Tacking "proa" - what's the penalty?

[champ0815]

Well, this topic appears from time to time as minor part of the discussion in different threads, but although i try to understand all the arguments made by the proa supporters i couldn't figure out what is actually the penalty for tacking with a proa and not shunting it.
I understand that a true proa is not able to tack because of the statics of the rigging, so, for the correctness of the terminology, we make the thought experiment proa vs outrigger.
We keep all the parameters of the two boats identical, just the rigging differs in the way that the outrigger can tack and the proa can shunt. These two boats have the task to sail to windward on a relatively narrow strip of water maybe even against some current.
I would expect the shunting proa to be significantly slower than the tacking outrigger, since the stopping of the proa while shunting costs much more time than the slowing of the outrigger on the "wrong tack" due to larger wetted area in "atlantic proa mode".
Ok, the pacific proas are made for wide bodies of water where shunting is required not so often but even there: especially for larger cruising boats, where the flying of the second hull is rather undesired so that the argument of wetted area doesn't count so much, would the outrigger not be the faster option - as fast as the proa on the "pacific proa tack", significantly faster during tacking and only slightly slower on the "atlantic proa tack"?
Background for this question is the often pronounced wish for a multihull cruiser which can be trailered within the legal (width) limits. A cat with two symmetrical hulls has to be narrow to meet this requirement, a tri needs three hulls to maintain (and build), so the proa with one wide hull and a smaller ama to fit in folded configuration under the wide one looks like a possible solution. Therefore the question: is the additional complication of the shunting rig (and rudder/board configuration) worth the effort?

 

[spidennis]

My suggestion is to head over to one of the proa discussion groups on yahoo where there's plenty of proa fans to give you some experienced answers. Most here don't understand proas and you'll end up explaining more than getting the answers you are looking for. No disrespect guys, but am I not right?


http://groups.yahoo.com/group/proa_file/


http://au.groups.yahoo.com/group/harryproa/



http://groups.yahoo.com/group/proadesign/

edit:
http://www.proafile.com/

 

[Petros]

what if you made a combo/hybrid where it sails like a trimaran but with only one outrigger, when you tacked into the wind the outrigger would swing around the stern to end up on the down wind side of hull when the tack is completed.

seems a simple parallelogram linkage with a small sail on the out rigger could make it self tacking. it could be mounted so the outrigger stay aft of the transom so none of the swinging support beams cross over the hull.

To tow it you simply align the outrigger to the centerline.

 

[spidennis]

that sounds like one tricky and tippy operation. I'd love to see that happen, but not enough to make a working model of it.

 

[champ0815]

@Petros
Very ingenious!
But i question the general necessity of all the complications of a shunting proa design, if there is not much (if any) gain in it over a tacking outrigger.

 

[champ0815]

Well, to put it the other way round:
What's the penalty in removing one ama from a trimaran (without compromising the integrity of the rigg)?

 

[marshmat]

I would suggest that two of the main reasons for using a proa configuration are:
- Can optimize the windward and leeward hull shapes differently
- Relatively simple and light structure, so hulls can be slender (therefore efficient)

If it has to sail well on both tacks, the first point is lost. (On the flip side, you gain the option of fore/aft asymmetry- but you have that in cats and tris too.)

If it has to sail well on both tacks, your crossbeams and rig have to be able to perform equally well from either direction. That precludes many of the weight-saving techniques used by ama-to-windward shunters, which are only required to stay intact (instead of fully stiff and usable) if caught with the wind on the wrong side.

So, while I do appreciate the ingenuity and fun value of some tacking outrigger designs, I do think a cat or tri is a fundamentally better layout if tacking is required.

Shunting is not complicated or difficult with a well-designed rig. There are plenty of videos around of shunting proas doing their end-for-end thing in only a few seconds and a couple of boat lengths.

 

[Timothy]

I think the answer may be that the primary purpose of the wind ward ama on a traditional proa is for ballast rather than flotation It is more akin to the "canting keel" a misnomer since it is not a keel but a ballast bulb connected to the hull by a strut. A tacking proa thus has the ballast on the wrong side on one tack. To compensate they usually have racks on the aka for the crew to climb out on when on the wrong tack. Without movable ballast a tacking proa needs the hulls to be of near equal size as both will be required to support the entire displacement and more of both the boat and crew. With both hulls the same size the catamaran configuration becomes the better configuration. Its is interesting to note that I believe the evolution of the multihull in the pacific was tacking out rigger first. catamaran second and flying proa last. The modern trimaran has all its ballast to windward at all times and thus has by far the greatest righting moment of all multis but at a cost and weight penalty.

 

[champ0815]

Thanks, Timothy! Interesting and valid argumentation.
But nevertheless: a trimaran has to carry two amas which have a volume usually larger than the displacement of the whole boat - to avoid submersion when driven hard.
Cut one off, maybe leave a bit of structure to secure the rigg and you can make the remaining ama smaller due to the reduced weight of the boat without compromising the righting moment on the ama-to-leeward-tack.
Okay, maybe this is a bit more difficult - the windward ama of a tri has its part in the righting moment of the vessel. To compensate this, we leave the volume of the ama as it is in the tri - it is still a nice compact package when folded without the penalty of building and maintaining three hulls, constructing shunting solutions or compromising the tacking ability.

 

[cavalier mk2]

I like the idea of a proa set up to shunt and tack. For long tacks sailed Pacific but for short boards and maneuvering tacked to save the shunting time. The hulls can still be optimized for the long tack but in tight areas the need to frequently tack won't slow things down as much. The idea of the structure strong enough to tack or shunt seems a good thing to me for cruising.

 

[champ0815]

@marshmat
I follow your argumentation: with tacking you loose the static advantages of the proa design - loads only in one direction.
This is the case for all tacking multihulls, so the thought experiment with the cutting off of one ama of a tri is still valid - the beam structure is designed to take the loads on both tacks.
And even with the initial thought experiment with the identical proa/outrigger on their way to windward on a narrow strip of water: shunting means coming to a still stand, therefore time to brake, time to stand still and time to accelerate. In my opinion an unnecessary loss of momentum in comparison to tacking.

 

[warwick]

Does any one know how the arrangement of sling shot would have worked out as a tacking proa. A sliding a rack with a mini float at each end.

 

[cavalier mk2]

You guys are talking outriggers, not proas. Proas are double ended to travel in both directions.....

 

[Timothy]

Exactly. An out rigger may be a better candidate for a tacker as it is not symmetrical fore and aft and therefore the center of gravity can like a tri be further aft. However unlike a tri that can almost be square with little danger of pitch poling and because only two hulls at most are in the water at any time it can tack, I am dubious that an out rigger of sufficient beam to have an RM equal to a tri of the same length could tack at all.

 

[champ0815]

Exactly, that's the point!
Is there a need for a RM equal to a tri to sail fast and secure in a broad range of conditions or would an "amputated tri" suffice for that?
The ability to tack would be equal or better than the complete tri, the stability on the one tack the product of width and ama volume, on the other of width and ama weight.