Aftmast rigs???

That sail is the early '70s tallboy. Lots of people tried them, lots of people gave up on them. It may been indirectly because of cost (1 extra sail, a bunch of extra fittings, very little extra all-round speed) but mostly they just didn't seem to work most of the time.

They may well work in South California (steady winds, flat water) but they were quite twitchy to sail with in most places. The slightest over-sheeting destroys the flow over the larger and more efficient sails.

It's a bit like leaving the headsail up while reaching under spinnaker. Even on boats with comparatively long poles (like Etchells or FFs) where the headsail will not clog up the slot, it's still often faster to drop the headsail, even when you have the time to trim it intensively.

The Whitbread maxi ketches used to use them on long courses. They are quite fun. Whether they are "a secret weapon" is another issue

 

my MainStaySail vs ordinary Staysails/Tall Boys

CT249, I agree with some of your 'tall boy' criticism, and that Gentry PDF supports those views as well.

But my staysail (I term mainstaysail) is quite different in operation. It is intended to act like a mainsail (aerodynamically), and its leach location/orientation is quite different than the ordinary staysail or tall boy.

I repeat a portion of my posting above.

 

Upper Regions of my Genoa, mast tube helps genoa?

I firmly believe that in the regions of overlap between my mainstaysail and the genoa (all the region below the attachment point of the staysail and the mast), the aero principles laid out by numerous experts will appy,....that is that the genoa will benefit increasingly from the trailing staysail, just as the headsail benefits from the mainsail in more traditional sailplans.

Now what happens at the upper regions of the my genoa configuration,...that non-overlapping region at its tip?? This is a good question.

Perhaps we could just say it will suffer as a productive sail area just as does the upper tip area of the mainsail on a fractional rig.

But my hope and expectation is that having that mast section located at the trailing edge of the genoa's upper tip area will actually help the genoa sail in that region.

What do I base this hope on?,...2 things.
1) There is a slot created between the mast tube and the genoa sail in this region. And just as in the case where a trailing mainsail helps the headsail gain in productivity, I'm hoping the trailing mast tube creates a slot that helps the upper tip area of the genoa gain in productivity.

2) AMO Smith describes a situation where a trailing edge obstruction, a circular tube I believe it was, increased the aero lift of the airfoil in front of it. I'll have to try and find that reference as I don't have it handy.

Ah ha, I found it:
View attachment AIAA-59830-918_AMOSmith_High-LiftAerodynamics.pdf

PAGE #520

 

"AMO Smith describes a situation where a trailing edge obstruction, a circular tube I believe it was, increased the aero lift of the airfoil in front of it."

So the foil in front gets more lift. But how does the total L/D look when you include the pole and foil together as a whole?

 

Extra Drag of Bare Mast

Dennis, to tell the truth I do not know the answer to your specific question there, And even reading deeper into AMO Smith's theory may not give the definitive answer.

Here is my take on the question of the extra drag of my bare mast. Yes the bare mast will LIKELY create substantially more drag than if it had a sail attached to it. But then in what instances is this going to be of great concern?,...perhaps mostly beating up-wind? Please remember I am touting this rig design as a cruiser's delight. What is that old saying, "gentlemen do not sail upwind", or something like that. They wait for more favorable conditions to make their passages. I believe somewhere else in this long subject thread I quoted another gentleman's years of ocean cruising, and his observations on the % of time he encountered hard upwind conditions. Perhaps I'll look up that posting.

There have been plenty of conventional KETCH rigged cats that have made admirable voyages in all types of weather including upwind ones. And they have fared perfectly fine. Now my rig does not have all of the extra windage of that second mast and its supporting rigging,...a single-masted ketch.

I also carry a mast that is substantially shorter than a conventional fractional rigged cat with the same sail are,...so a little less drag there from the shorter rig.


Here is another consideration from a posting long ago...

I think my twin headsails also channel that wind to hit my aftmast in a much narrower range of angles when sailing upwind.
Beam reach or off the wind, I don't have to be too concerned.

Finally I have in the past suggested a 'fairing strip' of some sort attached to my bare mast. I have imagined it as some sort of twin very thin flexible sheet materials that would attach to the mast in some fashion and have trailing edges that would slid against one another such as to provide a short trailing flap (for various tacks and wind angles) to that bare mast section. Maybe they could be twin sheets that would be glued to either side of the mast tube and form into some tear-drop shape at their trailing edge. Or maybe something that would slide into a slotted base material glued to the aft side of the mast tube, in lieu of a more complicated mast tube with a slot built into itself.
At any rate it would be an interesting exercise in reducing the drag of the bare mast tube that could be carried out with 'glued-on' attachments at relatively small expense,...for those sailors looking to upgrade their aftmast rig for lower windage. Could be fun and interesting exercises.

 

Seems I must be missing the point, because the way I have understood the issue, it seems a little contradictory. How can you make a point to say the sail in front of the pole gets good extra performance, then say, well if even if the total performance is worse, it doesn't really matter anyway because cruisers don't sail to windward*? Either total performance matters or it does not.

*I have disputed this point elsewhere in the thread in more detail. If you ask someone who's boat cant sail to windward how many miles they have sailed to windward, of course they will say they never sail upwind. Go ask people who's boats actually sail well to windward how often they do it. Sure its still nice to avoid it, but many people enjoy it in calm conditions, or will do it if they have to. But its only possible if the boat can do it, and the worst it performs the less it will be done.

Also only retired people have the time to wait for the winds to be perfect. By far the most people just sail for the weekend or a couple of weeks at a time.

