cat ketch downwind

[matoi]

In an old thread to wich I'm not able to reply any more, Boatmik made an observation on running a cat ketch boat downwind:

Originally Posted by BOATMIK

Doing it the right way you let the mizzen flip to the side it wants and run the main by the lee - sounds scary but on an unstayed rig you can let the main out 10 degrees forward of the normal running position. Use lots of vang on both sails and watch her go. If you have attached flow (it will be reversed from back of sail to the front - Laser style

I would appreciate if someone could clarify this for me - is the main supposed to be let out 10 degs forward from abeam, or is it supposed to be trimmed so that the airflow runs from leech to luff?

Does any of the two boats in my sketch do it properly?

Thank you, and sorry for being a bit slow on this - English is not my native language and I've never sailed a cat ketch boat, but am interested to learn about it.

Best wishes,
Mato

[Timothy]

I sail mine as in the sketch on the right,the main by the lee

[tom28571]

Your right hand diagram is the correct way to sail an unstayed cat ketch downwind. I've never measured Mik's 10 degrees and probably go further than that pretty often. It is a nice stable attitude. I think Mik misstates it as "by the lee" though, as it is just the opposite.

[matoi]

Thank you both!

[Eric Sponberg]

Matoi,

Yes, the diagram on the right is the way to sail a free-standing cat ketch downwind. You can read more about sailing cat ketches on my website in two locations. The first is on the state of the art of free-standing rigs:

http://www.sponbergyachtdesign.com/StateoftheArt.htm

The second is on my design Eagle/Globetrotter 45:

http://www.sponbergyachtdesign.com/EagleNews.htm

and

http://www.sponbergyachtdesign.com/Globetrotter45.htm

See the renderings on this last link to see the G45 in both the upwind and downwind configurations.

I hope that helps.

Eric

[matoi]

Eric, thank you. Project Amazon is really an amazing project. It's a pity the owners didn't manage to compete in the race for which you designed it for.
In photos, and article I noticed the gap between the mainsail and mizzen. This reminded me of Dashew's writings about importance of having the main and mizzen well separated on ketches - does it hold true for cat ketches as well or did I confuse something?

Back to running downwind, based on experience with this boat or with other cat ketch boats like NIS boats, what would be approximately the performance difference one could expect - between sailing with the sails in the 'obstruction to wind mode' and letting them forward a bit and having them generate some aerodynamic lift?

[Eric Sponberg]

Originally Posted by matoi

Eric, thank you. Project Amazon is really an amazing project. It's a pity the owners didn't manage to compete in the race for which you designed it for.
In photos, and article I noticed the gap between the mainsail and mizzen. This reminded me of Dashew's writings about importance of having the main and mizzen well separated on ketches - does it hold true for cat ketches as well or did I confuse something?

Back to running downwind, based on experience with this boat or with other cat ketch boats like NIS boats, what would be approximately the performance difference one could expect - between sailing with the sails in the 'obstruction to wind mode' and letting them forward a bit and having them generate some aerodynamic lift?

Thanks for your compliments. One reason Project Amazon made it only part way through the first leg of the 1998 Around Alone is lack of money. The vast majority of people who build boats think that they can do so with small amounts of money. They greatly underestimate the costs and either don't finish the project, or they build it to very low quality or poor functionality. In Project Amazon's case, the owner began with huge amounts of money--but being tied to the gold market of the era, by the start of the race, the gold scandals around the world reduced his fortune considerably, to the point where he could not pay his own way in the race. So he retired.

As for the spacing of the masts, of course, widely spaced masts reduce the interference of the main's downwash on the mizzen. This applies to all two-masted rigged boats. In order for the two masts and their sails to have a combined circulation effect, such as a jib with a main, the masts would have to be very close together, so close that they would be impractical to use--the main boom would not have a space for itself. So you place them far apart so that the mizzen is in cleaner air and removed as far as possible from the downwash of the main.

As to your last question, the performance of "drag-only" downwind sailing and "lift" downwind sailing is very pronounced. I can (and have before) report the experience of Eric Hall, owner of Hall Spars, who in the mid 1990s equipped his J-90 sloop with a sloop-rigged free-standing wingmast with the same total sail area and then raced it against other conventionally rigged J-90s. He could swing the boom forward of the beam so that the rig generated lift downwind. He found that upwind, both boats (his and the others) sailed comparably--same speed, same pointing ability. However, downwind, he found that he was about twice as fast as the other boats--that's right, twice as fast downwind.

Lift is always much more powerful than drag, usually on the order of 5 to 10 times more powerful, more if highly refined for the application--just think about the lift to drag ratios of aerofoils and airplane wings. The boom-forward arrangement that a free-standing cat ketch allows you makes the sails operate in lift, not drag. Now, of course you don't get 5 to 10 times more speed, the physics and balance of forces of the rig and the hull don't allow that, but you do get much more power, and the boat speed downwind for such boats is significantly greater than that for conventionally rigged boats where boom movement is restricted and where spinnakers--drag only sails--are the best devices they can deploy.

