Aftmast rigs???

No need to make that 3 stay a permanent one, particularly since you would utilize it so seldom...why work around that obstruction. Just make it a detachable stay, liking a running backstay or such. Many boats have featured such detachable inner stays, often to lend addition support to the mast when sailing under a reefed mainsail.

 

Thanks Guillermo!

 

Oh My. This is a perfect example of how old wives tales take on a life of their own on the internet.

I'm sure I will be reading about the "lifting" aspects of the aft mast rigs in posts written by others in the next 5 years, due only to someone else reading this nonsense.

Please provide a vector diagram showing this lifting component! Unless the rig is canted to windward there is no vertical lift. Raking the forestay more than a normal rig does not change the direction of the wind as well.

Mis-reading the writing of Capt. Nat doesn't make a fairy tale come true.

 

What can I say, except I was there...

Manie sailed with me once, maybe he can offer an understandable explanation.

Q. Why does a kite go up in the air ?

It could be our sails work off the vacuum created by it instead of the air pushing onto the sail, very similar to an aeroplane wing.

A 45 degree stay will do as much upward sailing as it would do foreward sailing.

 

Yep this is 90% myth. But to be strictly accurate there is a tiny vertical component to any fore and aft sail with a raked leading edge which is sheeted off the centreline. It just isn't enough to make much practical difference. The one that is 100% myth is the one about such sails lifting the *bows*.

Always difficult to know what to do with a little correction like that, because you know that a percentage of the flat earthers are going to take it out of context and claim that its justification for their nonsense, but OTOH if you don't put the correction in then if they ever come across it then that's evidence of the conspiracy against their wonderful insights...

 

The amount of lift may be more than some think.

For a foresail, the center of lift is much closer to the front of the sail than the rear of the sail. When you are sheeted out way on a downwind run imagine a force vector perpendicular to the sail surface at say 35% back from the front edge.

With minimal forstay rake (very rare), this force vector will be nearly horizontal.

But if the rake is a lot, say 45°, the force vector will be way off vertical. The combination of rake and how far out your are sheeted is what produces a lift vector with an increasing upward component. With enough of a combination, you can get vertical lift that is close to or more than the forward lift. Go look at some of the 12' skiff videos on youtube (http://www.youtube.com/watch?v=_G7PlPrjkrw for example).

 

I agree with your argument P Flados, but not completely with that video as an example. In the video they are running, but I undestand what you're trying to show. The aft mast sail makes some lift in any direction you sail. I doubt it's going to be so much that it will lift the boat out of the water so you can parashute up an down, but the sail has two effects, there is defenately lift and it has more of a righting effect on the boat. I also doubt if this sail can be used to it's potential on a monohull, but on a multihull, yes.

 

The physics is exactly the same. Orbital velocity the works. These tanks are very sophisticated pieces of equipment, wave tank testing is mandatory for seakeeping characteristics of many vessels these days.

 

Yes, physics is the same.

For seakeeping, there is concern with surface tension, but if model is wider than 0.3m it usually is considered appropriate.

 

Aftmast rigs made me remember the early 70s when a partner and I were re-rigging a fifty foot, 1901 SF bay oyster smack as a schooner. We'd stepped the mainmast and the bowsprit, but the foremast was still being hewn when we got a job in Vallejo, 30 miles away. The boat's the shop so....
No engine, no foremast.... we set the mainsail, strung 3 jibs and it worked fine, sailing downwind north to Napa river, then working our way up to anchor. Beating to windward down San Pablo bay also worked when the job was over. A week later we were back home and finishing the foremast and stepping it.
In Port Townsend WA is a 24' cabin aluminum motorboat which has had the engine removed, an outboard added, and a stern-mounted bipod mast with a huge roller genoa set up. It seems to work but I haven't examined it closely.

 

Yeap! They can analyze behaviour in breaking waves, i.e.

