Herreshoff's catamaran reasoning

[brian eiland]

(courtesy of Claas.vanderLinde on another forum)

Here is what Herreshoff himself wrote about it in 1877:
"... In the fall of 1875 I was thinking and thinking how to get great speed out of single hulled boats, of the kind in common use. To get great speed, thought I, one must have great power, one must have a great sail, you must have something to hold it up, and that something must be large and wide, and have a large sectional surface, and also a great deal of frictional surface.

These properties in a hull to give stability are not compatible with attaining great speed. Indeed, the more one tries to make a stiff, able hull the less speed will be attained, even if corresponding additions are made to the sail.

So then, there are two important principles of speed which constantly work against each other. If we increase the power to get more speed we must increase the stability of the hull correspondingly. An increased hull has more resistance, both from sectional area and surface friction. So what we would fain gain one way we needs must lose in the other.

Well, a boat must have width, and the wider she is, generally speaking, the more stable she will be. But a wide boat cannot have great speed, apply what power you will to her, so the next thing that is to be done is to decrease the sectional area and in a measure retain stability; the boat would have power to lift at a distance each side of the keel, where it would do great work. I kept on following this principle, getting the keel higher and higher, until by and by the keel came out of the water, when, lo and behold! there was the double boat!

Nothing else to be done but take a saw and split her in two, spread it apart a little way, and cover all with a deck, and there you are! That was the rough road which I traveled, and having arrived thus far I abandoned my ill-shaped hulls, and in their place substituted two long, narrow, very light boats and connected them at the bow, stern and middle. The boats (hulls) must be far enough apart, so that the water which they displace in moving will not crowd against them on the inside. To hold them apart at such a distance rigidly would be difficult, and not only difficult but useless; indeed, just exactly the thing which should not be done. So the fore tie beam and the after tie beam must be connected to the hulls by universal joints, and the main beam in the centre must be joined to the hulls in such a manner as to give them freedom in a slight lateral motion, for it will be seen that if the bow of one boat pitches she must pitch in a circle round a centre, and in doing that the two hulls are drawn bodily together, slightly, to be sure, but enough to pull them to pieces if provision is not made for it. ..."

(Source: Herreshoff, Nathanael Greene. "The Amaryllis. How the Yachting Wonder of 1876 Was Conceived and Built." New York Herald, April 16, 1877, p. 5.)

By 1877 this looked like http://www.voilesnews.fr/dossier_ca...istorique_des_multicoques/Amaryllis_II-02.JPG

For more info one might visit the museum site http://www.herreshoff.org/frames/mmframe.htm

 

[Skippy]

I kept on following this principle, getting the keel higher and higher, until by and by the keel came out of the water, when, lo and behold! there was the double boat!

To hold them apart at such a distance rigidly would be difficult, and not only difficult but useless; indeed, just exactly the thing which should not be done. So the fore tie beam and the after tie beam must be connected to the hulls by universal joints, and the main beam in the centre must be joined to the hulls in such a manner as to give them freedom in a slight lateral motion, for it will be seen that if the bow of one boat pitches she must pitch in a circle round a centre, and in doing that the two hulls are drawn bodily together, slightly, to be sure, but enough to pull them to pieces if provision is not made for it. ..."

That's neat brian. Do you have any idea where I can find a further discussion of these structural stresses? I'm very interested in them, but I still haven't figured them out.

Also, have you ever seen Malcolm McIntyre's hyddroplane from the 1930s? Bernard Smith has a picture of it in "The 40-Knot Sailboat". It's also a catamaran with the adjustable beams, and it has a twin rig mounted on an A-frame. The windward sail is tilted leeward so it pushes down on the windward half of the boat, and the leeward sail conversely is tilted windward so it lifts up the leeward sid of the boat. Together, they produce very litttle heeling moment, maybe even none, even at high speed. The daggerboards are canted inward for the same reason, and the hulls are made to plane.

 

[tspeer]

If you want to see some aerodynamic results from A-frame rigs, go to http://www.tspeer.com/landyachts/twin/dragstudy.htm

The key is even though you divide the area into two panels, you don't want to reduce the height of the rig compared to the equivlent single wing rig. And the rig is best suited to rigid wings, because you get the best performance by sheeting out the leeward panel when the boat starts to become overpowered, but keep the windward panel driving. Naturally, operating a soft sail at low or zero lift will make it flog, so you need a rigid wing. Theory says it's actually beneficial to backwind the leeward panel instead of backing off on the windward panel if the craft is still overpowered.

CA in this figure is the axial (fore-aft) force coefficient - negative is goodness. CA/Cl is the axial force divided by the heeling moment - positive is goodness. These data consider only the inviscid lift and induced drag. Adding the same parasite drag to both cases won't change their relative ranking with regard to drive, but will amplify the differences in net drive/heeling moment.