Eric

[Paul B]

Originally Posted by Eric Sponberg

where spinnakers--drag only sails--

Eric

Hmmmmm...

I don't think most people who know how to sail use spinnakers as "drag only" sails.

[Tad]

Originally Posted by Eric Sponberg

downwind, he found that he was about twice as fast as the other boats--that's right, twice as fast downwind.

And the skipper's switched boats a few times to make it a fair comparison of the boats rather than skippers sailing ability??.....and the weight difference between boats was???.....and the conventional rigs were flying spinnakers?...or not?

[Paul B]

Originally Posted by Tad

And the skipper's switched boats a few times to make it a fair comparison of the boats rather than skippers sailing ability??.....and the weight difference between boats was???.....and the conventional rigs were flying spinnakers?...or not?

This claim has been challenged before. Eric will come back saying he isn't making the claim, he is simply repeating what Eric or Ben said.

Even if the conventional rig was not using a kite it still sounds off. The J90 is a pretty quick boat, so it wouldn't take much breeze to get one to sail 4 knots downwind under main and jib. That means the wing mast rig whould have to be going 8 knots in the same breeze. If the conventional rig was doing 5 knots the wing mast would have to be doing 10, 6 vs 12. I think the term "not bloody likely" fits here.

Now if it was really light, drifting conditions and the conventional rig was doing 1 knot, maybe we could believe the wing mast was doing 2. But then was it the rig, or the local puff, or the angle?

The only thing I know for sure about the J90 with the wing mast is there was an article about it a few years ago. I think it was in Sailing World magazine. I think it was Ben who was being interviewed and he said the boat could not point with the conventional boats upwind, so couldn't hold a lane, and that made tactical sailing pretty difficult.

[Perm Stress]

"Lift is always much more powerful than drag, usually on the order of 5 to 10 times more powerful, more if highly refined for the application--just think about the lift to drag ratios of aerofoils and airplane wings. The boom-forward arrangement that a free-standing cat ketch allows you makes the sails operate in lift, not drag. Now, of course you don't get 5 to 10 times more speed, the physics and balance of forces of the rig and the hull don't allow that, but you do get much more power, and the boat speed downwind for such boats is significantly greater than that for conventionally rigged boats where boom movement is restricted and where spinnakers--drag only sails--are the best devices they can deploy."

You probably talk about lift to drag ratio of wings with properly organized attached flow at lee side, as opposed to something with separated flow on lee side. L/D of modern gliders indeed could be as high as 60/1 (here I refer to F Bethwaite book "High performance sailing"). While L/D of ordinary sail top out at ~6/1 (my own research by studying published wind tunnel data for ordinary sails).

When talking about pure total force without any respect for its direction or dividing to components, than flat plate total force coefficient ~1, while theoretical maximum of lift coefficient for wing is either 4 or 6 (Here I refer to A.M.O. Smith paper "High lift aerodynamics", can't remember an exact number just now); it is, lift in pure force terms is only about 3 times more powerful as drag in practical applications (here I again refer to F Bethwaite book "High performance sailing"). This is still a world of difference, however.

To sum up:
* in terms of LIFT OT DRAG ratio, lift is 5 to 10 times more powerful as drag in practical applications;
* it must be kept in mind, that highest L/D ratios are achieved at very small angles of attack in the order of few degrees;

* In terms of PURE FORCE, lift is up to 3 times more powerful as drag in practical applications;


Best regards,
Gvidas

[CT 249]

Originally Posted by Eric Sponberg

As to your last question, the performance of "drag-only" downwind sailing and "lift" downwind sailing is very pronounced. I can (and have before) report the experience of Eric Hall, owner of Hall Spars, who in the mid 1990s equipped his J-90 sloop with a sloop-rigged free-standing wingmast with the same total sail area and then raced it against other conventionally rigged J-90s. He could swing the boom forward of the beam so that the rig generated lift downwind. He found that upwind, both boats (his and the others) sailed comparably--same speed, same pointing ability. However, downwind, he found that he was about twice as fast as the other boats--that's right, twice as fast downwind.
Eric

Eric, with respect, can a performance gain of such size be remotely possible?

A J/90 rates about the same as a Farr 30 PHRF. Some polars (including Farr's) say that in 12 knots true wind, a Farr 30's best downwind VMG is around 7- 7.3 knots at around 160 true.

So, that means that if we fitted a freestanding wingmast on a Farr 30, and it had the same boatspeed boost as you report (and it should since the boats are similar) then the boat would be getting something like 14 knots downwind in 12 knots of breeze.

That means that a wing-masted Farr 30 would be hitting two knots more speed than a Volvo 70, which (according to polars) has a boatspeed of around 11.3nots at 141 true when going for VMG in 12 knots of breeze.

