Aft-mast Origination & Justification
Over the past number of years I have attempted to make a positive case for my aft-mast {mast-aft} sailing rig configuration, both within, and aside of the technical discussions of sail aerodynamics. I now see some renewed technical discussions of the headsail/mainsail interaction, and lift/drag factors of cruising boat rigs under the
Sail Aerodynamics subject thread; and some interesting new participants with respectable technical backgrounds.
http://boatdesign.net/forums/showthread.php?t=457&page=18
I’m going to refrain from re-entering these technical micro-flow discussions at this time, and watch where they go. Rather I will choose to do a review of why I pursued my alternative rig configuration for cruising vessels based upon the real-time, observable phenomena that we experience as sailors.
So here are a few other observations I based my thought processes on:
POINTING CAPABILITIES
If properly set up with reasonably good sail shapes and a tight forestay, the jib-headed sailing rig will outpoint the uni-rig vessel. Overwhelmingly I believe the majority of sailors would agree that the jib and/or genoa headed vessel will go to windward better than under mainsail alone….just good old practical observations from multiple sources. Hopefully we’ve come to the understanding that this phenomena is a result of the interaction of the leading sail and its following sail….the two sail combination producing a more favorable wind flow to the headsail that allows it to point slightly higher….again supported by multiple sources of which I will only sight two at this time.
I’ve referenced this experimental work by Hall Spars before, but I will again for a re-emphasis of their findings: Eric responds, “ first, of course, the boat would be improved upwind with a No.1 jib. Generally, we could not point as high as the others here (Block Island) and therefore had difficulty holding lanes.”
http://boatdesign.net/forums/showpost.php?p=5685&postcount=3
…again, good solid real-life experimental results from a respectable source
2) In racing situations we’ve often seen, or made use of, this ‘extra lift’ phenomena when we sought to make use of the ‘safe leeward position’ and force our windward opponent into a pinching situation. Even while we are just leeward of him in the race, the flow disturbance off his rig gives us a slightly better lift to weather.
3) Numerous other examples are available that I just didn’t have time to reference.
So, like any respectable sailor, I wanted at least the ‘capability’ of good weatherly performance to come to my rescue when I really might need it. I believe this aftmast design is capable of good windward performance, perhaps even exceptional in some cases.
SIZE OF THE HEADSAIL
Here is where a real argument will arise….mastheaded vs. fractional, big genoa vs. jib, how much overlap, etc.
From practical experiences I would be willing to bet that most all of us would prefer a fairly good size genoa sail in light-airs, for either pointing, reaching, or even running (particularly if we have no dedicated downwind sails). A Bermuda rigged cruising boat without a good size headsail or furling reacher is destined to be a motorboat in anything other than substantial breezes.
So I wanted my new design cruising rig to have a good size headsail, a genoa. And since I was not intending to utilize a rotating mast, why not make it a mastheaded sail where I could get the max sail area in the lowest CE form. On a multihull vessel I’ve got a good wide sheeting base to make better use of this genoa on a reach or a run.
Again, from practical experiences and theory both, I recognized the effectiveness of a good leading edge sail. I wanted one, even at the expense of a self-tacking headsail. This was to be a cruising design, not a racing vessel requiring multiple tacking capabilities. Fractional rigged designs are a requirement if you intend to have a rotating spar, or a self-tacking jib. I did not want to limit my effectively good headsail size in difference to the smaller self-tacking jib nor rotating mast. Besides, since I was looking to eliminate the mainsail, I saw no need for a rotating mast.
Please note I utilized the word “headsail” to speak of the most forward of the sails on my rig, even thought some would say I have a second ‘headsail’…a staysail.
