The Ketch-Rigged Cat

[beiland]

Originally Posted by Nordic cat

Brian,
I will have rotating wingmasts that are unstayed, as does a Kelsall designed cat called Cool Change.

Sorry about that, I left those out of my thoughts at the time. I hope you are successful with your bi-rig, it will offer an interesting alternative rig.
__________________________________________________ _________

You could do boom furling with an unstayed mast which is nearly as easy as roller furling, but still lets you use battens for sail shape and less flogging of the sails.

Boom furling is not actually as easy to get right in real life as it first appears, and might be even more difficult on a free-standing, uni-rig. There are quite a number of accounts of problems getting boom furling fine tuned to work properly on conventional rigged boats.
__________________________________________________ _____

All those pieces of wire/rope and adjoining fittings are the weak points of stayed rigs. Lose one piece and you risk the whole rig. The safest rig is an unstayed rig, and it also offers the lowest windage

I’ve seen numerous references to the ‘hundreds of weak points’ of conventional rigs by those advocates of free-standing rigs. And yes there are many potential points of failure. BUT, I might suggest you look at the statistics of the situation. There are 100’s thousand sailboats out there in the world that have conventional rigs with 100’s of failure points. Yet how many really end up suffering failures?? Percentage wise I would guess very low, even among old boats that have never had their rigging replaced ever….pretty good track record statistically.
__________________________________________________ ___

Every meter of ½"/12 mm wire adds about 1 lb of drag at 40 knots! You rigging would add what would be the same drag as towing 6-8 tenders would!

My single-masted ketch should not produce more drag than a conventional two-masted ketch…maybe even less? Yes it will produce more drag than a free-standing rig, but not much more than a std ketch rig.

Your posting did prompt me to go back to some old thoughts I had on creating a simple shroud cover that might wrap around any round sectioned ‘rigging wire, rope, etc’ and cut its drag production substantially. I posted this ‘forum project’ subject thread as a challenge to many forum members to come up with some new ideas.
__________________________________________________ ______

The Prout rig has the main bulkhead to take most of the loads, and would be just as easy to handle as your rig, if done with boom furling. The loads would be more manageable

I think I addressed the bulkhead subject in my previous posting.

Boom furling is a possibility, but as I mentioned its not full proof, and it certainly gets in the way of my fishing cockpit.

[Whimsical]

I read that link and found Tom Speers post where he regurgitates some info I had previously come across.
"Sharp corners are very draggy. Hoerner has data on cylinders of various shapes. Starting with a rectangular cross-section and rounding the corners, a radius of 20% of the height of the cylinder can reduce the drag by 80%. There is not a lot of additional drag reduction from there to completely rounded."
Wouldn't applying this to sheerlines provide a far greater reduction to drag than reducing rigging drag?
If this is true why do designers persist in having sharp sheers and cabin tops? A 1.75 meter high hull would need about 350mm round on the top edge, or maybe 700mm to allow for the single sided rounding

Mike

[Nordic cat]

My guess is that the "square edged" boats are alot easier to build. Look at your own boat, and the effort per m2 needed for the rounded edges compared to the flatter parts.

Builders using moulds will probably need more moulds or more time consuming deck to hull joints if they followed this route, and then you lose some deck area.

Others complain of the difficulties of coming alongside with very rounded shapes, and claim poorer safety when walking on the side decks, this is not a big issue in my book, but might be partially true.

Shuttleworth and Schionning (to a lesser extent) as well as other designers adress this issue in their designs, Lagoons seem to be at the opposite end of this spectrum, but again it's "horses for courses".

Many of the very streamlined coachroof designs have major heat problems due to the large flattish window surfaces, and need to use some kind of external shading material, that probably also adds quite a bit of windage, so how bifg is the nett gain?

Boat design is the art of compromise....