 

Unfortunately you don't really understand what you are looking at. The 12s have an enormous pitch down pressure from their spinnakers: that's why they're hanging off the back of the boat. They even have the gunwhales extended aft of the transom in order to get the weight back far enough. If you let the spinnaker flap the bow lifts. If a spinnaker genuinely lifted the bow - as a kite sail can do - then when a gust hit the crew would be rushing the weight forward to keep the bow down.

 

As an engineer I have dealt with vector force balances on numerous occasions. I have also been reviewing sail / wing aerodynamics quite a bit for some time. I referred to the clip just to show the angle of the spinnaker while it was running. I understand the the overall force balance on this boat is requiring the crew to be a far aft as possible. I am pretty confident that my statements were factual as presented.

Much of the time, the force vector for the spinnaker was probably pretty close to being in alignment with the center of force for the water drag acting on the hull and boards. If the overall spinnaker force vector passes through a point forward of the drag forces resisting forward movement, the sail would tend to lift the nose. If it is more to the rear, the reverse could be true.

Note that in the context presented, lift is force directed straight up that tends push the overall boat up out of the water. Pitching moment is a force acting on a moment arm that tends cause rotation of the boat (bow up or bow down). A sail can provide lift and at the same produce a pitching moment that pushes the bow down. I have been on a run where the stern was lifted out of the water with the nose not being pushed down all that much.

However, for the specific example of the boat in the clip, it also has other sails that are providing minimal lift but are providing a lot of forward pitching moment. The overall force balance for the boat is the vector sum of all wind induced forces on the sails, boat and crew above the water line; the vector sum of the hull drag, hull planing, dagger board and rudder forces below the water line and the weight balance.

These forces are in a constant state of change. At times it looks like the lower portion of the sails are powered up and the upper regions are doing less. Other times it is reversed. This shifts the force vectors all over the place. The water forces on the hull are also unstable. As is obvious, the overall instability is causing a whole lot surging and rocking.

 

Vertical Lift by Sails

Unfortuntionately I have to believe what my eyes are telling me. When the spinnaker in that video is flailing (no power), the bow of the boat settles down. When the spin is really under power it is lifting the bow. Its center of effort is far enough forward of the boat's drag center, such that the vertical lift that is being created by the spinnakar is lifting the bow. This lift is NOT to be confussed with 'aero lift' we refer to in sailing aero discussions as that 'perpendicular plane' force that the vertical wing of our sails creates and we see manifested in forward driving forces or heeling forces.

Yes the spinniker is driving the boat forward from a center of effort that is well above the drag center of the boat, and logically we say that the driving fwd force coupled with the drag force of the boat produces a bow down rotation force. So why isn't this boat driving its bow down?? (and don't tell me its just because of the crew weight on the rear).

Here is what I think. This boat is light enough that its drag center is well aft in this situation, and its drag is relatively small as it planes off (notice that ballasted monohulls will NOT experience this). So the bow never gets a chance to dig in and allow the fwd driving force of the spinnaker to rotate the bow down to any great degree. Surely as soon as they let that bow dig in that driving force of the spinnaker will rotate the bow down. Stay on plane boys!!

But I also would be willing to bet that if we could see the flow stream lines of the air passing over that spinnaker we would see a downward deflection of the air created by that top portion of that spinnikar. 'For every action there is an equal and opposite reaction' . A portion of that opposite reaction is vertical lift by that sail, and its well forward of the boats contact center with the water. That sail is producing vertical LIFT.

 

Rigging Force Review for Aft-Mast or Mast-Aft

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............................................1000 kg
Backstay Force...........................................1260 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??

_______________________________________________________

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.

Comments, suggestions, corrections welcomed
(I've attached a 'forum posting size' sketch that I used in this vector analysis. If you want a full size to-scale sketch, send me an email at brian.eiland@gmail.com)




REFERENCE:
**(two recent inquires: 1) a daysailing charter operator in a trade wind area who struggles several times a day with his full battened mainsail, and 2) another gentleman who recognizes the future of high fuel prices and desires to do a 55 foot ‘gamefishing-under-sail’ charter vessel)