STAYSAIL or MAINSTAYSAIL
Some sailors might term my second sail a ‘second headsail’, or an inner ‘staysail’ as one would find on a cutter rig. I’ve chosen to call it the ‘main-staysail’ because I have no traditional mainsail to perform the functions of the ‘following sail’ subject I addressed above (the interaction of the leading sail and its following sail....the two sail combination producing a more favorable wind flow to the headsail ). In recognition of this need for a good helper sail, I chose a cutter type configuration with a good parallel slot arrangement. Regardless of what you believe about the ‘slot theory’, etc, eventually you will have to come to the conclusion that with a multiple-sail vessel, the leading sail is helped more by the trailing sail than vice-versa.
So I wanted my second trailing sail to 1) help my leading edge genoa, 2) be self-tacking, and 3) be of a size and disposition that it be easy to handle. I had had experiences with a standard staysail on my personal ketch/cutter-rigged vessel, and I knew I needed to improve upon the staysail’s self-worth. I knew it needed to be separated some greater distance from the headsail to be more usable in a greater number of conditions, so it needed to be moved back. Could it be made a bit bigger and substitute for the combination of staysail and mainsail of the traditional cutter rig?? Why not, particularly since I desired to get rid of the traditional mainsail. And lets make it a bit bigger, locate it over the center of the vessel such that it could be utilized alone in higher wind conditions, make it self-tacking, and make it roller furl.
At this stage I realized I had a net overall lost of sail area compared to the traditional sloop or cutter. But wait a minute; I always had a deep appreciation for the ketch rig for a cruising vessel design…..lets add a mizzen.
KETCH RIG ATTRIBUTES
To those cruising sailors that have had the pleasure of utilizing a ketch rig, I do not think I need to sell them on the concept. In fact I think most of them would join me in the praise of this rig configuration. Besides the full sail configuration, they are happy to be sailed under genoa/mizzen, with the mainsail stowed or reefed, and under mainsail alone, or with a combo of genoa, mizzen staysail and mizzen sail. This is a balanced and very versatile cruising rig.
Of course they do require two masts and correspondingly some extra amount of rigging.
Could I add a mizzen sail onto my double-headsail configuration and come up with a ketch style rig?? Wow, I believe so!! I could even term it a ‘
single-masted ketch’ !. The idea was born.:idea:
There is another big plus for a ketch rig on a multihull. The overall center of effort, CE, is lowered by a considerable amount compared to the sloop rig, and particularly the fractional sloop rig. Have a look at the illustrations below, and the illustration I will be providing for a big tri project I was consulting on. The rig heights can be a good 25% lower, and the overturning moments considerably reduced.
The overall sail area could even be increased on this lower aspect ratio rig. Besides, too much emphasis has been placed on hi-aspect ratio sailplans that are really only good for windward work. Marchaj, et al, have shown the virtues of low-aspect ratio sails for off-wind sailing….often two times more efficient!! Lets design cruising rigs for cruising sailors that….
…rarely want to go out bashing to windward. Here is an interesting first hand ‘cruiser’s analysis’. A Liveaboard Cruiser for the Real World
http://www.boatdesign.net/forums/showpost.php?p=35945&postcount=27
Having voyaged over 30,000 miles I have come to the conclusion that idyllic trade wind sailing with steady winds of 15 to 25 knots for 24 hours is a dream, or a myth made up by writers of cruising stories.
. And here a Capt’s experience with multiple head sails (cutter rig)
http://www.boatdesign.net/forums/showpost.php?p=39964&postcount=80
Chris White has very high praises for a ketch rig onboard a multihull. Have a look at this
separate posting below. I excerpted a few observations by Chris White during one of his voyages onboard his client’s 63’ foot ketch rigged catamaran, Concept 63.
