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Old 28-04-2013, 14:15   #31
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Re: New Revolutionary Rig ?

It's cool. I guess I dont see why it's any different than a staysail schooner with a high aspect main in back that acts as a foil or "air rudder"...?
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Old 28-04-2013, 15:22   #32
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Re: New Revolutionary Rig ?

The notion that you could sail sideways is not controversial, Jedi.

I think what people are struggling with here is that Chris appears to be suggesting this could be done directly to windward. (eg when pinned to a dock by a wind abeam)

Let's unpack this. It's appealing to think that a boat on port tack could throw a rope to a boat, similarly closehauled, on stbd.

The keel of the boat on port is trying to stop it slipping to stbd, and vice versa. Surely you could unload the keels on both boats by mutual cooperation? And would the combined boat-conglomerate therefore, hopefully, sail directly upwind?

I tried to talk myself out of this by reasoning that the lift vectors would cancel, and all you would be left with was the drag, which would be double the amount for one boat, acting directly downwind.

However that's an unconvincing argument, once you consider that the resultant of lift and drag has a upwind, rather than a downwind, component, for any boat capable of sailing to windward.

It's an interesting proposition, but I've not got any further with it in the time I can spare, and regretfully hand it over to others who might have time to debunk it or argue for it.
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Old 28-04-2013, 18:11   #33
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Re: New Revolutionary Rig ?

As often happens, when I got on with life (and, more urgently but arguably less importantly, work) the answer popped unbidden into my head.

But I won't spoil the fun by spilling the beans as I currently see them, in case someone else wants to puzzle this out.
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Old 28-04-2013, 19:55   #34
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Re: New Revolutionary Rig ?

Not true vector analysis, but the way I perceive the answer to the thought problem, is to consider three forces that are important in the scenario.

First, there will be no difference between true wind and apparent wind such as occurs as a boat sails to windward. Therefore, the sails cannot approximate the efficiency demonstrated by those of two boats sailing close hauled (on opposite tacks).

Second, the hulls and keels are being dragged against their broadest surfaces through the water, so they will oppose this direction of travel through the water. Similarly, above the water, the true wind is shoving the boat to leeward with much greater force than when slicing into an apparent wind which is shifted forward.

Third, the ability of a boat's keel to prevent leeway is augmented by forward travel through the water, and so I think that forces of leeway would overcome any realized moment to windward.

Finally (albeit somewhat outside the thought experiment's perfect conditions) the boats being exposed broadsides to wind and waves would result in chaotic motion compounded by the two boats tugging and pulling on each other; this action will disturb any organized airflow over the sails, rendering all discussion of lifting forces moot.

The only way this scenario ends well is with a sharp knife!
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Old 28-04-2013, 20:09   #35
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Re: New Revolutionary Rig ?

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Originally Posted by phillysailor View Post
Not true vector analysis, but the way I perceive the answer to the thought problem, is to consider three forces that are important in the scenario.

First, there will be no difference between true wind and apparent wind such as occurs as a boat sails to windward. Therefore, the sails cannot approximate the efficiency demonstrated by those of two boats sailing close hauled (on opposite tacks).

Second, the hulls and keels are being dragged against their broadest surfaces through the water, so they will oppose this direction of travel through the water. Similarly, above the water, the true wind is shoving the boat to leeward with much greater force than when slicing into an apparent wind which is shifted forward.

Third, the ability of a boat's keel to prevent leeway is augmented by forward travel through the water, and so I think that forces of leeway would overcome any realized moment to windward.

Finally (albeit somewhat outside the thought experiment's perfect conditions) the boats being exposed broadsides to wind and waves would result in chaotic motion compounded by the two boats tugging and pulling on each other; this action will disturb any organized airflow over the sails, rendering all discussion of lifting forces moot.

The only way this scenario ends well is with a sharp knife!
Hmmm.... I had the same thoughts as your points 2 & 3, but I was able to dismantle those objections to my own satisfaction by theorising that both boats would be able to leave their boom at the same angle relative to the wind, while pivoting their bows directly into the wind.
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Old 28-04-2013, 20:20   #36
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Re: New Revolutionary Rig ?

Okay, so sweeten the chances of success by putting an Aero rig on each vessel, so that they are able to delink the orientation of the sails from the orientation of the boat. Both are then steered directly upwind.

I actually don't see why the bastard wouldn't go dead upwind. I'm confident it still ends in tears, but so what... if MythBusters is paying, I say go for it!
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Old 29-04-2013, 16:25   #37
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Re: New Revolutionary Rig ?

It's fairly simple.

The keels angle of attack while moving forward thru the water provides a vector with an upwind force.
The sails overall vector is slightly downwind but less than the keels upwind force.
Net result is you can make headway upwind.

When you tie the two boats together you kill both boats keel driven upwind force and are left with two sail driven downwind forces.
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Old 29-04-2013, 16:58   #38
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Re: New Revolutionary Rig ?

valhalla

I bet you're one of these skeptics who don't believe in perpetual motion, either.

<broad grin>
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Old 29-04-2013, 17:10   #39
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Re: New Revolutionary Rig ?

Now if you mounted the masts directly to high aspect keels with a reverse orientation and set them up in slots with a reversing mechanisim so they slide back and forth in the opposite direction, you would get the upwind vector from the keels...

Wait a minute, if it's blowing 30kts, I'm probably just going to relax at the dock.
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Old 29-04-2013, 17:29   #40
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Re: New Revolutionary Rig ?

