Multi-masted sailing cats

[rzj7l2]

Just came accross a new Design by Bernd Kohler. Just a preliminary image, design is not yet ready.
Design brief: A simple & cheap 12m go-anywhere cat with accomodation of a 10m boat, just 2 doubles.

Masts to be home built.

[sigurd]

for the biplane cat, this is not the case and you approach a position where during a beam reach the stagger completely blankets the following wing.

Put up a lee screecher...

[mdcf]

Originally Posted by sigurd

Put up a lee screecher...

I'm starting to think that you would mostly rig just one of the masts, the exception would be DDW and sailing close to windward. I could see on a reach setting the full windward sail and a small triangle of sail on the other mast (to take pressure off the steering) With reliable mast/boom furling this wouldn't be so hard. With the rig size correctly you could have it close to fully powered up with just one rig set... when two sails are set you'd probably run with a reef in each unless you were racing (scaring you senseless). If you can get away from setting foresails it would allow for fully rotating masts

[ancient kayaker]

The biplane rig may have some advantages. When running you have a laterally balanced rig with each side goose-winged for maximum area, when beating the rigs work together and do not interfere to any great extent. sail blanking should only occur on a reach over a particular range of angles to the apparent wind, when the windward rig is at it's most effective and the lee rig is not needed except in light airs. This should not happen too often.

Every type of sail rig has its shortcomings in certain conditions. I think it's a valid and interesting concept.

[mdcf]

Originally Posted by ancient kayaker

The biplane rig may have some advantages. When running you have a laterally balanced rig with each side goose-winged for maximum area, when beating the rigs work together and do not interfere to any great extent. sail blanking should only occur on a reach over a particular range of angles to the apparent wind, when the windward rig is at it's most effective and the lee rig is not needed except in light airs. This should not happen too often.

Every type of sail rig has its shortcomings in certain conditions. I think it's a valid and interesting concept.

I agree the more I think about it the more versitile it becomes. Possibly the biggest down side being having to trim both sails, so potentially harder for short handed sailing. It also looks nice ...

[sigurd]

If you can get away from setting foresails it would allow for fully rotating masts

You can do it with easyrigs but I think it is also possible with other rotating masts, using some types of foresails that don't need a tight luff. Except if you need past 180' ?

[tspeer]

Originally Posted by ancient kayaker

...I assume since positive stagger is by far the most common it is considered to be optimal. Presumably with positive stagger the upper wing is moved ahead of the turbulence coming off the lower wing, but the turbulence off the upper wingtip would then be more likely to influence the lower wing. I never did figure out the pros and cons. ...

According to Munk's stagger theorem, if the wings are carrying the same lift distribution, it makes no difference to the induced drag whether the stagger is positive or negative. That's a big "if", though.

There are actually a couple of reasons why positive stagger was more popular than negative stagger. Positive stagger is stabilizing in pitch, as opposed to negative stagger, and the wings were also generally set up with more incidence in the forward wing.

Another big reason for positive stagger is downward visibility. When you're flying a tail-dragger from behind the wings, you can look down and ahead of the lower wing leading edge on landing if there is positive stagger. But the lower wing is in exactly the wrong spot to see down and ahead when landing. The most notable negative stagger airplane, the Beech Staggerwing was a cabin biplane with the pilots sitting where they could look up ahead of the upper wing leading edge. They were far enough forward that the lower wing wasn't such a hindrance when looking downward.

[mdcf]

Originally Posted by tspeer

According to Munk's stagger theorem, if the wings are carrying the same lift distribution, it makes no difference to the induced drag whether the stagger is positive or negative. That's a big "if", though.

I would assume that this is only true when the relative distance between the wings is fixed and the stagger varied and for low alpha?

The problem as I see it for a biplane wing is that as the wind moves aft the vertical seperation of the 'wings' reduces and the blanking effect increases until you end up with a tandem wing cinfiguration.

Excuse the ASCII art

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   ^           ^           ^
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            Wind
 

I still think it has lots of interesting properties thoough.

[tspeer]

Originally Posted by mdcf

I would assume that this is only true when the relative distance between the wings is fixed and the stagger varied and for low alpha?

Yes.