Regards

Alan

[Whimsical]

Nordic
I see that round side decks adds time to the home builder and in my case I would estimate 300 hours. In a mould I wouldn't think it adds a lot and would probably make the hull to deck joint easier. I totaly agree with the downsides to the sloped cabin sides which I will get around by using sunbrella shades at anchor.
Some of the other posts in the link had numbers for hull drag and the gains to be had are substantial and far greater than the total rigging drag, which I have also disposed of. It just seems to me that if someone is looking to reduce drag it will be more obvious to look first at the areas with the greatest gain potential. As an add on to an existing boat reducing shroud drag is a valid endeavour.
I do find it funny that amongst the mountain of discussion on designing boats the subject of reducing air drag seems miniscule when it can be the major component of the total, especially with cats.

Mike

[Nordic cat]

Originally Posted by Whimsical

Nordic

Some of the other posts in the link had numbers for hull drag and the gains to be had are substantial and far greater than the total rigging drag, which I have also disposed of. It just seems to me that if someone is looking to reduce drag it will be more obvious to look first at the areas with the greatest gain potential. As an add on to an existing boat reducing shroud drag is a valid endeavour.
I do find it funny that amongst the mountain of discussion on designing boats the subject of reducing air drag seems miniscule when it can be the major component of the total, especially with cats.

Mike

Hi Mike,

I agree fully, air drag is often ignored in catamaran design for charter boats (mostly)

How are your bi-rig plans coming along? I hope to have my rig drawings ready in a few weeks, maybe we can look at the 2 designs and steal good ideas??

Regards

Alan

[beiland]

Originally Posted by Whimsical

If this is true why do designers persist in having sharp sheers and cabin tops? Mike

Originally Posted by Whimsical

It just seems to me that if someone is looking to reduce drag it will be more obvious to look first at the areas with the greatest gain potential. As an add on to an existing boat reducing shroud drag is a valid endeavour.

I do find it funny that amongst the mountain of discussion on designing boats the subject of reducing air drag seems miniscule when it can be the major component of the total, especially with cats.

I believe that one of the most obvious drag factors are the big square deckhouses, both unto themselves, and how they act as an end plate to the bottom of the sailing rig. I've taken my clue from Peter Wormwood and tried to give the forward face of the cabin deckhouse a shape more inline with the apparant wind in a close-hauled situation.

[beiland]

Another example

[beiland]

Originally Posted by beiland

Dear sandy,
Are you aware there are sailing rigs out there with backstays of less than:
1) 15 degree angles
2) 10 degree angles
3) even 0 degrees (no backstays)
Have you heard of the B&R sail rigs?? They are commonly ultilized on the Hunter line of sailboats. They might be termed 'strange' or certainly 'different' as they do away with backstays (NO BACKSTAYS).

The Bergstrom & Ridder:
The B&R rig takes the swept back spreader a step further. The angle on this rig is a massive 30 degrees. The idea behind the rig was to contain rigging loads as much as possible within the mast structure itself and avoid loading the deck and hull. This allowed builders to make lighter boats and not have to reinforce deck and hull structures for strength, reducing construction costs. This also allows builders to use a lighter mast section, reducing the cost of the rig. More recent developments of the rig includes reinforcements by incorprating rigin struts between chainolate and teh gooseneck. The purpose being to distubute the compression loads on teh mast reducing the anmount of reinforcement the deck needs to take the download pressure of the mast.

The big advantage of this rig is that it allows more roach in the leech of the mainsail increasing sail area for better downwind performance. The swept back spreaders also give a great amount of fore and aft support to the mast, eliminating the need for a backstay.

There are a number of discussions that can be found on this subject:
H260 Standing Rigging
.....and related subjects of swept back spreaders:
Loads for swept spreader rig - Boat Design Forums

I've added a few other observations on this B&R rig subject I brought up earlier in this subject thread:

Shallow-angled Backstays of B&R rig

Originally Posted by RHough
...I have read that the B&R rig has a narrow shroud base ... this is not correct, or the rig is not a B&R. B&R rigs use small, non-overlapping jibs. Inboard shrouds (that serve to increase compression loads) are not required. Since the shrouds can be at the gunwale, the shroud base is wide, not narrow.

I agree with you here, the B&R rig does NOT have a narrower shroud base. It makes you wonder why several web site references discussing the B&R rig repeat this error? Maybe it’s the same phenomena that kept promulgating the incorrect explanation for the ‘slot effect’ for so many years??