My own experience; I have been the owner of a few sailboats, and one of them was a ketch-rigged vessel…more specifically a staysail-cutter ketch. For a cruising boat I really liked this ketch rig. It broke my total sail area down into more manageable size sails, it lowered my overall rig height, it allowed for helm balance by ‘tweaking’ the mizzen sail, it allowed for ‘mainless’ sailing under headsail/mizzen combo, and, had it been roller furling, it would have been even easier to sail single-hand. I went thru a particularly nasty offshore storm by initially running off downwind with just the small staysail, then upon full fatigue, lying slightly upwind under a backed staysail and reefed mizzen. Here’s a subject thread on going ‘mainless’
http://www.boatdesign.net/forums/showthread.php?t=21274
What did I dislike? I always thought the cutter staysail jib was too small, and it was marginalized by its too-close proximity to the headsail. And I was disappointed in the strip-area of the mainsail behind the spar that appeared to do nothing for forward drive. Now remember this was back in 1973, 35 years ago. I searched out as much info as I could find on cutter-rigged vessels, and I had heard of some aft-mast experiments in AYRS. My
webpage “Sail Propulsion” describes a few other items/processes that influenced my selection of this aft-mast configuration.
So now I’d come up with a single-masted ketch that I thought even looked very presentable….not too radical. But it had a mast canting forward!! That would prove to be a VERY tough sell to the conservative sailor. Sailors are a very conservative bunch that cling to tradition.
Just by happenstance a sailing friend/professional captain came across a magazine clipping of a vessel named
Diomedea Exulans. That clipping is in the archive section of my website, and I’ve posted it below. Here was a vessel with almost the same mast inclination as mine. This vessel’s rig design reinforced my thoughts about the possibilities of a forward raked mast
I had chosen a 10-degree forward cant to keep things in proportion, and allow for fairly good size twin headsails that would not overlap the spreaders. This cant could be reduced. In fact there was a problem with adapting this style rig to an existing in-build trimaran where I decided to reduce this cant to 6-degrees, and increase the size of the mizzen sail. The point is, the 10-degree cant is not set in stone, nor is the placement of the mizzen sail on the vertical backstay, nor do the spreaders need to be straight athwartships verses possibly raked back, etc, etc. There may be extra-light weight synthetic runners added to the tip of the aft jumper strut to counter the mizzen’s head from adding a torque force thru the aft strut to the mast at the hounds. And there could be other rigging additions as well.
What is needed is a ‘mapping of the rigging forces’ in this rig design, which might lead to subtle modifications that would optimize this rig. Personally I do not have the computer skills to set this program up. I had hoped that an adventurous client would come along with the funds to do this stress mapping and FEA as required to optimize this rig concept.
Is it worth the effort?? I would argue that it is a form of the ketch rig with lots of virtues for the cruising vessel, and particularly the multihull vessels. All three sails can be roller furled for great ease of handling. It should go the weather, and reach very well. It may well have more rigging members added to it than I have shown in the preliminary drawings, but it should still have less rigging than a conventional two-masted ketch.
BACKSTAY TENSIONS
The single biggest concern voiced about this mast-aft rig configuration is the large amount of backstay(s) tension it will experience as a result of the geometry and the need to maintain tight forestays, and then the extra compression loading this will impart to the mast.
This subject alone will consume more time and space than I care to give it with this individual posting here…so I will delay it for awhile till I post some other material I have collected on the aft mast subject.
But let me leave you with the analogy that came to my mind when I considered this aft-staying subject. When I looked at the profile view of the upper portion of my mast aft rig I saw what appeared to be an analogous situation to the athwartship staying of a mast by conventional spreaders and shrouds. My aft ‘jumper strut’ was acting as a spreader, and my masthead backstay was the upper shroud wire….and like a conventional rig, my lower backstay was a ‘diagonal’. The genoa’s headstay pull was analogous to the aft/sideways force of a twisted-off mainsail headboard, and the cutter sail headstay loading could be considered analogous to the headboard pull of a conventional reefed mainsail. These forces can be accounted for in a conventional rig, so why not with my aftmast arrangement??
In fact I can utilize a larger ‘cap shroud angle’ for my upper backstay than might be considered prudent with a conventional rig as I don’t experience the same degree of torsional instability that a conventional rig might experience. A larger ‘backstay angle’ could possible represent a smaller backstay load.
Oh well….more later. (sure would be nice to do a three dimensional space frame computer modeling of the rig where lots of the variables could be changed around to arrive at the most ideal form of this rig).