The sentence in my earlier post #32 which misled me, causing me to temporarily reject my earlier true statement (similar to valhalla's closing statement in #37), was this:

<< the resultant of lift and drag has a upwind, rather than a downwind, component, for any boat capable of sailing to windward. >>

and my realisation, referred to in my next post, was that what I should have thought, and written, was this:

<< the resultant of lift and drag has a forwards component, for any boat capable of sailing to windward. >>

So the thing is this: forwards, for a typical modern sailboat capable of sailing to windward is (let's say) 45 degrees to the true wind direction.

I think 'sailing to windward' is a potentially misleading concept. We're only sailing to windward in any meaningful way when we combine the progress made on opposite tacks. At any given moment, we're sidling to windward, rather than sailing to windward.



Because of this major difference between the forwards direction and the windward direction, I temporarily lost track of the important truth I thought I'd got to grips with in boyhood, namely there is NO windward resultant from the combination of lift and drag. If there was, perpetual motion would be straightforward.

Like albatrosses, sailing boats use a sophisticated and cunning tradeoff to create the illusion of something for nothing. (just to be clear: it's not the same tradeoff: the albatross's one is infinitely more subtle and sophisticated than ours)

When my girlfriend first took me sailing, I couldn't enjoy the experience, because it made no sense to me. How could we possibly be travelling in the opposite direction to the motive force we were using?

I was intrigued but frustrated at my inability to explain it, and being very young and extremely impatient I had to cheat by stopping off at the library when I was biking home afterwards, to demystify the physics.

And, exactly as valhalla points out, it is the keel which singles out the forwards-directed component of the windforce (which with an efficient modern keel, is directed nearly beam-on when sailing hard on the wind, and accordingly has a large DOWNWIND-directed component)

The more efficient the keel, the better it is at singling out a small component.

It's perhaps a bit like a kid using a stick to push a dried item of roadkill off the road. The stick will have to be slanted a lot flatter on a dry road with a coarse surface than for smooth wet concrete, and if there's a layer of ice, the stick can be almost vertical.

There will only be a tiny component of the thrust directed forwards as opposed to downwards, but the interface between road and kill will separate out this component, and an icy interface will minimise the reaction force of drag in the opposite direction.

<PS: my inbox just went bing-bong with Valhalla's capitulation to the forces of perpetual motion, beating me to the punch while I was dreaming of roadkill: Yikes! what say ye, members of the jury ? >
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Old 29-04-2013, 17:43   #41
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Re: New Revolutionary Rig ?

Amplifying the thought experiment of the kid and the small previously furry animal:

Imagine the kid's trying to push the kill up a steep road. (45 degrees steep: this kid likes a challenge) It's also icy (this kid has crampons)

The stick is tilted back at 5 degrees from perpendicular to the road, trailing the direction the road kill is travelling. So the kid end of the stick is 40 degrees above the kill end, relative to the horizon.

The thrust from the stick is therefore directed substantially downwards, AND YET the altitude of the possum is steadily increasing.

If there is a hairpin zig in the road, the possum will end up directly above where it began, despite the application of a force which is, on the face of it, a down- rather than an up- force.
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Old 29-04-2013, 19:58   #42
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Re: New Revolutionary Rig ?

I guess it would just muddy the waters to mention that Chris White has designed a trim tab/flap/whatever you want to call it at the trailing edge of the keels.
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Old 01-05-2013, 06:03   #43
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Re: New Revolutionary Rig ?

dont let this topic stop. I love cwd catamarans. my dream boat.
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Old 01-05-2013, 16:12   #44
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Re: New Revolutionary Rig ?

I'm reminded of the problem of whether or not things fall at the same speed, if they have no resistance. It was thought that lighter objects fell slower than heavier objects as they were heavier, but they do fall at the same speed.

The reason why they do is quite simple: If lighter objects fell slower, you could attach a lighter object to a heavier, and it would slow down the heavy object. In other words, if you took a jug of water and added a glass of water to it, it would then fall slower. Therefore; all objects fall at the same speed, unless forces acting upon them (i.e. wind resistance/atmosphere).

This to me is the same sort of thing - I don't see how the foils could make it go directly into the wind, since the drag will overcome any lift. To suggest that they could pull the boat directly into the wind is like suggesting perpetual motion (free energy, in other words) as someone else hinted at.
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Old 01-05-2013, 16:19   #45
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Re: New Revolutionary Rig ?

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Originally Posted by Caracal View Post
I'm reminded of the problem of whether or not things fall at the same speed, if they have no resistance. It was thought that lighter objects fell slower than heavier objects as they were heavier, but they do fall at the same speed.

The reason why they do is quite simple: If lighter objects fell slower, you could attach a lighter object to a heavier, and it would slow down the heavy object. In other words, if you took a jug of water and added a glass of water to it, it would then fall slower. Therefore; all objects fall at the same speed, unless forces acting upon them (i.e. wind resistance/atmosphere).

This to me is the same sort of thing - I don't see how the foils could make it go directly into the wind, since the drag will overcome any lift. To suggest that they could pull the boat directly into the wind is like suggesting perpetual motion (free energy, in other words) as someone else hinted at.
I think your Perpetual motion reference is flawed, there is an external source of energy, the wind, so there is no "perpetual Motion" at work here, equally your analogy on gravity, while interesting is of no relevance to the issue at hand.

Having said that I dont dismiss the issues people are raising, just some of tangental references.
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