I brought this ‘narrow base’ subject up for another reason. I’ll begin by quoting Chris Mitchell of AES, Applied Engr Services of NZ, “Rigs with No Backstays”;

Rigs with no backstays are not so common. Most small dinghys under 20 feet have no backstays and no runners and no checkstays. They do however normally have some 30 to 35 degrees of spreader sweep. At larger sizes engineering requirements do not work out terrible well. The sidestays have to compensate by generating the fore/aft loads also, generally from a fairly small staying base in a fore/aft sense. Sailing up wind the rig can manage without the sidestays being enormous, given 30 degrees of spreader sweep and chainplates on the gunwhales.

Now if I take the drawing (attached) for a B&R rig from this site
http://pdf.nauticexpo.com/pdf/selden...-6227-_52.html
…and I search for this ‘effective fore/aft base’, I come up with this attached sketch impressed upon the original drawing. In a fore/aft sense the ‘backstaying portion’ of the shroud is working at a 12 degree angle measured with respect to the mast….pretty shallow for a backstay.

How did I arrive at this 12 degree angle? Looking at the upper spreader zone, a spreader raked at 30 degrees (normally quoted for the B&R rig) results in an actual spreader length of dimension “A”, and thus the effective aft reach of dimension “C”. Viewing this from the side view, we see an effective aft angle of 12 degrees.

This shallow backstay angle was always a concern of mine when evaluating the conventional 3-point staying arrangement of many multihull rigs. Here we often see a 3 legged staying arrangement of one forestay and two ‘shroud-backstays’ splayed out at 120 degrees to each other. Interestingly we might even draw an analogue between the multihull rig and a ‘spreaderless B&R rig’ as you might imagine from a portion of the description at this website:
http://kobernus.com/hunter260/rigging/rigging.html

A three-legged stool is more stable than a four-legged one. The B & R rig has the same approach. To accomplish this, the rig utilizes 30 degree swept-back spreaders creating 120 degrees between each rigging point. This tripod arrangement is similar to the huge radio towers you see from the highways.

Both the 3 point multihull rig, and a B&R rig, are taxed with maintaining reasonable forestay tensions under the burden of shallow-angled backstays.

[beiland]

Originally Posted by brian eiland
.....In a fore/aft sense the ‘backstaying portion’ of the shroud is working at a 12 degree angle measured with respect to the mast….pretty shallow for a backstay.

So shallow backstay angles are workable if we consider the number of B&R rigs and 3-point multihull rigs out on the water.

Rather interestingly these shallow backstay angles are very close to same magnitude as that shallow backstay on my aftmast rig. From a recent posting of mine:
http://boatdesign.net/forums/showpos...&postcount=110


.... let me point out a couple of critical items I see in my aftmast rig design:

2) Hounds Loading: Here is were I will experience some problems. As drawn at present There is only a 10 degree angle between the lower backstay and the mast....thus significantly higher compression loads to the mast. But if I attach this backstay to the front of the mast and run it to the very sterns of the vessel I get a 15 degree angle...much better. And if I chose a 6 degree rake for the mast rather than the 10 degrees shown, things change again...likely for the better. This is a portion of the 'stress mapping' I am seeking, and this is Chris' forte.

Now before someone jumps down my throat, let me say that I do realize that my shallow backstays are working against larger forestay angles and loads, and this puts them at a greater disadvantage The point I sought to make was that shallow-angled backstays by themselves should not a sole argument against my aftmast rig concept.

Here is an interesting B&R style rig on a Tennant catamaran. The adaptation of this concept combined with my mast-aft rig might produce an interesting arrangement. I’ll think about this, and maybe sketch it up in the future

[beiland]

Originally Posted by sandy daugherty

Finite element analysis is not necessary. Freshman level Statics and Dynamics are sufficient to calculate the loads of Mr Eiland's rig. They are enormous. They are not impossible. They just offer no advantage substantial enough to justify the engineering studies, load testing lab time, and (most importantly) the loss of interior space to the massive trusses required to carry these loads. Why bother? Make the mast 5' taller to compensate for the aerodynamic "Loses" of the mast.