The problem as I see it for a biplane wing is that as the wind moves aft the vertical seperation of the 'wings' reduces and the blanking effect increases until you end up with a tandem wing configuration. ...

Yes, I agree. When the cross-wind separation is small, the two demi-wings would have the induced drag of a single wing of the same span.

Most biplane rig designers use two sails of approximately the same aspect ratio that they would for a single rig, with mast heights 70% of the single rig mast height. This lowers the center of effort, but it also doubles the induced drag when the sails have a small crosswind separation. Only if the sails were so widely separated that their interference was negligible would their induced drag get back to the same value as for the single rig with 40% more span.

However, if the biplane rig maintained the same mast height as the single rig, then any crosswind separation would be beneficial to reducing the drag. The aspect ratio of each demi-rig would be twice that of the single rig, of course, because the chord would be cut in half. I've never seen the two rigs compared on this basis, but I think it's the correct one. The center of effort of the single rig could be lowered by cutting its span by 30%, too, with much the same effect on the induced drag.

[yipster]

found Aerade has interesting biplane docs with Munk's theory
variatons on section, chord, gap, stagger, sweep and decalage
and even tests on endplates

[mdcf]

Originally Posted by yipster

found Aerade has interesting biplane docs with Munk's theory
variatons on section, chord, gap, stagger, sweep and decalage
and even tests on endplates

Nice find, especially as these go up to 48 degrees. {EDIT} Ah although they are testing a different parameter, e.g. they are letting the foils stall at high alpha, and they are not testing a resverse stagger

[mdcf]

Originally Posted by tspeer

However, if the biplane rig maintained the same mast height as the single rig, then any crosswind separation would be beneficial to reducing the drag. The aspect ratio of each demi-rig would be twice that of the single rig, of course, because the chord would be cut in half. I've never seen the two rigs compared on this basis, but I think it's the correct one. The center of effort of the single rig could be lowered by cutting its span by 30%, too, with much the same effect on the induced drag.

Have to say I like the idea of two high aspect ratio wings and looking at it in that form. Put a couple of free standing rotating rigs up and you could end up with a very efficient setup.

Although it might be better to have these in tandem if the chord is small enough and the seperation enough. You'd preserve the reaching and at 45 degrees off the wind you would not get that much shadow. The chalenge would be to make the sail controls easy (and make the boat balance.)

[ancient kayaker]

I plan to test an interesting concept in the Summer on a small scale (kayak sailing rig). It will probably use a triplane wing assembly pivoted at the center of lift on a common mast, with zero stagger, the alpha being automatically controlled by a tailplane arrangement.

The angle between the wings and tailplane will be reversed to change tack, so the entire wing rotates to take the wind from the opposite beam. I have seen it tried with a single wing but not, as far as I know, with multiple wings.

It should work very well into the wind and on a reach. A downwind heading will involve either tacking downwind

I admit to straying away from the topic a bit here, but thought you would be interested given the biplane discussion above.

[mdcf]

Originally Posted by ancient kayaker

I plan to test an interesting concept in the Summer on a small scale (kayak sailing rig). It will probably use a triplane wing assembly pivoted at the center of lift on a common mast, with zero stagger, the alpha being automatically controlled by a tailplane arrangement.

The angle between the wings and tailplane will be reversed to change tack, so the entire wing rotates to take the wind from the opposite beam. I have seen it tried with a single wing but not, as far as I know, with multiple wings.

It should work very well into the wind and on a reach. A downwind heading will involve either tacking downwind

I admit to straying away from the topic a bit here, but thought you would be interested given the biplane discussion above.

That sounds interesting, I assume you are using fixed wings? do you have any thought on how to reef the rig? Are you going to the triplane to keep the center of pressure low?

I'm particularly interested in the notion of balanced rigs, the loads that typically run through the running rigging seem unnecessary and a point of failure that could easily be avoided.

[sigurd]

Somewhere on the net, there was a multihull model with four or five masts standing on a horizontal boom, which rotated on a bearing at midship. Stagger was adjusted by rotating the boom contra the individual sheets. Can't remember for sure how but I think the whole boom was balanced by a tail.
Hope somebody can find that link.