The A-frame rig poses some interesting structural problems, too, since the compression on the leeward panel increases without bound as the apparent wind speed increases. You can't asume the stability of the craft will cap the loads because you're compensating for the heeling moment aerodynamically.

 

[Skippy]

1. If you want to see some aerodynamic results from A-frame rigs, go to www.tspeer.com/...

2. The key is even though you divide the area into two panels, you don't want to reduce the height of the rig compared to the equivlent single wing rig.

3. Theory says it's actually beneficial to backwind the leeward panel instead of backing off on the windward panel if the craft is still overpowered.

4. The A-frame rig poses some interesting structural problems, too, since the compression on the leeward panel increases without bound as the apparent wind speed increases. You can't asume the stability of the craft will cap the loads because you're compensating for the heeling moment aerodynamically.

1. Thanks Tom, I'll check that out.

2. I would expect that to depend on the boat's beam, since some minimum angle is necessary to eliminate the heeling moment.

3. Wow, very counter-intuitive. I would think you'd get a wicked lee helm. Also extra displacement. In fact, I thought maybe sheeting out the windward surface would be a good way to tack.

4. I see the windward mast pushing down on the top of the leeward mast, is that what you mean? But I don't understand how that's a stability issue. The other thing that worries me is that by relying on canted sails & keels instead of ballast, the vessel is subject to catastrophic failure (heeling) if the windward keel is ever jostled out of the water.

 

[tspeer]

When I was looking at the A-frame rig, I had a landyacht in mind. For that application, downforce is useful, since it increases the traction of the tires.

I set the apex angle such that skidding and heeling happened at the same time. If the base is wide enough, then you can get the condition where the line of action of the windward wing passes through the contact point of the lee wheel. Such a rig would not heel at all with the leeward wing sheeted out all the way. But this is wider than necessary, because there is some stability from the yacht's weight that can be used and because such a non-heeling yacht can still skid. The optimum apex angle turned out to be around 18 degrees, which is quite reasonable.

For a boat, you might want to sheet out the windward wing, which would effectively cant the rig to windward. However, the heeling moment of the leeward wing about the leeward hull is not much reduced from that of a single wing rig.

As for lee helm, I didn't try to estimate the effects on steering trim. I believe you're right that there would be considerable lee helm. Whether or not that's a problem with a landyacht, I don't know. It may actually be beneficial, in that to have steering you have to place some of the weight on the front wheel. That weight represents side force that's not available from the lee wheel. If the yacht has lee helm, then the front wheel is carrying a sideload in the right direction.

The relationship between stability and lee panel compression is the same as the relationship between stability and mast compression on a boat. The limit of stability caps the lift on the rig, and this caps the compression needed to resist the shroud tension. If there's no llimit to the stability, there's no limit to the compression.

There is one other advantage to the A-frame rig: it eliminates one stay. Only fore and aft stays are needed. This is a significant reduction in parasite drag (windage), especially for a yacht that will experience triple digit apparent wind speeds. There's also the potential for weight reduction in the body and axle, since these don't have to carry significant bending moments. The axle only serves to keep the wheels apart. The body weight is actually slung like a hammock from the apex of the rig.

An A-frame rig doesn't presuppose canting keels or hydrofoils for stability. It could be put on a conventional catamaran configuration, for example. But I don't necessarily think it's a good rig for a watercraft because of the downforce if the leeward panel is sheeted out, or the unstable nature of a windward canted rig if the windward panel is sheeted out.

 

[CT 249]

Am I being too harsh when I think that Bernard Smith is a bit of a whacko? I've looked at a lot of his stuff and there seems to be repeated examples of some crude rig shoved onto a Canadian canoe and sailed in 4 knot breezes on a lake for an hour. There always seems to be some deep reason why he can't get out and get his creations working. Hell, he's been trying for decades....Surely some time in the last 40 years he's had time to get it all together if the basic ideas were ever going to succeed.

Frank Bethwaite used to try his Smith type boats in front of my house when I was a kid. He describes them in HPS, and points out the basic flaws. He went on to make the HSP which was great buzz to sail.

The planing cat and inclined rig have been tried many. many times, with very little success AFAIK. Lots of people say windsurfers go fast because they derive lift from windward cant, but the rigs are actually very close to vertical most of the time. Hell, look at the pics of Eric setting the new world speed record; his rig is about as upright as he can get it. I have been coached by the Olympic bronze medallist in windsurfers and he got me to stick the rig as vertical as I could. Upwind, when we have the rig raked to windward, boards are less competitive than downwind when the rig sits pretty much vertical (in the sideways plane). On boards we can easily cant our rigs miles to windward; we don't because we know it's slow (as I had driven home to me when I tried short harness lines in the last raceboard nationals.....ohhh the pain and ignominy).