The compression load on an exemplar 40' sloop's mast base is 15 tons. move the mast base aft to fifteen degrees from the backstay nearly quadruples the load, to somewhere around 110,000 pounds. In a double backstay arrangement which seems to fit a catamaran vessel, the backstays would need to carry 75,000 pounds apiece, (50% safety factor) plus athwartship loads, with a single headsail. Adding a second sail and the load required to maintain less than 1% deflection in the luff would represent a geometric projection in the loads on the backstays. Now you have to build a boat to carry those loads. It quickly becomes apparent that you have spent a lot of money designing and building a boat that is very heavy, uses exotic materials to carry the loads in rough seas, and it doesn't go very fast as a result. What most people would expect to be a big open bridgedeck salon is cut up into small odd shaped compartments divided by walls hiding massive trusses. And the beam under that mast will be an absolute engineering marvel. But yes, it can be done.

Brian Eiland has made some beautiful drawings though!

I’ve promoted this aft-mast rig for a significant number of years, without a great deal of success in getting it into a full-scale application. I’ve not found that really enthusiastic person willing to spend the money (not that much really) on the in-depth force mapping study I had hoped to conduct to search for the most optimum configuration. But I continue to get quite a few enthusiastic inquiries from sailors who feel this approach to a ‘main-less’ sail plan is just what they have been searching for**.

So with no stress mapping study at hand, lets go back to the logic I used when first developing my rigging configuration for the aft-mast sail plan. Lets re-look at the force-vector diagram analysis, and see if it is really as ‘overstressed’ as some naysayers have touted.

MASTHEAD
Lets start at the masthead. There is the primary forestay, a backstay, and two shrouds…all rather traditional. I’ve chosen to represent the force in the forestay with a 5cm long vector. Lets say this vector represents 1000 force units, thus each 1cm on the force diagram will represent 200 force units.

At the masthead the forestay force is broken down into two perpendicular forces, the compression load down the mast and the forward pulling force. The fwd force needs to be offset by the aft pull of the backstay. The backstay is at a more shallow angle so it must pull a bit harder to exert its rearward force. At present we know:

Forestay Force............................................1 000 kg
Backstay Force...........................................12 60 kg
Forestay compression Load in Mast.................820 kg
Backstay Compression Load in Mast..............1150 kg
Total Compression Load in Mast....................1970 kg
(at very upper portion & disregarding shroud loads)

.....nothing very unusual....very conventional. The mast is experiencing compression loads from both the forestay and the backstay force components acting in the vertical direction. And it’s doubtful that those compression loads imparted to the mast by the backstay and forestay are much greater than in the case of a purely vertical standing sloop rig mast.
My masthead backstay then passes over an aft jumper strut that redirects its force down to the base structure supporting the mast.

[IMPORTANT NOTE] This backstay that originates at the masthead DOES NOT reattach to the base of the mast itself, but rather to a structure of the vessel, …and preferably to the structure that accepts the compression loads of the mast to the vessel.]

So we have one of the backstays that delivers a force of 1260 kg to the vessel.

AFT JUMPER STRUT
The masthead backstay now bends over the outer tip of an aft jumper strut that I’ve placed at the mast hounds location, and pushes in on the mast tube. Just as with a conventional spreader element the aft jumper strut is set to bisect the angled turn of this backstay. By vector analysis the backstay exerts two equal forces of 360 kg each push on the aft jumper strut.

Aft Jumper Strut Push Load to Mast.................720kg
Aft Jumper Compression Load in Mast.............negligible

FORWARD JUMPER STRUT
Now I propose that we offset this entire cross-load pushing by the aft jumper strut with an opposing forward jumper arrangement. In order to accommodate the inner forestay and its sail this fwd jumper fixture will likely assume a ‘V’ configuration that is somewhat conventional in form.

BUT, the assembly is also unconventional in form. In the first place it is not set perpendicular to the mast tube, but rather in-line with the push of the aft jumper strut. And the included angle between the two struts might well be 60 degrees rather than the more common 90 degrees. AND it will NOT consist of two individual jumper stays (wire cables), but rather will be fashioned of a continuous loop of ‘cable’ that would wrap around the back-side of the mast at its lower ‘termination’, and might even do so at its upper ‘termination’.

