I am sure this is one of those areas where people have firm views.

I am presuming that an asymmetric hull will not point as close as a similarly rigged hull with a deep asymmetric board. Is my assumption correct? Could anyone put a figure on the difference in pointing ability?

My question is fuelled by the tests I have been doing with the lug rig with controllable CE and how well this might sit on a minimal proa platform with asymmetric hull

Thanks

Tink

One should not compare a high aspect board with an asymmetric hull, as they are very different animals in every way. Speed and angle towards the wind are very small parts of the big picture.
If I design a proa for cruising and poking into uncharted coves, then an asymmetric hull will be much safer, much more robust and cheaper to build.
A longer and more slender asymmetric hull will be faster and more closewinded than a shorter hull with a board.
A high aspect board can and will stall in some conditions. It does not contribute as much to course-stability which puts much more strain on the rudder.
High aspect boards do not like beaches, sandbars, semi-submerged logs, underwater rocks and everything else that is normal day stuff for a cruiser. An asymmetric hull does not care much about those things. It will easily glide over a lot of stuff with only minor scratches in the paint.

I don’t have any hard factual numbers based on any measurements, but my experience with my models have shown me that an asymmetric sharpie-hull is very closewinded. I estimated it to sail at about 30 degrees to the wind. This is with a badly sheeted crab claw sail.

The green asymmetric ASP is the most close-winded model I have tested yet.

This is my view on this complex subject.

Cheers,
Johannes

Tink,

Surely the answer if you want the most high winded boat you can get is to do both???

You still have Johannes’ “cruising” option if you lift or leave out the board. Although I do recall Sven Stevens saying that asymmetric hulls were not worth bothering about and that they could be a control issue at high speeds?

Johannes is also right. Relying solely on foils which can stall or break can bring on unwanted and possibly catastrophic events.

Rob

Thanks Johannes and Rob,

I think a CAD model and a physical model are in order plus a bench marking Matrix. It is obviously too simple to think of close windedness in isolation but I am curious to know what the difference is.

Must go to work now but I will leave a small part of brain mulling over proas

TINK

When you say close winded, do you mean pointing angle or VMG? Faster boats often point lower than slower boats, in part due to apparent wind angle, but the VMG of the faster boat is usually better (but not always).

In general, high aspect boards combined with a low wetted surface optimised hull will provide the best VMG. As evidence, no development multihulls that I know of opt for asymetric hulls only. If you want to race your proa, go for boards. If you want to cruise it, the robust simplicity of an asymetric hull has a lot going for it.

The actual performance deficit for asymetric hulls may be in the order of 10%, all else being equal, but very difficult to determine exactly, as Johannes says, they are very different beasts with different objectives. How do you compare them? You could, for instance, compensate for the lack of boards by making the hull a bit longer.

Mal,

VMG at the end of the day is the only thing that really matters…...If you can afford to make the hull longer,you should do this anyway for lots of other reasons as well.

If you want options (regardless of length), do both, asymetric hull and high aspect asymetric boards. You don’t have to use the boards, you can even leave them ashore if you want to whilst cruising,

But what about high speed control???  Russ Brown’s latest collaboration with Paul Beiker (Jester 32) has a symetrical vaka,  I think Madness’ is symetric also, whereas Jzerro’s is asymetric?????

Perhaps Russ could comment??

Rob

It’s not a given that asymmetry is necessary without boards, so there are actually four possible combinations, asymmetric or non asymmetric hulls, with or without boards. One school of thought is that asymmetry was introduced to help with the weather helm issue, rather than to improve the L/D ratio. I’m not aware of any studies which investigate the effects of asymmetry.

To provide lateral resistance, all you really need is a hull deep enough to provide the required lateral area. The penalty is wetted surface area. The deeper the hull the better the lateral resistance but the more skin friction you have. High aspect ratio boards are more efficient, so you can get the same lift with less wetted surface area. With a boardless boat, you have the option of pointing the same but going slower, or going the same speed and pointing lower.

I’m not sure what the high speed control issue is with asymmetric hulls. I would think that this is a detail design issue and that you may or may not have control issues with either boarded or boardless hulls. There are to many variables to be able to make a generalisation that asymmetric hulls have high speed control issues.

Mal,

Attached below is an extract from Sven Steven’s post No 13 dated 28 Nov 2011: Testing with Models:

“Both 15 meter designs were made in January 1999 , The reason for testing in 2005, ; was to see how much “fatness” the windward side of a pacific proa needed to have ( see below models), to counteract the ama induced weatherhelm.  I spare you the results, as I now firmly believe that it is not worth bothering about making the vaka A-symmetrical at all, ( unless for the purists, who seek to unveil the historic Pacific Proa’s true nature ) 
Why, because the A-symmetrical behaviour of a Vaka hull is largely depended on speed.  The quicker the Vaka travels, the more force is generated to bear away to leeward.
Now ,the faster you travel the less weather helm “correcting “ force is needed, as the ama is scimming now anyway, generating barely any resistance to the platform.  See also Russell’s Sept 2011 video at 7min.
It should be; the quicker you go , the less force you need to have to bear away to leeward. Matter of fact , the quicker you travel , the less force you want anywhere, its directional balance you seek, and minimal steering corrections. 
Note; Pacific Bee most times sails with Autohelm 2000 , and it barely needs to correct the path of intended travel. 
So that A-symmetrical test route was back then considered busted, that Vaka model discarded.”

