Mast-aft Sailing Rig, Single-masted 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