Now, I may be conservative and unaware of wing masts (which would be odd, considering we've had about half a dozen of them in the family) but it does seem odd that a Farr 30 could hiss past a VO 70, Wild Oats, Maximus, Speedboat and all the rest. What's even more disconcerting is that that means that a Farr 30 with a wingmast would be faster than Maximus when Maximus HAD a wingmast (which it dropped because of lack of performanmce IIRC).

It's also interesting to see what would happen to other boats if they had the same sort of speed boost. While other boats may not have the same amazing reaction as the J/90, it's still interesting to consider that if we doubled an Opti's speed with a wingmast it would be faster downwind than a 49er or an International 14. It would probably scare the world's best A Class cats downwind with that sort of speed boost...... which is sort of odd, since the A Class has a wingmast itself and therefore one would have thought it wasn't a complete dog.

And what of the boats that HAVE wingmasts? The Tasar is fast for a boat of its type, but it actually has no speed advantage over many older boats downwind in the light. And what of the Finn? It has a stayless wingmast, and it's rated 113.5. If the stayless wingmast was responsible for half of its downwind speed, how come that's not reflected in a vast performance jump when the wingmast was introduced and how come the Finn is still such a slow boat? If the stayless wingmast was responsible for half of the Finn's downwind speed, that means that the Finn would have been the slowest downwind boat in the world by about 30% before it got its wing.

Sorry, the notion that a stayless wingmast can increase downwind boatspeed by 100% doesn't seem to tie in with observed performance on the water. Wing masts are cool (I own and use a couple) but the real life evidence of a vast performance increase, certainly in monos, just doesn't seem to be there.

[Timothy]

While I think the "double the speed downwind" is an exaggeration I can say that on my boat that while sailing wing on wing if the booms are eased past 90 degrees the sails do generate lift and that there is a marked improvement in performance. I should say that is the case when sailing dead downwind or very close to dead downwind, and at any other angle I am faster over the ground if I cover more ground and gibe.

[Eric Sponberg]

Okay, Okay, I can see you guys will not let up. You are probably right to do so to keep wild-ass claims from becoming legend. First, Paul B, yes, this was as reported to me by Eric Hall. He and I did have conversations leading up to the trial sail in question, which occurred late November 1999. I went back through my notes of that time, and I never wrote down, "twice as fast", although I do remember him making statement when we were on board. What I did write down, to quote Eric Hall, was "equal upwind performance" and "offwind kicked crap." I also have a note that says "5' taller P, 4' shorter E." This implies a higher aspect ratio rig, which of course would account for some performance increase, but certainly not 2 to 1. I don't have any note about the overall sail area, whether the jib was reduced accordingly. I also don't have any note regarding the mainsail shape, whether it had a bigger, squarer roach. I expect that it would since this is one of the advantages of a free-standing rig--no backstay, no interference with a big roach.

Also, in Eric Hall's experience, I do not know if at the time he and the other boats were flying spinnakers or not. I don't have any notes in that regard. Certainly, some spinnakers can be carried on the wind, but I think it is safe to assume that traditional spinnakers are designed and used to sail before the wind, and in that regard, while they may generate some lift, their primary function is to operate by drag.

Tad, wouldn't it be nice to do perfect tests? This rarely, rarely happens--only in the America's Cup and perhaps in other high-budget campaigns. Certainly, Eric Hall may have been more able to do better tests with nearly identical boat weights and switched skippers in constant ideal conditions. I don't know if he did that. Maybe so, maybe not. I guess we'd have to ask him. But simple logistics and costs usually prevent this from happening. In my case, my clients never want to do measured tests--"Just give me the boat please, I'm off!" It costs money to do tests, and no client of mine has ever wanted to spend the money and time to do it properly, even with fancy new rigs.

So, in the end, this experience is anecdotal, not fully and repeatably empirical. Do wingmasts offer better performance on other boats? I think so--all the elements of increased lift and increased efficiency are there. Perhaps a claim of "twice as fast" is hyperbolic, and I am sorry if that is the way my statement across. Certainly, different boats are going to behave differently, and one has to sort out all of the variables of hull and rig to determine what causes what, and to identify the true effects of wingmasts on performance. I will stand by my belief that they are beneficial, and better than stayed rigs.

Eric

[Paul B]

Originally Posted by Eric Sponberg

I think it is safe to assume that traditional spinnakers are designed and used to sail before the wind, and in that regard, while they may generate some lift, their primary function is to operate by drag.
Eric

I can't believe someone in the business would make this statement.

Perhaps a statement like this could have been true in 1964. I can tell you in the past 30+ years I have very rarely seen a spinnaker used in the way you describe. It is not fast to do so. Only for short periods where you need to soak for a tactical reason would the spinnaker be a mostly drag sail. At other times, especially in lighter windspeeds, the sail is being used to generate a lot of lift.