So all that stuff that Smith goes on about......hell, I dunno. It's been tried for decades without success. When theory fails in practise despite many efforts for decades, isn't that an indication that the theory is wrong?

 

[FAST FRED]

"When theory fails in practise despite many efforts for decades, isn't that an indication that the theory is wrong"

Perhaps , but the Law of Unintended Consiquences sometines intervenes.

Mr H was all for increrased stability for sail carring power , yet the Cars work by generating small bow waves that can be overcome.

He got FAST boats , but NOT for the reasons he built them for.

GRAND!!


FAST FRED

 

[CT 249]

Oh yeah, wasn't knocking Herreshoff....Nat and L Francis were damn smart. Really damn smart.

I was just casting aspersions at Smith.

 

[Skippy]

Am I being too harsh when I think that Bernard Smith is a bit of a whacko?

I've looked at a lot of his stuff and there seems to be repeated examples of some crude rig shoved onto a Canadian canoe and sailed in 4 knot breezes on a lake for an hour. There always seems to be some deep reason why he can't get out and get his creations working.

Hell, he's been trying for decades....Surely some time in the last 40 years he's had time to get it all together if the basic ideas were ever going to succeed.

Oh CT, be nice! That's funny, Smith does have some goofy ideas. Is he still alive?

The thing about invention is it's better to be crazy than to be narrow-minded. If you have a truly stupid idea, you just stick it in the round file and move on. But if you just keep tweaking the parameters on some old outdated albatross and you never get beyond that, you may wind up never coming up with anything at all. And Smith really was an inventor first, at least in the books of his that I've read. He was making prototypes, not polished commercial designs. Like they say, all pioneers seem nuts at the time, until their crazy ideas are accepted by the mainstream, at which point the ideas suddenly become "obvious".

I think I got the idea of what Smith was trying to do, and I also think it's impossible. He wanted absolute maximum possible speed, and he wanted it on both tacks. The "sailloons" I would agree do seem like a silly idea, but even they have two qualities of a good airfoil: they're light and they have some thickness. The original "flip tackers" didn't work, but a similar design is the Aeroskimmer, a catamaran with a single canting wing mounted on an A frame. And the 3-point proa-like design was used by Yellow Pages Endeavor (see Radical and Advanced Sailing Vessels or innovoile.free.fr). And foil designs have evolved a lot since the 1960s.

But the basic idea of having a geometry that is somehow ideal for all points of sail on both sides of the wind just seems too hard to me. The proa might be the best solution if you can live with shunting, and the trimaran might be optimal if you had super-ultra-lightweight materials and low-windage ballast that you could suspend as far windward as you needed (kind of like a sailboard with training wheels). But I think one thing Smith's work proves is the old saying that all design is compromise.

And my final objection is his emphasis on equilibrium over stability. I just don't like the idea of relying on hydrofoils generating downward lift when they can be yanked out of the water, and even the canted rig can be dangerous in heavy weather. Do you have any links or references to Bethwaite's work? I'd like to see that.

 

[CT 249]

Well, I dunno....I don't think that anyone who has stuck with his own ideas for 50 years (having checked the site) despite a fairly significant lack of success, can be called "open minded". In that time, the dinghies have gone from wooden hiking boats to carbon skiffs, the windsurfers have arrived and gone through enormous changes, cats have been developed into wing masted fliers, conventional keelboats have gone from being S&S designs to Open 60s.

The thing that beats me is that there are thousands of windsurfers out there who COULD easily incline their rigs, like Smith wants us to, and we know it's often slower to do so and never all that much faster. So his theory must be wrong. It's been tried in C Class cats (a US cat and a French one) as well....no success. Both were uncontrollable, as Bethwaite also found. The idea just doesn't work in the real world AFAIK.

There's been an enormous amount of development in all that stuff, much more than "tweaking an albatross". I've been looking at dinghy design pretty seriously, and one thing I notice is that the breakthroughs normally come from people looking around them, keeping in contact with the outer edges of the mainstream, and taking ideas from one area and intelligently applying them to another to create something different and great. The leaps (wings, trapezes, cats, windsurfers) come from people who are willing to look at everything they see, not people who spend 50 years on the vain pursuit of one idea.

I don't know whether Cunningham copied Smith's layout for YPE. By 1959, when Smith launched his first unsuccesful boat, Charlie Cunningham (father of Lindsay) was already a very succesful and innovative designer. His Gwen 12 of 1943 was arguably the world's first "modern" dinghy; lighter than most current boats, trapeze powered, fully-battened sail. Lindsay helped work on the Gwen 12, but he and Charlie moved to cats in the late '40s/early '50s. The Cunninghams were among the leading innovators in cat design for decades, AND designers of many of the best raceboats. While Smith was playing with canoes in drifters on lakes, Lindsay was building 3 stage wing masts and winning the Little America's Cup with the fastest of all small boats.