But I do agree that for many charter style cats a rig that goes well to windward is a waste as the hull wont do it anyway. This type of craft is the best selling too! Lagoon noticed, and I like thier aft mast rig.

I met a guy who sailed 38-40' lagoons from France to AU to sell with his own money. He said these things pretty much don't sail to windward at all. Better motorsail or you will never get there. I'm not hating, this is exactly what he told me and he done this big trip several times with various lagoons. But on the milk run, windward performance is not that important. What a shame we all cant just sail around the world in the tradewinds.

I done a Pacific crossing on a Hunter Legend 40.5 we purchased in the USA (the GFC was good for some ). Nice boat and faster than most on this run. We almost never sailed upwind, but once we got home and cruised like we usually do around our local areas, we needed upwind performance again and wished it had slightly better performance, even though it was not terrible.

 

Dennis, I was a little disappointed with your posting above. It sounds as though you are suggesting that I have said that my rig will not be effective going to windward?,...and primarily because of the drag of my bare mast??

I would first point out that there are degrees of weatherlyness, and just because some vessels won't point as high as others does NOT preclude then from being considered as viable cruising boat rig that can go to weather.

There are many many ketches that go to weather in a fine fashion and likely carry more rigging drag than mine.

Then there is this example of a twin foresail rig with bare mast that seems to do pretty good as well....

Lyra's rig

Or Kojii's wishbone aftmast (bare mast) that has been seen on this subject thread a number of times,...here is just one...
http://www.boatdesign.net/forums/sailboats/aftmast-rigs-623-39.html#post677981


Then how about this recent testing of a double headed rig with an A-frame mast (again a bare mast) that did very well in the testing
Wind tunnel and CFD investigation of unconventional aftmast rigs

That's just a few examples I could find most immediately. I'll have more to add to this drag discussion later. I am most interested in reducing the drag of my mast, because I am also a sailor that can never be satisfied with poor windward performance.

 

Sorry Brian, that's not what I meant. I know your rig would go to windward well enough even if not optimally. I sail to windward fine with only my jib up with the drag of a mast and mainsail in a bag. I agree optimizing it for windward at the expense of many other things may not be the best thing to do for many people. I even used examples to how this is true. look at the charter cats. They dont sail to windward well at all but are the best selling cat. I'm sure your rig on a lighter boat with dagger boards would be far better than a charter cat to windward, even though a charter cat rig is better for windward work. The whole rest of the boat is so heavy and with lar keels it wont sail to windward well anyway. Its a mismatch of rig to boat in a way.

Your rig on these exact boats will still not sail to windward well, but not because of the rig. In fact the rig would be perfect on those boats because that slight performance loss to windward would be a totally irrelevant point on this type of boat, even from a performance standpoint to windward since the hull wont do it anyway. Cruising speed would be higher most of the time as people can furl more sails out rather than raise the main so over all it could be a better solution.

My issues were that you mentioned cruisers don't sail to windward, which I disagree with. The other issue is I found it contradictory to mention one thing as a performance benefit, as if performance is important, then turn around and say well performance doesn't really matter anyway as cruisers don't sail to windward if that performance benefit turns out not to be true.

 

Another Aft Mast Concept

Hey Dennis,
Look what I ran across this morning on this subject thread:
http://www.boatdesign.net/forums/projects-proposals/need-composite-engineer-design-lamination-schedule-52644.html

Interesting :idea:

This gentleman started out with some other interesting illustrations/ideas that got poo-pooed by the traditionalist..
http://www.boatdesign.net/forums/boat-design/frame-vs-single-mast-rig-loads-weight-cat-51951.html

 

I think that guy is quite smart and came up with decent design concept. I do think a regular style rig, with perhaps a smaller main and 2 permanent furling headsails would have been fine. More like the pic in the first thread. (link to description http://www.boatdesign.net/forums/bo...-rig-loads-weight-cat-51951-2.html#post724432) I agree with what is being said in that second thread about the complicated unconventional rig being a bad idea mainly because its only small light 35 foot boat. The sails will be no issue to handle anyway. Those other rig options only make sense to me on a much larger heavier boat IMO.

Also it looks like he done a decent job and done the right thing asking for professional help to do the final scantlings and adjustments to the design. His concept was fairly good I thought. I don't see why he needed to be poopooed so hard. He did indicate he had already purchased a lot of the material, so I hope he comes back and post more if it goes ahead.

 

Check this out Dennis :!:

 

Thank you for this very interesting report, I missed it when you first published it.
There is a lot of reading I missed while I was cruising in the last 5y, but I saw a lot of sunsets and sunrises....

 

I have seen that before. The paper straight out says the aft mast version with 2 jibs has better performance that conventional sloop version. When tests like this seem to "prove" convention wrong it makes me think there is something wrong with the test. How else can you explain people with unlimited budgets who only care about winning are not using this setup if its faster?

 

Maybe they're suggesting that in cruising mode where Lagoons (and other overweight catamarans) have mains'ls the size of hankies and jibs of little consequence, that the aft mast rig provides better possibilities. We're not talking racing rigs but cruising rigs where performance parameters vary enormously. The wonderful thing about the English language is you can be as precise or as vague as suits your purpose.

 

Remember it is just one test, and it is on a model rather than a full size vessel.

So lets just accept it as one positive sign for this rig configuration/concept, and not a full endorsement of this concept over a full history of conventional rigs. I don't want to start a 'rigging war' here ,...just entertain intelligent/thoughtful contributions to the potentials of alternative rigs.

(I was VERY glad to see some actual wind tunnel testing done on these alternative configurations, ..... :!:)