The actual jumper stay ‘cable’ itself will be constructed from one of the new-age synthetic rigging materials such as Dyneema, Spectra, PBO, LCP, Aramid, C-6 carbon tow, etc. Ideally this stay material will have NO pre-stretch requirements thus no pre-loading. It should be very strong upon immediate application of force, and in a minimal diameter that it can be looped around the mast section in a continuous manner, at least on one end, maybe both. As a continuous loop, ‘both sides’ will always be carrying ½ the total load, rather than one side under load, while the other might be slack. There will also be a minimum of ‘fittings’ required to attach them to the mast (less weight, less failure pts). I imagine a simple ‘block’ of material attached to the mast around which the loop of this jumper stay can not slide any further along the mast….and one end needs to be adjustable

I call this whole assembly a ‘modernized diamond jumper’. It needs to offset the 720 kg force of the aft jumper with its four 4 cables….thus 180 kg each in their horizontal force component:

Front Jumper Push to Mast..........................................720 kg
Divided by 4 Strands........................................... .......180 kg each
Vertical Force Each Strand............................................ 340 kg each
Total Compression Load in Mast Tube.........................1360 kg
(in between the upper and lower jumper cable turning blocks)

INNER FORESTAY
The inner forestay is approx 75% the length of the primary forestay, so to keep things equally taunt should require about 75% of the load of the forestay (maybe even less since the inner foresail is much smaller than the primary genoa).

Front Forestay Force............................................. ...1000 kg
Inner Forestay Force............................................. ....750 kg
Inner Forestay Compression Load in Mast...................650 kg
Inner Forestay Fwd-Pulling Force..............................~350 kg

LOWER BACKSTAY(s)
Here is where we really load things up due to the shallow angles of the lower backstay(s). Lets explore 4 options:

1) Shallow angle backstay as originally drawn (about 10 degree angle with mast):
Backstay Load.............................................. ............1940 kg
Compression Load to Mast.........................................1900 kg

2) Broader angle backstay to sterns of vessel (about 14.5 degrees)
Backstay Load.............................................. ............1340 kg
Compression Load to Mast.........................................1280 kg

NOTE: Both of the two conditions above are based upon using the lower backstay(s) to resist the entire forward load of the inner forestay.
BUT, what if the forward jumper strut could accept some additional horizontal loading to help offset some of the forward pull by the inner forestay? Wouldn’t that take some loading requirements away from those lower backstays? (….to be explored in another posting).

__________________________________________________ ___________
Let's review the forces we’ve added to the mast column at this point due to the fore/aft rigging arrangement:

a) At the upper tip in the masthead area we’ve added virtually no
additional compression forces over those experienced by a standard straight standing mast under the traditional loading of tight forestay and tight backstay.

b) In the hounds region we’ve added considerable additional compression loading associated with the diamond jumper stays ’pulling together’ from their upper and lower ‘turning block terminations’. These create extra compression loads within the mast column itself, but they cancel each other in terms of adding extra compression loading to the lower mast and the stepping base. This panel of the mast is relatively short, and the rigging is such that it is not easily drawn out of column, so a reasonably strong mast section for this panel section at the hounds should be able to sustain these higher compression loads.

c) The lower panels of the mast suffer from the higher compression loads exerted by the shallow lower backstay(s), but not nearly as much as some have exaggerated.

Compression Loads to Mast Column by Fore/Aft Rigging
1970 kg.........................Forestay + Backstay......................1970 kg
..660 kg.............................Inner Forestay............................660 kg
......0 kg.................Shallow angle Lower Backstay.................1900 kg
1280 kg.................Broader angle Lower Backstay.......................0 kg
3910 kg.................................Totals......... ..........................4530 kg

These figures don’t appear to be that excessive…certainly no where near the 4 to 6 times loading that some naysayers have claimed. And certainly something that can be dealt with relative ease.
Have I made any errors in those figures above??
I can send anyone a full size sketch to scale if they wish to review it.
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SHROUD LOADING
One of the other primary reasons I sought to develop this aftmast rig idea was to end up with a rig that could make optimum use of those nice headsails in sailing upwind without resorting to narrow spreaders….besides narrow spreaders on a multihull craft can really load up the mast. If the sails don’t overlap the mast, I can make use of the nice wide shroud angles at both the upper spreader and at the base.