Bethwaite's work is "High Performance Sailing", which is available in most good bookstores. It's a must-read, although his history can be a bit wonky, and his assessment of cat and windsurfer performance is a bit out. As he says, his experience with Smith's ideas lead him to a study of wind which I find extremely interesting.

 

[Skippy]

Well CT, as far as I can tell, I agree with everything you said. I wasn't trying to suggest that nobody else does creative innovation, just that a lot of good new ideas seem stupid at first, so it's dangerous to criticise people just because some of their ideas don't work. In 1899, the US Secretary of Commerce stated his opinion that all possible inventions had already been made and nothing new would be discovered in the future!

And I thought the Smith website was pretty much just historical, not intended to be up to date. It says he's still alive and he's in his 90s! I had tons of fun reading his first book, and learned a lot from it, including the things that I disagreed with. It's the book that got me started, and I found it very interesting. Thanks for the Bethwaite reference, I'll look for it.

 

[brian eiland]

Herreshoff patent reference

That's neat brian. Do you have any idea where I can find a further discussion of these structural stresses? I'm very interested in them, but I still haven't figured them out.

Brian wrote:
Skippy, I think this info is more along the lines of what you were asking about; the stresses in the catamaran structure, verses the A-rig.

....excerpted again from another forum....
Tom Speer writes:
>The biggest problem I see with compliant hull connections is diagonal
>stability. Basically, it has to come from the windward hull, not the
>leeward hull. The windward shroud tension will keep the windward bow in
>the water, but the center of buoyancy of the leeward hull will always be
>at the pivot. So the boat will rotate about a line between the lee hull
>pivot and a point somewhat aft of the windward bow. That's a drastic
>reduction in diagonal stability. I suppose the leeward shroud can keep
>the lee hull from wracking too much. But then it's not really following
>the waves any more.
>
>There's also the issue of pretension in the shrouds to get forestay
>tension. You basically can't, because you'll lift up the sterns. So
>the forestay tension has to come mainly from main sheet tension. This
>may be why you see Amarylis constructed with a strong backbone.
>
>There's a fundamental tradeoff between stability and sensitivity to
>disturbances. I think I'd rather go for stability.

Claas responded:
Not quite. Rig and hulls are totally independent on the Herreshoff
catamarans. Forestay, shroud and mainsheet tensions are transferred not
into the hulls but into an independent and rigid truss system
including the strong backbone to which Tom correctly refers. The hulls
are attached to this with a separate system of trusses and balljoints.
Very complicated stuff.

Having said that, diagonal stability was indeed an issue with the
Herreshoff catamarans, but not a big one. The hull's pivot point was
aft of its center of gravity and as the windward hull began to lift it
would do so stern first requiring one of the crew to go aft on the
windward hull and act as ballast.

For those who are really interested in this, here is a link to the
original patent by Herreshoff which explains everything in detail.

http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=US189459&F=0&QPN=US189459

Fascinating reading, but note you need to be willing to invest some
time if you really want to understand it.

 

[Skippy]

...what you were asking about; the stresses in the catamaran structure
...a link to the original patent by Herreshoff which explains everything in detail.

Yes, thanks. I'll be a while reading it.

 

[Skippy]

1. ...there would be considerable lee helm. Whether or not that's a problem with a landyacht, I don't know. It may actually be beneficial, in that to have steering you have to place some of the weight on the front wheel. That weight represents side force that's not available from the lee wheel. If the yacht has lee helm, then the front wheel is carrying a sideload in the right direction.

2. If there's no llimit to the stability, there's no limit to the compression.

3. I don't necessarily think it's a good rig for a watercraft because of the downforce if the leeward panel is sheeted out, or the unstable nature of a windward canted rig if the windward panel is sheeted out.

1. That sounds possible, depending a lot on the geometry. If you're not designing for general use, is an asymmetric configuration like YPE feasible?

2. Okay, I get it.

3. I notice that modern twin masts are upright, maybe with a slight tilt inward but not much. That should increase the sail efficiency by decreasing the interaction, and also help tacking from the greater increase in wind helm when the windward sail is sheeted out, since the sails are farther apart. The cat plus twin rig is definately my favorite combination right now for a low-displacement application.


Thanks for your comments Tom, they've been very helpful.

 

[DSmith]

whacko?

Nearly every great inventor through the ages has been called whacko at some stage?

.......31 knots with a soft rig and lots more potential doens't sound too bad for a failed idea.

www.whbs.demon.co.uk/sr2/