I can tell you from personal experience helming a 36 ft planing monohull at 16 knots plus with excessive lee-helm, courtesy of an over balanced rudder is highly disconcerting and more prone to stalling. As Sven says, the quicker you travel…............

Rob

Rob,

I agree that the turning moment will be proportional to speed, but that may be a bad thing or a good thing, depending on the type of rig, the course you are sailing and many other variables. One set of tests is not enough data to make this sweeping conclusion.The only thing you might conclude is that asymmetry was not useful for Sven’s particular configuration.

Any boat will be more difficult to control at high speed than at low speed. Lee helm caused by an overbalanced rudder is actually weather helm, but feels like lee helm and yes it is highly disconcerting. However, these issues are separate to the effects of asymmetry. Asymmetry will cause a tendency for the hull to yaw in one direction, which may or may not be useful. The yaw force will be proportional to speed but will be relatively constant as speed generally does not vary wildly or instantaneously, unless you have an an extremely light boat in wild conditions, in which case you have much more than asymmetry to worry about:-)

Mal and Rob,
Thank you very much for the interesting information. I have lot to consider, and lots of actions to think about. I think it very useful to test TP03 with varying board depths and see the effect on VMG. I think some CAD models to compare wetted area will useful. Then I think some simple sailing models.

I quite like the idea of deep symetical sharpe hull with a small board on a linear track for steering. On TP01 which had an asymetric hull I found the CLR move to about 1/4 the hull length from the bow, I wonder if with a symmetrical hul it will remain nearer the centre. 

Thanks again

TINK

I think a flat lee-side is more important than asymmetry in it self. I have noticed that every model I have tested that has a flatter lee-side will track very straight, especially through surf and breaking waves. I think my very crude and simple Barge-proa is a good example of this. I post a picture from the three picture series on page 11 in my “Advanced Sharpie Proa” thread.
I was very surprised when I saw my barge-proa sail right through a nasty breaking wave without breaking a sweat.
I does not have any dagger/lee-board. It is tracking straight with only a rudder hold with a rubberband and the hard straight lee chine.

I have read somewhere (but I can not find it now) that the asymmetry was mainly to get straight tracking when sailing through surf to or from the beach.
I think it really helps the boat behave nice and predictable in rough seas.

Cheers
Johannes

Johannes
Some very sound logic here, I like it, as I posted before I need some models.

On a side issue I like your lug rig much better than my quick and dirty effort. What have you made your sail out of? is it flat? or did you attempt to get some camber in it? How is attached to the spars?

TINK

Thanks Tink!

My lug is made out of 8 mm wooden round spars, hand-sewn spinnaker-cloth and cable-ties. It is very simple and crude, but it works really good.
I cut out the shape I want with an additional 3 cm on all edges. I fold the sail-cloth over a thin cord and sew a pocket. I have two 15 mm “darts” in the forward part of the sail to make some camber.

Polytarp sail

Cheers,
Johannes

Johannes

Thanks, sometimes my quick and dirty is just too quick and dirty, I will try again along your lines when I get a chance

Tink

A deep V hull works reasonably well in a flat sea with a decent wind.  In a lumpy sea with not so much wind boards win every time as they bite in the less disturbed water lower down.
The other problem with a deep V is that the centre of pressure on the hull is close to the bow, so the rig has to be move far more. Lift is generated at the leading edge.  It is the same for low aspect keels,  I suspect once moving they would work as well if shorter,  the length only helps when starting to move,  worth an experiment if you where considering them.

Mark

The other problem with a deep V is that the centre of pressure on the hull is close to the bow, so the rig has to be move far more. Lift is generated at the leading edge.

I suppose this is why the Walaps of the Marshall Islands have such a deep rocker. I have found deep rocker to make the hulls behave better, especially in waves. This spreads out the center of lateral resistans over a greater lenght of the vaka, and makes it more stable. Very similar to swept back delta wings. The Polynesian and Micronesian where using advanced swept back delta wings both as lifting bodies and sails, millenia before Nasa and Lockheed-Martin even existed.

I think this is a “problem” with deep V shaped tri and catamaran hulls, compared to deep V proa/walap vakas.
The walap and proa needs for and aft symmetrical hulls, and they have a lifting sail keeping the vaka from pitching to much in waves. The tri and catamaran has two or three hulls that can excite pitching, and with the right (wrong) wavelength they will hobbyhorse and behave badly.

For every “wrong” in a catamaran trimaran or monohull, the proa has at least two “rights” instead.
I just keep getting more and more impressed by their advanced technology. No wonder why people have had a hard time understanding the proa. We need to go through 50 years of space-exploration before we in the west start really appreciate the delicate superadvanced “stone age crafts” of Polynesia and Micronesia.

Cheers,
Johannes