I’ve chosen to go with a 20 degree cap shroud angle at the masthead. This cuts the compression loading to the mast by a full 100% (literally in half) of the loading at 10 degree angles used by many racing sloops. This can be very significant considering the ‘infinite nature’ of the stability/righting moments of big cruising multihulls.

And not only is this broader shroud angle effective at reducing mast loading in the top panels of the mast, but it propagates down at each spreader level.

HALYARD LOADING
Often ignored are the significant ‘duplication loads’ imposed by the halyards. All three of my sails are designed to be roller-furling (maybe even roller-reefing on some cruising vessels using modern sail materials and furlers). As such I definitely contemplate the use of halyard locks to hold the sails up rather than a traditional halyard tail back down the mast to just increase compression loads even more….again it can cut those mast compression loads in half.

[beiland]

I just became aware that the columns of figures presented in this rigging force review above are skewed out of line...I guess due to some formating difference between forums?.

So here is a little better view
Aftmast rigs??? - Page 23 - Boat Design Forums

[beiland]

Originally Posted by Nordic cat

....I see that someone is building one of your mast aft designs in Thailand, it will be interesting to see how that works out..

Regards,Alan

Two have been completed,.... with two more on order..

...lots of photos here
HK40' - Power Sailing Catamaran.

The second one utilizes a little taller rig than the first one, ...and neither one makes use of the mizzen sail, but these are 'powersailers', not sailing vessels.

[beiland]

Originally Posted by glyphics

Hi Brian,
There is an interesting sailboat concept called the Universal Hull that uses an aft-mast rig and a single staysail. I wonder if you have seen it. This site has a reprint of an article the July issue of the British magazine, "Seahorse" (alas, without the referenced diagrams), but the YouTube video shows good performance from a small rig.

My Wooden Boat of the Week

That was an interesting posting. I had not seen it. I've made a copy and reread it several times. It has more to do with hull design, but it was interesting that he used that mast-aft rig with the single foresail as a beginning,....and maybe more interesting once he experiments with a more std rig, as he mentions doing.

I also enjoyed this little quote from the article,
"For the purposes of the article, I submit that the best type of research science has no fear of being wrong, because we often learn as much from a theory which is proved wrong as from one which is proved to be right. Instead, we make progress when we make bold and clear predictions which are sufficiently explicit to be tested to destruction."

Youtube videos of aft mast sailing

http://www.youtube.com/watch?v=OUQHQmmdSZc

YouTube - ‪Universal Sail - stability and power‬‏

Even though the breeze was light, you can also see a little of the explosive acceleration when the boat heels and the chine touches the water....towards end of video

[beiland]

Not exactly a ketch, but a two masted rig on a cruising cat.

Chris White Atlantic 47, MastFoil rig


Here is an interesting discussion
Chris White Atlantic 47, MastFoil - Boat Design Forums

[beiland]

I was sent a reference to this test document just recently by some designers in Holland.


INTERESTING RESULTS
I'm going to summarize some of this paper here in case it should be taken down, or disappear from the web, as many things tend to do nowadays.

Wind tunnel and CFD investigation of unconventional aftmast rig
Wind tunnel and CFD investigation of unconventional rigs (PDF Download Available)

ABSTRACT
This paper presents research activities carried out by the authors to investigate aerodynamic behavior of several unconventional sailplans,... in comparison to the sloop traditional solution. In particular an 'A-shaped' mast placed in the stern area of the yacht has been considered in single-jib and double-jib configurations. Wind Tunnel tests and performance prediction analyses have been performed in order to compare different configurations.

...a most interesting solution, including major potential development, appears to be the one configured on an “A” shaped mast, placed in the stern area of the yacht. In this way, what was the mainsail is now transformed into another jib (in the double jibs configuration) or even completely removed (single jib configuration).

