mast-aft rig. Their short vertical mast at the aft position resulted in a highly sloped forestay that was both too long for the rig’s overall height, and it could over-exert a forward pulling force on the masthead that was indefensible by the shallow-angled backstays…..result, big time sag, sails too full. This rig had other problems as well….not an example of a successful mast-aft rig, but a case study in some things to avoid.

How about the use of a bi-pod or A-frame mast as I have hinted at previously. The slickest example I can reference was onboard Olaf Harken’s innovative “Procyon” (photos avail). But I’ll save this discussion for another time, as I just received a whole packet of new info on this subject.

My apologies Tom for not ‘showing you the numbers’. My observations are still more qualitative than quantitative, but I think I’ve put together enough qualitative material to justify some serious wind tunnel testing time, finite element analysis and/or CFD. And even though I may not go racing with this rig, the cruising potential of this ketch style rig is a very viable option. Chris White certainly appreciates a ketch rig. Check out his Concept 63 ‘Sailplan’, ‘Sailing Report’, and ‘Whitbread Comparison’ on his website
<www.chriswhitedesigns.com/concept63/index.php>



*Note: I’ve just learned that Mr.Ollier has designed a new boat for the next ‘RACE’, and it is longer. No details have yet been released as to its details and in particular its rig.
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  #14  
Old 12-19-2004, 11:40 PM
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brian eiland brian eiland is offline
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Raked Cutter Rig

Brian wrote:
I found this discussion today on the web that I took the liberty to add to the forum discussion of cutter rigs, and to this aft mast discussion that features two raked headsails acting in a somewhat similar manner

Raked Cutter Rig by WindSpeedYachts.com

History.
Ever since 1851 when the Schooner America with her 11.5 degree mast rake won the America's cup, mast rake has been used to improve the performance of sail power as a propulsion method. Every high speed craft including land yachts, ice boats, windsurfers and racing Multihulls use mast rake to improve performance.
Yachts that race to a handicap don't use mast rake because it is penalized by the handicap rules. In handicap racing the fastest yacht is not the winner, the yacht that sails closest to her handicap is the winner. Our yachts are not designed to any racing rules they are designed to be safe, efficient and comfortable.
The cutter rig has historically been the rig of choice for the monohull cruising sailor.

The Modern Raked Cutter Rig.
In 1991 we proudly redeveloped and introduced the modern, efficient raked cutter rig into cruising Multihulls.

The Raked Cutter Rigs features:
+A 7.5 degree mast rake, the key to the whole modern cutter rig working so well:
+Parallel fore stays for the greatest cutter effect.
+The distance between the forestays is spaced for maximum sail efficiency.
+The Jib easily tacks between the stays.
+Low aspect ratio rig to increase stability, reduce rig loads and reduce pitching.
+Low centre of gravity, low wind resistance cruising rig.
+The stay sail is close to the deck which improves efficiency and pressurizes +the windward nets and lifts the leeward nets reducing on board spray and increases stability.
+With the roller furling Jib on the composite bowsprit, which is wide enough to walk on and the fully battened stay sail on the fore beam, the fore triangle is broken into two easily handled sails.
+The large fully battened mainsail is reduced in area by the mast rake.
+This long low sail plan increases stability, reduces the sail handling effort required by the crew and off the wind increases the power to propel the yacht.

Notes
This rig is the complete opposite approach to the trend for high aspect ratio fractional rigs being installed on cruising yachts.
We have observed the trend by designers and builders to increase the height of masts to "improve performance" however we are informed by boat owners that increased speeds are achieved by reefing the mainsail. This is not surprising, tall masts raise the centre of effort of the sailplan, this increases the heeling force which pushes the leeward hull further into the water increasing hull drag. If you look into the hull shapes, for example a 12 m catamaran with fat hulls 1.2 to 1.4 m wide on the water line and fixed keels, the drag on these hulls when the leeward hull is depressed increases considerably. This type of hullshape is happy with both hulls evenly floating while motoring or sailing down wind not being pressed hard to windward or reaching.
The high aspect ratio racing rig works exceptionally well on racing yachts however it does not make slow hull shapes fast hull shapes.
How does mast rake increase yacht speed?

The more horizontal the fore stay angle the greater the vertical lift generated by the jib. Effectively lifting the bows which reduces the yachts displacement which reduces wave drag and increases the yachts speed. Raking the mast aft has a similar effect to fore stay angle.

With a vertical mast the forces generated by the mainsail are forward and downwards but by raking the mast aft these forces are rotated to become forward and upward resulting in increased speed due to decreased displacement.

The effect of mast rake is apparent sailing to windward but far more obvious when reaching and running.

Because the bows are not driven down by sail pressure we can use finer bows which further reduce drag and pitching.

Another advantage of mast rake is reduced pitching caused by the lack of the pendulum effect.
With a vertical mast in a sea way the masts mass and momentum forces the bows and sterns up and down alternately greatly increasing the pitching.
With a raked mast in a sea the masts mass and momentum forces are rotated to force the bows forward and the sterns up and down alternately greatly reducing the pitching.

The combined effect of the lift generated from the raked cutter rig, the lift generated from the under wing, the extra buoyancy and dynamic lift generated by the inboard flair and the stabilizing effect of the rudder mounted foil combine to reduce heeling, under wing wave impacts, yacht motion and increases average speeds.

The Raked Cutter Rig of the WindSpeed 40 has proved to achieve higher average speeds than any other rig aboard a liveaboard cruising Multihull yacht yet encountered.

Test results
Test have been undertaken on many yachts over many years but the most convincing test was aboard a 2000 kg 10 metre racing Multihull with a 15 metre rotating mast. This yacht started life with a vertical mast, and over two years the mast was raked progressively aft with consistently improving performance. The lift generated by the raked mast allowed us to carry more sail area in stronger winds off the wind and when sailing to windward, the crew of four would sit forward of the mast to keep the bows down.

On a 1.5m long radio controlled test model trimaran the mast was raked so far aft that the model capsized bow over stern from the lift generated from mast rake!

Note: We were testing the feasibility of using surface effect wings for racing trimaran beams at the time and these foils may have contributed in the capsizes [tested in 1981]

Conclusion
Our theoretical calculations and practical testing indicate that the raked cutter rig on the WindSpeed 40 generates 240 kg of vertical lift in 18 knots of apparent wind and sailing to windward at 8 knots this reduction in displacement equates to approximately 0.1 knots extra yacht speed! The lift generated by the raked cutter rig increases as the sheets are eased which increases the improvements in downwind yacht speeds

(opinions of www.windspeedyachts.com.au)

Brian added a note: It doesn't appear from their posted drawings that the mast rakes back 7.5 degrees?
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  #15  
Old 12-20-2004, 02:31 AM
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Quote:
Originally Posted by brian eiland
Brian wrote:

Because the bows are not driven down by sail pressure we can use finer bows which further reduce drag and pitching.
Betcha the bows are always driven down by sail pressure. (well unless you are using a kite anyway) Draw the force triangles. However the stern can be effectively lifted, and if significant this "displacement reduction" will of course affect the way the hull goes though the water
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