....Many attempts to reduce drawbacks of aerodynamic interference of the mast on sails, both in sloops and in multiple mast sailplans, have been made in the past. On the experimental yacht Amoco Procyon by O. Harken
.....
(BE note: They were apparently quite knowledgeable about the Procyon rig, and several others, but made no mention of my numerous postings on the subject of aftmast?)

So far the purpose of this research, conducted by a cooperation between Mechanics Department of Politecnico di Milano and IDEA Department of Facoltà di Architettura di Pescara, was to compare through wind-tunnel tests, a traditional sloop sail plan with a (similarly sized) “A” shaped stern mast sail plan, both in the single-jib and the double-jib configuration.



Two complete scaled models for both single rigged yacht and traditional sloop yacht have been built and tested in the Politecnico di Milano Twisted Flow Wind Tunnel.
The yacht model, consisting of yacht hull body (above the waterline) with deck, mast, rigging and sails, is mounted on a six component balance, which is fitted on the turntable of the wind tunnel. The turntable is automatically operated from the control room enabling a 360° range of headings. In order to correlate force measurement readings and the sail shape and in order to provide input data for CFD calculations, an in-house photogrammetric measuring system has been developed to recover flying shapes during tests.

The traditional sloop yacht rig used as a reference is a Comet 51’a Vallicelli Yacht Design & Co 51 feet IMS cruiser-racer sailing yacht, winner of 2007 IMS Italian Championship.
Wind tunnel tests were performed using a 1:10 scaled model of this yacht where a mainsail with the maximum IMS rule allowed roach and 100% non overlapping jib have been used.














Apparent wind angles were chosen to be 22°, 27°, 32° and 42° which cover the upwind range. Tests were conducted in upright condition. For each apparent wind angle tested the first task was to determine the maximum driving force potentially achievable At the same time the influence of the sails trimming changes was observed using the data acquisition program that visualizes the forces acting on yacht model in real time.

At the end, some runs were performed on the bare hull and rigging (without sails) for both yacht models at different apparent wind angles and in different heeling conditions in order to measure windage. These values are subtracted from each of the measured data points in order to produce the sail force coefficients.


From a pure aerodynamic point of view the relative performance of different rigs can be compared by comparing the driving force at similar apparent wind
angles and heeling moment. From these figures unconventional rigs seem to perform better than the standard sloop configuration.

As can be seen at closer AWA unconventional solutions are better than the standard sloop and in particular the two jib configuration with overlap seems to be able to produce higher driving force (at dynamic pressure =1) at the same heeling moment.

As can be seen unconventional configurations have aerodynamic centre of effort which are lower than the standard sloop and in particular the two jibs without overlap sailplan has the lowest values.

In order to gain further understanding of the sailplans aerodynamic behavior numerical simulations have been carried out using RANS method.

For each design scenario performance prediction have been carried out in 4-20 Knots true wind speed. Figure 24 shows a comparison in terms of optimal VMG in close hauled condition between standard sloop and
unconventional rig with reference to both single and double jib configurations. In particular figure 24 refers to full scale case with the same sails tested in the scaled model.

The results obtained confirm that the double jib configuration performance is better than the standard sloop up to 10 knots TWS, while the single jib
performance is pretty similar to sloop configuration. In windier conditions all the unconventional rig solution are faster and in particular the double jib with overlap gives the best performance.

CONCLUSIONS
In the present paper an unconventional rig has been investigated in comparison with a standard sloop rig by means of wind tunnel tests. The traditional sloop yacht rig used as a reference is a Comet 51’a Vallicelli Yacht Design & Co 51 feet IMS cruiser-racer. Several unconventional configurations have been tested, all characterised by an “A” shaped stern mast without mainsail in single-jib and double-jib configurations. Aerodynamic data available from experiments have been used to perform some performance prediction at full scale by means of a VPP code.

Both experimental tests and VPP calculation show that the double jib
configuration with overlap gives the best performance and also the same configuration without overlap gives better results in comparison with the standard sloop solution.

Numerical investigation have been carried out using RANS simulation in order to better understand the aerodynamic differences resulting from the experimental tests. Simulation results put in evidence a slat effect in the overlapping jibs configuration leading to more attached flow on the aft jib allowing for an higher pressure drop on the sailplan.